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.    .    LIBRARY    .    . 

Connecticut 
Agricultural  College. 

VOL lA.X.RX 

CLASS    NO .4?.....v?   >-' 

COST      ^HN" 
DATE Zr^-i  ...  3  .  19/4, 


Digitized  by  the  Internet  Archive 
in  2009  with  funding  from  • 
Boston  Library  Consortium  IVIember  Libraries 


http://www.archive.org/details/geologyoflakesupOOvanh 


UNITED  STATES  GEOLOGICAL   SURVEY 

GEORGE  OTIS  SMITH,  Director 


THE  GEOLOGY  OF 
THE  LAKE  SUPERIOR  REGION 


BY 

t 

CHARLES  RICHARD  VAN  HISE 

AND 

CHARLES  KENNETH  LEITH 


5SC£/MLHij  DtPARTMENT 
RECEIVED 

JAN  2     1C6? 

Wilbui  Cross  Library 

Univeisity  ci  Connecticut 


WASHINGTON 

GOVERNMENT    PRINTING    OFFICE 
1911 


M  1  ^X. 


CONTENTS. 


Page. 

Chapter  I.  Introduction 29 

Outline  of  monogra]5h 29 

Acknowledgments •. 30 

Geography 30 

Topography 33 

Relief '. 33 

Drainage 33 

Chapter  II.  History  of  Lake  Superior  mining 35 

The  Keweenaw  copper  district  of  Michigan 35 

Copper  mining  on  Isle  Eoyal  and  elsewhere 37 

Marquette  iron  district 38 

Menominee  iron  district 39 

Crystal  Falls,  Florence,  and  Iron  River  iron  districts 39 

Gogebic  iron  district 40 

Vermilion  iron  district 40 

Mesabi  iron  district 41 

Accounts  of  the  district  before  its  opening 41 

Opening  and  development 42 

Cuyuna  iron  district 44 

Baraboo  iron  district ., 45 

Less  important  developments 45 

Clinton  iron  ores  of  Dodge  County,  Wis 45 

Paleozoic  iron  ores  in  western  Wisconsin .■ 45 

Iron  ores  of  the  north  .''hore  of  I^ake  Superior 45 

Silver  mining  on  the  north  shore  of  Lake  Superior 46 

Lake  Superior  gold  mining 46 

General  remarks 47 

Industrial  changes 47 

Smelting 47 

Influence  of  physiography  on  industrial  development 48 

Production  of  iron  ore 49 

Chapter  III.  History  of  geologic  work  in  the  Lake  Superior  region 70 

General  statement 70 

Work  of  individuals 70 

Growth  of  geologic  knowledge 72 

Bibliography 73 

Michigan 74 

Northern  Wisconsin 77 

Minnesota 78 

Ontario 81 

Lake  Superior  region  (general) 83 

Chapter  IV.  Physical  geography  of  the  Lake  Superior  region,  by  Lawrence  Martin 85 

Topographic  provinces 85 

The  Lake  Superior  highlands 85 

Topographic  development 85 

The  broad  uplands -  - . .  89 

Position,  relief,  and  sky  line 89 

Relation  of  original  and  present  topography 89 

Monadnocks 90 

Valleys  in  the  peneplain 90 

■     5 


6  CONTENTS. 

Chapter  IV.  Physical  geography  of  the  Lake  SuiJerior  region,  }>y  I^awrence  Martin — Continued.  Page. 
The  Lake  Superior  highlands — Continued. 
The  broad  uplands — Continued . 

Soil  and  glacial  topogra])h  y 91 

Description  of  dif trict.s  in  detail 91 

Gabbro  ])lateau 91 

St.  Louis  plain 92 

Vermilion  district 94 

Rainy  Lake  and  Lake  of  the  Woods  district 92 

Hunters  Island  and  Thunder  Bay  region 94 

Region  north  of  Lake  Superior 95 

Region  northeast  of  Lake  Superior 95 

Michipicoten  district 95 

Region  north  of  Sault  Ste.  Marie 95 

Marquette  district 96 

Menominee  district 96 

Crystal  Falls  district 96 

Keweenaw  Point 97 

Northern  ^Wisconsin 97 

Central  Wisconsin 98 

Northeastern  Wisconsin 98 

Linear  monadnocks  and  other  ridges 98 

General  description 98 

Keweenawan  monoclinal  ridges 99 

General  statement , 99 

Northeastern  Minnesota 99 

Isle  Royal  and  Michipicoten  Island 99 

Keweenaw  Point  and  northern  Michigan  and  Wisconsin 100 

Keweenawan  mesas 100 

Huronian  monoclinal  ridges  and  valleys 102 

Gunflint  Lake  district 102 

Penokee  Range 102 

Giants  Range 103 

Marquette  district 105 

Menominee  district 106 

Crystal  Falls  district 107 

North-central  Wisconsin 107 

Northwestern  Wisconsin 108 

The  lowland  plains 108 

Area 108 

Character  and  structiu-e 108 

Denudation 109 

The  belted  plain 109 

The  Minnesota  lowlands 110 

The  basin  of  Lake  Superior 110 

General  character  and  origin ' 110 

Description  of  escarpments 112 

Duluth  escarpment 112 

Keweenaw  escarpment 115 

Escarpment  of  northern  Wisconsin  (Superior  escarpment) 115 

Isle  Royal  escarpment 115 

Age  of  escarpments 116 

Bearing  of  escarpments  on  age  of  peneplain 116 

Chapter  V.  The  Vermilion  iron  district  of  Minnesota 118 

Location,  area,  and  general  geologic  succession 118 

Topography 119 

Archean  system 119 

Keewatin  series : 119 

Ely  greenstone 119 

Distribution 119 

Appearance  and  structiu-e 119 

Mineral  constituents 120 

Clastic  rocks 121 


CONTENTS.  7 

Chapter  V. — The  Vermilion  iron  district  of  Minnesota — Continued.  Page. 
Archean  system — Continued. 
Keewatin  series — Continued. 
Ely  greenstone — Continued. 

Acidic  flows 121 

Intrusive  rocks 121 

Extension  of  Ely  greenstone  beyond  district 122 

Soudan  format  ion 122 

Distribution 122 

Deformation 123 

Lithology 124 

Origin 126 

Relations  of  Ely  greenstone  and  Soudan  formation 126 

Laurentian  series 128 

Porphyry 128 

Granite  of  Basswood  Lake 128 

Algonkian  system 129 

Huronian  series 129 

Lower-middle  Huronian .• 129 

General  statement 129 

Ogishke  conglomerate 129 

Distribution 129 

Deformation 129 

Lithology 130 

Greenstone  conglomerate 130 

Granite  conglomerate 130 

Porphyry  conglomerate 131 

Chert  and  jasper  conglomerate 131 

Common  Ogishke  rock 131 

Metamorphism 131 

Relations  to  adjacent  formations 132 

Thickness 132 

Agawa  formation 132 

Knife  Lake  slate 132 

General  statement 132 

Lithology 133 

Microscopic  character '...  134 

Deformation 135 

Relation  to  adjacent  formations 135 

Thickness 135 

Intrusive  rocks 135 

Upper  Huronian  (Animikie  group)  and  Keweenawan  series 136 

Theironoresof  the  Vermilion  district,  Minnesota,  by  the  authors  and  W.  J.  Mead 137 

Distribution,  structure,  and  relations 137 

Chemical  composition 139 

Mineral  composition  of  the  ores  and  cherts 140 

Physical  characteristics  of  Vermilion  ores 140 

Texture 140 

Density 141 

Porosity 141 

Cubic  contents 141 

Secondary  concentration  of  Vermilion  ores 141 

Precedent  conditions 141 

Mineralogical  and  chemical  changes 142 

Sequence  of  secondary  alterations  and  development  of  textures 142 

Volume  change  in  Ely  ore 142 

Distribution  of  phosphorus , 143 

Chapter  VI.  The  pre-Animikie  iron  districts  of  Ontario 144 

Lake  of  the  Woods  and  Rainy  Lake  district 144 

Introductory  statement 144 

Archean  system 144 

Keewatin  series 144 

Laurentian  series 145 


8  CONTENTS. 

Chaptek  VI.  The  prc-Animikie  iron  districts  of  Ontario — Continued.  Page. 
Lake  of  the  Woods  and  Rainy  Lake  district.— Continued. 

Algonkian  sy.-ftoin 146 

Iluronian  series 14(5 

Steep  Rock  Lake  district 147 

General  geology 147 

Iron  ores 149 

Atikokan  district 149 

Kaministikwaa  and  Matawdn  district : 149 

Michipicoten  district 150 

Geography  and  topography 150 

Succession 150 

Archcan  system 151 

Keewatin  series 151 

Gros  Cap  greenstone 151 

Distribution 151 

Petrographic  character 151 

Wawa  tuff 151 

Distribution 151 

Petrographic  character 151 

Structure  and  thickness 1.52 

Helen  formation 152 

Distribution 152 

Structiu'e  and  thickness 153 

Petrographic  character 1,53 

Relations  to  other  formations 153 

Eleanor  slate 154 

Laurentian  series 154 

Algonkian  system 154 

Huronian  series 154 

*                            Lower-middle  Iliuonian 154 

Dore  conglomerate 154 

Distribution,  topography,  and  structure 154 

Petrographic  character 154 

Thickness 1.55 

Relations  to  underlying  rocks 155 

Michipicoten  extensions 155 

The  iron  ores  of  the  Michipicoten  district,  by  the  authors  and  W.  J.  Mead 156 

General  statement 156 

Chemical  composition 156 

Mineral  composition 156 

Physical  characteristics 157 

Color  and  texture 157 

Density 157 

Porosity 157 

Cubic  feet  per  ton 157 

Secondary  concentration  of  the  Michipicoten  ores 157 

Chapter  VII.  The  Mesabi  iron  district  of  Minnesota 159 

General  description 159 

Archean  system  or  "Basement  Complex" 160 

Distribution 160 

Kinds  of  rocks 160 

Structure 161 

Algonkian  system. . .'. 161 

Iluronian  series 161 

Lower-middle  Huronian 161 

Distribution ^ 161 

Graywackes  and  slates 161 

Conglomerates 162 

Giants  Range  gi'anite 162 

Relations  of  Giants  Range  granite  to  the  lower-middle  Htu-onian  sediments  and  of  both  to  other 

rocks 162 

Structure  and  thickness 163 

Conditions  of  deposition : 163 


CONTENTS.  9 

Chapter  VII.  The  Mesabi  inm  district  of  Minnesota — Continued. 
Algonkian  system — Continued . 
Huronian  series — Continued. 

Upper  Hm-onian  ( Aniniikie  group) Igg 

General  character  and  extent 163 

Pokegaraa  quartzite 1(;4 

Biwabik  formation Ig4 

Distribution Ig4 

Kinds  of  rocks Ig5 

Greenali'te  rocks 165 

Ferruginous,  amphibolitic,  sideritic,  and  calcareous  cherts 168 

Siliceous,  ferruginous,  and  amphibolitic  slates 170 

Paint  rock 171 

Sideritic  and  calcareous  rocks 171 

Conglomerates  and  quartzites 171 

Thickness ; 171 

Alteration  by  the  intrusion  of  Keweenawan  granite  and  gabbro 171 

Virginia  slate 172 

Distribution 172 

Slate 173 

Cordierite  homstone  resulting  from  the  alteration  of  the  Virginia  slate  by  the  Duluth  gabbro.  173 

Relations  to  the  Biwabik  formation 174 

Structure 174 

Thickness I74 

Structm-e  of  the  upper  Huronian  ( Animikie  group) 175 

Relations  of  the  upi)er  Huronian  (Animikie  group)  to  other  series 176 

Conditions  of  deposition  of  the  upper  Huronian  (Animikie  group) 176 

Keweenawan  series 177 

Duluth  gabbro I77 

Diabase 177 

Embarrass  gianite 178 

Cretaceous  rocks 178 

Distribution  and  character 178 

Fossils 179 

Pleistocene  glacial  deposits I79 

The  iron  ores  of  the  Mesabi  district,  by  tlie  authors  and  W.  J.  Mead 179 

Distribution,  structure  and  relations I79 

Chemical  composition  of  ferruginous  cherts  and  ores 180 

Analyses ISO 

Representation  by  means  of  triangular  diagram 182 

Mineralogical  composition  of  ferruginous  cherts  and  ores 183 

Physical  characteristics  of  the  ores 183 

Texture 183 

Density 184 

Porosity 184 

Cubic  contents 185 

Magnetic  phases  of  the  iron-bearing  formation 185 

Occurrence 185 

Chemical  composition 185 

Secondary  concentration  of  Mesabi  ores 186 

Structural  conditions 186 

Original  character  of  tlie  iron-bearing  formation 186 

Alteration  of  sideritic  or  greenalitic  chert  to  ferruginous  chert  (taconite ) 187 

Chemical  change 187 

Mineral  change 187 

Volume  change 187 

Development  of  porosity 187 

Alteration  of  ferruginous  cherts  (taconite)  to  ore 188 

Volume  changes 188 

Method  of  expressing  volume  changes  by  triangular  diagram 189 

Data  used  in  triangle 189 

Consideration  of  the  triangular  diagram 190 

Alterations  of  associated  rocks  contemporaneous  with  secondary  alteration  of  the  iron-bearing  for- 
mation   191 


10  CONTENTS. 

CuAiTKR  \'II.  The  Mcsalii  iron  dit-trift  of  Minnesota — Continued.  Page. 
The  iron  ores  of  I  lie  Mesabi  district,  liy  the  authors  and  W.  J.  Mead— Continued. 

Pliosplionis  in  Me.s;il)i  ores 192 

Dislrilnition  in  the  iron-bearing  formation '. 192 

Secondary  concentration  of  pliosphorus 194 

Explanation  of  phosphorus  in  the  paint  rock •. .  19.5 

Phosphorus  in  the  amphibole-inagnetite  phases  of  I  lie  iron-bearing  formation 195 

Minerals  containing  phosphorus 195 

Detrital  ores  in  the  Cretaceous  rocks 190 

Sef[uence  of  ore  concentration  in  the  Mesain  district 197 

Chapter  VI 11.  The  G\inflint  Lake,  Pigeon  Point,  and  Animikie  iron  districts  of  Minnesota  and  Ontario 198 

Gunllint  Lake  district 198 

Geography 198 

Succession  of  rocks 198 

Algonkian  system 198 

Huronian  series 198 

Upper  Huronian  (Animikie  group) 198 

General  description 198 

Gunflint  formation 199 

Distribution ' 199 

Structure 199 

Potrographic  character 200 

Contact  metamorphism 200 

Thickness 200 

Eove  slate 200 

Distribution 200 

Structure 201 

Petrographic  character 201 

Contact  metamorphism 201 

Thickness 201 

Keweenawan  series 201 

Duluth  gabbro. 201 

Logan  sills 202 

Relations  of  the  Keweenawan  rocks  to  one  another  and  to  adjacent  formations 202 

Geologic  relations 202 

Topography  as  related  to  geology 20,3 

The  iron  ores  of  the  Gunflint  Lake  district 203 

Chemical  composition 204 

Physical  characteristics 204 

Pigeon  Point  district 204 

Animikie  or  Loon  Lake  district  of  Ontario 205 

Location  and  general  succession 205 

Archean  system 205 

Algonkian  system 205 

Huronian  series 205 

Lower-middle  Huronian 205 

Kinds  of  rocks 205 

Intrusives 206 

Upper  Huronian  (Animikie  group  i 206 

General  description 206 

Iron-bearing  formation 206 

Conglomerate 206 

Lower  iron-bearing  member 207 

Interbedded  slate 207 

Upper  iron-bearing  member 207 

Upper  black  slate 207 

Keweenawan  series 207 

General  description 207 

Logan  sills 208 

Structural  features 208 

General  topographic  features  in  their  relations  to  geology 208 

Westward  extension  of  the  Animikie  district 209 


CONTENTS.  11 

Chapter  VIII.  The  Gunflint  Lake,  Pigeon  Point,  and  Animikie  iron  districts  of  Minnesota  and  Ontario — Con.  Page. 
Animikie  or  Loon  Lake  district  of  Ontario — Continued. 

The  iron  ores  of  tlie  Animikie  district  of  Ontarii i 209 

Occurrence 209 

Character  of  the  ore 210 

Secondary  concentration  of  tlie  Animikie  ores 210 

Structural  conditions 210 

Original  character  of  the  iron-bearing  formation 210 

Nature  of  alterations 210 

Sequence  of  ore  concentration 210 

Chapter  IX.  The  Cuyuna  iron  district  of  Minnesota  and  its  extensions  to  Carlton  and  Cloquet,  and  the  Minne- 
sota River  valley  of  southwestern  Minnesota 211 

Cuyuna  iron  district  and  extensions  to  Carlton  and  Cloquet 211 

Geography  and  topography 211 

Succession  of  rocks 211 

Algonkian  system 212 

Huronian  series 212 

Upper  Huronian  (Animikie  group) 212 

■'                                    General  statement 212 

'                                  Distribution  and  structure 212 

Lithology  and  metamorphism 213 

Correlation 213 

Keweenawan  series  (?) 215 

Cretaceous  rocks 215 

Quaternary  system 216 

Pleistocene  glacial  deposits 216 

The  iron  ores  of  the  Cuyuna  district,  by  the  authors  and  Carl  Zapffe 216 

Distribution,  structure,  and  relations 216 

Character  of  the  ores ? 219 

General  appearance 2*19 

Chemical  composition 220 

Mineralogical  composition 221 

Texture 223 

Secondary  concentration  of  Cuyuna  ores 223 

Structural  conditions 223 

Original  character  of  the  Deerwood  iron-bearing  member 223 

Mineralogical  and  chemical  changes 223 

Phosphorus  in  Cuyuna  ores 224 

Minnesota  River  valley  of  southwestern  Minnesota 224 

Chapter  X.  The  Penokee-Gogebic  iron  district  of  Michigan  and  Wisconsin 225 

Location,  succession  of  rocks,  and  topography 225 

Archean  system 226 

General  statement 226 

Keewatin  series 226 

Laurentian  series 226 

Relations  of  Keewatin  and  Laurentian  series • 227 

Algonkian  system 227 

Huronian  series 227 

Lower  Huronian 227 

Sunday  quartzite 227 

Lithology  and  distribution 227 

Relations  to  adjacent  formations 228 

Bad  River  limestone 228 

Distribution 228 

Lithology 228 

Metamorphism 228 

Relations  to  adjacent  formations 228 

Upper  Huronian  (Animikie  group) 229 

General  statement 229 

Palms  formation 229 

Distribution 229 

Lithology 229 

Relations  to  adjacent  formations 230 


12  CONTENTS. 

Chapter  X.  The  Penokee-Gogebic  iron  district  of  Michigan  and  Wisconein — Continued.  Page. 
Algonkian  system — Continued. 
Huronian  scries — Continued. 

Upper  Ilitfonian  (Aniraikie  group) — Continued. 

Ironwood  formation : 230 

Distribution. 230 

Lithology 231 

Relations  to  adjacent  formations 232 

Tyler  slate 232 

Distribution 232 

Lithology 232 

Metamorphism 232 

Relations  to  adjacwil  formations 233 

Upper  Huronian  (Animikie  group)  of  the  eastern  area 233 

Keweenawan  series 234 

General  description 234 

Relations  to  adjacent  series 234 

Cambrian  sand.^tone 235 

The  iron  ores  of  the  Penokee-Gogebic  district,  by  the  authors  and  \V.  J.  Mead 235 

Distribution,  structure,  and  relations 235 

Chemical  composition  of  the  ferruginous  cherts  and  ores 238 

Mineralogical  composition  of  the  ferruginous  cherts  and  ores 240 

Physical  characteristics 240 

General  appearance -40 

Density 240 

Porosity 241 

Cubic  contents 241 

Texture 241 

Magnetitic  ores 24 1 

Secondary  concentration  of  Gogebic  ores 242 

Structural  conditions 242 

Original  character  of  the  iron-bearing  formation 243 

Alteration  of  cherty  iron  carbonate  to  ferruginous  chert 243 

Chemical  change 243 

Mineral  change 243 

Volume  change 243 

Development  of  porosity 243 

Alteration  of  ferruginous  chert  to  ore 244 

Triangular  diagram  illustrating  secondary  concentration  of  Gogebic  ores : 246 

Alteration  of  rocks  associated  with  ores  during  their  secondary  concentration 240 

Occurrence  of  phosphorus  in  the  iron-bearing  formation 247 

Phosphorus  content 247 

Minerals  containing  phosphorus 248 

Behavior  of  phosphorus  during  secondary  concentration 249 

Sequence  of  ore  concentration  in  the  Gogebic  district 2.50 

Chapteh  XI.  The  Marquette  iron  district  of  Michigan,  including  the  Swanzy,  Dead  River,  and  Perch  Lake 

areas 251 

Marquette  district 251 

Introduction -• 251 

Location,  succession,  and  general  structure 251 

Archean  system 253 

Northern  area 254 

Keewatin  series 254 

Laurentian  series 255 

Southern  area 255 

Isolated  areas  of  Archean  rocks 256 

Algonkian  system 256 

Huronian  series 2.5G 

Lower  Huronian 256 

Mesnard  quartzite 2.56 

Name  and  distribution 256 

Lithology 256 

Metamorphism 257 


CONTENTS.  •  13 

Chapter  XI.  The  Marquette  iron  district  of  Michigan,  etc.— Continued.  Page. 

Marquette  district— Continued. 
Algonliian  system— Continued. 
Huronian  series — Continued. 

Lower  Huronian — Continued. 

Mesnard  quartzite — Continued. 

Relations  to  adjacent  formations 257 

Thickness ^^^ 

Kona  dolomite 

Name  and  distribution ~^^ 

Lithology ~^^ 

Metamorphism ""_ 

Relations  to  adjacent  formations '-^^ 

Thickness -^* 

We  we  slate -^^ 

Distribution 258 

Lithology 2^^ 

Metamorphism '''^^ 

Relations  to  adjacent  formations 259 

Thickness 259 

Middle  Huronian 2o9 

Ajibik  quartzite ■ 

Name  and  distribution "^^ 

Deformation " 

Lithology'. 260 

Metamorphism """ 

Relations  to  adjacent  formations -  ■  ■  •  260 

Thickness 261 

Siamo  slate "" 

Name  and  distribution 2G1 

Deformation 261 

Lithology 261 

Metamorphism 261 

Relations  to  adjacent  formations 262 

Thickness 262 

Negaunee  formation -"2 

Name  and  distribution 262 

Deformation -^'- 

Lithology,  including  metamorphism 263 

Relations  to  adjacent  formations 264 

Thickness 264 

Intrusive  and  eruptive  rocks 264 

Upper  Huronian  ( Animikie  group) 265 

Goodrich  quartzite -"^ 

Distribution  and  structure 265 

Lithology,  including  metamorphism 265 

Relations  to  adjacent  formations 265 

Thickness 265 

Bijiki  schist 266 

Name  and  distribution 266 

Lithology,  including  metamorphism 266 

Relations  to  adjacent  rocks 266 

Thickness 267 

Michiganime  slate " ' 

Name,  distribution,  and  correlation 267 

Deformation -"' 

'  Lithology 267 

Metamorphism -"'  . 

Relations  to  adjacent  formations -"''' 

Thickness 268 

Clarksburg  formation -"^ 

Distribution 268 

Lithology 268 


14  •  CONTENTS. 

Chapter  XI.  The  Marquetto  iron  flistrii-l  of  Michigan,  etc. — Continued.  Page. 
Marquette  district — Continued. 
Algonkian  system^Continued. 
Huronian  series — Continued. 

Upper  Huronian  (Animikie  group) — Continued. 
C'lark.slmrg  formation — Continued. 

Relations  to  adjacent  formations 268 

Thickness 268 

Intru.'ive  igneous  rocks 268 

Cambrian  .•<and.-^tono 269 

Quaternary  deposits 269 

The  iron  ores  of  the  Marqtieltc  district,  by  the  authors  and  \V.  J.  Mead 270 

Distribution,  structure,  and  relations  of  ore  deposits 270 

Chemical  rompo.'rition  of  Marquette  ores 273 

Chemical  composition  of  iron-bearing  Negaunee  formation 273 

Mineral  compo.^iition  of  Marquette  ores 274 

Physical  charac'teristics  of  Marquette  ores 274 

Secondary  concentration  of  Marquette  ores 275 

Structural  conditions 275 

Chemical  and  mineralogical  changes  in  secondary  concentration  of  Marquette  ores 276 

Volume  changes  in  secondary  concentration  of  Marquette  ores 276 

Representiition  of  ores  and  jaspers  on  triangular  diagram 278 

Sequence  of  ore  concentration  in  the  Marquette  district 278 

Occurrence  of  phosphorus  in  the  Marquette  ores 279 

Distribution  of  phosphorus 279 

Mineralogical  occurrence  of  phosphorus 281 

Phosphorus  in  relation  txj  secondary  concentration 281 

Swanzy  district 283 

Geography  and  topography 283 

General  succession  and  structure 283 

Archean  system 283 

Algonkian  system 285 

Huronian  series 285 

Upper  Huronian  ( .\nimikie  group') 285 

Goodrich  quartzite 285 

Michigamme  slate 285 

Paleozoic  sediments 285 

Quaternary  deposits 285 

Correlation 286 

The  iron  ores  of  the  Swanzy  district,  by  the  authors  and  W.  J.  Mead 286 

General  description ; 286 

Secondary  concentration  of  Swanzy  ores 286 

Dead  River  area , 287 

General  succession 287 

Archean  system 287 

Keewatin  series 287 

Laurentian  series 287 

Algonkian  system 287 

Huronian  series 287 

Middle  Huronian 287 

Upper  Huronian  (Animikie  group) 288 

Perch  Lake  district  (including  western  Marquette) 288 

Geography  and  topography 2§8 

General  succession 288 

Archean  system 288 

Laurentian  series 288 

.Vlgonkian  system 289 

Hmcmian  series 289 

Middle  Huronian 289 

Upper  Huronian  (.Vnimikie  group) 289 

Quaternary  deposits 290 


CONTENTS.  15 

Chapter  XII.  The  Crystal  Falls,  Sturgeon,  Felch  Mountain,  Calumet,  and   Iron  River  iron  di.strict.'i  of  Michi-  Page. 

gan  and  the  Florence  iron  district  of  Wisconsin 291 

Crystal  Falls  iron  district 291 

Location  and  area 2!J  1 

General  succession  and  structure .- 291 

Archean  system 293 

Laurentian  series 293 

Algonkian  system 293 

Huronian  series , 293 

Lower  Huronian 293 

Sturgeon  quartzite 293 

Randvillo  dolomite 293 

Middle  Huronian  (?) 294 

Hemlock  formation 294 

Distribution  and  general  character .• 294 

Area  south  and  west  of  the  westernmost  Archean  oval 294 

Fence  River  area 29.5 

Other  areas  of  the  Hemlock  formation 29.5 

Iron-bearing  slate  member  ("Man.sfield  slate")  of  the  Hemlock  formation 29.5 

Negaunee  (?)  formation 296 

Magnetic  belts  northeast  of  Fence  River 296 

Negaunee  (?)  formation  at  Michigararae  Mountain  and  in  the  Fence  River  area 296 

Ferruginous  quartzite  associated  with  iron-bearing  formation  north  of  Michigamme 

Mountain 298 

Upper  Huronian  (Animikie  group) 298 

•Michigamme  slate 298 

General  character 298 

Vulcan  ii-on-bearing  member '. 298 

Intrusive  and  extrusive  rocks  in  upper  Huronian 299 

Relations  of  the  upper  Huronian  to  underlying  rocks 300 

Cambrian  sandstone 300 

Sturgeon  River  district 300 

Location  and  area 300 

General  succession 300 

Archean  system 301 

Laurentian  series 301 

Algonkian  system 301 

Huronian  series 301 

Lower  Huronian 301 

Sturgeon  quartzite 301 

Randville  dolomite 301 

Middle  Huronian  (?) 301 

Negaunee  ( ?)  formation 301 

Igneous  rocks 301 

Keweenawan  series  (?) >  301 

Felch  Mountain  district 302 

Location,  structure,  and  general  succession 302 

Archean  system 302 

Laurentian  series 302 

Algonkian  system 302 

Huronian  series ; 302 

Lower  Huronian 302 

Sturgeon  quartzite _. 302 

Randville  dolomite .' 302 

Upper  Huronian  (Animikie  group) 303 

Felch  schist 303 

Vulcan  formation 303 

Keweenawan  series  (?) 304 

Intrusive  rocks 304 

Paleozoic  sandstone  and  limestone 304 

Correlation 304 

Laurentian  series 304 

Lower  Huronian 30.5 


16  CONTENTS. 

CiiAi'iKH  XII.  The  Crystal  Falls,  Sturgeon,  Felch  Mountain,  Caluiiiel,  and  Iron  River  districts,  etc. — Conta.  Page. 
Fclcli  Mdunlain  di.-Jtrict — Conlinur-d. 
Correlation — Continued. 

Upper  Huronian  (Animikie  group) 305 

Keweenavvaii  series  (?) .• 30.5 

Calumet  district 306 

Location  and  general  succession 30C 

Arehean  system 306 

Laurentian  series 306 

Algonkian  system 306 

Huronian  series 306 

Lower  Huronian 306 

Sturgeon  ([uartzite 306 

Randville  dolomite 306 

Upper  Huronian  (Animikie  group) 307 

Felch  schL^^t 307 

Vulcan  formation 307 

Michigamme  slate 307 

Paleozoic  limestone  and  sandstone 307 

Correlation 307 

Iron  River  distrirt,  by  R.  C.  Allen 308 

Location  and  extent 308 

Topography  and  drainage 308 

( 'haracter  of  the  glacial  drift 309 

General  succession 309 

Arehean  (?)  system 309 

Keewatin  series  (?) 309 

Algonkian  system 310 

Huronian  series 310 

Lower  Huronian 310 

Saunders  formation 310 

Distribution 310 

Lithologic  characters 310 

Structure 311 

Thickness 311 

Relations  to  adjacent  formations 311 

Upper  Huronian  (Animikie  group) 311 

Michigamme  slate 311 

Distribution  and  general  characters 311 

General  structure 312 

Vulcan  iron-bearing  member 313 

Distribution  and  exposures ■  ■  ■  313 

Relations  to  Michigamme  slate 313 

Thickness  and  structure 314 

Lithologic  characters 314 

Distribution  and  local  structure 315 

Local  magnetism  in  the  Vulcan  iron-bearing  member 317 

Intrusive  and  extrusive  rocks  in  the  upper  Huronian  (Animikie  group) 318 

Relations  of  upper  Huronian  (^nimikie  group)  to  luiderlying  rocks 318 

Ordovician  rocks 319 

Florence  (Commonwealth)  iron  district  of  Wisconsin 320 

Location  and  general  succession 320 

Algonkian  system 321 

Hiu'onian  series 321 

Upper  Huronian  (Animikie  group) 321 

Michigamme  slate 321 

General  character  and  distribution 321 

Vulcan  iron-bearing  member , 321 

Intrusive  and  extrusive  greenstones  and  green  schists 322 

Quinnesec  schist 322 

Intrusive  and  extrusive  greenstones  and  green  schists  other  than  Quinnesec 322 

Granite  and  gneiss  intrusives 323 

Paleozoic  sandstone 323 

Quaternary  deposits 323 


CONTENTS.         .  17 

Chapter  XII.  The  Crystal  Falls,  Sturgeon,  Felch  Mountain,  Calumet,  and  Iron  River  districts,  etc. — Contd.  Page. 

The  iron  ores  of  the  Crystal  Falls,  Iron  River,  and  Florence  districts,  by  the  authors  and  \V.  J.  Mead 323 

Distribution,  structiu-e,  and  relations 323 

Chemical  composition : 324 

Mineral  composition • ■ 325 

Physical  characteristics .• 325 

Secondary  concentration 320 

Structural  conditions 326 

Chemical  and  mineralogical  changes 326 

Time  of  concentration 326 

The  iron  ores  of  the  Felch  Mountain  and  Calumet  districts,  by  the  authors  and  W.J.  Mead 326 

Felch  Mountain  district 327 

Cahmiet  district 327 

Secondary  concentration  of  the  Felch  Mountain  and  Calumet  ores 328 

Structural  conditions 328 

Chemical  and  mineralogical  changes 328 

Chapter  XIII.  The  Menominee  u'on  district  of  Michigan 329 

Location  and  extent 329 

Topography 329 

Succession  of  formations 329 

Archean  system 330 

Laurentian  series  and  unseparated  Keewatin 330 

Algonkian  system 331 

General  character  and  limits 331 

Huronian  series 332 

Lower  Huronian 332 

Succession  and  distribution 332 

Sturgeon  quartzite 332 

Distribution 332 

Lithology 332 

Deformation 332 

•                             Relations  to  adjacent  formations 332 

Thickness 333 

Randville  dolomite 333 

Distribution 333 

Lithology 333 

Deformation 334 

Relations  to  adjacent  formations 334 

Thickness • 334 

Middle  Huronian 334 

Upper  Huronian  (Animikie  group) 335 

Vulcan  formation , 335 

Subdivision  into  members 335 

Distribution '. 336 

Traders  iron-bearing  member 337 

Brier  slate  member 337 

Curry  iron-bearing  member 337 

Deformation 338 

Relations  between  the  members  of  the  Vulcan  formation  and  the  ilichigamme  slate 338 

Thickness 339 

Michigamme  ("Hanbury  ")  slate 340 

Distribution 340 

Name 340 

Lithology 340 

Defor^iation 341 

.   Thickness 342 

Relations  of  Upper  Huronian  to  underlying  rocks ._ 342 

Relations  between  Vulcan  formation  and  the  lower  Huronian 342 

Relations  between  Michigamme  ("Hanbury")  slate  and  the  middle  or  lower  Huronian...  343 

Igneous  rocks  in  the  Algonkian 344 

Quinnesec  schist 344 

Green  schists  at  Fourfoot  Falls 345 

47517°— VOL  52—11 2 


18  .       CONTENTS. 

Chapter  XIII.  The  Menominee  iron  district  of  Michigan — Continued.  Page. 

Paleozoic  rocks 345 

Cambrian  system 346 

Lake  Superior  sandstone 346 

Lithology 346 

Relations  to  adjacent  formations 346 

Cambro-Ordovician 346 

Hermansville  limestone 346 

The  iron  ores  of  the  Menominee  district,  by  the  authors  and  W.  J.  Mead 346 

Distribution,  structure,  and  relations 346 

Chemical  composition  of  the  ores 350 

Average  iron  content  of  the  iron-bearing  formation 351 

Mineral  composition  of  the  ores 351 

Physical  characteristics  of  the  ores 352 

Iron  ore  at  base  of  Cambrian  sandstone 353 

Secondary  concentration  of  the  Menominee  ores 353 

Structural  conditions ■ 353 

Mineralogical  and  chemical  changes 354 

Sequence  of  ore  concentration  in  the  Menominee  district 354 

Chapter  XIV.  North-central  Wisconsin  and  outlying  pre-Cambrian  areas  of  central  Wisconsin •    355 

Northern  Wisconsin  in  general 355 

Wausau  district 355 

Location,  area,  and  general  geologic  succession 355 

Archean  (?)  system 356 

Algonkian  system 356 

Huronian  series 356 

Middle  Huronian  (?) 356 

Rocks  intrusive  in  middle  Huronian  (?)  and  Archean  (?) 357 

Upper  Huronian  (?) 357 

Cambrian  system 357 

Barron,  Rusk,  and  Sawyer  counties 357 

Vicinity  of  Lakewood 358 

Necedah,  North  Bluff,  and  Black  River  areas 358 

Baraboo  iron  district 359 

Location  and  general  geologic  succession 359 

Archean  system 360 

Laurentian  series 360 

Algonkian  system 361 

Huronian  series • 361 

Middle  Huronian  (?) 361 

Baraboo  quartzite 361 

Seeley  slate 361 

Freedom  dolomite 361 

Upper  Huronian  (?) 361 

Paleozoic  sediments 361 

Quaternary  deposits 362 

The  iron  ores  of  the  Baraboo  district,  by  the  authors  and  W.  J.  Mead 362 

Occurrence 362 

Chemical  composition 362 

Mineralogical  character 363 

Physical  character 363 

Secondary  concentration 363 

Structural  conditions 363 

Original  character  of  the  iron-bearing  member 363 

Mineralogical  and  chemical  changes ^ 364 

Waterloo  quartzite  area 364 

Fox  River  valley 365 

Chapter  XV.  The  Keweenawan  series 366 

General  characteristics 366 

Distribution 366 

Succession 366 


CONTENTS.  19 

Chapter  XV.  The  Keweenawan  series — Continued.  Page. 

Black  and  Nipigou  bays  and  Lake  Nipigon 3G7 

Lower  Keweenawan 367 

Middle  Keweenawan 368 

Black  and  Nipigon  bays  and  adjacent  islands 368 

Lake  Nipigon 368 

Relations  of  the  Keweenawan  of  Black  and  Nipigon  bays  to  other  rocks 369 

Northern  Minnesota 370 

The  Keweenawan  area 370 

Lower  Keweenawan 370 

Middle  Keweenawan 371 

Effusive  rocks 371 

Intrusive  rocks 372 

A.            Basic  rocks 372 

■^                    Duluth  laccolith 372 

Area  and  character 372 

Relations  to  other  formations •. 372 

The  Beaver  Bay  and  other  laccoliths  and  sills 373 

Anorthosites 374 

'                        Basic  dikes 374 

Acidic  rocks ' 374 

Keweenawan  rocks  in  the  Cuyuna  district  of  north-central  Minnesota 375 

Thickness  of  the  Keweenawan  of  Minnesota 375 

Northern  Wisconsin  and  extension  into  Minnesota 376 

Distribution 376 

Structiire 376 

Lower  Keweenawan 376 

Middle  Keweenawan 377 

Upper  Keweenawan 378 

Relations  of  the  Keweenawan  to  other  series 378 

Keweenawan  granites  of  Florence  County,  northeastern  Wisconsin 379 

Northern  Michigan 380 

Distribution 380 

Keweenaw  Point 380 

Succession  and  correlation 380 

Lower  and  middle  Keweenawan  of  Keweenaw  Point 381 

Order  of  extrusion 381 

Presence  of  basic  intrusive  rocks 381 

Acidic  intrusive  rocks 382 

Nature  and  source  of  detrital  material 382 

Variations  in  thickness  of  sedimentary  beds 382 

Faults 383 

Upper  Keweenawan 383 

Relations  to  Cambrian  rocks 384 

Main  area  west  of  Keweenaw  Point,  including  Black  River  and  the  Porcupine  Mountains 384 

The  South  Range 385 

Rocks  of  possible  Keweenawan  age  in  outlying  areas 386 

Thickness  of  the  Keweenawan  of  Michigan 386 

Eagle  River  section 386 

Portage  Lake  section 387 

Black  River  section 388 

Relations  of  the  Keweenawan  of  Michigan  to  underlying  and  overlying  formations 388 

Isle  Royal 389 

Michipicoten  Island 390 

East  coast  of  Lake  Superior 391 

General  consideration  of  the  Keweenawan  series 393 

Lower  Keweenawan ■■ 393 

Middle  Keweenawan 394 

Igneous  rocks 394 

Varieties 394 

Review  of  nomenclature  of  Keweenawan  igneous  rocks,  by  A.  N.  Winchell 395 

The  grain  of  Keweenawan  igneous  rocks — the  practical  use  of  observations 407 

The  extrusive  masses 408 


20  CONTENTS. 

Chapter  XV.  The  Keweenawan  series — Continued.  Page. 

General  consideration  of  the  Keweenawan  series — Continued. 
Middle  Keweenawan — Continued. 
Igneous  rocks — Continued. 

The  intrusive  masses 410 

Source  of  lavas '"1 

Sedimentary  rocks 412 

Source  and  nature  of  material 412 

Extent  of  sediments 413 

Upper  Keweenawan 413 

Relations  to  underlying  series 414 

Relations  to  overljdng  series 415 

Conditions  of  deposition 416 

Thickness  of  the  Keweenawan  rocks 418 

Areas  of  Keweenawan  rocks 419 

Volume  of  Keweenawan  rocks 419 

Length  of  Keweenawan  time 420 

Jointing  and  faulting 420 

The  Lake  Superior  synclinal  basin 421 

Metamorphism '■  423 

R6sum6  of  Keweenawan  history 424 

Chapter  XVL   The  Pleistocene,  by  Lawrence  Martin 427 

The  glacial  epoch 427 

Plan  of  presentation 427 

Ice  advances 427 

Driftless  Area :■  = 429 

Retreating  ice -  - -. ^"^ 

Contrasted  general  effects  of  glaciation - 430 

Destructive  work  of  the  glaciers trj 430 

Removal  of  weathered  rock ^,..- 430 

Striae  and  roches  moutonn^es -  - 431 

Broadened  and  deepened  valleys 431 

Glacial  rock  basins 431 

Transporting  work  of  glaciers 432 

Constructive  work  of  glaciers 433 

Ground  moraine 433 

Drumlins 433 

Eskers *134 

Terminal  moraines 434 

Kames 435 

Recessional  and  interlobate  moraines 435 

Drainage  of  drift-covered  areas 435 

Differences  between  younger  and  older  drift 435 

Effect  of  nunatak  stages  on  distribution  of  drift 436 

Variation  of  deposits  with  slopes 436 

Outwash  deposits 437 

Pitted  plains ■ 438 

Loess - 438 

Valley  lakes  due  to  variation  in  stream  load 438 

Distribution  of  glacial  drift 439 

Marginal  lakes 441 

Glacial  Lake  Agassiz 442 

Marginal  glacial  lakes 442 

Lake  Nemadji 443 

Lake  Duluth 444 

Intermediate  glacial  lakes 445 

Lake  Algonquin 446 

Nipi.ssiiig  Great  Lakes 447 

Effect  of  tilting  on  glacial  lakes 448 

Present  iiosition  of  raised  beaches 449 

Glacial-lake  deposits 452 

The  four  Pleistocene  provinces 453 

Grounds  for  distinction 453 


CONTENTS.  21 

Chapter  XVI.  The  Pleistocene,  by  Lawrence  Martin — Continued.  Page. 
The  four  Pleistocene  provinces — Continued. 

Drif tless  Area 454 

Area  of  older  drift 454 

Area  of  last  drift 454 

Areas  of  glacial-lake  deposits 454 

Postglacial  modifications 455 

Modifications  on  the  land 455 

Modifications  in  and  around  the  Great  Lakes 456 

Summary  of  the  Pleistocene  history 459 

Chapter  XVI I .  The  iron  ores  of  the  Lake  Superior  region,  by  the  authors  and  W.J.  Mead 460 

Horizons  of  iron-bearing  formations 460 

General  description  of  ores  of  the  Lake  Superior  pre-Cambrian  sedimentary  iron-bearing  formations 461 

Introduction 461 

Kinds  of  rocks  in  the  iron-bearing  formations 461 

Chemical  composition  of  the  iron-bearing  formations 462 

Ratio  of  ore  to  rock  in  the  iron-bearing  formations 462 

Structural  features  of  ore  bodies 462 

Shape  and  size  of  the  ore  bodies 475 

Topographic  relations  of  the  ore  bodies 476 

Outcrops  of  the  ore  bodies 476 

Chemical  compo.sition  of  the  ores 477 

Mineralogy  of  the  ores 479 

Physical  characteristics  of  the  ore 480 

General  character 480 

Cubic  contents  of  ore 481 

Range  and  determination 481 

Use  of  the  diagram 482 

Construction  of  the  diagram 482 

Effect  of  porosity 482 

Effect  of  moisture 483 

Moisture  of  saturation 433 

Excess  of  moisture  handled  in  mining 434 

Exploration  for  iron  ore 434 

Magnetism  of  the  Lake  Superior  iron  ores  and  iron-bearing  formations 486 

Manganiferous  iron  ores 433 

Iron-ore  reserves 433 

Data  available  for  estimates 433 

Availability  of  ores 433 

Reserves  of  ore  at  present  available 439 

Estimates 43g 

Life  of  ore  reserves  at  present  available 499 

Reserves  available  for  the  future 49]^ 

Estimates 492 

Comparison  of  Lake  Superior  reserves  with  other  reserves  of  the  United  States 492 

Lowering  of  grade  now  discernible 493 

Effect  of  increased  use  of  low-grade  ores 494 

Comparison  with  principal  foreign  ores 495 

Tra,usportation 4g5 

Mine  to  boat 495 

Docks 496 

Boats 497 

Dock  to  furnace 497 

Total  cost  of  transportation 497 

Methods  of  mining 497 

Rates  of  royalty  and  value  of  ore  in  the  ground 499 

Origin  of  the  ores  of  the  Lake  Superior  pre-Cambrian  sedimentary  iron-bearing  formations 499 

Outline  of  discussion 499 

The  iron  ores  are  chiefly  altered  parts  of  sedimentary  rocks -.  500 

Conditions  of  sedimentation 5OO 

Iron-bearing  formations  mainly  chemical  sediments 5OO 

Order  of  deposition  of  the  iron-bearing  sediments 5OX 

Are  the  iron-bearing  formations  terrestrial  or  subaqueous  sediments? 5OI 


22  CONTENTS. 

Chapter  XVII.  The  iron  ores  of  the  Lake  Superior  region,  by  the  authors  and  W.  J.  Mead — Continued.  page. 
Origin  of  the  ores  of  the  Lake  Superior  pre-Cambrian  sedimentary  iron-bearing  formations — Continued. 
Conditions  of  sedimentation — Continued. 

Pog  and  lagoon  origin  of  part  of  tlie  iron-bearing  rocks 502 

Hypothesis  of  bog  and  lagoon  origin  not  applicable  to  the  main  masses  of  the  iron-bearing  sediments. .  502 

Hypothesis  of  glauconilic  origin  not. applicable 503 

Iron-bearing  sediments  not  laterite  deposits 503 

Iron-bearing  sediments  not  characteristic  transported  deposits  of  ordinary  ero-sion  cycles .503 

As.-!Ociation  of  iron-bearing  sediments  with  rontemporaneotis  eruptive  rocks ,506 

Association  of  iron-bearing  sediments  and  eruptive  rocks  outside  of  the  Lake  Superior  region 508 

Significance  of  ellipsoidal  structure  of  eruptive  rocks  in  relation  to  origin  of  the  ores 510 

Eruptive  rocks  associated  \yith  iron-bearing  sediment;*  of  Lake  Superior  region  carry  abundant  iron.  512 

Genetic  relations  of  upper  Huronian  slate  to  associated  eruptive  rocks 513 

Main  mass  of  iron-bearing  sediments  probably  derived  from  associated  eruptive  rocks 513 

Direct  contributions  of  iron  salts  in  hot  solutions  from  the  magma 513 

Contribution  of  iron  salts  from  crystallized  igneous  rocks  in  meteoric  waters 514 

Contribution  of  iron  salts  by  reaction  of  hot  igneous  rocks  with  sea  water 515 

Conclusion  as  to  derivation  of  materials  for  the  iron-bearing  formations 516 

Variations  of  iron-bearing  formations  with  different  eruptive  rocks  and  different  conditions  of 

deposition 516 

Chemistry  of  original  deposition  of  the  iron-bearing  formations 518 

Natiu'e  of  the  problem ; 518 

Formation  of  iron  carbonate  and  limonite 519 

Nature  of  carbonate  precipitate 520 

Precipitation  of  greenalite 521 

Processes 521 

Nature  of  greenalite  precipitate 522 

Source  of  alkaline  silicates  necessary  to  produce  greenalite 525 

Reactions  betwaen  greenalite  and  iron  carbonate,  or  carbon  dioxide 526 

Source  of  carbon  dioxide  for  reactions  with  greenalite 527 

Deposition  of  hematite,  magnetite,  and  silica  directly  from  hot  solutions 527 

Deposition  of  iron  sulphide 527 

Correlation  of  laboratory  and  field  observations 527 

Secondary  concentration  of  the  ores 529 

General  statements 529 

Chemical  and  mineralogical  changes  involved  in  concentration  of  the  ore  under  surface  condi- 
tions    529 

Outline  of  alterations 529 

Oxidation  and  hydration  of  the  greenalite  and  siderite  producing  ferruginous  chert 530 

Alteration  of  ferruginous  chert  to  ore  by  the  leaching  of  silica,  with  or  without  secondary  intro- 
duction of  iron 537 

Processes  involved 537 

Conditions  favorable  to  leaching  of  silica 538 

Solution  of  silica  favored  by  alkaline  character  of  waters ^. 538 

Transfer  of  iron  in  solution 539 

Secondary  concentration  of  the  ores  characteristic  of  weathering 539 

Mechanical  concentration  and  erosion  of  iron  ores 540 

General  character  of  mi  ne  waters 540 

Localization  of  the  ores  controlled  by  special  structural  and  topographic  features 544 

Quantitative  study  of  secondary  concentration 545 

Alterations  of  iron-bearing  formations  by  igneous  intrusions 546 

Ores  affected 546 

Possible  contributions  from  igneous  rocks 546 

Temperature  at  which  contact  alterations  were  effected 549 

Character  of  iron-bearing  formations  at  the  time  of  intrusions  of  igneous  rocks 549 

Chemistry  of  alterations 550 

Banding  of  amphibole-raagnetite  rocks 551 

Recrystallization  of  quartz 552 

High  sulphur  content  of  amphibole-magnetite  rocks 552 

Secondary  iron  carbonate  locally  developed  at  igneous  contacts 552 

Contact  alterations  not  favorable  to  concentration  of  ore  deposits 552 

SiU'face  alterations  of  amphibole-magnetite  rocks 553 

Summary  of  alterations  of  iron-beaiing  formations  by  igneous  intrusions 554 


CONTENTS.  23 

Chapter  XVII.  The  iron  ores  of  the  Lake  Superior  region,  by  the  authors  and  W.  J.  Mead — Continued.  Page. 
Origin  of  the  ores  of  the  Lake  Superior  pre-Cambrian  sedimentary  iron-bearing  formations — Continued. 

Alteration  of  iron-bearing  formations  by  rock  flowage 554 

Cause  of  varying  degree  of  hydration  of  the  Lake  Superior  ores 555 

Sequence  of  ore  concentration 557 

Origin  of  manganiferous  iron  ores 5(j0 

Part  of  the  metamorphic  cycle  illustrated  by  the  Lake  Superior  iron  ores  of  sedimentary  type 500 

Titaniferous  magnetites  of  northern  Minnesota 561 

Magnetites  of  possible  pegmatitic  origin 562 

Brown  ores  and  hematites  associated  with  Paleozoic  and  Pleistocene  deposits  in  Wisconsin 562 

Ores  in  the  Potsdam 562 

Brown  ores  in  "Lower  Magnesian  "  limestone 562 

Geology  and  topography 565 

Oilman  brown-ore  deposit 565 

Cady  brown-ore  deposit 565 

Origin  of  Spring  Valley  brown-ore  deposits 566 

Postglacial  brown  ores 566 

Clinton  iron  ores  of  Dodge  County,  Wis 567 

Occurrence  and  character 567 

Origin  of  the  Clinton  iron  ores 568 

Summary  statement  of  theory  of  origin  of  the  Lake  Superior  iron  ores 568 

Other  theories  of  the  origin  of  the  Lake  Superior  pre-Cambrian  iron  ores 569 

Genetic  classification  of  the  principal  iron  ores  of  the  world 571 

Chapter  XVIII.  The  copper  ores  of  the  Lake  Superior  region,  by  the  authors,  assisted  by  Edward  Steidtmann.  573 

The  copper  deposits  of  Keweenaw  Point 573 

General  account 573 

Transverse  veins  of  Eagle  River  district 575 

Dipping  veins  of  Ontonagon  district 576 

Amygdaloid  deposits 576 

Copper  in  conglomerates 577 

Composition  of  copper-mine  waters .579 

Copper  in  Keweenawan  rocks  in  parts  of  the  Lake  Superior  region  other  than  Keweenaw  Point 580 

Origin  of  the  copper  ores 580 

Common  origin  of  the  several  types  of  deposits 580 

Previous  views  of  nature  of  copper-depositing  solutions  and  source  of  copper 580 

Outline  of  hypothesis  of  origin  of  copper  ores  presented  in  the  following  pages 581 

Association  of  ores  and  igneous  rocks 581 

Ore  deposition  limited  mainly  to  middle  Keweenawan  time 581 

Deposition  of  the  copper  accomplished  l)y  hot  solutions 582 

Nature  of  gangue  minerals 582 

Nature  of  wall-rock  alterations ; 582 

Paragenesis  of  copper  and  gangue  minerals 585 

Contrast  with  present  work  of  meteoric  solutions : 585 

Source  of  thermal  solutions 586 

Three  hypotheses 586 

Were  the  thermal  solutions  derived  from  extrusive  or  from  intrusive  rocks? 587 

Significance  of  sulphides  of  copper  in  the  intrusives  and  lower  effusi^■es 588 

Conclusions  as  to  source  of  copper-bearing  solutions 588 

Chemistry  of  deposition  of  copper  ores 589 

Cause  of  diminution  of  richness  with  increasing  depth 591 

Relation  of  copper  ores  to  other  ores  of  the  Keweenawan 591 

.  Chapter  XIX.  The  silver  and  gold  ores  of  the  Lake  Superior  region 593 

Silver  ores 593 

Production T 593 

Silver  Islet 593 

General  account  of  silver  in  the  Animikie  group 594 

Origin  of  silver  ores  in  the  Animikie  group 595 

Gold  ores 595 

Chapter  XX.  General  geology 597 

Introduction 697 

Principles  of  correlation 597 

General  character  and  correlation  of  the  Archean 599 

Keewatin  series 599 


24  CONTENTS. 

Chapter  XX.  General  geology — Continued.  Page. 
General  character  and  correlation  of  the  Archean — Continued. 

l.aurentian  scries 600 

General  statements  concernini;  the  Archean  system 601 

General  statements  concerning  the  Algonkian  system 602 

Character  and  subdivisions 602 

Northern  Huronian  subprovince 602 

Lower  middle  Huronian 602 

Lithology  and  succession 602 

Igneous  rocks 603 

Conditions  of  deposition 603 

Correlation 603 

Upper  Huronian  (Animikie  group) 604 

Lithology  and  succession .' .604 

Igneous  rocks 604 

Conditions  of  deposition 605 

Correlation 605 

Southern  Huronian  subprovince 605 

Lower  Huronian 605 

Lithology  and  succession 605 

Igneous  rocks 605 

Conditions  of  deposition 606 

Correlation 606 

Middle  Huronian 607 

Lithology  and  succession 607 

Igneous  rocks 607 

Conditions  of  deposition 607 

Correlation .  i 608 

Upper  Huronian  (Animikie  group) 608 

Lithology  and  succession 608 

Igneous  rocks 609 

Conditions  of  deposition 609 

Correlation 609 

General  remarks  concerning  the  upper  Huronian  ( Animikie  group)  of  the  Lake  Superior  region 610 

Character 610 

Conditions  of  deposition  of  the  upper  Huronian  (Animikie  group) 612 

Keweenawan  series 614 

Lithology  and  succession 614 

Igneous  rocks 615 

Conditions  of  deposition 615 

Correlation 615 

Paleozoic  rocks •  •  -  ■  615 

Cretaceous  rocks 616 

Pleistocene  deposits 617 

Pre-Cambrian  volcanism 61" 

Pre-Cambrian  life 617 

Unconformities 617 

Unconformity  lietween  the  Archean  and  lower  Huronian 617 

Unconformity  between  the  lower  and  middle  Huronian 618 

Unconformity  at  the  base  of  the  upper  Huronian  ( Animikie  group  i 619 

Unconformity  at  the  base  of  the  Keweenawan 619 

Unconformity  at  the  base  of  the  Cambrian 619 

Deformation  and  metamorphism 620 

General  conditions , 620 

Principal  elements  of  structure 621 

The  Lake  Superior  basin 622 

R6sum6  of  history 623 

Index 627 


ILLUSTRATIONS. 


Page. 

Plate      I.  Geologic  map  of  the  Lake  Superior  region,  wilh  sections In  pocket. 

II.  Relief  map  of  the  Lake  Superior  region,  showing  the  larger  topographic  features 86 

III.  .4,  Pre-Oambrian  peneplain  in  Ontario,  near  Michipicoten;  B,  Jasper  Peak,  near  Tower,  Minn.  ...         88 

IV.  A,  Topographic  map  of  Rib  Hill,  Wis.;  B,  Typical  monoclinal  ridge  topography,  U\e  Royal,  Mich..         90 
V.  A,  The  Duluth  escarpment  and  even  upland  of  peneplain  on  Duluth  gabbro  in  Minnesota;  B,  Lake 

shore  escarpment  of  Archean  schists  and  Iluroniauquartzite  near  Marquette,  Mich.  .' 112 

VI.  Geologic  map  of  the  Vermilion  iron-bearing  district,  Minn 118 

VII.  A,  Ellipsoidal  parting  in  Ely  greenstone;  B,  Ellipsoidally  parted  Ely  greenstone,  showing  spheru- 

litic  development ^ 

VIII.  Geologic  map  of  the  Mesabi  iron-bearing  district,  Minn In  pocket. 

IX.  Sharp  folding  of  beds  of  iron-bearing  Biwabik  formation  in  Mesabi  district,  Minn.;  A,  Hawkins 

mine;  B,  Monroe  mine ^"^ 

X.  Typical  cross  section  through  iron-bearing  Biwabik  formation,  Mesabi  district,  Minn.,  from  drill 

records ■.■■'.■■' 

XI.  A,  Panoramic  view  of  the  Mountain  Iron  open-pit  mine,  Mesabi  district,  Minn.;  B,  Panoramic  view 

of  the  Shenango  iron  mine,  Mesabi  district,  Minn 180 

XII.  Geologic  map  of  Pigeon  Point,  Minn • -O'l 

XIII.  Geologic  map  of  the  Animikie  iron-bearing  district,  north  of  Thunder  Bay,  Ontario 206 

XIV.  Map  of  central  Minnesota,  including  Cuyuna  district 212 

XV.  Map  of  part  of  the  Cuyuna  iron  district  of  Minnesota,  showing  magnetic  belts 212 

XVI.  Geologic  map  of  the  Penokee-Gogebic  district 226 

XVII.  Geologic  map  of  the  Marquette  iron-bearing  district,  Mich In  pocket. 

XVIII.  Map  of  Carp  River  fault,  sees.  4,  5,  and  6,  T.  47  N.,  R.  25  W.,  Mich 252 

XIX.  Detailed  map  of  quartzite  ridges  of  Teal  Lake,  showing  faulting  and  unconformity  of  Ajibik  and 

Mesnard  formations -^'* 

XX.  Geologic  map  of  Dead  River  area,  Mich 286 

XXI.  Map  of  Perch  Lake  district,  Mich.,  showing  distribution  of  outcrops In  pocket. 

XXII.  Geologic  map  of  the  Crystal  Falls  district,  Mich.,  including  a  portion  of  the  Marquette  district.    In  pocket. 

XXIII.  Geologic  map  of  the  Calumet  district,  Mich 306 

XXIV.  Geologic  map  of  the  Iron  River  district,  Mich -' In  pocket. 

XXV.  Geologic  map  of  the  Florence  iron  district,  Wis In  pocket. 

XXVI.  Geologic  map  of  the  Menominee  iron  district,  Mich In  pocket. 

XXVII.  Vertical  north-south  cross  sections  through  the  Norway-Aragon  area,  Menominee  district,  Mich., 

illustrating  geologic  structure 346 

XXVIII.  Geologic  map  of  the  Keweenaw  Point  copper  district,  Mich - .       380 

XXIX.  A,  Hanging  valley  near  Helen  mine,  Michipicoten;  B,  Lake  clay  overlying  stony  glacial  till  in 

Mountain  Iron  open  pit,  Mesabi  range,  Minn 43- 

XXX.  .4,  Terminal-moraine  and  outwash-plain  topography  in  glaciated  area  of  western  Wisconsin;  B, 

Glaciated  valley  of  Portage  Lake  on  Keweenaw  Point,  Mich,,  with  hanging  valley  of  Huron  Creek.       434 
XXXI.  A,  Characteristic  Driftless  Area  topography  in  northern  Wisconsin;  B,  Characteristic  muskeg  and 

ground-moraine  topography  in  glaciated  area  of  Minnesota 436 

XXXII.  Jaspilite  from  Marquette  district,  Mich 464 

XXXIII.  A,  Folded  and  brecciated  jaspilite  of  the  Soudan  formation,  Vermilion  district,  Minn.;  B,  Hema- 

titic  chert  from  Negaunee,  Marquette  district,  Mich 466 

XXXIV.  Ferruginous  chert  and  slate  of  iron-bearing  Biwabik  formation,  Mesabi  district,  Minn 468 

XXXV.  .4,  Amphibole-magnetite  chert  from  Republic,  Mich.;  B,  Sideritic  magnetite-grunerite  schist  from 

Marquette  district,  Mich 4/0 

XXXVI.  .4,  Jaspery  filling  in  amygdules  from  ellipsoidal  basalt  of  the  Crystal  Falls  district,  Mich.;  B,  Cherty 

siderite  from  Marquette  district,  Mich.;  C,  Cherty  siderite  from  Penokee  district,  Mich 472 

XXXVII.  Greenalite  rock  from  Mesabi  district,  Minn 474 

25 


26  ILLUSTRATIONS. 

Page. 

Plate  XXXVIII.  Characteristic  specimens  of  iron  ores 480 

XXXIX.  CharacteriHtic  specimens  of  iron  ores 480 

XL.  Diaf^ram  showing  relation  of  density,  porosity,  and  moisture  to  cubic  feet  per  ton 480 

XLI.  yl.  Ore  dock.s  at  Two  Harbors,  Minn.;  B,  Excavations  at  Stevenson,  Minn 496 

XLII.  Photomicrosiraphs  of  natural  and  artificial  greenalite  granules,  cherty  siderite,  and  concre- 

t  ionary  ferruginous  chert 524 

XLIII.  Photomicrographs  of  greenalite  granules 532 

XLIV.  Photomicrographs  of  ferruginous  chert  showing  later  stages  of  the  alteration  of  greenalite 

granules 534 

XLV.  Photomicrographs  of  granules  and  concretionary  structtires  in  Clinton  iron  ores 536 

XLVI.  A,  Ore  and  jasper  conglomerate  from  Marquette  district,  Mich.;  B,  Ferruginous  chert  from 

Marquette  district,  Mich 542 

XLV II.  Photomicrographs  of  ferruginous  and  amphiboli tic  chert  of  iron-bearing  Biwabik  formation 

near  contact  with  Duluth  gabbro 548 

XLVIII .  Ferruginous  chert  or  jasper,  of  possible  pegmatitic  origin,  in  basalt 564 

XLIX.  Map  showing  location  of  copper-bearing  lodes  and  mines  on  Keweenaw  Point 574 

Figure  1.  Key  map  showing  location  of  Lake  Superior  region 31 

2.  Sketch  map  of  the  Lake  Superior  region,  showing  iron  districts,  shipping  ports,  and  transportation 

lines - 32 

3.  Diagram  showing  annual  production  of  iron  ore  in  Lake  Superior  region  since  the  opening  of  the  region.  49 

4.  Generalized  topographic  map  of  the  Lake  Superior  region 87 

5.  The  topographic  pro\ances  of  the  Lake  Superior  region,  with  some  subdi^dsions  of  the  peneplain 88 

6.  True-scale  cross  section  of  Keweenawan  monoclinal  ridges  near  the  end  of  Keweenaw  Point 99 

7.  Hypothetical  cross  section  showing  relation  of  secondary  lowlands,  mesas,  monoclinal  ridges,  etc., 

to  peneplain - 101 

8.  Graben  or  rift  valley  of  western  Lake  Superior,  showing  escarpments  on  either  side  and  peneplain 

above 112 

9.  The  drainage  of  the  St.  Louis  and  Mississippi  headwaters  before  the  stream  captures  along  the  Duluth 

escarpment 113 

10.  The  drainage  of  the  St.  Louis  and  Mississippi  headwaters  at  present,  after  stream  captures  and 

diversions 113 

11.  Structure  profile  in  northern  Wisconsin,  showing  the  south  edge  of  the  peneplain  on  the  pre- 

Cambrian  rocks  and  the  northern  part  of  the  belted  plain  of  the  Paleozoic 116 

12.  Diagram  to  illustrate  folding  of  "drag"  type,  common  in  the  Vermilion  and  other  ranges 123 

13.  Section  across  jasper  belt  in  sees.  13  and  14,  T.  62  N.,  R.  13  W.,  Vermilion  iron  range,  Minn 123 

14.  Transverse  sections  of  Chandler,  Pioneer,  Zenith,  Sibley,  and  Savoy  mines,  Vermilion  district, 

Minn 138 

15.  Diagram  illustrating  volume  changes  involved  in  the  alteration  of  jasper  to  ore  at  Ely,  Minn 142 

16.  North-south  cross  section  of  an  ore  deposit  on  the  Mesabi  range  near  Hibbing,  Minn 180 

17.  Triangular  diagram  showing  composition  of  various  phases  of  Mesabi  ores  and  ferruginous  cherts 182 

18.  Section  through  iron-bearing  Biwabik  formation  transverse  to  the  range,  showing  nature  of  circula- 

tion of  water  and  its  relations  to  confining  strata 186 

19.  Dia"-ram  showing  volume  changes  observed  in  the  alteration  of  ferruginous  chert  to  ore 188 

20.  Graphic  representation  of  the  changes  involved  in  the  alteration  of  greenalite  rock  to  ferruginous 

chert  (taconite)  and  ore 189 

21.  Triangular  diagram  representing  volume  composition  of  the  various  phases  of  ferruginous  cherts  and 

iron  ores  of  the  Mesabi  district 190 

22.  Diagram  showing  relation  of  phosphorus  to  degree  of  hydration  in  Mesabi  ores 192 

23.  Diagram  showing  relative  amounts  of  phosphorus  and  lime  in  Mesabi  ores 196 

24.  Cross  section  of  iron-bearing  Gunflint  formation  east  of  Paulson  mine,  Gunflint  district,  Minn 199 

25.  Plan  and  cross  section  of  the  iron-ore  deposit  in  sec.  12,  T.  43  N.,  R.  32  \V.,  Crow  Wing  County,  Minn.  218 

26.  Triangular  diagram  showing  mineralogical  composition  of  various  phases  of  iron  ores  and  ferruginous 

cherts  of  the  Cuyuna  district,  Minn ■ 2_1 

27.  Triangular  diagram  showing  volume  composition  of  various  phases  of  iron  ores  and  ferruginous 

cherts  of  the  Cuyuna  district,  Minn ""- 

28.  Cross  section  showing  the  occurrence  of  ore  in  pitching  troughs  formed  by  dikes  and  quartzite  foot- 

wall,  in  the  Gogebic  district -36 

29.  Ore  depo.sits  of  the  Penokee-Gogebic  district ; 237 

30.  Triangular  diagram  showing  chemical  composition  of  various  jihases  of  Gogebic  ores  and  ferruginous 

cherts - ■ ^39 

31.  Diagrammatic;  rei)re.sentation  of  the  changes  involved  in  the  alteration  of  cherty  iron  carbonate  to 

ferruginous  chert  and  ore,  Gogebic  district "^ 


ILLUSTRATIONS.  27 

Page. 
FiGtJRE  32.  Triangular  diagram  showing  volume  composition  of  the  ferruginous  cherts  and  iron  ores  of  the  Gogebic 

range 245 

33.  Diagram  showing  relation  of  phosjihorus  to  degree  of  hydration  in  Gogebic  ores 248 

34.  Diagram  showing  relative  amounts  of  phosphorus  and  lime  in  Gogebic  ores 249 

3.5.  Idealized  north-south  section  through  the  Marquette  district,  sho^ving  abnormal  type  of  synclinorium.  2.53 

36.  Ore  deposits  of  the  Marquette  district .• 270 

37.  Graphic  representation  of  the  volume  composition  of  the  principal  phases  of  the  iron-bearing  Negaunee 

formation 276 

38.  Triangular  diagram  showing  the  volume  composition  of  the  several  grades  of  ore  mined  in  the  Mar- 

quette district  in  1906 277 

39.  Diagram  showing  relation  of  phosphorus  to  degree  of  hydration  in  Marquette  ores 280 

40.  Diagram  showing  relative  amounts  of  phosphorus  and  lime  in  Marquette  ores 282 

41.  Outcrop  map  of  Swanzy  district,  Mich 284 

42.  Geologic  map  of  west  end  of  Marquette  district,  Mich 289 

43.  Sketch  map  to  show  general  relations  of  iron-bearing  rocks,  principally  upper  Huronian,  in  Crystal 

t-                         Falls,  Iron  River,  Florence,  and  Menominee  districts 292 

I  44.  Section  showing  roughly  the  succession  of  beds  in  the  Vulcan  iron-bearing  member  near  Atkinson, 

in  the  Iron  River  district,  Mich 318 

45.  Geologic  map  and  cross  section  of  Iron  Hill,  Menominee  district,  showing  relations  of  lower  and  mid- 

dle Hiu-onian 3^5 

46.  Horizontal  section  of  the  Aragon  mine  at  the  first  level,  Menominee  district,  Mich 347 

47.  Horizontal  section  of  the  Aragon  mine  at  the  eighth  level,  Menominee  district,  Mich 348 

48.  Vertical  north-south  cross  section  through  Burnt  shaft.  West  Vulcan  mine,  Menominee  district,  Mich. .  349 

49.  Sketch  to  show  pitch  of  a  drag  fold  in  a  monoclinal  succession 350 

50.  Triangular  diagi-am  representing  the  volume  composition  of  the  various  grades  of  ore  mined  in  the 

Menominee,  Crystal  Falls,  and  neighboring  districts  in  1907 352 

51.  Sketch  map  showing  occurrence  of  quartzites  of  Huronian  age  in  Tps.  33  and  34  N.,  Rs.  15,  16,  and 

17  E.,  Wis ■ 358 

52.  Sketch  map  showing  occurrence  of  Huronian  quartzite  near  Necedah,  Wis. 358 

53.  Sketch  map  showing  Baraboo,  Fox  River  valley,  Necedah,  Waushara,  and  Waterloo  pre-Cambrian 

areas  of  south-central  Wisconsin 359 

54.  Generalized  cross  section  extending  north  and  south  across  the  Baraboo  district 360 

55.  Vertical  section  of  Illinois  mine 364 

56.  Section  on  south  cliff  of  Great  Palisades,  Minnesota  coast " 371 

57.  Sketch  showing  unconformable  contact  between  Keweenawan  diabase  porphyry  and  Cambrian  sand- 

stone at  Taylors  Falls,  Minn 379 

58.  Diagrammatic  section  illustrating  the  assigned  change  of  attitude  of  a  series  of  beds,  like  the  Kewee- 

nawan, from  an  original  depositional  inclination  to  a  more  highly  inclined  attitude 419 

59    Map  of  the  Lake  Superior  basin,  designed  to  show  the  structure  and  e.xtent  of  the  Keweenawan 

trough 422 

60.  Sketch  map  showing  the  glaciation  of  the  Lake  Superior  region,  giving  names  of  lobes  and  probable 

general  directions  of  ice  flow 428 

61.  Sketch  showing  the  glacial  cirque,  the  rock  basins,  and  the  hanging  valley  near  the  Helen  mine, 

Michipicoten 432 

62.  Sketch  showing  the  origin  of  the  drift  deposits  overlying  the  ore  in  the  Mesabi  iron  range 443 

63.  Glacial  Lake  Nemadji 444 

64.  Glacial  Lake  Duluth 445 

65.  Hypothetical  intermediate  stage  vrith  the  expansion  of  glacial  Lake  Chicago  and  the  later  stage  of 

glacial  Lake  Duluth 446 

66.  Glacial  Lake  Algonquin 447 

67.  Part  of  Nipissdng  Great  Lakes 448 

68.  Sketch  map  shoiving  Driftless  Area  and  regions  of  older  drift,  last  drift,  and  lake  deposits 453 

69.  St.  Louis  Ri\-er  at  the  stage  when  it  cut  its  valley  and  emptied  directly  into  Lake  Nipissing 456 

70.  The  present  St.  Louis  River,  which  has  been  converted  into  an  estuary  by  post-Nipissing  tilting 457 

71.  Triangular  diagram  showing  chemical  composition  of  all  grades  of  iron  ore  mined  in  the  Lake  Supe- 

rior region  in  1906 478 

72.  Textures  of  Lake  Superior  iron  ores  as  shown  by  screening  tests 481 

73.  Diagram  showing  relation  between  estimated  ore  reserves  of  the  Lake  Superior  region  and  rate  of  pro- 

duction    490 

74.  Diagram  representing  decline  in  grade  of  Lake  Superior  iron  ore  since  1889 493 

75.  Cross  section  of  Keweenaw  Point  near  Calumet,  showing  copper  lodes  in  conglomerates  and  amyg- 

daloids ^"4 

76.  Triangular  diagi-am  comparing  the  amomita  of  undecomposed  silicates,  quartz,  and  residual  weathered 

products  in  different  kinds  of  muds,  shales,  and  weathered  rocks 612 


THE  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


By  C.  R.  Van  Hise  and  C.  K.  Leitii. 


CHAPTER  I.   INTRODUCTION. 
OUTLINE  OF  MONOGRAPH. 

The  Lake  Superior  rejjion  is  a  part  of  the  southern  margin  of  tlie  great  pre-Cambrian  shield 
of  northern  North  America.  It  is  bordered  and  overlapped  on  the  south  by  Paleozoic  rocks  of 
the  iEssissippi  Valley  and  on  the  southwest  by  Cretaceous  deposits.  The  pre-Cambrian  rocks  . 
of  the  area,  which  may  be  divided  into  a  considerable  number  of  lithologic  and  time  units, 
contain  the  great  iron  and  copper  deposits  by  which  the  region  is  most  widely  known.  The 
great  development  of  the  mineral  industry  in  this  region  has  afforded  the  geologist  unusual 
opportunity  for  study,  as  it  has  not  only  made  the  region  more  accessible  but  has  justified 
larger  expenditures  for  geologic  study  than  would  otherwise  have  been  made.  Tliis  fortunate 
combination  of  a  field  containing  an  exceptionally  full  record  of  a  little-known  part  of  the 
geologic  column  \vith  the  means  of  studying  it  has  warranted  tlie  study  of  the  pre-Cambrian 
with  a  degree  of  detail  that  has  been  practicable  in  but  few  other  significant  pre-Cambrian 
regions. 

Geologic  surveys  of  various  parts  of  the  Lake  Superior  region  have  been  conducted  under 
national,  state,  and  private  supervision  almost  without  interruption  since  the  early  part  of  the 
nineteenth  century,  especially  since  the  opening  of  the  mining  industry  in  the  middle  of  the 
century.  The  later  reports  have  naturally  been  more  adequate  than  the  earlier  ones,  because 
they  have  included  the  results  of  the  earlier  work  and  have  gained  the  advantage  derived  from 
the  greater  accessibility  of  the  district.  The  reports  thus  far  issued  have  dealt  with  small  parts 
of  the  region  or  with  certain  phases  of  its  general  geology.  State  and  private  surveys  have  neces- 
sarily worked  within  jirescribed  areas,  so  that  notwithstanding  the  multiplicity  of  reports 
certain  parts  of  the  region  have  not  yet  been  adetpiately  covered.  It  has  been  the  proper  func- 
tion of  the  United  States  Geological  Survey  to  make  detailed  surveys  designed  to  accomplish 
the  uniform  treatment  and  correlation  of  the  several  ore-bearing  districts,  and  finally  to  publish 
a  monographic  report  on  the  region  as  a  whole.  Work  under  a  general  plan  for  these  surveys 
was  begun  in  the  early  eighties  under  the  direction  of  Prof.  R.  D.  Irving,  whose  monograph  on 
the  copper-bearing  rocks  of  Lake  Superior  "  appeared  in  1883,  though  it  was  partly  prepared  at 
an  earher  date,  while  he  was  connected  with  the  Wisconsin  Geological  Survey.  The  develop- 
ment of  this  plan  has  since  been  continuous.  Until  1888  the  work  was  in  charge  of  Professor 
Ir\ang:  since  that  time  it  has  been  under  the  direction  of  Dr.  Charles  R.  Van  Hise,  the  senior 
author  of  this  monograph.  Detailed  monographs  on  the  live  leading  iron  ranges  have  been 
published  and  also  papers  covering  different  phases  of  the  general  geology  of  the  region. 

This  monograph  represents  the  first  attempt  to  give  a  connected  account  of  the  geology  of 
the  Lake  Superior  region  as  a  whole,  with  special  reference  to  the  iron  and  copper  bearing  for- 
mations.    Attention  is  dii'ected  primarily  to  general  features  of  correlation  of  the  formations, 

o  Mon.  U.  S.  Geol.  Survey,  vol.  5,  1883. 

29 


30  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

to  the  geologic  history  of  the  region,  and  to  tlio  origin  of  the  iron  and  copper  ores.  In  addition, 
brief  chapters  are  presented  on  several  parts  of  the  district  which  had  not  yet  been  reported 
on  by  the  United  States  Geological  Survey.  No  attemj)t  is  made  to  give  details.  For  these 
tlie  reader  is  referred  to  the  pul)lications  of  the  United  States  Geological  Survey  and  of  state 
geological  surveys  and  to  other  sources  specified  in  ai)j)ropriate  places  in  this  volume. 

Tiiough  tills  monograph  may  be  regarded  as  completing  a  stage  in  the  jjrogress  of  the 
geologic  survey  of  tlie  region,  and  lience  may  be  considered  final  in  one  sense,  it  may  also  properly 
be  regarded  as  only  the  first  of  a  series  of  general  studies  of  tlie  tlistrict.  The  area  is  so  large 
and  the  record  is  so  complex  that  this  monograph  will  accomplish  its  purjjose  if  it  discloses  the 
elements  of  some  of  the  major  j)rol)lcms  of  the  region  and  affords  a  basis  for  a  better-directed 
attack  on  them  than  has  heretofore  been  possible.  Future  monographs  will  untloubtedly  be 
written  on  each  of  the  many  phases  of  subjects  that  are  barely  touched  upon  in  this  monograph, 
such,  for  instance,  as  the  petrography  and  consanguinity  of  the  igneous  rocks  of  different  periods, 
the  conditions  of  sedimentation  of  various  series,  the  relations  of  volcanism  to  ore  deposition, 
and  the  correlation  of  major  and  minor  structural  features  of  the  Lake  Sujjerior  region  with  one 
another  antl  with  the  various  structural  features  of  Xortli  America.  Besides,  certain  areas  not 
yet  fully  reported  on  will  require  detailed  monographic  description.  It  is  hoped  that  the  work 
of  the  United  States  Geological  Survey  in  the  Lake  Sujierior  region  may  be  continued  along 
the  lines  indicated. 

Parts  of  the  region  have  been  studied  at  different  times  by  men  occup3'ing  different  view- 
points. Some  areas  which  have  recently  become  commercially  prominent  have  not  yet  been 
adcciuately  studied  in  detail.  Finally,  mining,  drilling,  and  various  public  and  private  surveys 
are  so  rapidly  extending  the  knowleclge  of  the  geology  of  the  region  that  it  is  practically  impos- 
sible at  the  present  time  to  write  a  monograph  that  will  not  require  modification  in  some  par- 
ticulars almost  before  it  comes  from  the  press.  Because  of  these  facts  this  work  shows  inequal- 
ities and  inadequacies  of  treatment  for  different  parts  of  the  region  and  for  different  phases 
of  the  subject.  It  is  hoped,  however,  that  the  monograph  will  be  measured  by  the  advance  it 
represents  over  previous  available  knowledge  and  especially  by  its  attempt  to  bring  out  sig- 
nificant general  features  of  the  geology  not  heretofore  discussed,  and  not  by  its  deficiencies, 
of  which  the  writers  have  a  lively  appreciation. 

The  parts  of  the  report  written  partly  or  wholly  by  others  than  the  authors  bear  the 
names  of  the  writers.  It  will  be  understood  that  any  chapter  or  section  for  which  no  names 
are  given  has  been  written  by  C.  R.  Van  Hise  and  C.  K.  Leith. 

ACKNOWLEDGMENTS. 

The  completion  of  tliis  monograph  and  the  detailed  studies  leading  up  to  it  have  been 
facilitated  by  the  cordial  cooperation  of  the  mining  men  of  the  region.  To  attempt  to  mention 
the  names  of  all  who  have  gone  out  of  their  way  to  render  aid  in  these  studies  would  involve  the 
publication  of  a  list  including  the  greater  number  of  local  mining  men,  and  even  from  such  a  list 
some  names  would  probably  be  inadvertently  omitted.  Especially  valuable  has  been  the  infor- 
mation furnished  by  the  Ohver  Iron  Mining  Company  (United  States  Steel  Corporation),  which 
has  a  most  highly  developed  and  efficient  engineering  and  geologic  staff.  Valuable  aid  has  been 
given  by  state  and  provincial  surveys  and  by  the  Minnesota  tax  commission.  To  all  these 
men  and  organizations  we  express  our  indebtedness  and  thanks. 

We  are  indebted  to  Messrs.  W.  J.  Mead,  Lawrence  Martin,  Alexander  N.  Winchell,  A.  C. 
Lane,  R.  C.  Allen,  and  Edward  Steidtmann  for  sections  of  this  report  bearing  their  names, 
and  to  numerous  other  men  mentioned  in  the  report  who  have  contributed  in  ilifferent  ways. 
Not  the  least  of  our  indebtedness  is  to  Mr.  A.  C.  Deming  for  efficient  clerical  service. 

GEOGRAPHY. 

The  Lake  Superior  region  comprises  parts  of  Michigan,  Wisconsin,  Minnesota,  and  Ontario- 
adjacent  to  Lake  Superior.     (See  figs.  1    and   2.)     The  accoiii])anying  general  geologic  map- 


INTRODUCTION. 


31 


(PI.  I,  in  pocket)  covers  the  area  between  parallels  44°  and  49°  north  and  meridians  84°  and 
95  west,  comprising  ai)proximately  1,81,000  s(|uare  miles — an  area  almost  equal  to  that  of  the 
sLx  New  England  States  and  New  York,  New  Jersey,  Pennsylvania,  and  Maryland,  or  that  of 
Sweden  and  Belgium. 


v^-O      KENTUCKYy-^j^^j^i^ 


'OKLAHOMA    I  )    ''" 


200  300  WILES 


FiGiTRE  1. — Key  map  showing  location  of  Lake  Superior  region. 


The  region  includes  several  ore-bearing  districts  of  comparatively  small  area — the  Ke- 
weenaw copper-bearing  district  of  Keweenaw  Point,  Michigan,  about  1,350  square  miles;  the 
Marquette  iron-bearing  district  of  Michigan,  extending  westward  from  the  city  of  Marquette 


82 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


on  tlic  hike  slioro,  about  .330  s(|uare  miles;  the  Menominee  iron-hearir^s^  district,  e.\tendin<^ 
from  Iron  Mountain  in  Michigan  eastward  alon»  Menominee  River,  a<!;gre<^ating  112  square 
miles:  tlie  Crystal  Falls  iron-l)earin<;  district  in  MicJiigan,  in  tiie  vicinity  of  tlie  town  of  Crystal 
Falls,  540  square  miles;  the  Iron  River  district,  west  of  the  Crystal  Fails  district,  in  the  vicinity 
of  the  town  of  Iron  River,  210  square  miles;  the  Florence  iron-bearing  district,  in  Wisconsin, 
west  of  the  Menominee  district,  75  square  miles;  the  Calumet  and  Felch  Mountain  iron-bearing 
districts  of  Micliigan,  in  Dickinson  County,  aggregating  200  square  miles;  the  Fenokce-Gogebic 
iron-bearing  district,  in  Michigan  and  Wisconsin,  aliout  450  square  miles:  tiie  Vermilion  and 
^lesabi  iron-bearing  districts  of  Minnesota,  trending  east-northeast  in  parallel  areas  along  the 
nortliern  boundary  of  the  State,  1,400  s(|uare  miles:  the  Cuyuna  iron  district  of  Minnesota,  in 


Figure  2.— Sketch  map  of  Lake  Superior  region,  shovvinj:  iron  districts,  shipping  ports,  and  transportation  lines. 

the  vicinity  of  Brainerd,  about  300  sf|uare  miles;  the  Micliiijicoten  district  of  the  northeast 
shore  of  Lake  Superior,  aggregating  140  square  miles,  and  others  of  less  importance.  The 
total  area  of  these  principal  ore-bearing  areas  is  thus  less  than  3  per  cent  of  that  of  the  entire 
Lake  Superior  region,  but  these  districts  are  commercially  important  and  the  remaining  por- 
tions are  not;  for  the  most  part  also  they  include  a  fuller  succession  of  pre-Cambrian  rocks  than 
the  intervening  areas,  and  the  detailed  geologic  mapping  has  been  largely  confined  to  them. 
For  these  reasons  the  tei'm  "Lake  Superior  region"  is  commonly  used  as  a  collective  designa- 
tion of  the  ore-bearing  districtG,  notwithstanding  the  fact  that  they  comprise  only  a  small 
percentage  of  the  entire  Lake  Superior  region. 


INTRODUCTION.  33 

TOPOGRAPHY." 
RELIEF. 

The  principal  topographic  feature  of  tlie.Lake  Superior  region  is  the  Lake  Superior  basin, 
which  has  a  general  easterly  and  westerly  trend.  Most  of  the  ridges  and  valleys  in  the  adjacent 
areas  lie  parallel  to  the  axis  of  the  Lake  Superior  syncline,  and  are  due  to  the  erosion  of  parallel- 
trending  folds,  faults,  and  cleavage  produced  during  deformations  parallel  to  the  axis  of  the 
Ijake  Superior  basin. 

The  topography  has  been  modified  by  glacial  action.  Ridges  have  been  smoothed  and 
rounded  and  some  of  the  valleys  deepened,  and  the  features  have  been  then  masked  under  a 
varying  thickness  of  glacial  drift. 

Lake  Superior'  covers  about  17  per  cent  of  the  area.  Its  mean  water  level  is  about  602 
feet,  about  21  feet  higher  than  Lakes  Michigan  and  Huron,  whose  mean  level  is  581  feet.  The 
basin  of  Lake  Superior  descends  978  feet  below  lake  level,  nearly  400  feet  below  sea  level.  The 
greatest  depth  in  upper  Lake  Michigan  is  870  feet,  or  about  289  feet  below  sea  level. 

On  the  several  sides  of  Lake  Superior  the  land  rises  abruptly,  reaching  elevations  of  1,400 
to  1,700  feet  (locally  1,900  feet)  in  northern  Michigan  and  Wisconsin  on  the  south;  1,.300  to 
1,700  feet  (locally  l,90Qto  2,200  feet)  in  northern  Minnesota  on  the  northwest;  and  1,100  to  1,300 
feet  (locally  1,700  to  2,100  feet)  in  Ontario  on  the  north  and  northeast.  The  range  of  eleva- 
tion (from  a  maximum  of  2,230  feet  in  the  Cook  County  region  of  Minnesota  to  376  feet  below 
sea  level  northeast  of  Keweenaw  Point  in  Lake  Superior)  is  2,606  feet,  but  the  actual  observable 
relief  is  about  1,628  feet,  from  the  level  of  Lake  Superior  to  the  high  point  in  Cook  County 
northwest  of  Grand  Marais,  Minn. 

As  the  topographic  map  (fig.  4,  p.  87)  shows,  the  Lake  Superior  region  falls  into  tlu-ee  natural 
divisions — the  uplands,  the  lowlands,  and  the  lake  basins.  All  of  the  Upper  Peninsula  of  Michigan 
from  Marquette  eastward  to  Sault  Ste.  Marie  is  lowland,  nowhere  rising  more  than  900  feet 
above  sea  level  or  300  feet  above  Lake  Superior.  A  similar  very  narrow  lowland  belt  skirts 
the  south  shore  of  Lake  Superior,  with  many  interruptions,  from  Marquette  westward  to  the 
head  of  the  Lakes  at  Duluth  and  Superior.  Elsewhere,  except  at  some  less  important  points, 
the  upland  borders  the  lake  closely,  and  it  includes  the  remainder  of  the  Lake  »Superior  region, 
lying  between  1,000  and  1,700  feet  (except  locally)  above  sea  level,  400  feet  higher  than  the 
lake.  In  this  upland  division  are  situated  nearly  all  the  mining  districts.  Parts  of  Lakes 
Superior,  Michigan,  and  Huron  occupy  the  depressions. 

DRAINAGE.c 

Lake  Superior  is  situated  south  of  tlie  Height  of  Land,  near  the  intersection  of  three  major 
drainage  systems.  It  is  near  the  watersheds  of  the  Hudson  Bay,  the  St.  Lawrence  River,  and 
the  Mississippi  River  drainage. 

A  large  part  of  the  Lake  Superior  region  is  tributary  to  Lake  Superior  and  Lake  Michigan, 
and  hence  to  the  St.  Lawrence  River  drainage  system.  The  principal  streams  flowing  into 
Lake  Superior  are  Carp,  Ontonagon,  Black,  Brule,  Bad,  Nemadji,  and  Montreal  rivers  in  Michi- 
gan and  Wisconsin,  on  the  south  side  of  the  lake,  St.  Louis  River  of  Minnesota  on  the  west  side 
of  the  lake,  and  Kaministikwia  and  Nipigon  rivers  on  the  north  side  of  the  lake.  St.  Marys 
River,  discharging  from  Lake  Superior  into  Lake  Huron,  carries  a  larger  volume  than  any  other 
stream  in  the  area.  It  has  been  estimated"*  to  carry  86,000  cubic  feet  of  water  a  second  past 
Sault  Ste.  Marie. 

a  For  detailed  account  of  topography  and  drainage,  see  Chapter  IV. 

I>  The  general  topography  of  this  lake  has  been  reviewed  by  M.  W,  Harrington  (Nat.  Geog.  Mag.  vol.  7, 1S9G,  pp.  111-120).  who  hasalso  studied 
the  currents  in  the  Great  Lakes  in  detail  (Bulletin  B,  Weather  Bur.,  U.  S.  Dept.  Agr.,  1895). 

c  The  physical  geography  of  a  part  of  this  region  was  described  in  its  larger  aspects  in  1S50  by  Foster  and  Whitney,  Report  on  the  geology 
and  topography  of  a  portion  of  the  Lake  Superior  land  district  in  Michigan,  vol.  1,  pp.  lS-83. 

liSeliermerhom,  L.  Y.,  Am.  Jour.  Sci.,  3dser.,  vol.  33,  1887,  p.  282.  • 

47517°— VOL  52—11 S 


34  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

A  number  of  short  streams,  such  as  Manistique,  White,  and  Escanaba  rivers,  flow  south- 
ward into  Lake  Michigan  and  Green  Bay.  Menominee  River,  which  forms  the  Michigan- 
Wisconsin  boundary,  flows  southeastward  into  Green  Bay,  receiving  as  tributaries  the  Paint 
and  the  IMichigamme.  Peshtigo  and  Wolf  rivers  drain  northeastern  Wisconsin.  A  number  of 
small  streams  drain  the  northeastern  part  of  the  Lower  Peninsula  of  Micliigan. 

Another  large  part  of  the  Lake  Superior  region  in  Wisconsin  and  Minnesota  is  tributary 
to  the  Mississippi  and  so  to  the  Gulf  of  Mexico.  The  principal  tributaries  in  tliis  area  are 
Wisconsin,  St.  Croix,  Black,  Chippewa,  Swan,  and  Prairie  rivers. 

A  third  large  part  of  the  Lake  Superior  region,  in  northern  Minnesota  and  western  Ontario, 
is  tributary  to  Lake  Winnipeg,  and  hence  to  Nelson  River  and  Hudson  Bay.  This  system  com- 
prises the  numerous  large  lakes  occupying  a  large  portion  of  the  area  of  northern  Minnesota 
and  western  Ontario,  including  Lakes  Rainy  and  Vermilion  and  Lake  of  the  Woods. 

The  divide  between  the  St.  Lawrence  and  the  Mississippi  drainage  systems  extends  from 
Portage  in  central  Wisconsin,  between  Wisconsin  and  Fox  rivers,  north  to  the  Wisconsin- 
Michigan  boundary  (fig.  4,  p.  87)  thence  northwest  and  west  into  Minnesota,  and  thence  north 
between  upper  Mississippi  River  and  St.  Louis  River  to  the  Giants  Range.  The  Giants  Range, 
extending  east-northeast  across  the  northern  part  of  Minnesota,  separates  the  Mississippi  and 
the  St.  Lawrence  systems  on  the  southwest  and  southeast,  respectively,  from  the  Nelson  River 
and  Hudson  Bay  system  on  the  north.  The  areas  of  these  three  large  drainage  systems  within 
the  Lake.  Superior  region  are  as  follows:  St.  Lawrence,  107,000  square  miles;  Mississippi, 
52,000;  Hudson  Bay,  22,000. 

As  a  whole  the  drainage  of  the  Lake  Superior  region  is  very  imperfect.  The  rxumerous 
lakes,  swamps,  waterfaUs,  and  rapids  are  features  of  an  immature  drainage. 


CHAPTER  II.   HISTORY  OF  LAKE  SUPERIOR  MINING. 

THE  KEWEENAW  COPPER  DISTRICT  OF  MICHIGAN  (1844).° 

The  existence  of  copper  was  known  to  the  Chippewa  Indians  met  in  the  Lake  Superior 
region  bj^  the  earliest  explorers.  They  exhibited  crude  ornaments  of  native  copper  but  seemed 
to  make  no  further  use  of  their  knowledge.  There  is  evidence  that  mining  was  carried  on  at 
a  far  earlier  period. 

■^Tiether  the  mining  was  done  by  ancestors  of  the  aboriginal  tribes  discovered  in  possession  of  the  Lake  district 
by  the  earliest  white  explorers,  or  by  some  antecedent  people  of  higher  civilization,  is  a  point  that  archaeologists  and 
ethnologists  are  still  arguing.  Whatever  may  have  been  the  derivation  or  fate  of  that  prehistoric  race  of  copper  miners 
vaguely  termed  "mound  builders,"  it  is  certain  that  they  enjoyed  at  least  a  rudimentary  civilization  and  were  suc- 
cessful metallurgists,  for  they  possessed  the  art  of  tempering  copper.  Weapons  for  the  chase  and  war  and  domestic 
utensils  of  good  finish  and  style  and  highly  tempered  are  dug  from  mounds  and  found  in  sand  dunes  along  the  southern 
shore  of  Lake  Superior  from  time  to  time. 6 

The  existence  of  native  copper  on  Keweenaw  Point  was  reported  by  La  Garde  in  1636,  by 
the  Jesuit  missionaries  in  the  "Relations,"  extending  from  1632  to  1672,  by  Baron Le  Houtan  in 
1689,  by  P.  de  CharlevoLx  in  1721,  and  by  Jonathan  Carver  in  1765.  The  report  of  Captain 
Carver  led  to  the  formation  of  a  mining  company  which  actually  mined  copper  ore  in  1761  and 
1762,  but  \vithout  commercial  success.  In  1771  Alexander  Henry,  an  Englishman,  began 
mining  operations,  but  he  desisted  in  1774.  The  copper  ores  were  noted  in  1819  by  H.  L.  School- 
craft and  in  1823  by  Major  Long,  both  of  them  conducting  explorations  for  the  Government. 
The  first  systematic  survey  and  study  of  the  copper  ores  was  made  by  Douglass  Houghton 
for  the  first  Micliigan  Geological  Survey.  In  1830,  in  company  with  Gen.  Lewis  Cass,  he 
first  visited  the  copper  region,  and  some  years  later  began  combined  geologic  and  topographic 
surveying,  for  which,  by  considerable  effort,  he  had  procured  support  from  the  Michigan 
legislature.     His  first  report  was  published  in  1841. 

Previous  stories  of  mineral  wealth  on  the  southern  shore  of  Lake  Superior  had  been  too  vague  and  confused  to 
interest  capitalists  sufficiently  to  venture  their  money  in  attempts  at  mining  in  a  country  which  was  then  much  farther 
from  the  centers  of  wealth  and  population  than  is  Cape  Nome  to-day,  measured  by  time  and  transportation  facilities. 
This  apathy  was  dispelled  by  Dr.  Houghton's  first  report,  which  was  clear  and  concise  and  bore  upon  its  face  the 
stamp  of  truth.  He  told  the  world  that  vast  stores  of  copper  existed  upon  the  southern  shore  of  Lake  Superior.  Pressure 
was  brought  to  bear  upon  the  Federal  Government,  and  in  1843  an  arrangement  was  concluded  with  Dr.  Houghton 
by  which  he  was  to  combine  a  linear  survey  for  the  United  States  with  a  topographical  and  geographical  survey  he  was 
then  making  for  the  State  of  Michigan.  It  was  necessary  that  the  linear  survey  be  made  before  mining  locations  could 
be  granted  by  the  federal  authorities,  as  there  were  no  boundaries  other  than  those  of  nature  before  that  time.  The 
work  was  begun  in  1844,  and  during  that  and  the  following  year  rapid  progress  was  made.  Dr.  Houghton's  career 
was  brought  to  an  untimely  end  by  his  accidental  drowning  in  Keweenaw  Bay  in  the  late  fall  of  1845,  but  his  work 
was  then  so  far  advanced  that  it  was  taken  up  and  pushed  to  early  completion  by  competent  successors." 

The  first  actual  copper  mining  at  Lake  Superior  was  done  in  1844,  and  the  first  product  secured  was  a  few  tons 
of  oxide  ore — not  native  copper — taken  from  a  fissure  vein  near  Copper  Harbor,  Keweenaw  County,  by  the  Pittsburg 
and  Lake  Superior  Mining  Company,  which  later  developed  the  Cliff  mine,  nearly  20  miles  to  the  southwest.  The 
Minnesota  mine,  in  Ontonagon  County,  was  opened  shortly  after. <* 

The  subsequent  history  of  the  copper  district  is  one  of  continuous  rapid  growth  with  only 
minor  fluctuations. 

o  In  the  following  history  of  the  Keweenaw  copper  district  the  authors  have  drawn  freely  on  the  excellent  brief  account  of  early  conditions 
in  the  Copper  handbook,  by  Horace  J,  Stevens. 

(■Stevens,  H,  J.,  Copper  handbook,  vol.  6,  1906,  p,  14, 
tidem,  vol,  2,  1902.  pp,  16-17, 
rfldem,  vol,  0,  1906,  p,  17. 

35 


36 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


The  following  table  of  annual  iiroduclion  sliows,  in  amount  ami  in  percentage,  tin'  relation 
of  Lake  Superior  shipments  to  those  of  the  United  States: 

Anmuil  production  of  Lake  Superior  copper  district,  compared  with  annual  production  of  United  States,  1850  to  1907. "^ 


Lake  Superior 

Lake  Superior 

Lake  Su 

>erior 

or  Michigan  dis- 

or Miciiigau  dis- 

or Mictiigau  dis-  | 

trict 

trict. 

trict 

Year. 

United 
States. 

Year. 

United 

States. 

Year. 

United 

States. 

Per- 

Per- 

Per- 

Amount. 

cent 
age. 

Amount. 

cent- 
age. 

Amount. 

cent- 
age. 

Long  tons. 

Lomi  tons. 

LoTUi  Ions. 

Long  tons. 

Long  tons. 

Long  tons. 

1850 

MO 

572 

88 

1871 

13.000 

11.942 

91 

1892 

154,018 

54.999 

30 

1851 

900 

779 

86 

1872 

12.500 

10.961 

87 

1X93 

147.0.3:! 

50,270 

34 

1852 

1,100 

792 

72 

1873 

15.500 

13.433 

86 

1894 

158.120 

51,031 

32 

1853 

2.0OO 

1,297 

65 

1874 

17,  .500 

15,  ,327 

87 

1895 

109.917 

57,7.37 

34 

1854 

2.250 

1,819 

81 

1875 

18,000 

16,089 

89 

1896 

205,384 

63,  418 

31 

1855 

3.000 

2,593 

86 

1876 

19,000 

17,085 

89 

1897 

220,571 

«i,706 

29 

1856 

4,000 

3.666 

91 

1877 

21,000 

17,  422 

83 

1898 

2.35.050 

06,056 

28 

1857 

4.800 

4.255 

88 

1S78 

21,500 

17,719 

82 

1899 

253.870 

fo,(103 

20 

1858 

5.500 

4,088 

74 

1879 

23,  OOO 

19, 129 

83 

1900 

269,111 

63.461 

24 

1859 

6.300 

3.985 

63 

I8.S0 

27.000 

22,  204 

82 

1901 

268,522 

09,501 

26 

1860 

7,200 

5,388 

74 

1881 

32,000 

24,363 

76 

1902 

294.297 

76. 050 

26 

1861 

7,500 

6,713 

89 

1S.S2 

40, 467 

25,439 

62 

1903 

311.582 

86,848 

27 

1862 

9.000 

6,005 

67 

1883 

51,574 

26. 663 

51 

1904 

.362. 739 

93,001 

26 

1863 

8,500 

5,797 

68 

1884 

64, 708 

30, 961 

47 

1905 

402,704 

102.874 

25 

1864 

8,000 

5,576 

69 

1885 

74,052 

32,209 

43 

1900 

409. 414 

102.514 

25 

1865 

8,500 

6,410 

75 

1S86 

70,4.30 

36, 124 

51 

1907 

386. 655 

96, 480 

25 

1866 

8,900 

6,l38 

69 

1887 

81,017 

33.941 

42 

1908 

420.953 

99,  408 

23 

1867 

10,000 

7,824 

78 

1888 

101,054 

38.604 

38 

1909 

502,  425 

103,290 

20.  S 

1868 

11,  (»0 

9,346 

80 

1889 

101,239 

39.364 

38 

1910 

493,705 

99,545 

20 

1869 

12,500 

11, .886 

95 

1890 

115,966 

45.273 

39 

1870 

12,600 

10,992 

87 

1891 

126,839 

50,992 

40 

c  Stevens,  H.  J.,  op.  cit.,  vol.  9,  1909,  p.  1594.     Production  for  1909  and  1910  from  Engineering  and  Mining  Journal, 

For  many  years  the  district  held  first  place  as  a  producer  of  copper  ore  in  the  United  States, 
and  in  total  production  it  is  stUl  first;  but  in  1887  and  later  years,  except  1891,  its  annual  ship- 
ments have  been  surpassed  by  those  of  the  Butte  district  of  Montana  and  since  1904  by  the 
copper  districts  of  Arizona. 

The  deposits  first  to  be  developed  were  the  transverse  fissure  veins,  rich  m  mass  copper, 
cutting  across  the  strike  of  the  beds  in  the  Eagle  Harbor  region,  at  the  northeast  end  of  the 
district.  The  Cliff  mine  was  discovered  by  Charles  T.  Jackson  in  1845.  Production  contmued 
in  this  district  until  1895.  It  is  now  inactive  but  has  been  newly  explored  with  a  view  to  a 
reojienmg. 

Next  to  be  developed  were  the  vein  or  mass-copper  deposits  following  the  trend  of  the 
Keweenaw  beds  in  Ontonagon  County,  at  the  southwestern  end  of  the  district.  The  presence 
of  copper  in  this  district  was  known  for  many  years,  but  systematic  mining  was  not  started 
until  a  few  years  after  the  Eagle  River  district  was  opened.  The  principal  mines  were  the  Min- 
nesota (now  the  Michigan),  the  National,  and  the  Mass.  The  Minnesota  was  discovered  in  1847 
by  S.  O.  Knapp,  through  surface  indentations  of  ancient  workings.  In  one  of  these  was  found 
a  mass  of  copper  weighing  6  tons,  together  with  rotted  timbers  on  wMch  it  had  been  supported. 
The  first  shipment  from  tints  mine  was  made  in  1848,  and  for  fourteen  years  70  per  cent  of  the 
ore  was  "mass."  The  opening  of  the  Minnesota  mine  was  followed  by  that  of  the  National, 
Mass,  and  other  mines.  The  district  is  still  actively  producing,  but  prmcipally  from  the  ain^g- 
daloidal  beds,  mass  copper  at  present  (1908)  constituting  only  about  25  per  cept  of  the  ore 
produced. 

The  am3-g(ialoid  deposits  of  the  central  part  of  the  district  were  the  next  to  receive  atten- 
tion. The  first  of  these  deposits  was  discovered,  in  1848,  on  the  present  Pewabic  location,  and 
the  second  on  the  Isle  Royal  location.  The  Quincy  had  been  opened  in  1847  on  a  transveise 
vein,  but  the  Quincy  ain3-gdaloid  was  not  found  until  1S5(),  the  same  year  that  the  main 
"Pewabic"  bed  was  found.  During  1856  the  Quincy  proihiced  13,462  pounds  of  cojiper.  liut 
it  did  not  become  profita])le  until  ISOO.  In  1877  tlie  Osceola  amygdaloid  was  discoveretl,  and 
that  3'ear  the  Osceola  mine  pi-oduced  2,744,777  pounds  of  coj)per.  The  'Wolvei'ine  was  opened 
before  1890  but  was  not  profitable  until  1S97.     The  Atlantic  niuie  was  openeil  in  1872.     The 


HISTORY  OF  LAKE  SUPERIOR  MINING. 


37 


richest  amygdaloid  bed  in  the  district  is  tire  "Baltic,"  whicii  was  ffrst  f)r()ved  valuable  by 
the  Baltic  mine  in  1S97,  and  a  few  years  later  was  discovered  on  the  Champion  location. 

The  amygdaloid  deposits  are  now  the  most  numerous,  and  in  1907  produced  73.1  per  cent 
of  the  total  copper  ore  of  the  district,  of  which  about  75.5  per  cent  came  from  Houghton  County. 
A  larger  proportion  of  the  production  will  come  from  the  amj^gdaloids  in  the  future. 

The  last  of  the  inincipal  types  of  deposits  to  be  discovered  were  those  in  the  Allouez  con- 
glomerate and  the  Calumet  and  Hecla  conglomerate.  Both  conglomerates  were  discovered  by 
E.  J.  Hulbert  and  associates.  The  Allouez  conglomerate  was  found,  in  1S59,  at  tlie  site  of  the 
Allouez  mine,  and  was  worked  for  a  short  time,  but  soon  proved  to  be  unj)roductive,  in  this 
locality  at  least.  Later  it  was  found  to  be  productive  farther  south,  on  the  Boston  and  Albany 
location,  later  the  Peninsula  and  now  the  Franklin  Junior.  The  Allouez  conglomerate  has 
jdelded  but  little  profit. 

The  site  of  the  Calumet  and  Hecla  was  bought  by  Hulbert  in  1860,  the  evidence  being  a 
number  of  copper-bearing  conglomerate  bowlders  and  a  few  depressions,  sucii  as  in  other  parts 
of  the  district  were  found  to  indicate  ancient  workings.  In  1S69  Hulbert  and  liis  associates 
returned  to  the  spot  and  dug  through  an  amygdaloid  into  the  conglomerate  bed.  The  Calumet 
and  Hecla  paid  their  first  dividends  in  1869  and  1870.  Up  to  January,  1910,  ths  dividends  of 
the  Calumet  and  Hecla  have  aggregated  .1110,550,000  on  a  capital  of  .S2, 500,000. 

The  table  below  shows  the  relation  in  percentage  of  the  annual  production  of  the  Calumet 
and  Hecla  mine,  from  1867  to  1908,  to  the  amiual  production  of  the  Alichigan  district  for  the 
same  period. 


Percentage  of  total  Michigan  copper  production  produced  by  the  Calumet  and  Hecla  mine,  1867  to  1908." 


1867. 
1868. 
1869. 
1870. 
1871. 
1872. 
1873. 
1874. 
187.5. 
1876. 
1877. 
1878. 
1879. 
1880. 


7 

5 

24 

4 

46 

0 

57. 

0 

61 

0 

66. 

0 

62 

5 

5S 

5 
5 

59. 

56. 

5 

60. 

0 

63. 

5 

61. 

0 

63. 

5 

1881 

57.5 

1882 

56.0 

1883 

52.5 

1884.. 

58.0 

1885... 

.  65.'5 

1886 

62.5 

1887 

60.5 

1888 

58.0 

1889 

55  0 

1890 

. .  59.  0 

1891 

56.  5 

1892 

4fi.  0 

1893                     53  5 

1894 

53.5 

1895. 
1896. 
1897. 
1898. 
1899. 
1900. 
1901. 
1902. 
1903. 
1904. 
1905. 
1906. 
1907. 
1908. 


61.5 
63.0 
58.0 
58.5- 
61.0 
54.5 
53.5 
47.6 
39.8 
38.7 
41.2 
43.6 
37.9 
36.6 


The  onh"  other  mine  now  operating  on  the  conglomerates  is  the  Tamarack,  opened  m  1881. 
COPPER  illNING  ON  ISLE  ROYAL  AND  ELSEWHERE. 

Isle  Royal  is  unusually  rich  m  interesting  evidences  of  prehistoric  copper  mining.  The 
first  minmg  of  historical  record  was  begun  soon  after  the  opening  of  Keweenaw  Point,  in  1844, 
culminating  in  1847  and  1848  and  wanmg  in  1855,  when  the  island  was  again  without  perma- 
nent inhabitants.  Another  brief  period  of  development,  from  1871  to  1883,  resulted  in  the 
opening  on  the  island  of  the  Saginaw  and  Minong  mines,  with  a  combined  production  of  less 
than  10,000  tons  of  copper.  Since  the  nineties  exploration  has  been  going  on  intermittently, 
but  without  success.  No  mines  are  operating  at  the  present  time.  The  ores  are  essentially  the 
same  as  those  of  Keweenaw  Point.  As  mined  they  were  low  grade,  probably  less  than  1  per 
cent.     They  occur  principally  in  fissure  veins  m  the  traps. 

The  copper-bearing  formation  has  been  found  elsewhere  in  the  Lake  Superior  region,  but 
the  copper-mining  industry  has  practicallj"  not  extended  beyond  Keweeiraw  Point  and  Isle 
Royal.     The  southwestern  extension  of  the  Keweenaw  district  in  Wisconsin  and  Minnesota  is 


a  Calculated  from  data  in  Stevens's  Copper  hand  book,  vol.  9, 1909. 


38  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

being  extensively  exploted  and  opened  for  mining,  l)ut  thus  far  the  production  has  not  been 
important.  As  the  copper-producing  area  has  been  restricted  to  that  of  the  early  discoveries 
and  as  the  co]iper-mining  industry  has  developed  evenly,  it  is  unnecessary'  for  our  purposes  to 
follow  its  lustory  in  greater  detail. , 

IVLiRQUETTE  IRON  DISTRICT  (1848). 

Iron  was  first  discovered  in  1844  near  the  site  of  Negaunee  by  the  Government  linear 
surveying  party  in  charge  of  William  A.  Burt,  himself  under  the  direction  of  Douglass 
Houghton.  The  Michigan  legislature  having  failed  in  1843  to  renew  appropriations  for  the 
Michigan  Survey,  Dr.  Houghton  had  turned  to  the  Federal  Government  and  had  succeeded 
in  procuruig  an  additional  allowance  per  mile  for  geologic  work  in  connection  with  the  linear 
survey  of  the  Upper  Peninsula,  which  liad  already  been  begiui,  and  he  iiimself  took  the  contract 
for  the  linear  survey  in  order  to  have  the  direction  of  the  work. 

In  1848  iron  ore  was  mined  by  the  Jackson  Association  (subsequently  the  Jackson  Iron 
Companj^)  and  carried  by  team  to  a  Catalan  forge  which  they  had  constructed  near  Carp  River. 
The  project  was  not  commercially  successful  and  was  closed  in  1850.  The  ilarquette  Iron  Com- 
pany opened  the  Cleveland  mine  near  the  present  town  of  Ishpeming  in  1849  and  carted  its  ore 
to  a  forge  at  Marquette.  This  also  was  a  financial  failure  and  was  discontinued  in  1854.  In 
1850  and  again  m  1852  a  few  tons  of  ore  were  shipped  from  the  district  to  Pemisylvania  for 
trial  in  Pennsylvania  furnaces.  The  openmg  in  1855  of  the  ship  canal  along  St.  Marys  River, 
connecting  Lake  Huron  and  Lake  Superior,  was  followed  in  1856  by  the  first  regular  shipments 
of  iron  ore  from  the  Marquette  district  to  the  lower  lakes,  amoimtmg  to  6,.343  tons.  Up  to  that 
time  the  local  forges  had  consumed  about  25,000  tons  of  ore.  The  completion  in  1857  of  the 
Iron  Mountain  Railway  (later  the  Houghton,  Marquette  and  Ontonagon  Railway  and  ulti- 
mately the  Duluth,  South  Shore  and  Atlantic  Railway)  between  Marquette  and  the  mines 
gave  easy  transit  to  the  lake,  and  22,876  tons  were  shipped  in  1858  and  five  times  that  amoimt 
in  1860. 

From  1855  to  1862  transportation  facilities  were  so  far  improved  as  to  make  it  possible  to  get  ore  out,  but  the 
mines  had  not  yet  been  really  brought  into  relation  with  the  iron  market.  Therefore  the  companies  met  with  no  real 
success  whether  they  tried  to  make  iron  themselves  or  to  send  their  ore  down  to  the  furnaces  of  Ohio  and  Pennsyl- 
vania. The  Lehigh  Valley,  and  not  Pittsburg,  was  still  the  iron  center  of  the  United  States.  The  war  suddenly 
changed  the  whole  outlook.  A  great  demand  sprang  up  for  all  kinds  of  iron  goods,  and  both  mining  and  iron  making 
on  the  Upper  Peninsula  received  a  strong  impetus.  Shipments  increased  from  49,000  tons  in  1861  to  five  times  that 
amount  in  1864,  while  the  companies  made  fabulous  profits.  *  *  *  The  year  1865  marked  a  slight  retrogression, 
but  the  eight  years  following  saw  a  wonderful  growth,  the  boom  in  iron  and  steel  reflecting  the  rapid  industrial  develop- 
ment of  the  country,  and  from  1870  to  1873  registering  its  speculative  excitement.  *  *  *  In  1863  but  three  mines 
shipped  ore;  in  1864,  five;  in  1865,  seven;  in  1866,  nine;  1868  added  tour  more  mines,  1870  three  more,  while  in 
1872  the  table  of  shipments  increases  the  total  number  of  mines  by  11  to  29,  and  in  1873  no  less  than  40  are 
represented.  The  total  shipments  of  1866  were  just  below  300,000  tons;  those  of  1873  almost  exactly  foiu-  times  that 
amount." 

The  opening  of  the  Republic,  Michigamme,  and  Spurr  mines  in  1872  practically  completed 
the  area  of  the  Marquette  district  as  known  at  present,  though  a  few  discoveries  of  importance 
have  been  made  within  the  area  since  that  time.  Exploration  is  still  vigorous.  The  field 
for  deep  exploration  opened  by  recent  discoveries  is  a  large  one. 

The  necessary  increase  in  means  of  shipment  was  made  by  the  building  of  the  Chicago  and 
Northwestern  Railway  from  Negaunee  to  Escanaba  and  bj^  increase  in  the  capacit}'  of  the 
docks  already  built  at  Marquette.     As  a  result  of  the  panic  of  1873 — 

development  work  ceased,  production  fell  off  almost  25  per  cent  in  1874  and  yet  further  in  1875,  and  the  number  of 
mines  reporting  shipments  declined  from  40  in  1873  to  33  in  1874  and  to  29  in  1875.  The  working  force  of  those 
that  continued  operations  was  largely  reduced,  and  only  five  mines  showed  a  larger  output  in  1874  than  in  1873. & 

a  Mussey,  H.  R.,  Combination  In  the  mining  industry:  Studies  in  history,  economics,  and  public  law,  Columbia  Univ.,  vol.  23,  No.  3, 1905, 
pp.  55,57,  59. 
I>  Idem,  p.  73. 


HISTORY  OF  LAKE  SUPERIOR  MINING.  39 

Returning  prosperity  brought  an  increase  in  shipments  of  80  per  cent  between  1878  and 
1882,  and  tlie  number  of  producing  mines  increased  from  29  in  1875  and  1877  to  48  in  1882. 
The  foUowmg  year  saw  a  considerable  depression  because  of  overproduction,  but  thenceforth 
the  production  showed  a  general  increase  until  1891,  with  a  minor  depression  in  1885.  The 
years  1891  and  1893  saw  another  falling  off  in  production,  the  latter  contemporaneous  with  the 
general  panic  of  1893.  From  that  time  to  the  present  there  has  been  a  general  increase  in 
production,  with  shght  recessions  in  1904  and  1906.  The  Lake  Superior  and  Ishpeming  Railway 
was  constructed  in  1896  to  carry  the  ores  of  the  Cleveland-Chffs  Iron  Company  from  the  Ish- 
peming district  to  Lake  Superior. 

The  table  of  production  of  iron  ore  from  the  Marquette  range  (pp.  51-60)  summarizes  the 
development  of  the  district. 

The  Swanzy  district,  southeast  of  the  Marquette  district  proper,  is  reached  by  the  Chicago 
and  Northwestern  Railway;  its  production  is  usually  credited  to  the  Marquette  district.  The 
district  was  first  explored  in  1869,  and  the  Smith  (later  the  Cheshire  and  Princeton)  mine  was 
opened  in  1871.  Systematic  exploration  by  drilling,  begun  in  1902  by  the  Cleveland-Cliffs 
Company,  greatly  extended  the  ore  reserves  and  determined  the  probable  limits  of  the  district. 
A  largely  increased  production  may  be  looked  for. 

MENOMINEE  IRON  DISTRICT  (1872). 

The  Marquette  district  had  been  the  sole  producer  of  iron  ore  in  the  Lake  Superior  region 
for  nearly  thirty  years  when  its  first  competitor,  the  Menominee  district,  entered  the  field. 

The  first  practical  discovery  of  iron  ore  in  Menominee  County  was  made  by  the  brothers  Thomas  and  Bartley 
Breen  some  time  previous  to  1867,  though  the  veteran  explorer  S.  C.  Smith  claims  to  have  been  and  probably  was 
aware  of  its  existence  in  that  section  as  early  as  1855,  in  which  year  he  traversed  what  he  called  a  new  range,  south 
and  east  from  Lake  Michigamme  to  Escanaba,  locating  what  is  now  the  estate  of  the  Republic  Iron  Company  on  the 
way.  The  first  practical  work  in  the  way  of  development  was  done  by  N.  P.  Hulst  for  the  Milwaukee  Iron  Company 
at  the  Breen  and  Vulcan  mines  in  1872,  and  by  John  L.  Buell  at  the  Quinnesec  the  following  year." 

The  existence  of  ore  in  shipping  quantity  had  been  demonstrated  in  1874,  but  the  distance 
of  the  district  from  the  Great  Lakes  and  the  lack  of  facilities  for  shipment  prevented  its  further 
development  until  the  extension  of  the  Menominee  branch  of  the  Cliicago  and  Northwestern 
Railroad  from  Escanaba  to  Quinnesec.  This  was  carried  through  to  Iron  Mountain  in  1880,  and 
thence  northwest  to  Iron  River  and  the  Gogebic  range.  The  Chicago,  Milwaukee  and  St.  Paul 
Railway  entered  the  district  in  1886  and  the  Wisconsin  and  Michigan  Railway  in  1903.  When 
shipment  had  once  started  it  increased  much  more  rapidly  than  that  of  the  Marquette  district. 
The  first  year's  output  of  10,405  tons  jumped  to  95,221  tons  the  following  year,  to  269,609 
tons  the  third  year,  and  to  592,086  tons  the  fourth  year,  and  reached  the  million  mark  in  1882. 
In  1901,  1902,  and  1903  the  Menominee  surpassed  the  Marquette  range  in  shipment,  but  for 
the  most  part  in  later  j'ears  it  has  been  producmg  about  the  same  amount  yearly  as  the  Mar- 
quette district.  Its  total  slupment  to  the  end  of  1909  is  71,212,121  tons  as  compared  with  a 
total  of  91,838,558  tons  from  the  Marquette  district.  The  table  (pp.  61-65)  includes  shipments 
from  the  outlying  Florence,  Iron  River,  and  Crystal  Falls  districts  to  the  northwest. 

CRYSTAL  FALLS,  FLORENCE,  AND  IRON  RIVER  IRON  DISTRICTS  (1880). 

The  Crystal  Falls,  Florence,  and  Iron  River  districts  may  be  regarded  as  northwesterly 
outliers  of  the  Menominee  range,  and  they  are  included  in  it  in  tables  of  production. 

For  a  number  of  years  after  the  opening  of  the  Menominee  range  prospectors  worked  in  various  places,  among 
others  in  the  vicinity  of  Crystal  Falls,  seeking  to  follow  the  iron  range  west  of  the  Menominee  River.  As  a  result 
of  this  endeavor,  the  deposits  at  Florence,  Wis.,  and  then  those  farther  north  and  west  at  Crystal  Falls,  Mich.,  were 
in  turn  located.  It  was  not  until  1881  that  sufficient  exploratory  work  had  been  done  at  Crystal  Falls  to  warrant  a 
belief  in  the  future  of  this  iron-bearing  area.    In  April,  1882,  the  Chicago  and  Northwestern  Railway  completed 

o  Swineford,  A.  P.,  .\nnual  review  of  the  iron  mining  and  other  industries  of  the  Upper  Peninsula  for  the  year  ending  December  31, 1881;  Mar- 
quette, 1882,  p.  119. 


40  GEOLOGY  OF  THE  LAIvE  SUPERIOR  REGION. 

its  branch  to  Crystal  Falls,  and  the  shipment  of  ore  began.  The  Amasa  deposits  were  not  exploited  to  any  great  extent 
until  the  year  1888,  when  the  Chicago  and  Northwestern  Railway  built  a  branch  from  Crystal  Falls  to  Amasa.  The 
Chicago,  Milwaukee  and  St.  Paul  Railway  in  1893  completed  a  line  from  Channing  to  Sidnaw,  which  runs  through 
Amasa." 

These  districts  have  as  a  whole  (hn-cloped  slowly  as  compared  witli  tlio  other  principal 
iron  districts  of  the  Lake  Superior  country,  partly  because  of  the  slightly  lower  grade  of 
many  of  the  ore  bodies  and  partly  because  of  the  lack  of  exposure,  making  exploration  difricult 
and  costly.  Consequently  large  areas  remain  to  be  tested  underground.  The  increasing 
demand  for  iron  ore  of  the  lower  grades  has  brought  about  a  revival  of  exploration  in  this  area 
during  the  last  few  years.  This  is  one  of  the  most  promising  fields  of  exploration  yet  remain- 
ing ui  the  Lake  Superior  region,  and  the  next  few  years  are  likely  to  see  large  developments. 

GOGEBIC  IRON  DISTRICT  (1884). 

The  Gogebic  range  of  Michigan  and  its  extension,  the  Penokee  range  of  Wisconsin,  some- 
times referred  to  together  as  the  Penokee-Gogebic  district,  were  long  known  to  explorers  and  had 
been  mapped  by  the  geologists  of  the  Michigan  and  Wisconsin  surveys  prior  to  their  opening 
in  1884.  The  first  recorded  notice  of  their  discovery  appears  on  the  plats  of  the  township 
surveys.  It  is  remarkable  that  subsequent  discoveries  have  been  restricted  to  the  areas 
first  determined  by  the  geologic  mappmg.  Early  exploration  was  largely  confined  to  the  weU- 
exposed  magnetic  portions  of  the  formation  at  the  west  end  of  the  range,  which  have  been 
less  productive  than  the  central,  less  well  exposed  portions  of  the  iron-bearing  formation. 
In  1884  the  first  shipment  of  1,022  tons  was  made  from  the  Colby  mine  to  Marquette.  In 
the  following  year  the  shipment  reached  119,860  tons,  owing  to  good  transportation  facilities 
and  to  the  remarkable  speculation  wliich  in  1886  and  1887  led  to  the  formation  of  mining 
companies  in  this  district  with  a  nominal  capital  exceeding  $1,000,000,000.  The  inevitable 
collapse  in  the  fall  of  1887  took  the  savings  of  smaller  investors  and  many  mines  were  closed 
down,  but  the  stronger  companies  weathered  the  storm  and  in  spite  of  the  speculative  failure 
the  production  of  ore  steadily  increased  until  1890,  when  for  a  period  of  several  years  the 
shipments  reflected  the  depressed  and  unstable  conditions  which  affected  the  Lake  Sui)erior 
region  as  a  whole.  In  the  autumn  of  1885  the  Milwaukee,  Lake  Shore  and  Western  Rail- 
way (subsequently  part  of  the  Chicago  and  Northwestern)  was  finished  froni  the  mines  to 
Ashland. 

The  Wisconsin  Central  Railway  crossed  the  range  at  Penokee  Gap  in  1873,  connecting 
with  Ashland,  and  in  1887  extended  a  branch  to  the  center  of  the  district.  The  Duluth,  South 
Shore  and  Atlantic  Railway  already  paralleled  the  range  on  the  north  at  the  time  of  its  dis- 
covery and  afforded  easy  connection  with  the  lake. 

VERMILION  IRON  DISTRICT  (1885). 

J.  M.  Clements  describes  the  opening  of  the  Vermilion  district,  in  Minnesota,  as  follows:'' 

The  first  mention  of  the  occurrence  of  iron  ore  in  the  Vermilion  district  was  made  by  J.  G.  Norwood,  who  obser^-ed 
it  during  his  explorations  in  1850  and  published  a  statement  concerning  it  in  the  report  accompanjdng  that  of  D.  D. 
Owen.c  The  iron  he  observed  is  that  which  occurs  near  Gunfiint  Lake,  at  the  extreme  ea.st  end  of  the  district,  and 
which  geologically  belongs  vnth  the  ores  of  the  Mesabi  range.  In  this  part  of  the  Vermilion  district  the  ores  have 
never  been  exploited  to  any  extent  and  are  at  present  of  little  commercial  importance. 

Interest  in  what  is  now  known  as  the  Vermilion  iron-bearing  district  was  aroused  in  the  .sixties  by  the  reported 
occurrence  of  gold  in  the  \-icinity  of  Vermilion  Lake.  There  was  considerable  excitement  tor  several  years  and  a 
small  rush  to  the  district.  Shafts  were  sunk  and  stamp  mills  were  erected,  the  machinery  ha\ing  been  jiacked  in 
from  Duluth  over  the  Vermilion  tr^il.  A  town  site  was  laid  out  near  Pike  River,  at  the  southwest  extremity  of  \'er- 
milion  Lake,  and  some  buildings  were  erected.  In  all  a  good  deal  of  money  was  fruitlessly  expended,  as  no  gold 
deposits  of  any  importance  were  found. 

a  Clements,  J.  M.,  and  Smyth,  H.  L.,  The  Crystal  Falls  iron-bearing  district  of  Michigan:  Mon.  V.  S.  Geol.  Survey,  vol.  30,  1899,  p.  175. 
kClcnients,  J.  M.,  The  Vermilion  iron-bearing  districi  of  .Minnesota:  Mon.  U.  S.  Geol.  Survey,  vol.  45,  1903,  pp.  213-215. 
c  A  report  of  the  geological  survey  of  Wisconsin,  Iowa,  and  Minnesota,  1852,  p.  417. 


HISTORY  OF  LAKE  SUPERIOR  MINING.  41 

Some  time  after  this,  in  1875,  the  first  exploration  for  iron  ore  in  this  district  was  taken  up  by  Mr.  George  R. 
Stuntz,  accompanied  by  Mr.  John  Mailman,  who  began  to  prospect  the  iron  formation  and  iron  ore  exposed  on  Lee 
Hill,  southwest  of  the  Bay  of  Vermilion  Lake,  which  is  now  known  as  Stuntz  Bay,  named  after  Mr.  Stuntz.  The 
ore  deposits  on  Soudan  Hill  were  then  discovered.  In  1880  Prof.  A.  H.  Chester  examined  the  Vermilion  Lake  iron 
f>>rmation  for  private  parties  and  Mr.  Bailey  Willis  studied  it  for  the  Census  Office.  Systematic  and  extensive  efforts 
were  made  in  the  late  seventies  and  the  early  eighties  to  develop  the  iron  ores.  By  this  time  the  Minnesota  Iron 
Company  had  been  organized  and  all  of  the  properties  which  at  that  time  were  known  to  contain  ore  and  great  stretches 
of  country  which  were  in  the  continuation  of  the  ore  range  had  been  purchased,  the  company  owning  over  20,000 
acres  of  land  on  the  Vermilion  range  proper  and  in  the  vicinity  of  the  good  harbor  on  Lake  Superior  known  now  as 
Two  Harbors.  On  August  1,  1884,  the  Duluth  and  Iron  Range  Railroad  was  completed  from  Two  Harbors  to  Tower, 
near  Vermilion  Lake.  This  road  was  72  miles  long.  At  a  later  date  it  was  connected  with  Duluth,  25  miles  away. 
During  the  first  year  (1884)  62,124  tons  of  ore  were  shipped,  some  of  this  having  come  from  the  stock  piles  which 
had  been  growing  during  the  years  of  development  preceding  the  opening  of  the  railroad. 

Prospectors  were  busy  in  the  years  prior  to  the  opening  of  the  railroad  in  prospecting  the  district  to  the  east 
of  Tower,  and  in  1883  outcrops  of  ore  were  found  by  Mr.  H.  R.  Harvey  in  sec.  27,  T.  63  N.,  R.  12  W..,  near  the  present 
town  of  Ely.  The  body  of  iron  ore  indicated  by  these  outcrops  was  further  tested  in  1885-6  and  led  to  the  opening 
up  of  the  great  deposits  at  Ely  on  which  are  now  working  the  Chandler,  Pioneer,  Zenith,  Sibley,  and  Savoy  mines. 
During  1888  there  were  shipped  from  the  Chandler  mine  54,612  tons  of  high-grade  ore. 

••  From  this  time  on  the  development  of  the  range  was  rapid  and  steady,  as  is  shown  by  the  annual  increase  in  the 
shipments  of  ore. 

The  Vermilion  range  was  thus  opened  at  about  the  same  time  as  the  Gogebic  range,  but 
its  mines,  in  contrast  to  those  of  the  Gogebic,  were  from  the  start  in  the  hands  of  a  strong 
company,  which  controlled  the  railroad  and  prevented  active  competition.  To  quote  from 
Mussey : 

A  comparison  of  the  output  of  the  two  ranges  by  years  discloses  an  interesting  contrast  between  centralized  control 
backed  by  adequate  capital  in  the  Vermilion  district  and  competitive  exploitation  based  on  small  undertakings  and 
insufficient  funds  in  the  Gogebic  district.  The  Gogebic  district,  which  was  not  really  opened  up  till  1885,  in  the  second 
year  following  produced  more  than  a  million  tons;  the  A'ermilion,  though  opened  a  year  earlier,  did  not  reach  the 
million  mark  till  1892,  when  the  Ciogebic  produced  almost  three  millions,  only  to  fall  off  to  less  than  half  that  amount 
the  next  year.  Production  on  the  Gogebic  moves  upward  by  leaps  and  starts,  one  season  rising  to  excess,  the  next 
sinking  back  to  deficiency;  the  output  of  the  Vermilion,  on  the  other  hand,  climbs  with  a  regularity  that  is  surprising, 
when  one  considers  the  variable  conditions  of  the  market  in  which  it  had  to  be  sold.'' 

MESABI   IRON   DISTRICT    (1891). 

ACCOUNTS    OF   THE   DISTRICT  BEFORE   ITS    OPENING. 

In  penetratmg  the  vast  wilderness  north  and  west  of  the  Great  Lakes  country,  the  early 
explorers  were  compelled  for  the  most  part  to  stick  close  to  the  waterways,  for  the  nature  of 
the  country  made  travel  for  long  distances  exceedingly  arduous  by  any  other  method  than 
'canoeing.  Three  of  tlie  canoe  routes  to  the  country  northwest  of  Lake  Superior  cross  the  Giants 
or  Mesabi  Range*  and  its  eastward  continuation.  Mississippi  River  and  its  tributaries,  Prairie 
and  Swan  rivers,  touch  the  western  portion  of  the  district.  Embarrass  Lake,  tributary  to 
St.  Louis  River,  and  thence  to  Lake  Superior  and  the  St.  LawTcnce,  crosses  the  Giants  Range 
near  its  east-central  portion.  Gunfiint  Lake,  one  of  a  chain  of  lakes  tributary  to  Rainy  River 
and  Nelson  River  and  thence  to  Hudson  Bay,  lies  far  to  the  east,  on  a  continuation  of  what  is 
now  known  as  the  Mesabi  district.  Hence  the  first  published  references  to  the  Mesabi  district 
concern  the  parts  of  the  district  immediately  adjacent  to  these  canoe  routes.  Brief  descriptions 
of  Pokegaana  Falls  on  Mississippi  River  and  of  adjacent  areas  were  made  by  Z.  M.  Pike  in 
1810,  by  James  Allen  and  Henry  R.  Schoolcraft  in  1832,  and  by  J.  N.  Nicollet  in  1841.  In 
1841  also  Nicollet  published  his  map  of  the  hydrograpliic  basin  of  the  upper  Mississippi,  on 
which  the  Giants  or  Mesabi  Range,  called  "Missabay  Heights,"  was  for  the  first  time  delineated, 

a  Mussey,  H.  R.,  Combination  in  the  mining  industry:  Studies  in  history,  economics,  and  public  law,  Columbia  Univ.,  vol.  23,  No.  3,  pp. 
90-91. 

t  The  name  "Mesabi"  has  been  variously  spelled  and  applied  with  various  limits  to  the  ridges  of  this  district,  and  the  use  of  the  same  term 
to  denote  the  iron-l>earing  district  as  such  has  added  to  the  confusion.  The  spelling  "Mesabi"  has  been  adopted  by  the  United  States  Geo- 
graphic Board.  It  has  become  usual,  for  the  sake  of  clearness,  to  speak  of  the  main  topographic  feature  as  the  Giants  Range.  In  this  report  the 
terms  are  definitely  distinguished,  Mesabi  range  being  applied  only  to  the  iron-bearing  district  that  occupies  a  linear  belt  of  low  sloping  land  at 
the  base  of  the  Giants  Range. 


42  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

by  hachures,  although  very  imperfectly.  In  18.52  J.  G.  Norwood  reported  the  occurrence  of 
iron-bearinj^  rocks  at  Gunflint  Lake  and  mentioned  fjranite  and  gneiss  seen  in  crossing  the 
range  at  Embarrass  Lake.  In  1866  Charles  'V\liittles<^y  reported  on  explorations  made  m 
northern  Minnesota  durmg  the  years  1848,  1859,  and  1864.  He  mentioned  Pokegama  Falls  and 
made  vague  reference  to  the  granitic  rocks  of  the  range.  "Mesal)i  Range"  was  used  in  an 
indefinite  way  to  cover  what  are  now  known  as  the  Giants  and  Vermilion  ranges.  In  1866,  also, 
Henry  H.  Fames,  the  first  state  geologist  of  Minnesota,  reported  granite  and  gneiss  seen  on  a 
trip  across  the  range  at  Emliarrass  Lake.  In  describing  the  ranges  of  the  northern  part  of  the 
State,  including  the  "Missabi  Wasju,"  he  stated  that  they  appear  to  be  traversed  by  metal- 
bearing  veins.  Presumably,  however,  this  statement  refers  mainly  to  the  Vermilion  range.  In 
a  second  report,  published  the  same  year,  Mr.  Fames  is  more  explicit,  and,  referring  to  the 
general  elevated  area  of  the  northern  part  of  the  State,  including  tlie  Giants  Range,  states: 
"In  this  region  are  found  also  immense  bodies  of  the  ores  of  iron,  both  magnetic  and  hematitic, 
occurring  in  dikes  and  associated  with  tlie  rock  in  which  it  is  found;  in  some  of  these  formations 
iron  enters  so  largely  into  its  composition  as  to  affect  the  magnetic  needle."  Pokegama  Falls 
and  Prairie  River  Falls  were  visited,  and  at  the  latter  place  the  presence  of  "iron  ore "  was  noted. 
Tlu'se  reports  of  Fames  contain  the  first  references  to  iron  ore  in  the  Mesabi  district  proper, 
although  iron-bearing  rocks  had  been  noted  by  Norwood  in  1852  at  Gunflint  Lake. 

From  this  time  on  desultory  exploration  work  was  done  in  certain  portions  of  the  district. 
It  was  confined  for  the  most  part  to  the  area  west  of  Birch  Lake,  in  Rs.  12,  13,  and  14  W., 
and  to  the  vicinity  of  Prairie  River.  No  published  accounts  of  the  earlier  portion  of  this  explora- 
tory work  are  to  be  found. 

The  first  examination  of  the  Giants  Range  by  a  mining  expert  with  particular  reference  to 
the  occurrence  of  iron  ore  in  workable  deposits,  noted  in  print,  was  made  in  1875  by  A.  H. 
Chester,  of  Hamilton  College,  New  York.  Reaching  the  Giants  Range  at  Embarrass  Lake,  he 
worked  eastward  toward  Birch  Lake.  In  his  report  (published  in  1884)  he  called  attention  to 
the  magnetic  character  of  the  iron  m  this  area  and  to  the  fact  that  the  alternating  iron  laj^ers 
are  not  thick  or  continuous.  The  percentage  44.68  was  given  as  a  fair  average  of  iron  in  the 
rocks  of  this  part  of  the  district.  In  general,  one  gathers  the  impression  that  he  was  not  favor- 
ably impressed  with  the  economic  prospects  of  this  area.  Between  the  time  of  Chester's  exam- 
ination of  the  range,  in  1875,  and  the  publication  of  his  report,  in  1884,  N.  H.  Winchell,  state 
geologist  of  Minnesota,  briefly  noticed  the  Mesabi  district  in  two  of  his  reports.  In  1879  he 
told  of  the  occurrence  of  iron  ore  in  R.  14  W.  and  published  analyses.  In  1881  he  told  of  a 
trip  from  Embarrass  Lake  east  to  range  14  and  noted  the  magnetic  character  of  the  iron-bearing 
formation  in  range  14,  as  well  as  its  similarity  to  the  formation  at  Gunflint  Lake.  Indeed,  the 
iron-bearing  formation  in  range  14  was  called  the  "Gunflint  beds."  In  1883  Irving  called  the 
iron-bearing  rock  series  in  the  Mesabi  district  Animikie,  a  term  which  had  been  applied  to  similar 
rocks  at  Thunder  Bay  and  westward  to  Gunflint  Lake,  and  correlated  the  Animikie  rocks  with 
the  original  Huronian  rocks  of  the  north  shore  of  Lake  Huron  and  with  the  iron-bearmg  forma- 
tion and  associated  rocks  of  the  Penokee-Gogebic  iron  range  of  Michigan  and  Wisconsin.  From 
this  time  on  the  term  Animikie  is  much  used  in  the  literature  on  the  Mesabi  range  to  designate 
the  iron-bearing  formation  and  associated  rocks.  In  1884,  in  the  same  volume  in  which  Chester's 
report  was  published,  N.  H.  WincheU  discussed  the  age  of  the  Mesabi  rocks,  assignmg  them  to 
the  "Taconic,"  then  regarded  as  Lower  Cambrian,  and,  following  Irving,  correlated  them  with 
the  iron-bearing  rocks  of  the  Penokee-Gogebic  district.  In  the  late  eighties  a  number  of  other 
reports  on  the  district  were  issued  by  the  Minnesota  Survey,  but  they  contain  no  important 
points  not  noted  in  reports  above  cited.     This  brmgs  us  to  the  openmg  of  the  district  for  minmg. 

OPENING   AND   DEVELOPMENT. 

Since  the  late  sixties  there  had  been  more  or  less  exploration,  particularly  along  the  eastern 
portion  of  the  district,  from  Embarrass  Lake  to  Birch  Lake,  and  the  presence  of  iron-bearmg 
rocks  had  been  recognized  and  discussed  in  the  reports  mentioned  above.     However,  not  a  single 


HISTORY  OF  LAKE  SUPERIOR  MINING.  43 

deposit  of  iron  ore  of  such  size  and  character  as  to  warrant  mining  had  been  revealed.  In  fact, 
the  range  had  been  "turned  down"  by  many  mining  men  who  had  examined  it.  This  was 
largely  because  of  the  fact  that  they  confined  their  attention  principally  to  the  eastern,  magnetic 
end  of  the  range,  where  exposures  of  the  iron-bearing  formation  are  numerous.  Even  up  to  the 
present  time  no  ore  has  been  fovmd  there  in  quantity.  Yet  the  impression  was  gradually  develop- 
ing that  iron  ore  in  large  quantity  was  to  be  found  in  this  district,  and  a  few  prospectors  were 
working  diligently. 

Among  the  more  persistent  of  the  Mesabi  range  explorers  were  the  Merritts — Lon  Merritt, 
Alfred  :\Ierritt,  L.  J.  Merritt,  C.  C.  Merritt,  T.  B.  Merritt,  A.  R.  Merritt,  J.  E.  Merritt,  and  W.  J. 
Merritt — of  Duluth,  Minn.  Their  faith  in  the  range  was  the  first  to  be  rewarded.  On  November 
16,  1890,  one  of  their  test-pit  crews,  in  charge  of  J.  A.  Nichols,  of  Duluth,  struck  iron  ore  in  the 
NW.  i  sec.  .3,  T.  58  N.,  R.  18  W.,  just  north  of  what  is  now  known  as  the  Mountam  Iron  mine. 
This  was  followed  in  1891  by  the  discovery  of  ore  in  the  area  now  covered  by  the  Biwabik  and 
Cincinnati  mines.  John  McCaskill,  an  explorer,  observed  iron  ore  clinging  to  the  roots  of  an 
upturned  tree  on  what  is  now  the  Biwabik  property.  Test  pitting  by  the  Meiritts,  in  charge 
of  W.  J.  Merritt,  led  to  the  discovery  of  the  Biwabik  in  August,  1891.  The  Cincinnati  mine 
was  opened  the  same  fall.  The  Hale,  Kanawha,  and  Canton  mines  were  opened  in  the 
spring  of  1892. 

The  discovery  of  ore  near  the  sites  of  the  present  towns  of  Virginia,  Eveleth,  McKinley,  and 
Hibbing  followed  in  rapid  succession.  The  excitement  followmg  the  fu'st  discovery  of  ore  at 
Mountain  Iron  was  greatly  augmented  by  each  succeeding  find,  and  in  1891  and  1892  there  was 
the  inevitable  rush  of  explorers. 

Up  to  October,  1892,  there  were  two  railways  touching  the  range— the  Duluth  and  Iron 
Range,  crossing  the  range  at  Mesaba  station  on  its  way  to  the  Vermilion  range,  and  the  old  Duluth 
and  Winnipeg  (now  the  Great  Northern),  reaching  the  range  at  Grand  Rapids.  Both  these  places 
were  far  removed  from  the  exploring  centers.  Most  of  the  explorers  went  through  Mesaba 
station.  Reaching  this  place  by  rail,  they  were  compelled  to  travel  12  to  50  miles  to  the  west 
along  "tote  roads"  which  were  all  but  impassable.  The  time,  money,  and  energy  needed  to 
conduct  even  modest  explorations  at  this  time  can  be  appreciated  only  by  those  who  have 
experienced  the  difficulties  of  inland  travel  in  the  Lake  Superior  region  away  from  railways. 
The  stories  of  this  "totmg"  period  contain  the  usual  records  of  misfortunes,  lucky  strikes,  and 
enterprise  incidental  to  a  mining  boom. 

The  railways  were  not  long  in  getting  into  the  field.  In  October,  1892,  two  lines  were  put 
in  operation.  The  Duluth,  Missabe  and  Northern  Railway  was  built  to  connect  the  Mountain 
Iron  mine  with  the  old  Duluth  and  Winnipeg  Railway  (now  the  Eastern  Railway  of  Minnesota,  a 
part  of  the  Great  Northern  system)  at  Stony  Brook  Junction,  and  later  was  extended  to  Duluth. 
Almost  immediately  after  the  connection  with  Mountain  Iron  a  branch  was  sent  out  to  Biwabik. 
About  the  same  time  the  Duluth  and  Iron  Range  Railroad  sent  out  a  branch  from  its  main  line 
to  the  group  of  mines  at  Biwabik.  Very  soon  thereafter  both  railways  got  into  Virginia.  Hib- 
bing was  reached  by  the  Duluth,  Mssabe  and  Northern  in  1893.  Eveleth  was  reached  by  the 
Duluth  and  Iron  Range  in  1894  and  by  the  Duluth,  Missabe  and  Northern  very  soon  thereafter. 
The  Mississippi  and  Northern  (Eastern  Railway  of  Minnesota)  about  the  same  time  projected  a 
spur  from  Swan  River  to  the  Hibbing  district. 

With  the  advent  of  railways  the  development  of  the  range  went  on  by  leaps  and  bounds. 
This  marvelous  development  has  continued  to  the  present  time.  The  only  considerable  check 
occurred  during  the  period  of  general  financial  depression  wliich  the  country  underwent  in  1894, 
1895,  and  1896.  Almost  an  untouched  wilderness  m  1890,  the  district  is  to-day  the  greatest 
producer  of  iron  ore  in  the  world.  The  rapidity  of  the  development  of  the  nuning  industry  of 
the  district,  carrying  with  it  all  the  prosperity  of  the  range,  can  not  be  better  told  than  by  the 
table  of  sliipments  from  the  district  (pp.  65-68). 

The  development  of  the  Mesabi  range  eastward  toward  the  magnetic  portions  of  the  iron- 
bearing  formation  has  been  less  satisfactory  than  that  to  the  west.     A  small  amount  of  ore 


44  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

was  opened  up  at  the  Spring  mine,  formerly  the  site  of  the  Mailman  mine,  leading  to  the  ron- 
struction  of  a  spur  railway,  and  minor  discoveries  not  yet  exploited  have  been  reported  frmii 
places  farther  east.  Also  certain  ore  deposits  have  been  developed  in  the  vicinity  of  the  town 
of  Mesaba,  near  the  Iron  Range  Railway  track.  The  last-named  depo-sits  mark  about  the 
eastern  limit  of  the  principal  mining  operations. 

The  most  noteworthy  developments  of  tiie  district  in  late  years  have  been  the  exploration 
and  exploitation  of  the  ores  in  the  western  part  of  the  district,  wliich,  because  of  their  content 
of  loose  quartz  grains,  giving  them  the  name  "sandy  taconite,"  were  long  regarded  as  worthless. 
As  a  result  of  elaborate  experiments  in  washing  tests  it  was  found  possible  to  utihze  these  ores, 
and  mining  operations  are  now  being  conducted  and  planned  on  an  enormous  scale.  Since  1900 
several  to^\^as  have  sprung  up  in  what  was  before  a  wilderness.  The  town  of  Coleraine,  built 
by  fiat  of  the  OUver  Iron  Alining  Company,  is  an  example  of  what  may  be  accomphshed  in  a 
short  time  by  large  capital  intelligently  expended  by  a  single  group  of  indi\nduals  working  on  a 
uniform  plan.  The  railways  have  followed  up  and  made  possible  much  of  the  development  of 
the  western  Mesabi  district.  It  is  reached  by  spurs  from  both  the  Duluth,  ilissabe  and  Northern 
and  the  Great  Northern  railways,  leaving  the  main  lines  south  of  Hibbing.  , 

Still  more  recent  has  been  the  extension  of  the  district  by  exploration  for  12  miles  or  more 
west  of  Pokegama  Lake,  near  Mississippi  River.  The  ores  have  been  found  to  be  lean  but 
probabty  merchantable.  The  iron-bearing  formation  pinches  out  at  the  southwest  end  of  the 
district,  the  overl^ang  slate  coming  into  contact  with  the  underlying  quartzite.  This  part  of 
the  district,  together  wdth  magnetic  belts  farther  west,  particularly  the  one  running  through 
Leech  Lake,  the  east  end  of  which  comes  within  12  miles  of  the  Mesabi  district,  affords  interesting 
possibiUties  for  exploration,  which  will  be  adequately  undertaken. 

CUTUNA  IRON  DISTRICT  (1903). 

The  development  of  the  Cuyuna,  the  newest  of  the  Lake  Superior  iron  districts,  in  the 
same  geologic  group  as  the  Mesabi  district,  is  unique  in  a  way.  The  other  iron  ranges  of  the 
Lake  Superior  region  were  all  discovered  through  more  or  less  conspicuous  surface  indications 
of  ore  bodies.  Outcrops  of  the  ore  or  of  iron-bearing  rocks  existed.  There  are  no  rock  out- 
crops in  the  Cuyuna  district,  the  drift  mantle  being  SO  to  3-50  feet  thick,  and  the  first  tliscovery 
of  magnetic  iron-bearing  rocks  in  tliis  region  was  made  with  the  dip  needle  by  Cuyler  Adams, 
about  1895. 

The  dip  needle  was  the  sole  factor  used  in  the  subsecpient  tracing  of  the  ore  formations  b}" 
Cuyler  Adams  and  afterward  by  others,  preparatory  to  drilling,  from  the  time  of  the  first 
discovery  of  magnetic  iron-bearing  formations  until  1907,  when  more  or  less  indiscriminate 
drilling  began. 

The  first  drilhng  was  done  hi  1903  at  a  point  just  south  of  Deerwood,  ^liim.,  by  Cuyler 
Adams,  and  has  continued  in  greatly  increasing  amount  to  the  present  time,  some  2,000  drill 
holes  and  two  shafts  having  been  put  down,  resulting  in  the  discovery  of  a  number  of  ore 
deposits.  (See  pp.  216-219.)  The  distribution  of  the  ore  bodies  and  the  limits  of  the  district 
are  yet  very  imperfectly  known. 

Extension  of  magnetic  surveys  to  the  west  and  north  have  shown  isolated  magnetic  belts 
at  several  places,  some  of  them  beyond  the  western  boundary  of  Minnesota.  The  ilistribution 
of  some  of  these  belts  is  showm  on  the  general  map.  Underground  exploration  of  these  belts 
has  just  begun.  The  next  few  years  wiU  see  rapid  exploration  of  the  Cuyuna  range  and  the  coun- 
try to  the  north  and  west. 

For  some  time  before  the  drilling  began,  geologists  had  suspected  the  existence  of  iron- 
bearing  formation  in  the  CuAnma  district.  The  general  geologic  map  of  Minnesota,  published 
by  the  Minnesota  Cieological  and  Natural  History  Survey  in  1901,  showed  this  area  as  occupied 
by  a  .southwestern  extension  of  the  slates  and  cjuartzites  of  the  Mesabi  district.  In  1903  C.  K. 
Leith  published  a  sketch  showing  tlie  hypothetical  extension  to  the  southwest  of  the  iron- 


HISTORY  OF  LAKE  SUPERIOR  MINING.  45 

bearing  formation  of  the  Mesabi  district  through  the  since  chscovered  Cuyuna  district.  A 
similar  view  of  the  geologic  possibilities  was  held  by  W.  N.  Merriam,  geologist  for  the  United 
States  Steel  Corporation. 

The  Northern  Pacific  Railway  extends  tliroughout  the  length  of  the  Cuyuna  district  and 
affords  easy  access  to  the  ores.  It  also  runs  near  some  of  the  magnetic  belts  west  and  north- 
west of  the  Cuyuna  district.  The  Minneapolis,  St.  Paul  and  Sault  Ste.  Marie  Railway  passes 
the  district  on  the  southeast  and  in  1910  completed  a  spur  into  the  district.  For  both  rail- 
ways the  lake  port  ^dll  be  Superior.' 

BARABOO  IRON  DISTRICT  (190.3). 

The  discovery  of  ore  in  the  outlying  and  relativel}'-  small  Baraboo  district,  in  Wisconsin, 
■was  not  made  until  190.3.  The  quartzite  ranges  here  conspicuously  exposed  had  long  been 
recognized  as  Iluronian,  and  suggestion  had  been  made  that  iron-bearing  rocks  might  be  asso- 
ciated with  them.  In  fact,  for  several  years  the  Cliicago  and  Northwestern  Railway  had  quar- 
ried small  amounts  of  paint  rock'  within  a  few  feet  of  what  is  now  known  as  the  Illinois  mine. 
Because  of  the  covering  of  Cambrian  sandstone  and  glacial  deposits  the  ore  deposits  them- 
selves escaped  detection  until  drilhng  was,  in  1900,  begun  by  W.  G.  La  Rue  in  the  vicinity 
of  the  Illinois  mine  near  North  Freedom.  Since  that  time,  as  a  result  of  almost  uninterrupted 
exploration,  ore  deposits  have  been  found  at  various  places  in  the  Baraboo  syncline.  Only 
three  shafts  have  been  sunk  and  ore  has  been  slupped  from  only  one,  the  Illinois  mine.  The 
development  of  the  district  has  not  been  rapid  because  of  the  relatively  low  grade  of  ore,  the 
considerable  cost  of  mining,  and  the  great  expense  of  deep  drilling,  although  these  factors  have 
been  partly  offset  by  lower  freight  rates  to  Cliicago.  Both  mining  and  exploration  in  the 
Baraboo  district  are  in  their  infancy. 

LESS  IIMPORTANT  DEVELOPMENTS. 

CLINTON  IRON   ORES    OF  DODGE   COUNTY,  WIS.  (1849). 

There  is  no  record  of  the  fu-st  discovery  of  the  Clinton  iron  ores  in  Dodge  County,  Wis.,  for 
they  are  exposed  at  the  surface  in  accessible  country.  Ore  was  first  mined  from  them  in  1849. 
The  ores  have  been  partly  used  in  local  charcoal  furnaces  at  Mayville  and  Iron  Ridge  and 
partly  sliipped  to  ^Milwaukee,  Cliicago,  and  adjacent  points.  Because  of  their  low  percentage 
of  iron,  liigh  phosphorus,  and  moderate  quantity,  they  have  not  figured  largely  in  Lake  Superior 
production. 

PALEOZOIC   IRON   ORES   IN   WESTERN   WISCONSIN  (1857). 

Small  hematite  deposits  scattered  through  the  driftless  portion  of  the  Cambrian  sandstone 
area  north  of  Wisconsin  River,  in  Wisconsin,  were  opened  up  about  the  time  of  the  discovery  of 
the  Marquette  district.  In  1857  a  charcoal  furnace  was  built  at  Ironton,  in  Sauk  County,  to 
use  these  ores.  Another  was  built  at  Cazenovia,  in  Richland  County,  in  1876,  and  torn  down 
in  1879.  None  of  these  ores  has  been  mined  since  1880.  Records  of  production  are  not  avail- 
able, but  before  1873  about  25,000  tons  of  ore  was  mined  from  these  deposits. 

Farther  north,  at  Spring  Valley,  in  Pierce  County,  Wis.,  brown-ore  deposits  dissociated 
■wdth  Ordovician  limestone  were  opened  about  1890,  and  a  charcoal  furnace  was  built  to  use 
these  ores  in  1893.     At  a  later  period  coke  supplanted  charcoal  as  a  fuel. 

IRON  ORES  OF  THE  NORTH  SHORE  OF  LAKE  SUPERIOR  (1900). 

Since  the  opening  of  the  Lake  Superior  region  for  mming  the  north  shore  has  been  more  or 
less  explored  and  a  considerable  number  of  iron-bearmg  belts  have  been  located  in  the  territorv 
extending  from  the  Lake  of  the  Woods  beyond  Michipicoten.  Only  three  ore  bodies  have  been 
found.     The  best  knowTi  of  these  is  the  Helen  ore  body,  which  was  discovered  in  1897  ui  the 


46  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Michipicoten  district,  on  tlie  northeast  side  of  Lake  Superior.  This  district  was  cortnected  with 
Lake  Superior  l)y  tlie  buildmjx  of  the  Algoma  Central  and  Hudson  Bay  Railway  (12  miles)  in 
1899  luul  bof^an  shii)nieMt  in  1900. 

Discovery  of  the  Helen  mine  led  to  rather  vigorous  exploration  in  tne  many  kno\ui  iron- 
bearing  belts  in  the  immediatel,v  adjacent  territory,  m  some  places  by  drilling,  but  without 
conspicuous  success.  A  small  body  of  ore  was  found  iit  the  Josephine  mine,  a  few  miles  north- 
east of  tlie  Helen  mine. 

The  Atikokan  ore  <le|K)sit  was  discovered  in  1889,  was  explored  by  tunnel  and  drilling  in 
1898  and  1899,  and  became  accessible  for  mining  when  the  Canadian  Northern  Railway  passed 
it  m  1901.  Utilization  of  the  ore  has  been  restricted  by  its  high  suljjhur  content.  A  furnace 
has  been  constructed  at  Port  Artlnir  on  Lake  Superior  for  the  pur[)ose  of  utilizing  this  ore,  but 
thus  far  little  has  been  actually  mined  and  smelted. 

At  Steep  Rock  Lake  a  small  botly  of  ore  was  discovered  m  1901  by  the  Oliver  Iron  Mining 
Company.     No  ore  has  yet  been  mined. 

The  presence  of  an  iron-bearing  formation  in  the  Animikie  group  in  the  vicinity  of  Port 
jVrthur  and  at  pomts  lying  east  and  west  of  that  place  for  several  miles  was  noted  by  the 
earliest  visitors  to  this  region.  A  considerable  amount  of  more  or  less  desultory  exploration 
has  shown  that  the  formation  as  a  whole  is  lean  and  unmarketable.  At  Loon  Lake,  about  25 
miles  east  of  Port  Arthur,  vigorous  explorations  begmi  in  1901-2  disclosed  a  few  thin  layers 
of  lean  ore  of  considerable  horizontal  extent,  which  may  be  mined  m  the  future. 

In  general  the  results  of  exploration  for  iron  ore  on  the  north  shore  of  Lake  Superior  have 
been  disappomtiiag.  The  returns  have  not  been  proportionately  so  large  for  money  expended 
as  they  have  been  on  the  south  shore,  partly  owing  to  the  fact  that  the  iron-bearing  formations 
are  mainh'  of  the  Keewatin  series,  which  on  the  south  shore  are  foimd  to  have  smaller  quan- 
tities of  iron  ore  than  those  of  the  upper  Huronian  (^\jiimikie  group).  On  the  other  hand, 
exploration  has  been  slight  relative  to  the  extent  of  the  known  iron-bearing  formation  and 
the  large  and  not  easil}^  accessible  areas  open  for  exploration;  moreover,  the  exploration  has 
been  largely  confined  to  the  surface.  Future  exploration  and  mining  in  this  territory  will 
be  greater  than  that  which  has  already  been  done. 

SILVER  MINING   ON   THE  NORTH   SHORE   OF  LAKE   SUPERIOR   (1868). 

One  of  the  interesting  incidents  m  the  development  of  the  Lake  Superior  region  was  the 
discovery  in  1868  of  silver  ore  on  Silver  Islet,  near  Thunder  Cape,  on  the  northwest  coast  of 
Lake  Superior.  Before  the  mine  closed  in  1884  it  had  j'ielded  about  S.3,250,000  worth  of  silver. 
The  island  is  so  small  that  it  was  necessary  to  inclose  parts  of  the  vein  by  a  cofferdam  to  prevent 
the  inflow  of  the  lake. 

This  development  was  followed  bj'  active  exploration  of  a  number  of  silver  veins  in  the 
Animikie  group  to  the  west.  The  principal  mines  were  the  Shmiiah,  Rabbit  Momitain,  and 
Silver  Mountain  groups,  which  have  yielded  approximately  .$1,885,000  worth  of  silver,  but 
which  are  not  now  active. 

LAKE   SUPERIOR   GOLD   MINING  (1882). 

Still  another  less  important  phase  of  mmmg  development  m  the  Lake  Superior  region 
has  been  the  production  of  small  cjuantities  of  gold.  Between  1882  and  1897  the  Ropes  gold 
muie  m  tlie  Marquette  district  of  Michigan  produced  about  §650,000  worth  of  gold.  On  the 
Ontario  side  of  the  lake  approximately  $2,250,000  worth  of  gold  has  been  mined,  prmcipally 
m  the  Raiu}^  Lake  district,  which  was  opened  in  tiie  early  nmeties  and  reachetl  its  greatest 
development  in  1899.  At  present  the  production  of  gold  in  the  Lake  Superior  region  is  prac- 
tically nil,  but  ex|)loration  contmues  active,  and  from  tune  to  time  considerable  sums  are  spent 
in  opening  up  muies  and  builduig  mills  on  low-grade  gohl  tleposits. 


HISTORY  OF  LAKE  SUPERIOR  MINING.  47 

GENERAL  REiMARKS. 
INDUSTRIAL  CHANGES. 

The  foregoing  chronologic  account  of  the  opening  of  the  Lake  Superior  mining  industry 
gives  no  adequate  idea  of  the  magnitude  and  difficulty  of  tlie  work  and  the  forces  involved. 
The  bare  statement  that  a  district  "had  been  known  to  exj)lorers  for  many  years  prior  to  its 
opening"  but  poorly  expresses  the  persistent  limit  of  many  explorers  for  many  years  at  the 
expense  of  money  and  bodily  fatigue  tlirough  a  wilderness  difficult  to  reach  and  superlatively 
difficult  to  penetrate  and  explore.  Since  the  openmg  of  the  first  mine  m  tlie  region  there  has 
been  no  time  m  which  such  men  have  not  been  vigorously  i)rosecutmg  the  search.  vSurface 
exploration  has  been  foOowed  in  favorable  localities  by  test  pittmg  and  drilling  at  enormous 
expense.  In  the  Vermilion  district  $2,000,000  is  probably  a  conservative  estimate  of  the 
amount  spent  in  exploration  with  the  drill,  much  the  largest  proportion  of  it  entirely  without 
success.  In  the  Mesabi  district  30,000  test  pits  and  drill  holes  have  been  sunk  in  exploration 
of  the  range.  The  total  expenditure  on  preliminary  underground  exploration  in  the  Lake 
Superior  region  is  probably  not  less  than  .122,000,000.     (See  p.  4.S5.) 

Since  the  advent  of  large  capital  into  the  region  exploration  has  been  systematized  and 
now  often  includes,  as  a  prelimuiary  or  accompanying  step,  the  complete  geologic,  topographic, 
and  magnetic  mappmg  of  the  areas  to  be  explored. 

The  early  development  of  the  Lake  Supsrior  iron  minhig  district,  from  the  openmg  of 
the  Marcjuette  range  to  1873,  was  for  the  most  part  accomj)lished  by  small  companies  and 
small  capital.  The  period  from  1873  to  1892  was  marked  by  the  presence  of  larger  companies 
with  moderate  capital;  and  since  1892  mines  have  been  operated  by  strong  companies  with 
large  capital.  This  increase  in  capital  has  been  accompanied  by  combmation  of  the  mming 
companies. 

At  present  considerably  more  than  half  of  the  Lake  Superior  iron-ore  reserve  is  controlled 
by  the  Oliver  Iron  Mming  Company,  the  mmmg  branch  of  the  Lhiited  States  Steel  Corporation. 
The  Minnesota  tax  commission's  report  for  1908  credits  the  Oliver  Company  with  76.6  per 
cent  of  the  reserve  of  u-on  ore  for  Mumesota.^  It  is  not  clear  tliat  the  company's  dominance 
in  Michigan  is  so  great  as  this,  but  in  view  of  the  fact  that  the  jMinnesota  reserve  is  so  far  in 
excess  of  that  m  Michigan  it  is  not  likely  that  the  Oliver  Company's  percentage  of  the  Lake 
Superior  reserve  is  far  short  of  that  given  for  Minnesota. 

SMELTING. 

The  Lake  Superior  iron  mines  were  openetl  at  the  time  when  anthracite  and  coke  first  began 
to  be  largely  used  in  the  smelting  of  iron.  Before  that  time  the  fuel  used  in  local  furnaces  was 
largely  charcoal.  Charcoal  was  surpassed  in  amount  by  anthracite  in  18.55  and  by  coke  in  1869. 
More  anthracite  than  coke  was  used  until  1875,  but  since  then  coke  has  gradually  but  almost 
completch'  replaced  anthracite  for  smelting.  The  use  of  anthracite  and  coke  made  possible 
both  a  large  increase  and  a  centralization  in  pig-iron  production,  and  the  growth  of  the  Lake 
Superior  iron-mining  industry  is  practically  concurrent  with  the  increased  use  of  these  substances 
instead  of  charcoal.  Since  the  opening  of  the  Lake  Superior  region  much  the  larger  part  of  its 
output  has  been  used  in  coke  and  anthracite  furnaces  of  the  lower  lake  region. 

The  smelting  of  iron  ore  within  the  Lake  Superior  region  itself  has  been  thus  far  on  a 
relatively  small  but  still  considerable  scale.  Detailed  figures  are  not  available,  but  it  is  roughly 
estimated  that  about  3  per  cent  of  the  total  production  has  been  locally  smelted.  Several  small 
forges  were  built  in  the  ilarquette  district  of  Michigan  in  the  decade  before  the  first  shipment 
was  made  to  the  lower  Lakes.  Since  then  about  25  charcoal  furnaces,  most  of  them  now  aban- 
doned, have  been  built  in  the  Upper  Peninsula  of  Michigan,  also  one  at  Ashland,  Wis.,  and  several 
in  the  northern  part  of  the  Lower  Peninsula  of  ]\Iicliigan,  using  almost  entirely  Lake  Superior  ores. 

<»  First  biennial  repoft  of  the  Minnesota  tax  commission  to  the  governor  and  legislature  of  the  State  of  Minnesota,  St.  Paul,  1908,  p.  122. 


48  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

In  addition  several  small  furnaces  in  Wisconsin,  built  principally  for  the  use  of  local  ores,  have 
used  small  amounts  of  Lake  Sui)erior  ores.  Coke  furnaces  have  been  established  at  Duhith, 
i^fhm.,  and  at  Sault  wSte.  Mario,  Ontario,  and  several  of  the  charcoal  furnaces,  on  account  of  tlie 
dopictiunof  tiie  charcoal  supi)ly  and  the  increase  in  the  availibihtyof  coke,  have  substituted  coke 
as  fuel.  Milwaukee,  Chicago,  Detroit,  and  adjacent  points  have  of  coui-se  been  hvrge  users  of 
iron  ore  in  cok(>  furiuues,  but  these  lake  ports  are  outside  of  the  region.  In  1908"  there  were  in 
operation  a  coke  furnace  at  JXduth,  Minn.,  tliree  in  Wisconsin  outside  of  Milwaukee,  five  char- 
coal furnaces  in  the  Upper  Peninsula  of  ^Michigan,  three  furnaces  in  the  northern  part  of  the 
Lower  J'eninsula  of  Michigan,  and  the  steel  plant  at  Sault  Ste.  Marie,  Ontario.  The  largest  plant 
yet  projected  for  the  local  use  of  iron  is  to  be  buUt  for  steel  making  in  West  Duluth  by  the  L'nited 
States  Steel  Corporation;  it  may  be  in  operation  in  1912. 

In  recent  years  there  has  been  an  attempt  to  recover  by-products  from  the  charcoal  burned, 
the  first  notable  project  being  the  Cleveland-Cliffs  furnace  at  Presque  Isle,  in  the  Marf(uette 
district.  This  plant  is  most  elaborately  eciuijiped  for  the  recover}-,  as  by-products,  of  wood 
alcohol  and  creosote.  The  Lake  Superior  Iron  and  Chemical  Company,  at  Ashland,  Wis.,  also 
has  a  well-equipped  by-product  plant.  The  Zenith  furnace  at  Duluth  has  been  rebuilt  on  a 
large  scale  to  recover  by-products  from  coke.  At  present  it  is  supplying  gas  to  the  city  of 
Duluth.  The  steel  plant  now  planned  at  Duluth  by  the  United  States  Steel  Corporation  will 
utUize  the  gases  as  fuel. 

With  increase  of  population  directly  tributary  to  the  Great  Lakes  it  is  very  likely  that  the 
local  smelting  of  the  ores  will  increase.  The  depletion  of  the  timber  \vill  prubabl}-  compel 
mcreased  use  of  coke  instead  of  charcoal.  Peat,  which  is  found  locally  in  large  quantities,  may 
be  considered  as  a  possible  fuel  for  the  future. 

INFLUENCE  OF  PHYSIOGRAPHY  ON  INDUSTRIAL,  DEVELOPMENT. 

One  of  the  principal  relations  between  the  physiography  and  history  of  the  industrial 
Lake  Sui)erior  region  seems  sufficiently  distinct  to  be  summarized  in  a  few  words.  The  early 
stages  of  development  were  closely  controlled  by  conditions  of  accessibility.  The  early  explor- 
ers, traders,  and  prospectors  were  confined  to  the  lake  and  river  shores  and  to  country  easily 
accessible  from  them.  Wlien  mining  and  lumbering  began  there  was  also  a  distinct  localization 
of  these  mdustries  in  accessi])le  jjlaces.  With  the  growth  of  theindustiy  and  the  introduction 
of  railways  the  influence  of  physiography  on  the  local  distribution  of  activity  gradually 
became  less  marked,  until  at  present  this  distribution  is  but  little  affected  by  the  configuration 
of  the  surface  and  tlrainage.  The  situation  of  ore  deposits  has  of  course  localized  the  mining 
development.  Favorable  conditions  of  access,  though  advantage  has  been  taken  of  them,  have 
been  subordinate  factore.  An  iron  deposit  would  be  utilized  whether  it  was  in  a  swamp  or  on 
a  mountain,  whether  easily  accessible  or  not.  In  other  words,  increased  demand  for  the  raw 
materials  of  the  Lake  Superior  region,  due  to  general  commercial  conditions  and  the  westward 
movement  and  increase  of  population,  has  gradually  overridden  and  more  or  less  obliterated  the 
natural  phj'siographic  channels  of  development. 

The  relation  of  the  Great  Lakes  to  cheapness  of  water  transportation  and  of  the  simple 
topography  of  the  region  to  ease  of  railway  construction  to  any  mineral-proilucing  district 
continues  to  be  an  important  physiographic  influence  and  one  that  is  unusual  in  a  mining 
district. 

<•  Map  of  the  United  States  showing  location  of  lilast  furnaces  in  19Wf,  compiled  by  W.  T.  Thom  from  Swank's  Iron  and  steel  works  directory  tor 
1908:  Mineral  liesources  U.  S.  for  1908,  pt.  1,  V.  S.  Geol.  Survey,  1910,  PI.  II. 


HISTORY  OF  LAKE  SUPERIOR  MINING. 


49 


PRODUCTION  OF  IRON  ORE. 

The  production  of  iron  ore  from  the  several  producing  ranges  of  the  Lake  Superior  region 
since  their  opening  is  given  in  the  following  table,  compiled  principally  from  the  Iron  Trade 
Review.  The  figures  refer  mainly  to  shipments  rather  than  to  production.  The  figures  of 
the  United  States  Geological  Survey  do  not  go  back  far  enough  for  the  purposes  of  this  table. 
The  facts  of  the  table  are  graphically  expressed  in  figure  3. 


45 ,000,000 


40,000,000 


35  ,000,000 


30,000,000 


(0 

O    25,000,000 

(D 

Z 

o 

-■    20,000,000 


15,000,000 


5 ,000,000 


1550 


1855 


1660 


1865  1870  1875  1880  I6S5  1890 

YEAR5 
FiGUKE  3.— Diagram  showing  annual  production  of  Iron  ore  in  Lake  Superior  region  since  the  opening  of  the  region. 


Table  of  Lake  Superior  iron-ore  shipments  from  the  earliest  shipment  to  date.<^ 

Gogebic  Range. 

[Gross  tons.] 


Name  of  mine. 

lSS-1. 

1SS5. 

1886. 

1887. 

1888. 

1889. 

1890. 

1891. 

1892. 

Ada.    (Included  in  Ironton.) 
Anvil 

10,075 
175,563 
1.369 
159. 252 
16, 101 
1,799 
21,721 
29,763 

24,676 
174, 183 

47,000 
257,915 

45.690 
435,949 

73 
267,439 

42,090 

6.741 

74,015 

231,896 

Atlantic 

5,422 

94,553 
4,788 

179,937 

199, 865 

246,695 

83,554 

319,482 

8,880 
2,697 

40,639 

53,  267 
56,542 

80,486 
152,878 

46,574 
131,896 

130,833 

119,676 

CastiJe 

Colby  c 

1.022 

84,302 

257,432 

258,518 

285,880 

136,833 

193,038 

1,497 

23,794 

21, 150 

9,619 

69,968 

21,754 

13,907 
6,778 

10.055 

8,515 

1,997 

Geneva 

o  Figures  for  1893-1909,  inclusive,  from  Supplement  to  the  Iron  Trade  Review,  vol.  46,  No.  9.  March  3,1910.  Figures  for  previous  years  compiled 
from  the  annual  tables  published  by  the  Iron  Trade  Review  and  from  "Annual  review  of  the  iron  mining  and  other  industries  of  the  Upper  Penin- 
sula for  the  year  ending  December,  1880,"  by  A.  P.  Swineford. 

b  Under  Norrie  group  after  1904. 

c  Includes  Tildeu  prior  to  1891. 

47517°— VOL  52—11 4 


50 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


Table  of  Lake  Superior  iron-ore  shipments  from  the  earliest  shipment  to  dale — Continued. 

Oogeblc  Range— Continued. 
[Gross  tons. J 


Name  of  mine. 

18S4. 

1885. 

1886. 

1887. 

1888. 

1889. 

1890. 

1891. 

1892. 

5,468 

19,734 

61,714 

53,918 
28,721 

30,000 

103,169 
76,545 

51,551 

52,000 
63,903 

110,368 

22,383 
15,759 

1,506 

4,283 

Iinju'iial.    (See  Federal.) 

161,635 

9,950 
551 

18,424 

2,249 

Iron  King.    (See  Newport.) 

24,762 

8,635 

6,247 

300 

3,944 

18,497 

52, 179 

1,228 

2,882 

10,144 

64,779 

Mik'iiio 

Montreal  f  Section  331              

23,013 

20,184 
4,105 
124,844 
13, 714 
17,979 

43,989 

75,660 
23,217 
237,254 
30, 475 
19,906 
1,414 

38,015 

69,145 

1,313 

412. 196 

5,412 

49,976 
9,725 

26,087 

116,094 
36,987 

143,691 
71,488 

108,684 
105,606 

73,409 

New  Davis.    (See  Davis.) 

165,962 

15,419 

674,394 
13,354 

116,376 

35,245 

574 

906,728 

1,005 

172,060 

50,004 

758,572 

985,216 

6,711 

Pabst  *>                                    

1,103 

130,226 
32,227 

113,245 

102,382 

Pike 

16,388 

45,000 

3,058 

9,472 

11,694 

913 

Section  33.    (See  Montreal.)           | 

2,912 

1,405 

10,963 

18, 137 

6,010 

64,902 
28.41S 

56,046 

Tilden  c 

233,356 

10,780 

12,764 

2,387 

10,683 

1,878 

Vaughn.    (See  Aurora.) 

14,576 

37,210 

97 

53,242 

Wisconsin.     (See  Davis.) 
Yflle  ^\Vc<;t  Colbvl 

1,022 

119,860 

753,369 

1,324,878 

1,437,096 

2,008,394 

2,847,810 

1,839,574 

2,971,991 

Name  of  mine. 

1893. 

1894. 

1895. 

1896. 

1897. 

1898. 

1899. 

1900. 

1901. 

Ada.    (Included  in  Ironton.) 

13,297 
83,020 

68,064 
126,096 

70, 989 
245,883 

57,483 
91, 149 
60,727 
187, 169 

5,037 
123,208 
38,058 
133,076 

1.101 

66,067 

111,625 
50,307 
166, 122 

154, 615 

19,964 

170,309 

232.961 
135,955 
193,111 

286.399 

190. 13.5 

179,028 

203, 152 

223.747 

Brotherton                    

18,905 
28,578 

47,148 
47, 156 

40,567 
52,349 

50,490 
38,821 

46,186 
37,308 

73. 198 
43,162 

78,858 
62,524 

89,804 
125,496 

103.109 

179,374 

Castile 

504 
48.492 

633 

32.572 
3,569 

59,346 
15,210 
31,385 

32,616 

22,921 

152,875 

103,239 
5,029 

23,475 

10,253 
26,105 

18,329 

4,544 

7,964 

1,015 

1,255 

986 
7,728 

54,664 

10,358 

21,475 

Imperial.     (See  Federal.) 

Iron  Ilelt                                    . 

23,976 

45,109 

148,228 

81,351 

96,763 

58,418 

105,934 

43,883 

Trnii  Chief  No  2 

Iron  King.    (See  Newport.) 

7,977 

25,047 
33.893 

1,651 

1,265 

19,988 

9,604 

11,782 

332 

10,324 

153,307 

263,711 

7.844 

1.090 

107,524 

217,201 

34,140 

Mikado 

4,788 
138,882 

157,821 

11.397 
191, 106 

150,979 

91.846 

Montreal  (Section  33) 

New  Davis.    (See  Davis.) 

34,299 
109,718 

46,037 
150,392 

131,531 
142,369 

270,776 
196,953 

72,945 
190,448 

472,062 
3,930 

104.510 
2,058 

621,608 

2,4.37 

206,074 

37,911 

738,480 

329,068 

604,281 

700,990 

714,069 

666,389 

660,965 

219.960 
46,905 

68,984 
114,108 
13,185 

220.496 

207, 153 

120 

223.891 
175,925 

263.869 
154,705 

239,242 
139,658 

198. 6S6 

7.603 

Pike 

3.434 

6,346 

1 

21.788 

Section  33.    (See  Montreal.) 

1 

12,196 

10, 102 

15,691 

11,819 

r, 

1 

1,950 

20.970 

418,188 

22,876 
135, 118 

34.323 
209,077 

89.441 
250,205 

45,815 
270,890 

i2,526 
500,830 

74.097 
481.909 

89.997 

Tildcnc 

287,203 

446,670 

o  Includes  Aurora  after  1904  and  Pabst  afti^r  1901. 
ftUniier  Norrio  ^roup  after  1901. 
eUnder  Colby  prior  to  1891. 


d  linder  Norrie  group  after  1904. 
'  Includes  Tilden  prior  to  1891. 


HISTORY  OF  LAKE  SUPERIOR  MINING. 


51 


Table  of  Lake  Superior  iron-ore  shipments  from  the  earliest  shipment  to  date — Continued. 

Gogebic  Range — Continued. 
[Gross  tons-l 


Name  of  mine. 


1894. 


1895.  1896. 


1897. 


1899. 


1900. 


1901. 


Trimble 

Tylers  Forks 

Upson 

Valley 

Vaiigim.    (See  Aurora.) 

Windsor  (now  Gary) 

Wisconsin.    (See  Davis.) 
Yale  (West  Colby) 


2,474 


11,438 


1,329,385 


1,809,468       2,547,976 


488 


2,875,295 


841 
12.836 


2,938,155 


Name  of  mine. 


Ada.    (Included  in  Ironton.) 

Anvil 

Ashland 

Atlantic 

Aurora  « 

Bessemer 

Blue  Jacket ' 

Brotherton 

Cary  (and  Superior) 

Castile 

Chicago , 

Colbyi) 

Davis  ( W'isconsin) 

Eureka 

Federal 

First  National 

Geneva 

Germania  ( Harmony) 

Hennepin ". 

Imperial.    (See  Federal.) 

Iron  Belt 

Iron  Chief 

Iron  Chief  No.  2 

Iron  King.    (See  Newport.) 

Ironton 

Jack  Pot 

Kakagon  (now  Cary) 

Meteor  (Comet) 

Mikado 

Montreal  (Section  33) 

New  Davis.    (See  Davis.) 

Newport 

Nimikon  (now  Cary) 

Norrie  group  f 

Ottawa  (Odanah) 

Pabstd 

Palms 

Pence 

Pike 

Puritan  (Ruby) 

Section  33.    (See  Montreal.) 

Shores 

Sparta 

Sunday  Lake 

Tilden'e 

Trimble 

Tylers  Forks 

Upson 

Valley 

Vaughn.    (See  Aurora.) 

Windsor  (now  Cary ) 

Wisconsin.    (See  Davis.) 
Yale  (West  Colby) 


1902. 


135, 502 
301,824 
190, 213 
402,981 


63,256 
136,896 


44,625 
22,526 
31,. WO 


20, 502 
36,383 


79,121 


8,555 
102 


19,117 
98.834 
136,354 

141,571 


1,080,032 
26, 141 


32,113 
"'6.' 343 


144,630 
468, 672 


11,065 


26,043 


11,309 
274, 138 
148, 385 
356, 365 


94,986 
89,221 


22,965 

54,915 

734 


7,108 

2,240 

862 

26,353 


16,875 
31,709 


6, 150 
108, 709 
93, 139 


790,346 
87,929 


60,800 


115 


91.383 
211,534 


46,211 


45, 595 
344, 102 

77, 224 
212,920 


84,870 
01,860 


81,141 
11,225 


23,364 


23,197 
6,638 


59, 587 
26,611 
163,021 


618,  638 
30, 420 


53,718 


50, 625 
204,681 


1905. 


82, 118 
409, 131 
208,039 


137,351 
146, 414 


S3. 736 
3,160 


2,973 
2,589 


140, 740 
107,854 


1, 627, 128 
21,980 


13,963 

'iiiiei 


79, 209 
188, 104 


1900. 


79, 493 

341,841 

97, 689 


147.281 

216. 992 

2,108 


113,001 


9.436 
6,768 

3,227 


106, 168 


154, 043 
139,202 


1,245,!>97 
57,219 


5,622 
'i7,'934 


86,879 
169,697 


56,667 


3,664,929   2,912,708   2,398,287   3,705.207   3.643,514   3.037,102   2.699.866   4.088,067 


1907. 


39, 496 
298,056 
91,759 


104.224 

209,407 

6, 157 


17,. 347 


190. 9(« 


163,891 
169, 763 


1,109,085 
46,424 


24,922 


101,899 
312,490 


38,010 


1909. 


35, 937 
2.59,611 
41.4I!6 


22,927 
269. 612 
124,846 


96,  776 
96,3.18 


103,090 

224,251 

26, 982 


68.305 
'i22,'324 


170,095 
'ii5,'6()2" 


2,508 


152 
44,560 


277,694 


80,617 
177.006 


99, 195 
191,611 


773,243 
33,893 


977, 054 
100, 223 


22, 174 


111,130 
111.184 


93.712 
154. 506 


14,874 


71,4.58 


Total. 


706, 962 

5,. 387, 166 

1,547,123 

3,961,683 

20,889 

1,799 

1,762,498 

2,289,618 

36, 247 

68,727 

2.450,347 

103, 961 

462,134 

36,443 

1,997 

7. 108 

422,239 

259,733 

1,186,602 

12. 199 

551 

848,986- 
99,090 
71,904 
216,307 
997, 0&5 
2,861.252 

5,845,039 

28,035 

17, 744, 658 

481,359 

2,360,583 

1,284,489 

40, 566 

98,  732 

109,572 

55.808. 

4.862 

1,306,975 

5. 088. 635 

25. 931 

10.683 

11,375 

1,878 

148,905 

373, 173 


60, 896, 457 


Marquette  Range. 

Name  of  mine. 

Years  un- 
known. 

1854. 

1855. 

1856. 

1857. 

1868. 

1859. 

1800. 

1801. 

1862. 

Bessemer.    (See  Lillie.) 

Beaufort  ( Ohio ) 

a  Under  Norrie  group  after  1904. 

t>  Includes  Tilden  prior  to  1891. 

c  Includes  Aurora  after  1904  and  Pabst  after  1901. 


d  Under  Norrie  group  after  1901. 

f  Under  Colbv  prior  to  1891. 

/  Under  Iron  Clifis,  1890-1895;  under  Cleveland-Cliffs  group  after  1895. 


52 


GEOLOGY  OF  THE  LAKE  SUPEKIOIt  REGION. 


Tabic  of  Lake  Superior  iron-ore  shipments  from  the  earliest  shipment  to  date — Continued. 

Marquette  Range— Continued. 
[Gross  tons.] 


Name  of  mine. 

Years  un- 
known. 

1854. 

1855. 

185C. 

1857. 

1858. 

1859. 

1860. 

1861. 

1802. 

Blue.    (See  Queen  group.) 

T> .    j/Mitohell -^,. 

Braastad|\^.i,„l,^„p;;;;;;;;--_'";;;;;;; 

Breitung  Uematite  No.  2 ■^.. 

BulTaloi 

Cambria 

...   . 

Cheshire.    (See  Princeton.) 
Chester.    (See  Rolling  Mill.) 

Cleveland  &.                                      .  . 

3.000 

1,44& 

6,343 

13,204 

7,909 

15, 787 

40,091 

11,793 

40  364 

Cleveland  Hematite.    (Included  under 
Cleveland.) 



Curry 

Dalliba  (Phenix) 

Detroit 

Dexter 

Dey 

East  New  York 

Edison 

Edwards.    (See  Samson.) 

Erie 

Etna 

Fitch 

Fosterd 

Foxdale 

Gibson 

Goodrich . . 

Grand  Rapids  ( Davis) 

Green  Bay.    (See  Bay  State.) 

Hartford 

Home  (P.  and  L.  S.)  (now  Volunteer),.. 

Tmpprial 

Indiana.    (See  Bay  State.) 

Iron  Cliffs  « 

30,000 

12,442 

10,309 

28,377 

41.295 

.  i2,9i9 

46,096 

Keystone.    (See  East  Champion.) 
Lake  Angeline 

Lake  Superior, 

4,658 

24,668 

33,015 

25,195 

37,709 

Llllie 

Maas 

Manganese  (Negaunee) . . . 

Mary  Charlotte 

Mesahi's  Friend 

Miphic;iTnTnp  e 

Miller 

Milwaukee 

Mitchell 

Moore 

Negaunee 

New  York  (York). . 

North  Champion.    (See  Hortense.) 

' 

Nortnwest 

Ogden    

1 

Palmer  (Cascade).    (See  Volunteer  ) 
Pioneer 

i 

Pittsburg    and    Lake    Angeline.     (See 

under  Lake  Angeline.) 
Piatt 

Portland 

Prince  of  Wales  <» 

Queeno 

a  Under  Queen  ^roup  after  1890. 

6  UndcT  ricvclaii.l-ClilTs  group  after  \HXi. 

c  Iiicliidos  Clovrhmd  nfti'r  1S.S3;  incUidos  nanuim.  Foster.  Iron  Clifls,  Uichigamme,  and  Salisbiuy  after  1895. 

drndcr  Iron  CliiTs,  1S91-1S;W:  under  rieveland-Clifls  group  alter  1S95. 

<  Under  Clcveland-riills  group  after  1895. 

/  Under  Winthrop  after  1892. 


HISTORY  OF  LAKE  SUPERIOR  MINING. 


53 


Table  of  Lake  Superior  iron-ore  shipments  from  the  earliest  shipment  to  date — Coiitiuued. 

Marquette  Range — Continued. 
[Gross  tons.] 


Name  of  mine. 

Years  un- 
known. 

1S54. 

1855. 

1856. 

1857. 

1858. 

1859. 

18l». 

1861. 

1862. 

Queen  group  o 

Republic 

Richards 

Riverside 

Roiling  MiU 

Saginaw 



Sam  Mitchell.    (See  Mitchell.) 

Schadt 

Section  12 

Smith.    (See  Prmcetou.) 

South  Buffalo  c 

Spurr 

Star  West  (Wheat) 

' 

St.  Lawrence.    (See  Nonpareil.) 

Sterling.    (See  American.) 

Taylor 

Teal  Lake.    (See  Cambria. ) 

Titan 

Washington ^ 

Webster < 

West  Republic 

Wheeling 

Wheat.    (See  Star  West.) 

30,000 

3,000 

1,449 

6,343 

25,646 

22,876 

08,83? 

114.401 

49,909 

124, 169 

Name  of  mine. 

1863. 

1864. 

1866. 

1866. 

1867. 

1868. 

1869. 

1870. 

1871. 

1872. 

American  (Sterling) 

Austin 

Bamum  « 

14, 385 

33, 484 

44,793 

45, 939 

38  381 

Bay  State 

Bessemer.    (See  Lillie.) 

Bessie     

Beaufort  ( Ohio) 

Blue.    (See  Queen  group.) 

T>        *-  J  (Mitchell  

197 

BraastadVi„jjj^„p 

3,  409 

11.088 

14,239 

Breitung  llematite  No.  2 

Butfalo  c 

Cambria 

6,255 

21,635 

73,161 

67,588 

68,408 

Cheshire.    (See  Princeton.) 
Chester.    (See  RolUng  Mill.) 

Cleveland  / 

40, S42 

.    - 

44, 959 

33, 355 

42,680 

75,864 

102, 112 

100, 133 

132,884 

142,058 

151, 724 

Cleveland  llematite.     (Included  under 
Cleveland.) 

roliiTTiIiia.  (Klmnani 

ClUTV 

Dalliba  ( Phenix) 

Detroit 

Dexter 

Dey 

East  Champion 



East  New  York 

Edison 

Edwards.    (See  Samson). 

Empire 

Erie 

-■> 

Etna 

Fitch 

6,000 

14,540 

23, 458 

13,532 

18,684 

Foxdaie                        .        

Goodrich 

Oreen  Bay.    (See  Bay  State.) 

Hartford 

"Includes  Buffalo,  Prince  of  Wales,  Queen,  and  South  Buffalo  after  1890. 

6  Under  Iron  Cliffs,  1891-1895;  under  Cleveland-Cliffs  group  after  1895. 

c  Under  Queen  group  after  1890. 

d  Prior  to  1890.  see  Braastad:  includes  Marquette  after  1892. 

«  Under  Iron  Cliffs,  1S:10-1S95;  imdor  Cleveland-CUas  group  after  1895. 

/  Under  Cleveland-ClilTs  group  after  18.83. 

s  Includes  Cleveland  after  1SS3;  includes  Bamum,  Foster,  Iron  Cliffs,  Michigamme,  and  Salisbury  after  1895. 


54 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


Table  of  Lake  Superior  iron-ore  shipments  from  the  earliest  shipment  to  date — Continued. 

Marquette  Range— Continued. 
[Gross  tons. J 


Name  of  mine. 

1803. 

18G4. 

1865. 

1860. 

1867. 

1868. 

1869. 

1870. 

1871. 

1872. 

1,160 

4,782 

15,150 

25,440 

35,757 

58,402 

79,702 

48,725 

38,841 

Indiana.    (See  Bay  State.) 

77,237 

83,905 

19,500 
86,763 

65,505 

20,151 
50,201 

92,287 

24,073 
08,002 

127, 491 

46,607 
119,935 

130,524 

27,051 
105,745 

125,908 

35,432 
131,343 

127,642 

.53, 407 
100,582 

132,297 

33,645 
158,047 

119,910 

Keystone,    (See  East  Champion.) 

35,221 

78,970 

185,070 

Lillie 

4,866 

15,942 

24,153 





Marv  Chirlotte                                       



141 

8,000 

12,214 

33,761 

43,302 

43,665 

71,456 

94,S09 
1,809 

70,381 
2,921 

68,950 

9,925 

North  Champion.    (See  Hortense.) 



. 



Pendill                                            

Palmer  (Cascade).    (See  Volunteer.) 

Pittsburg    and    Lake    Angeline.     (See 

under  Lake  Angeline.) 
Piatt                                               

Portland 

13,445 

1 

■ 



11,025 

Rolling  Mill                                    

236 

6,772 

18,503 

Salisbury  e                                           ... 

545 

SamMitcheU.    (See  MitcheU.) 

2,843 

4,928 

17,360 

19, 151 

24,232 

26,437 

28,380 

Schadt 

Section  12 

Smith.    (See  Princeton.) 
South  Buffalo  c 

Star  West  ('^Tieat) 

Sterling.    (See  American.) 

Teal  Lake.    (See  Cambria.) 

4,171 

39,495 

West  Republic                               

Wiiithrop  / 

Wheat.    (See  Star  West.) 

203,055 

243,127 

186,208 

278,796 

443,567 

491,454 

617,444 

830,934 

779,607         893,169 

a  Under  Cleveland-riiffs  group  after  1895. 

bUnder  Winthrop  after  IS92. 

c  Under  Queen  group  after  1890. 

d  Includes  Uullalo.  I'rince  of  Wales,  Queen,  and  South  Buffalo  after  1890. 

«  Under  Iron  Cliffs,  1891-1895:  under  Cleveland-Clifls  group  after  1895. 

/  Prior  to  1890,  see  Braastad;  includes  Marquette  after  1892. 


HISTORY  OF  LAKE  SUPERIOR  MINING. 


55 


Table  of  Lake  Superior  iron-ore  shipments  from  the  earliest  shipment  to  date — Continued. 

Marquette  Range— Continued. 
[Gross  tons.] 


Name  of  mine. 

1873. 

1874. 

1875. 

1876. 

1877. 

1878. 

1879. 

1880. 

1881. 

1882. 

797 

4,702 

8,006 

Amps 

48,076 

41,403 

43,209 

37,6,32 
8,583 

37,909 

26,680 

24.015 
3,336 

24.522 
2,208 

27,883 
583 

41,778 
1,236 

Bay  State... 

Bessemer.    (See  Lillie.) 
Bessie 

Beaufort  (Ohio) 

5,532 
18,245 

Blue.    (See  Queen  group.) 

6,478 
13,279 
45,247 

14,824 
21,146 
43, 630 

■D        *  J  (Mitchell. 

8,658 
33,456 

7,  .549 
7,549 

,      5, 696 
27,236 

3,898 
12,549 

4,259 
23,740 

11,131 
26,595 

33,396 

Braastadj^^.j^j^^^p  ■-;;•;;;;;;;;;■;;;;;; 

7,502 

23,005 

Butialo  & 

Camliria 

2.610 
47,097 

6,329 
66,002 

10,083 
70,883 

3,754 
73, 464 

6,724 
94,027 

949 
131,167 

6,958 
112,401 

2,415 

212,748 

19,246 
145, 427 

5,531 
198,569 

64,  .545 

72,782 

56,877 

1.59,009 

Cheshire.    (See  Princeton.) 
Chester.    (See  Rolling  MUl.) 
Chicago 

Cleveland  c... 

133,265 

105,858 

129,881 

140,393 

152, 188 

152,737 

206, 120 

Cleveland  Hematite.    (Included  under 
Cleveland.) 

21,065 

35,088 

8,059 

6,663 

11,158 

12,066 

Dalliba(Phenix)... 

10,986 

44,836 

Detroit 

5,402 

Dey 

10,426 

5,227 

3,346 

7,715 

14, 495 

5,401 

4,029 

10,217 

3,408 

4,002 

Edison 

Edwards.    (See  Samson.) 

Erie 

2,731 

Etna..  . 

Fitch 

18, 107 

4,719 

847 

i25 

4,804 

1,122 

3,011 

11,648 

Foxdale 

Gibson 

Goodrich 

/6,.338 

503 

7,547 

3,992 

11,131 

10,245 

9,998 

Green  Bay.    (See  Bay  State.) 

Hartford 

Hortense  (North  Champion) 

Home  ( P.  and  L.  S. )  (now  Volunteer) . . . 
Himiboldt  (Washington). 

21,498 
38,014 

1.362 
27,890 

1,225 
23,921 

492 
18,204 

285 
14,726 

9,642 

3,333 

16, 545 

20, 302 

43,463 

Indiana.    (See  Bay  State.) 

IronClifls? 

Iron  Moimtain 

Jackson 

130, 131 

43,933 
158,078 

105,600 

31,526 
114,074 

90,568 

26, 370 
129,339 

98,  480 

22.5.39 

111,766 

5,945 

17,276 

80,340 

19,112 
127.349 
10, 127 
19,691 

83,121 

28, 161 

109,674 

8, 506 

30, 180 

103,219 

25,321 
173,938 
22,380 
28,962 

i26,626 

14,928 

204,094 

18,347 

31,200 

118,939 

18,0150 

262.235 

16, 748 

28,051 

90, 830 

Keystone.    (See  East  Champion.) 
Lake  Angeline... 

14.326 

296,509 

Lillie 

27,494 

Lucy  (McComber) 

38,969 

2,642 

10,407 

40,406 

Maas 

Manganese  (Negaunee) 

Mesabi's  Friend 

29,107 

45,294 

44,763 

70,074 

28,238 

58,622 

56,970 

52,766 

57,272 

43, 712 

Miller 

Milwaukee 

941 

13, 142 

31,635 

40,891 

Mitchell     . 

National  . . 

4,191 

33,310 

29,351 

24,833 

23,366 

Negaunee  Construction  Works 

1,177 

New  York  (York) 

70,882 
6,629 

77,017 

70, 103 
987 

58,863 
556 

55,581 
3,307 

21,903 
4.547 

57,528 
2,609 

58,512 
2,192 

50,074 

56,806 

New  York  Hematite 

2,105 

North  Republic 

9,998 

18,880 

Pendill...                .                .          ... 

4,000 

12, 549 

3,959 

13,686 

9,987 

Palmer  (Cascade).    (See  Volunteer.) 

Pittsburg    and    Lake    Angeline.     (See 
under  Lake  Angeline.) 

a  Under  Iron  Cliffs,  1890-1895;  under  Cleveland-Cliffs  group  after  1.S95. 

'  Under  Qucon  group  after  1890. 

c  Under  Clevoland-Clifls  group  after  1883. 

d  Includes  Cleveland  after  18.83;  includes  Barnum,  Foster,  Iron  Cliffs,  Michigamme,  and  Salisbiu-y  after  1895. 

'  Under  Iron  Cliffs,  1S91-1.S95;  under  Cleveland-Cliffs  group  after  1895. 

/  Includes  shipments  for  prior  years. 

e  Under  Cleveland-Cliffs  group  after  1895. 

»  Under  Winthrop  after  1892. 


56 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


Table  of  Lake  Superior  iron-ore  shipments  from  the  earliest  shipment  to  date — Continued. 

Marquette  Range— Continued. 
[Gross  tons.] 


Name  of  in [nc. 

1873. 

1874. 

1875. 

1876. 

1877. 

1878. 

1879. 

1880. 

1881. 

1882. 

Piatt 

Portland 

Princeton  (Swauzey  or  Cheshire) 

9,328 

187 

225 

8,434 

16,924 

17,985 

13,202 

15,011 

31,498 

105,453 

122,639 

119,726 

120.095 

165,836 

176,221 

135,231 

235,387 

233,786 

235,109 

Rolling  Mill 

11,319 
37,138 
11,023 

38,968 

16. 643 
45.486 
6,730 

2,849 

37.806 
55.318 
4.571 

12,804 

53,265 
56.979 
20. 510 

19,330 

38, 121 
44.005 
37.869 

10,  419 

30. 773 
54.097 
52.155 

10,351 

10,039 
43,396 
39,293 

5,455 

15, 172 
35,059 
21,457 

1,668 
30.793 
43,690 

4,584 

163 

16,276 

42,243 

Sam  Mitchell.    (See  Mitchell.) 

12,421 

Schadt 

5,027 

330 

13,243 

3,287 

Smith.    (Sec  Princeton.) 

31.933 
1,091 

42.068 
2,139 

23,094 

20,276 

22,801 

2,225 

1,409 
851 

2,746 
9,040 

8,873 

Star  West  fWheatl 

3,323 

9,554 

St.  Lawrence.    (See  Nonpareil.) 

Sterling.    (See  vVmerican.) 

1,110 

10,559 

15,146 

Teal  Lake.    (See  Cambria.) 

1,778 

28,920 

18, 198 

4,071 

15,324 

20,211 

4.704 

24. 141 

38,596 

39,276 

41,456 

4,443 

7,354 

27,865 

1,777 

Wheeling 

Wheat.    (See  Star  West.) 

1,158,249 

919,257 

889,477 

1,006,785 

1,010,494 

1,023.083 

1,130,019 

1,384,010 

1,579,834 

1,829,394 

Name  of  mine. 

1883. 

1884. 

1885. 

1886. 

1887. 

1888. 

1889. 

1890. 

1891. 

1892. 

American  (Sterling) 

3,618 

2,916 

1,483 

13,099 

20,032 

21,000 

21,604 

15,076 

62.752 
631 

69,408 

47,458 

52,975 

16,123 

10,211 

12,835 

Bessemer.    (See  Lillie.) 

847 

Beaufort  (Ohio) 

18,976 
20, 190 

18,360 
2,218 

17. 166 

17,354 

12,829 

Blue.    (See  Queen  group.) 

7,017 
58,743 

16,419 
74,067 

4,091 
86,789 

Braastad|^/j'^^{{^^;j^-p 

50,143 

73,144 

53,913 

155,341 

10,860 
58,784 
137,593 

24,686 
41,130 
146,330 

30.801 
57.861 
174,680 

50.919 
72.780 
215,098 

100.464 

80.359 

223,442 

Cambria.  . 

47.508 
104,960 

117 
218.219 

59,742 
210, 180 

50,796 
173,915 

34.  (»2 
133.413 

41.549 

109.978 

Che^shire.    (See  Princeton.) 
Chester.     (See  Rolling  Mill.) 

Cleveland  TTematite.     (Included  under 
Cleveland.) 

225,674 

218,757 

203,664 

207,441 

184.316 

274,048 

331,713 

221,788 

310,907 

714 

Curry 

16,671 

Dalliba  ( Phenix) 

i.687 
12.314 
4,878 

1.605 
26,099 

Detroit. 

3.809 
16.202 
2.709 

19.125 
750 

39,400 

18.500 
1,821 

10,112 
3,895 

6,080 
9,130 

5.448 

13.000 

Bey.. 

5,039 

2.697 

29,739 

893 

East  New  York 

13.094 

36,431 

50,293 

35,175 

Edison 

Edwards.    (See  Samson.) 
Empire..  .                           .... 

Erie 

5. 405 

1.091 

Fitch 

16.550 
21,949 

15.093 

Foster  c... 

10.029 

9,675 

9,643 

Foxdale 

a  Under  Queen  group  after  1890. 

ftlnclu'ios  liulTalo,  Prince  of  Wales.  Queen,  and  South  Buffalo  after  1896. 

cUndor  Iron  C'lilTs.  1891-1895;  under  Cieveland-ClitTs  group  after  1895. 

d  Prior  to  IKOO.  see  Iira;istiid:  includes  Marquette  after  1S92. 

e Under  Iron  cliiVs.  Iviii  is'i.i;  inider  Clevcland-Clitfs  group  after  IS9.'). 

/  Under  Cli'veluiid-Clilfs  croiii)  afler  IKti. 

a  Includes  Cleveland  after  1S83;  includes  Bamum,  Foster,  Iron  Cliffs,  Michiganune,  and  Salisbury  after  1895. 


HISTORY  OF  LAKE  SUPERIOR  MINING. 


57 


Table  of  Lake  Superior  iron-ore  shipments  from  the  earliest  shipment  to  date — Continued. 

Marquette  Range— Continued. 
[Gross  tons.] 


Name  of  mine. 

1883. 

1884. 

1885. 

1886. 

1887. 

1888. 

1889. 

1890. 

1891. 

1892. 

1,515 

12,142 

2,700 

Grand  Kapids  (Davis) 

1,200 

11,611 

20,058 

566 
7,757 

26,426 

9,362 

22,823 

Green  Bay.    (See  Bay  State.) 
Hartford 

5.678 

886 

5,685 

16,246 

' 

TTnTTibnlHt.  (  Wn.shinfjr.nn)  .  ,  , 

31.866 

23,763 

11,766 

20,207 

19,873 

11,655 

15,866 

23,259 
38,460 

188,776 

19,879 
18,552 

278,270 

4,571 

7,194 

Indiana.    (See  Bay  State.) 
Iron  Cliffs  a  . . . 

87,346 

393 

109,906 

191, 120 

302, 909 

23,041 

12, 139 

78.520 

134,616 

289,395 

.Tnrt^nn 

71,278 

27, 259 

200, 799 

4,614 

14,678 

83,251 

86,922 

204,796 

2, 683 

68,657 

111,051 

226,040 

708 

89,370 

131,731 

267, 622 

3,957 

101,909 

223,600 
240, 225 
.32,692 
22,276 

128,891 

229,070 

288,784 

33,916 

32,982 

124,682 

261,680 

318,321 

31,812 

43,483 

92,979 

241,605 

308,831 

19, 551 

27,683 

92,507 

Keystone.    (See  East  Champion.) 
Lake  Angeline... 

287,517 

366, 715 

T;illip 

29.005 

26,326 

lU^ins 

397 

1,484 

3,111 

1,367 
5,229 

20,  441 

7,060 

70, 128 

23,  (,92 

16,802 

9,555 

Minhigammeo 

42,533 

25,935 

12,373 

48,790 

58,726 

36,448 

66,999 

80,777 

23, 169 

1,894 

Miller" 

805 

25,991 

38,465 

46,693 
8,823 

50. 490 
8,411 

48,908 
540 

52,727 

24,763 

Mitchell 

21,178 

13.987 

Negaunee 

5,259 

45,304 

78,318 

76,488 

64,218 

85,846 

Negaimee  Construction  Works 

10.394 
1,517 

43 
1.077 

1.094 

5,128 

12,844 

2,  422 

11,220 

New  York  Hematite. 

North  Champion.    (See  Hortense.) 

289 

ll,9i;i 

1,436 

Northwest.. 

1,687 

2,200 

3,553 

12,605 
1,594 

18,249 

10,072 

Pendill 

318 

Palmer  (Cascade).    (See  Vcflunteer.) 

5,140 

1,203 

9,060 

Pittsburg  and  Lake  Angeline.    (See  un- 
der Lake  Angeline.) 
Piatt               .... 

2,676 

Portland 

32,  -115 

Princeton  (Swanzey  or  Cheshire) 

13,730 

3,557 

8,328 

2,842 

7,  .301 

29, 403 

491 
66, 122 

Queen  c. . . 

5,527 

109.217 

479. 509 
191,127 

379.719 

Republic. . 

152,565 

277,757 

250,835 

241,161 

220,624 

87 

1,374 

235,062 
21,030 

287.390 
22, 122 

220,0(,5 
3,915 

167,991 

5,022 
402 

3,712 

6,783 

Rolling  Mill  . . 

1,528 
9,108 
17,028 

15,700 

1,820 

946 

26, 629 

1,334 

3,437 

4,403 

1,058 

4,320 

29, 503 

51,667 
1,133 

48,304 

74,947 
4,512 

72, 449 
2,796 

85,798 
1,218 

Sam  Mitchell.    (See  Mitcheli.) 

.^flm^nn  (Arf^lp) 

600 

Schadt " 

Smith.    (See  Princeton.) 

4,964 

24,706 

69,359 

146,383 

Spurr 

9,067 
6,625 

752 
15,867 

Star  West  (Wheat). 

6,824 

9,200 

17, 538 

4,987 

7,997 

15,141 

4,412 

St.  Law-rence.    (See  Nonpareil.) 

Sterling.    (See  American.) 

6,155 

13, 128 
19,414 

Teal  Lake.    (See  Cambria.) 

19,411 
11,748 

23,340 
5,679 

13,865 
24,034 

16,003 
47,486 

2,846 
56,321 

60, 1.56 

141,524 

92,699 

127, 130 

934 
19, 623 
4,585 
4,098 

6,229 
10,558 
10,756 

2,054 

12,872 

3,335 

74 

448 
1,510 
19, 679 

30,734 
2,777 

12,700 
5,887 
6,383 

9.861 
2,074 

Winthrop  / 

109,576 

122,042 

191,658 

Wheat.    (See  Star  West.) 

1,305,425 

1,558,034 

1,430,422 

1,627,380   1,851,634  !l, 923,727 

2,642,813 

2,993,664 

2,512,242 

2,666,856 

a  Under  Cleveland-Cliffs  group  after  1895. 
6  Under  Winthrop  after  1,S92. 
c  Under  Queen  group  after  1890. 


^Includes  Buffalo,  Prince  of  Wales.  Queen,  and  South  Buffalo  after  1890. 
e  Under  Iron  Cliffs,  1891-1895;  under  Cleveland-Cliffs  group  after  1895. 
/  Prior  to  1890,  see  Braastad;  includes  Marquette  after  1892. 


58 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Table  of  Lake  Superior  iron-ore  shipments  from  the  earliest  shipment  to  daJf-Continued. 

Marquette  Range— Continued. 
[Gross  tons.] 


Name  of  mine. 


American  (Sterling) 

Ames 

.\  list  in ; 

ll;irnilin  a * 

W.w  Slate 

I!.',,i'iiier.    (See  LtUie.) 

lic'.iufoi't  (Oliio) 

liluo.    (See  Queen  group.) 

Boston 

,    .(Mitchell 

nraastaci^^-inthrop 

Brc'iiiinp  Hematite  No.  2 

liiillalo'' 

C'aiubria 

Champion • 

Clioshire.    (See  Princeton.) 
Chester.    (See  Rolling  MUl.) 

Chiiaso 

Cleveland  "^ -.--.'•J j'" 

Clevfhiii.l  Hematite.     (Included  under 
Cli'velunrl.) 

Cleveland  l-clifls  group  i 

Cohiinliia  ( Kloman) 

Currv 

Dallllia  (Phenlx) 

Detroit 

Dexter 

East  Champion 

East  New  York 

Edison 

Edwards.    (See  Samson.) 

Empire 

Erie 

Etna 

Fitch 

Foster  « 

Foxdale 

Gibson 

Goodrich 

Grand  Rapids  (Davis) 

Green  Bay.    (See  Bay  State.) 

Hartford 

Hortense  (North  Champion) 

Home  (P.  and  L.  S.)  (now  Volunteer). 

Humboldt  ( Washington) 

Imperial ■ 

Indlaua.    (See  Bay  State.) 
Iron  Clifls  / 


1894. 


1,103 


30, 445 
61,648 


218, 105 


1896. 


5,195    . 


47, 218 
42,788 


143,706 


41,656 
100,398 


587 


95,086 
113,375 


1897. 


7,833         21,740 


911 


352 
6,513 


Iron  Mountain 

Jackson :  ■  ■  •  • 

Keystone     (See  East  Champion.) 

Lake  .\ngeline 

Lake  Superior 

Llllie 

Lucy  (McComber) 

Maas 

Magnetic  (stock  pile) 

Manganese  (Negaimee) 

Marquette  0 

Mary  Charlotte 

Mesabi's  Friend 

Mlchigamme  / 

Miller 

Milwaukee 

Mitchell 

Moore 

National 

Negaunee 

Negaunee  Construction  Works 

New  York  (York) 

New  York  Hematite 

North  Champion.    (See  Hortense.) 

North  Kepublic 

Nonpareil  (St.  Lawrence) 

Northwest 

Norwood 

Ogden 

Pascoe 

PendlU 

Palmer 

Palmer  (Cascade).   (See  Volunteer.) 


130,812 


51,009 

351,973 

329,010 

68. 861 

21,964 


221,153       513,119 


13,752         18,903 


12,073    6,764 
940  


253,760 


935 


69,732 
'25,'666 


32,288 

355,453 

344, 758 

78,388 


1,610 


132,581 
'  21,' 487 


259,042 


42,186 

313,555 
342,  439 
54,285 


5,503 
3,214 


67 
1,532 

'2,'297 


110,648 
141,728 


718, 408 


1,154 


102,623 
163, 190 


869,482 


1899. 


124,930 
215,074 


1,011,048 


80,710 

342,251 

469, 576 
107,532 


79, 102 

489.685 

376. 761 

112.781 

10,033 


1900. 


1901. 


80,432 
113,743 


881,021 


27,987 


90,882   175,394 


1,041 


182, 169 


55,012 

460, 333 

686, 563 

211,023 

11,846 


23,235 


88,230 

464,988 
682, 595 
196,200 


62, 321 


31,714 

389,128 
709, 143 
114,990 


4,338 


68,907 
99,026 


860,484 


31,696 


4,647 


1902. 


5,007 
59,781 


63,976 
205,721 


1,104,864 


38,761 


7,440 


38,271 

481,574 

635,642 

98,788 


191,330 


195,573 
"'6,' 642' 


4,648 

'126,' 829 

■3,' 327 


15,449 

304,125 

832,796 

79,919 


37,655 
'234,Vi3 


204,286 


Pioneer 

Pittsburg  and  Lake  .\ngeline.    (See  un- 
der Lake  Angeline.) 

a  Under  Iron  ClilTs.  1s;iii-1h;i.^,:  under  Clevelatid-Cliffs  group  after  1896. 

6  Under  Queen  gruiip  aflrr  IS'ill. 

SKdSae^la;^,!  ai^if  Is^S; 'IJSu^S-Barnum,  Foster,  Iron  Cli.K  Michigamme,  and  Salisbury  after  IS95. 

«  Under  Iron  CiiUs,  ISM   ls.i.5;  under  Clcveland-Cliils group  after  189o. 

/  Under  Cieveland-Cliils  group  after  1895. 

g  Under  Winthrop  after  1S92. 


HISTORY  OF  LAKE  SUPERIOR  MINING. 


59 


Table  of  Lake  Superior  iron-ore  shipments  from  the  earliest  shipment  to  date — Continued. 

Marquette  Kange— Continued. 
[Gross  tons.] 


Name  of  mine. 

1893. 

1894. 

1895. 

1896. 

1897. 

1898. 

1899. 

1900. 

1901. 

1902. 

Piatt 

5,448 

41,226 

13, 198 

11,296 

Portland 

Primrose 

6,040 

Prince  of  Walesa 

Princeton  (Swanzey  or  Clteshire) 

19,096 

•■■•' 

6,593 

25,247 

55,802 

75,037 

67,051 

118,048 

Quartz 

Queenu 

120,673 
64, 195 

232, 469 
105,719 

204,957  1     323,057 

242,293 
124,342 

61,022 
140,312 

342,978 
137,085 

398,298 
130, 126 

400,845 
104, 604 

418, 044 
157,646 

Republic  Reduction  Co 

Ricliards 

6,887 
4,630 

1,088 

24,464 

4,613 

51,303 

54^181 

50,041 

Riverside 

43 

Rolling  Mm 

3,975 

22,685 

22,815 

24,874 

Saginaw 

Salisbury  c 

SamMltclieU.    (See  MitclieU.) 

Samson  ( A  rgyle)                            

Scliadt 

1,261 

Section  12 

Smitli.    (See  Princeton.) 

Soutli  Buffalo  u 

Spurr 

Star  West  ( Wtieat) 

5,550 

51,207 

9,658 

942 

6,716 

15,987 

St.  LawTence.    (See  Nonpareil.) 

Sterling.    (See  American.) 

Steplienson 

Tavlor 

Teal  Lalie.    (See  Cambria.) 

Titan 

Volunteer  (see  also  Home) 

69,561 

26,946 

32,672 

53,216 

l',617 

29,983 

47,578 

32,736 

Waslilngton 

Webster 

20,797 

West  Republic 

Wlieeling 

180,071 

134,365 

119,120 

150,496 

106,894 

122,592 

171,318 

148,945 

109 

129,496 

Wlieat.    (See  Star  West.) 

1,835,893 

2,060,260   2,097,838   2,604,221 

1                   1 

2,715,035 

3, 125, 039 

3,757,010 

3,457,522 

3,245,346 

3,868,025 

Name  of  mine. 

1903. 

1904. 

1905. 

1906. 

1907. 

1908. 

1909. 

Total. 

American  (Sterling) 

419 

13,764 

23, 222 

90,001 

240,339 
0  ''98 

Ames 

Austin 

195, 950 

111,229 

125,858 

433,037 
801  851 

Barnum  e 

Bay  State 

16,037 

Bessemer.    (See  Liliie.) 

Bessie 

29,718 
134.648 

21.S79 
38,306 

1.646 

25. 781 

78,029 

01,035 

72,987 

Blue.    (See  Queen  group.) 

Boston 

02  542 

Ti       t.  J 1  Mitcliell 

Braastadj^^j^^^^gp 

831  445 

Breitung  Hematite  No.  2 

7,8.54 

9,809 

38,671 

59, 667 

55,849 

129,673 

301  583 

Buflaloa 

017  73U 

Cambria                                  ... 

41,168 
74,238 

84,852 
174 

81,791 
64,680 

40,628 
115,007 

i 35. 145 
107,577 

85,977 
313 

136,815 
11,199 

9  037  717 

4,394,335 

9,012 
2, son, 298 

15,239,906 
94,813 

111,(371 

5Q  114 

Cliesltire.    (See  Princeton.) 
Chester.    (See  Rolling  Mill.) 

Cleveland  / 

Cleveland   Hematite.    (Included  under 
Cleveland.) 

810,845 

743.263 

1,288,416 

1.330.944 

1,030,928 

438,379 

877,433 

Columbia  ( Kloman) 

Dalliba  (Plienix) 



140,841 

Dexter 

118  512 

Dey 

2,709 

East  Cliampion     

7tt  002 

East  New  York 

22,523 

7,299 

33.095 

Edison     ..                        

893 

Edwards.    (See  Samson.) 

Empire 

40,565 

53,637 

108,993 

203  095 

Erie .- 

8.136 

Etna...                                 

1,0*11 

Fitch 

31,817 

Fosterc 

171,893 

Foxdale 

5.053 

3,429 

3,303 

31.447 

Cibson 

16,357 

a  Under  Queen  group  nftor  1,890. 

6  Includes  [iuil'iilo.  I'riiicc  of  Wales.  Queen,  and  South  Buffalo  after  1890. 

cUniier  Iron  I'lilf^,  ]v,ll-]S95;  under  Cleveland-CliUs  group  after  1895. 

d  Prior  to  1S90,  see  Braastad:  includes  Marquette  after  1.S92. 

e  Under  Iron  Clills,  1S90-1,S95;  under  Clevcland-CliUs  group  after  1895. 

/Under  Cleveland-Cliffs  group  after  I8.S:!. 

e  Includes  Cleveland  after  1883;  includes  Barnum,  Foster,  Iron  Clifls,  Michigamme,  and  Salisbury  after  1895. 


60 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


Table  of  Lake  Superior  iron-ore  ahipmcnlsfrom  the  larliial  sldpmenl  to  date — Continued. 

Marquette  Range— Continuird. 
[Gross  tons.) 


Name  of  mine. 

1903. 

1904. 

1905. 

1906. 

1907. 

1908. 

1909. 

Total. 

49.754 



110,736 

Green  Bay.    (See  Bay  State.) 

Hartford                                ' 

20, 085 

179,980 

322,209 

364,801 

328,161 

278,366 

250,680 

1,760,951 

30,574 

26,022 

713,961 

727 

1,661 

6,076 

55,756 

48,231 

115,478 

376,  Ol 

Indiana.    (See  Bay  State.) 

1,700,537 

393 

5,409 

310,950 
604,829 
77,454 

33, 180 

374.183 

727,378 

9,868 

5,066 

269, 116 

635,671 

32,  781 

85 

61,345 

283.373 
674,066 
80.545 

11.060 

280,298 

349.435 

61,708 

1,672 

159, 197 

3,885,513 

Keystone.    (See  East  Champion.) 

262,480 
590. 339 
63,209 



220,410 

261,955 

8,632 

1.115 

29,030 

8,285,400 

14,931,563 

Lillie 

1,743,490 

519,031 

32,378 

220,611 

292 

292 

6.359 

152.907 

34,303 

4S.8S5 

221,738 

257.088 

155,633 

99,104 

240,433 

1,057,  IM 

16.043 

880,362 

375, 451 

MitrhplI                                                  

11,539 

29.319 

25,828 

68.131 

150.216 

224,665 

i45,i32 

239,554 

253,448 

196, 170 

232,219 

312,217 

3,61.2,127 

12,708 

Npw  York-  fYork") 

1,123,071 

37,587 

North  Champion.    (See  Hortense.) 

^ 

289 

23,395 

1,687 

5,753 

986 

59,806 

45,993 

13, 131 

14, 172 

Palmer  (Cascade).    (See  Volunteer.) 

15,409 

Pittsburg  and  Lake  Angeline.    (See  un- 
der Lake  Anceline.) 

73,844 

Portland 

79,652 

79,652 

, 

6,040 

32,415 

Princeton  ( S wanzey  or  Cheshire) 

84,223 

76, 461 

129,079 

166,894 

177,863 

36,033 

42,934 

1,271,761 
491 

180,866 

Queen  group  rf     

254,658 
155, 415 

311,479 
124,506 

263,377 
150,699 

221.096 
177,220 

309.917 
170,554 

104,098 
67,999 

237,509 
176,575 

5,315,998 

6,193,469 

47, 174 

8,261 

55,593 

68,134 

86,129 

89,563 

35, 156 

60,994 

102,566 

688,455 

16, 160 

Rolling  Mill                     

6,786 

28,766 

49,204 

52,147 

133,139 

578,916 

451,424 

686,411 

Sam  Mitchell.    (See  MVtcheli.) 

267,805 

Schadt 

1.261 

21,887 

Smith.    (See  Trinceton.) 

245.412 

165.244 

Star  West  fWheatl 

204, 649 

St.  LawTence.    (See  Nonpareil.) 

39,869 
64,075 

39,869 

Sterling.    (See  American.) 

Stephenson 

6,305 

52,588 

■      122,968 

.32.970 

Teal  Lake.    (See  Cambria.) 
Titan 

90,  .371 

7,395 

71,870 

100,281 

38,544 

10,022 

1,393.175 

20,625 

44,716 

65.341 

Webster 

34.905 

133.077 

50.870 

10..i55 

WinthroD  f 

72,433 

1,759.115 

Wheat.    (See  Star  West.) 

3,040,245 

2,843,703 

4,215,572 

4,057,187 

4,388,073 

2,414,632 

4,256,172 

91,83S,55S 

a  Under  Clcveland-Clifls  Rroup  after  1895. 
tUiKlor  Winl.hrop  nftor  isaii. 
c  Under  Queen  group  after  1890. 


d  Inclnde"!  Buffalo.  Prince  of  Wales,  Queen,  and  South  Buflalo  afler  1890. 
el'nder  Iron  ClilTs,  1891-1S95:  under  Clevelami  riifTs  p-oup  after  1895. 
/  Prior  to  1S90,  sec  Braastad;  mcludcs  Marquette  after  1S92. 


HISTORY  OF  LAKE  SUPERIOR  MINING. 


61 


Table  of  Lake  Superior  iron-ore  shipments  from  the  earliest  shipment  to  date — Continued. 

Uenomlnee  Range. 

[Gross  Ions.] 


Xame  ot  mine. 

1S77. 

1878. 

1S79. 

1880. 

1881. 

1882. 

I8,s:i. 

1884. 

1885. 

Alpha 

Antoinc  (Clitt'ord) 

Aragon 

Armenia 

Bauer 

Baltic 

Berkshire 

Beta 

Breen 

5,812 

4,796 

1,463 

5,359 

Brier  Hill 

10,593 

4,388 

Bristol  (Claire) 

Calumet 

5,847 

29,239 

3,627 

Caspian 

34,566 

134,521 

247,506 

265,830 

290,972 

Chatham 

Clifford : 

Columbia 

" 

15,948 
115,862 

4,3,34 
21,493 

6,774 
34,622 

r^imninnwpnlth 

9,643 
30,856 

97,410 
11,816 

42,947 

Cornell 

Crystal  Falls 

1,341 

Cuff 

Candy 

12, 803 
46, 168 

21,851 
14,368 

17,534 
12,644 

13,374 
18,287 

3,676 

22,675 

3,410 

10, 079 

24,099 

608 

4,897 
49,897 
9,880 

Cvclopsa 

6,028 

Delphic 

Dober  6 

T)iinTi 

Eleanor  (Appleton) 

Emmett 

12,397 

22,474 

31,136 

.  648 

Fairbanks  c 

8,045 
160,165 

455 
40,232 

Florence 

14, 143 

100, 501 

Fogarty 

Forest 

Genesee  (Ethel) 

Gibson 

Great  Western 

587 

22,826 

20,710 

Groveland 

Half  and  Half 

Hemlock 

Hersel 

Hiawatha 

Hilltop 

HoUister 

Hope 

4, 280 
29,115 

4,362 
100,369 

636 
52,684 

2,739 
56,693 

Iron  River  d 

James 

Keel  Ridge 

11,496 

19,611 

23,425 

6,033 

Kimball 

Lament  (Monitor) 

Lee  Peck  e 

Lincoln 

Loretta 

Ludington  / 

8,816 

3,374 

52,152 

102,632 

101,165 

124, 194 

Manganate 

Mansfield 

3,477 

18,677 

18,187 

McDonald 

Metropolitan 

23,854 

36,643 

27,577 

Michigan  Exploration  Co 

Millie  (Hewitt).. 

4,362 

9,500 

7,516 

7,927 

4,627 

Monongahela 

Munro 

2,480 

29  221 

7^202 

114,836 

5,973 

37,620 
10,004 
71,710 
11,652 

Northwestern 

7,276 

73,519 

198, 165 

137,077 

165,547 
6,515 

Penn  Iron  Mining  Co.  » 

Perry 

3,138 

Pewabic  (see  also  Walpole) 

Quinnesec 

25,925 

41,954 

52,436 

43,711 

44,240 

21,676 

16,996 

14  110 

Riverton   (see  also    Dober   and 
Iron  River)  h 

13,465 

49,196 

60,406 

73,648 

76,514 

38, 120 

18,020 

Selden 

Sheridan 

Shelden  &  Shafer  (Union).    (See 

Columbia.) 
South  Mastodon 

798 

23,089 

10,856 

Sturgeon  River 

Tobin 

Verona 

a  Under  Penn  Iron  Mining  Co.  after  1892. 
bUndor  Riverton  aficr  1900. 
c  Included  in  Paint  Hivrr  after  1S93. 
d  Under  Riverton  after  1892. 


e  Cherry  Valley  ore. 

/  Included  in  Chapin  after  1S94. 

g  Includes  Curry.  Cyclops,  Norway,  and  Vulcan  prior  to  1S93. 

ft  Includes  Iron  River  after  1S92;  includes  Dober  after  190U. 


62 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


Table  of  Lake  Superior  iron-ore  shipments  from  the  earliest  shipment  to  date — Continued. 

Menominee  Range — Continued. 
[Gross  tons.] 


Name  of  mine. 

1877. 

1878. 

1879. 

1880. 

1881. 

1882. 

1883. 

1884. 

1885. 

V  ulcan  " 

4,593 

38,799 

56,975 

86,976 

8o,2?4 

94,042 

79,874 

101,722 

124,125 

Walpoleb                        

6,198 

15,292 

8,344 

10,405 

95,221 

269,609 

592,088          739,635 

1,136,018 

1,(M7,415 

895,634 

690,435 

Name  of  mine. 

1886. 

1887. 

1888. 

1889. 

1890. 

1891. 

1892. 

.    1893. 

1884. 

1 

Antoine  fCliffordl 

1,745 
50,275 

46,609 
26,649 

96,829 



167,948 

127,901 

138,209 

^ 

Baltic 

jj 

1,585 

1,226 

1,400 

' 

Brier  Hill 

57,352 

9,612 

* 

Chapin  (see  also  Ludington) 

198,871 

336, 128 

290,871 

518,990 

742,843 

488,749 

660,052 

489,134 

235,895 

Columbia      .             

14,282 
51.189 
4,566 

2,377 
56,609 
2,064 

10,936 
61,818 

11,385 
108,515 

60, 133 

116,786 

70,770 
134,982 

57,682 
249,113 

22,426 
151,291 

10,300 

174,921 

Cornel! 

3,974 

CuH 

.     _ 

5,376 
14,693 

28, 722 
6,101 

72, 162 
7,361 

100,681 
10,599 

125,773 
1,697 

37,189 
17,648 

14,297 
2,272 

24,677 

118,096 

151,828 

156,963 

162,721 

133,666 
4,377 

58,590 
5,618 

24,538 

8,210 

79,399 

142,585 

196, 269 

218,570 

48,806 

48,246 

9,634 

2,726 

Genesee  (Ethel) 

« 16,357 

87,487 

22,267 

23,239 

21,860 

38,454 

72,546 

62,464 

1,049 

67 

58,197 

35,531 

661 

Half  and  Half 

5,961 
8,347 

1,496 
17,072 

872 

600 

8,801 

2,183 
65,459 

Hemlock 

11,323 

955 

Hiawatha 

1,683 

Hilltop 

HoUister 

2,020 

1,057 

1,021 
15,543 

Hope 

2,275 

5,854 
78,591 

83,018 

110,000 

179,238 

155,458 

59,345 

1,176 

5,997 

3,298 

Kimball 

12,348 

31,139 

26,226 

42,819 
2,844 
26,019 

13,777 

2,600 

Lincoln 

1,813 

8,757 

8,131 

109 

Loretta 

55,983 

74,454 

101,653 

61,883 

116,297 

97,355 

6,844 
18,303 
66,526 

141,303 

15,777 

354 

Mansfield 

49,836 
45,370 

69, 2,i9 
9,150 

69. 558 
23,485 

41,640 

48,792 

51,463 

63,511 

McDonalil 

Metropolitan 

6,393 

9,070 

3,490 

Michigan  Exploration  Co 

505 

77 

Millie  (Hewitt) 

5,517 

1,163 

11,124 

12,274 

39,232 

5,889 

6,780 

13,062 

Miinro       

5,400 

30,460 

5,744 

3,441 

13,200 

Northwestern 

93,878 
13,933 

95,726 
10,240 

87,260 
12,506 

68.044 
32,700 

61,717 
62,654 

4,089 
45,435 

44,767 
18,390 

Paint  Uivertseealso  Fairbanks). . 
Penn  Iron  Mininc  Co.  i 

280,450 

i75,274 

a  Under  Penn  Iron  Mininc  Co.  after  1892. 

dlnclu'leil  in  I'l'wabii- uficr  1,S91. 

c Under  Kiverion  after  I'.iimi. 

i  Included  in  Paint  River  after  1893. 

« Includes  shipments  lor  prior  years. 


/  Under  Riverton  after  1892. 

ff  Cherry  \';\IIey  ore. 

*  Included  in  C'hapin  after  I.'!94. 

» Includes  Curry,  Cyclops,  Norway,  and  \'uk-un  prior  to  1S93. 


HISTORY  OF  LAKE  SUPERIOR  MINING. 


63 


Table  of  Lake  Superior  iron-ore  shipments  from  the  earliest  shipment  to  date — Continued. 

Uenomlnee  Range— Conlinucd. 
[Gross  tons.] 


Name  of  mine. 

1886. 

1887. 

1888. 

1889. 

1890. 

1891. 

1892. 

1893. 

1894. 

Peny 

28,991 

64,  .507 

115,273 

165,745 

304,010 

13,442 

6,585 

2,249 

Riverton  (see  also  Dober  and  Iron 
River)  a                             ... 

12,853 
790 

10,834 
1,302 

1^684 

13, 354 

11,971 

1,102 
4,005 

595 
1,476 

7,137 

45,745 

2,234 

Shelden  &  Shafer  (Union).    (See 
Columbia.) 

2,722 

1,018 

3,589 
6,829 

7,800 

4,775 

Vivian 

Vulcan  b '        143, 930 

305,03ii 
1,740 

r29,541 
900 

153,900 
9,614 

104,996 
2,940 

78,967 
3,895 

179,904 

25,635 

34,418 

12,699 

44,400 

3,705 

1 

880,006 

1,193,343 

1,191,101 

1,796,754 

2,282,237 

1,824,619 

2,377,856 

1,466,197 

1,137,949 

Name  of  mine. 

1895. 

1896. 

1897. 

1898. 

1899. 

1900. 

1901. 

1902. 

Antoine  fCllfford)                   . .    . 

27.931 

183, 296 

2,045 

110,821 
95,809 

98,847 
149,694 

104,510 
295,821 

93,025 
337,807 

119,940 
404,645 

63,429 

477,212 

18,750 

110,993 

646,203 

100, 864 

Baker        

Baltic 

17,326 

64,664 

Beta 

Brier  Hill     ...           

Bristol  (Claire) 

80,915 

51,639 

36,593 

129,035 

Chapin  (see  also  Ludington) 

618,589 

420,318 

643,402 

724,768 

940,513 

929,937 

929,701 

966,812 

Clifford                 

70,867 
208,880 

87,202 
93,707 

24.623 
98,283 

14,199 
250,687 

126.290 
117,295 

97,531 
63,342 

19,963 
77,799 

186,798 

112,704 

Cr v^tal  Falls 

13,037 

44,526 

95,210 

128,233 

147, 346 
20,210 
100,902 

197, 770 
38,209 
141, 148 

230,614 

195,555 

Cuff       

3,395 

41,942 

76,877 

178,800 

183,052 

Delnhic                                

52 

5,009 
49,381 

'°:l^ 

49,203 

90,885 
2, 107 

47,081 

31,062 

2,816 

Fairbanks  « 

22,820 

35,136 

37,594 

93,663 

74,235 

36, 756 

15,395 

130, 798 

Forest                               

14,455 

Gibson                              

14,643 

33,851 

43,316 

98,550 

123,361 
11,444 

42,470 

7,699 

Half  and  Half 

949 

94,646 

96,032 

69,865 

110,209 

72,413 

149,966 

123,331 

Hersel                

1,201 

11,008 
6,410 

20,355 
2,503 

74,596 

3,496 

i 

3,373 

1 

. 

Keel  Ridse 

i9,44i 

4,900 

Kimball                   

1 

67,652 

31,323 

47, 267 

43,622 
,        64,824 

72.959 
61,219 

19,727 
54,985 

7,747 

53,160 

34,334 

64, 104 

68,447 

128,300 

1 

37,182 

1             60,739 

86,607                90,155 

74, 113 

31,181 

33,  733 

60 

McDonald 

"  Includes  Iron  River  after  1S92;  includes  Dober  after  1900. 
^  Under  Pfnn  Iron  Mining  Co.  after  1S92. 
f  Included  in  Pewabic  after  1891. 
d  Under  Riverton  after  1900. 


e  Included  in  Paint  River  after  1S93. 

/  Under  Riverton  after  1892. 

g  Cherry  Valley  ore. 

ft  Included  in  Chapin  after  1894. 


64 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


Table  of  Lake  Superior  iron-ore  shipments  from  the  earliest  shipment  to  dale — Continued. 

Menominee  Range— Continued. 
[Gross  tons.] 


Name  of  mine. 

1895. 

1896. 

1897. 

1898. 

1899. 

1900. 

1901. 

1902. 

1 

1.071 
10,924 

216 
10,374 

53,272 
25,935 

Rlillii-  <  Hewitt). 

21,815 

17,430 

15, 194 

14,922 

12,133 
2,397 

1 

1 

• 

1,324 

1,316 
197,606 

10,383 

290,622 

179,917 

237,886 

223,713 

229,651 

358,126 

273,443 

Pewabic  (see  also  Walpole) 

262,551 
761 

273,587 

279,855 

305,072 

530, 129 
11,050 

2,202 

374,043 
25,967 

71,004 

507, 786 
06,383 

119,860 

530,291 
62,531 

Eiverton  (see  also  Dober  and  Iron 
River)  <^                              

215,850 

2,161 

Selden                                         



16,754 

3,419 

146 

31,104 

8,063 

Shelden  A:  Shafer  (Union).    (See 
Columbia.) 

■ 



Tobin              

18,957 
11,475 

55,238 



5,143 

43,245 

40,384 

13 

661 

1,923,798 

1,560,467 

1,937,013 

2,522,265 

3,301,052 

3,261,221 

3,619,053 

4,812,509 

Name  of  mine. 

1903. 

1904. 

1905. 

1906. 

1907. 

1908. 

1909. 

Total. 

Alpha 

1,370 
107,886 
522,035 
31,901 

1,370 

Antrjinp  CClitlordl 

81,164 

374, 944 

16,577 

138,395 
423,098 

195, 855 

431,000 

27,882 

100,996 
441,636 
36,665 

l,a53,792 

Aragon        

226,354 

246,984 

5,836,279 

311,608 

45,003 
174,426 
34,295 

4o,«)3 

Baltic 

123,236 

151,114 

133,246 

186,495  1 

189, 119 

129,037 
3,440 

1,10s, 663 

37,735 

Beta                      1 

:::::;:;:;:::::::::;::;.:::; 

4,211 

16,625 

21,004  I 

20,366 

75,425 

Brier  Hill     

I 

14,981 



246,581 

132,420 

210,388 

298,031 
15, 773 
80,875 

943,425 

345,676 

51,646 

13S.S67 

855,308 

14,883 

190,300 
15,222 
102,628 
391,620 
45,826 

396,825 

2,18.5,367 

121,354 

2,088 
704,051 

4,242 
541,324 

10,248 
902,628 

189,023 

587, 647 

68,730 

103,626 

527.971 

Chapin  (see  also  Ludington) 

Chatham 

16,182.416 
129,  439 

Clifford              

103, 626 

27,883 
8,085 

942,703 

5,051 

1,617 

6,346 

50,787 

2,511,784 

49,302 

Crystal  Falls 

Cuff 

117,096 

180,983 

152,255 

111,871 

114,158 

296 

986 

1,735,251 

58,419 

iii,85i 

1,410 

5,512 

844,889 

416,928 

286,093 

Dclnhic 

33,770 

65,192 

5,365 

21,051 
1,819 

91,476 
3,121 

141,992 
1,677 

8,829 

193,3% 

1,521,871 

18,719 

66,655 

8,500 

95,877 

153,452 

233,858 

169,459 

178,905 
7,949 

140,354 
32,560 

231,191 
77,356 

2,718,019 

Fogarty 

117,Sll5 

Forest                     

11,988 
132,380 

11,9SS 

61,694 

77,370 

80,971 

38,984 

6S,,5S5 
36.246 
112,747 
24,933 

471,4.S9 

4,548 

124,246 

9,123 

57,151 

101),  751 
1,294 

68, sis 
4,737 

191,265 

311,218 

234,492 
13,913 

1,872,228 

Groveland  

74,092 

Half  and  Half. 

7,524 

96.072 

79,420 

136,232 

124,450 

106,437 

117,181 

83,834 

112,481 

1,589.818 

Hersel             

».t5 

53,828 

38,288 

9,704 

20 
7,820 

138,190 

136,739 

4SS.6I2 

Hilltop 

20, 229 

6,371 

10,671 

25,842 

46,;ts2 

7,339 

. 

2s,.i.10 

17.S71 

904,,i,s7 



2,  .360 

59,760 

90,851 

152.971 

Keel  Kidco 

] 

93, 101 

Kimball 

1 



16,224 

16,224 

a  Under  Penn  Iron  Minfne  To.  aripr  1892. 
t>  Includes  Curry,  ryclops,  Norway,  and  Vulcan  prior  to  1893. 
« Includes  IronRiv'i'r  iificr  1n:i2:  iucludes  Dobor  after  1900. 
d  Included  in  I'ewabic  alter  IS'.il. 


e  lender  Riverton  after  1900. 

/  Included  in  Paint  Kiver  after  1893. 

e  Under  Hiverton  after  1892. 


HISTORY  OF  LAKE  SUPERIOR  MINING. 


65 


Table  of  Lake  Superior  iron-ore  shipments  from  the  earliest  shipment  to  rfa<e— Continued. 

Menominee  Range— .ContiiiMcil. 
[Gross  tons.] 


Name  of  mine. 

1903. 

1904. 

1905. 

1906. 

1907. 

1908. 

1909. 

Total. 

Lament  (Monitor) 

43,736 

29,393 

74,991 

89,980 

42,090 

555,341 

2,841 

241,627 

1,195,020 

1,001,518 

6,844 

1,102,998 

425, 708 

1,144 

107,027 

153,797 

368,265 

9,310 

278,556 

373,765 

36,810 

1,291,352 

371  289 

Lee  Pecko 

Lincoln 

15,606 
87,939 

17,577 
54,720 

19,539 
118,738 

5,890 
140,390 

714 
99,779 

1,657 
96,613 

13,354 

Ludington  >i 

Manganate 

Mansfield 

51,440 

79,163 

38,584 

183, 532 

44,633 

118,713 

Mastodon 

McDonald 

1,144 

Metropolitan 

Michigan  E.vploration  Co 

58,088 

146 
36,815 

39,819 
18,091 

603 
3,322 

Millie  ( Hewitt) 

40,860 
6,913 
8,739 

10,887 

Monongaliela 

Munro 

32,332 
9,080 

92,183 
91,238 

47,454 
91,792 

46,834 
53, 778 

27,773 
306 

23,241 

Nanatmo 

Northwestern 

17,280 

Norway  c 

Paint  River  (see  also  Fairbanks).. 

9,863 
343,543 

ii,257 
141,948 

11,973 
423,244 

28,321 
496,582 

75,805 
381,128 

Perm  Iron  Mining  Co.  t^ 

176,211 

428,004 

4,837,348 

3,138 

6,917,700 

502,903 

1,141,098 

501,985 

2,092 

116.299 

8,203 

39,350 

19,404 

1,394,737 

130,973 

405,412 

1  668  654 

Perrv 

Pewabic  (see  also  Walpole) 

Quinnesec 

489, 175 
49,708 

97,633 

372,791 
33 

81,543 

633,413 

493,891 

457, 796 

365,341 

465,453 
3,147 

171,200 
19,994 

Riverton  (see  also  Dober  and  Iron 
River) «.. 

82,611 

161,704 
21,017 

90,358 
26,080 

47,073 
38,069 

Saginaw  (Perkins) 

Selden 

Sheridan 

Sheldon  &  Shafer  (Union).    (See 

Columbia.) 
South  Mastodon 

Stephenson 

Sturgeon  River 

Tobin 

45,386 
50,910 
12, 122 

113,669 
20,202 
81,354 

166,529 

235,867 

237, 781 

161,642 

359,668 

Verona 

Vivian 

90,426 

122,577 

48, 493 

10,056 

Vulcan  c 

Walpole/ 

19,089 
375,385 
161,425 

12, 135 

10,926 

47,583 

92,632 

70,094 

154,150 

Youngs  town 

Zimmprman, 

1,832 

10,303 

3,749,567 

3,074,848 

4,495,451 

5,109,088 

4, 964, 728 

2,079,156 

4,875,385 

71,212,121 

Mesabl  Range. 


Name  of  mine. 

1892. 

1893. 

1894. 

1895. 

1896. 

1897. 

1898. 

1899. 

1900. 

1901. 

59,141 

234,562 

170,738 

390,800 

720,474 

777,346 

829,118 

.Adriatic 

Agnew 

14,903 

64,218 

41,300 

Albany 

Alberta 

.\uburn 

108,210 
"96,' 048" 

376,970 
"'247,069" 

131,478 
'"242,"  .56.5' 

176,263 
'427,404' 

235.030 
"383,'i86' 

385.992 
'553,' 836' 

263, 692 
'  '924,"868' 

427,510 
'416,074 

Bessemer 

Biwabik...^  .             .... 

"isi.'soo' 

Brav 

Brunt 

Burt 

Canisteo 

Canton 

24.416 

213,853 

359,020 

10,261 

99,498 

Cass 

Chisholm 

34, 573 

26,372 

17,187 

57,324 

32,912 

246 

Clark 

63,071 

199, 506 
15,627 

66, 137 

7,213 

22,003 

60,798 

80, 494 

152,947 

278,416 

Crosby 

Cyprus 

Day 

18, 651 

1,975 

Diamond 

Duluth 

37.026 

112,155 
564 

166,435 
9,647 

128,587 
121,707 

150,024 
224,630 

Elba 

Euclid 

Fayal 

136,601 

248,645 

642, 939 

575, 933 

1,072,257 

1,252,504 

1  656  973 

Forest 

Fowler 

Franklin 

46,617 

223,399 

280,423 

231,080 

30, 128 

200.400 

(iO,000 

168,524 

39,299 

Franz 

Genoa., 

17, 136 

309,514 

279,077 

276. 559 

253,651 

332,022 

Gilbert 

o  Cherry  Valley  ore. 

!>  Included  in  Chapin  after  1894. 

<:  Under  Penn  Iron  Mining  Co.  after  1892. 

47517°— VOL  52—11 5 


d  Includes  Curry,  Cyclops.  Norway,  and  Vulcan  prior  to  1893. 
'  Includes  Iron  River  after  1892;  includes  Dober  after  1900. 
/  Included  in  Pewabic  after  1891. 


66 


GEOLOGY  OF  THE  LAKE  SUPElilOR  REGION. 


Table  of  Lake  Superior  iron-orr  shipments  from  the  earliest  shipment  to  date — Continued. 

Uesabl  Range— Continued. 

[Gross  loiis.l 


Name  o!  mine.                          1892. 

1893. 

1894. 

1895. 

1896. 

1897.          1898. 

1899.             1900. 

1901. 

Glen 

Grant ■. 

Hanna 

Hartley 

Hawkins 

Hector  (Hale) 

3,616 

24,167 

31,004 

70,006 

13.728 

18,«07 

32,901 

30  929 

Higpins  No.  2 

Hobart 

HoUanil 

i 

Hull 

Hull-Rust 

Jordon 

Kellogg 

Kinnev 

LaBelle..  .             

Lake  Superior  group 

58,123 

67,659 

259, 912 

135,404 

154,320 

284,023 

594,761 

Larkin  (Tpsoraj 

Laura '. 

Leonard..  .               

' 

Longyear. .              

Mahoning..   .         

117,8S4 

167,245 

519,892 

.520.  751 

750,341 
28,615 

911,021 
65,340 

705, 872 

126,299 

Mariska ■ 

Miller 

13.858 

2,140 

Minorca 

Monica .         

Monroe 

Morris 

Mountain  Iron  (and  Rath  and  Aetna) . . . 

4,245 

121,463 

573,440 

371,274 

159,744 

773,538 

650,955 

1,137,970 

1,001,324 

1,058,100 

Pearce 

Penobscot.    .               ... 

11,933 

29,652 

85,619 

146,  Ml 

221,080 

Pettit 

Pillsburv 

99,691 

106,487 
57,847 

101.032 
41,905 

120.723 

Rol)erts". 

18, 614 

42, 756 

Rust 

Sauntrv- Alpena. ..       .                     .  . 

53,004 

1)8, 560 

328,739 

Sellers 

47,433 

153,037 

112,765 

174,867 

56,280 

34,918 

Seville 

Sharon ... 

56,810 

Sliver.... 

66,722 

226,156 

237,143 

202,144 

156,426 

Spring 

5,628 

47,700 

96,280 

12,215 

1,621 

101,675 

279,515 

st.ciair        ;:": r 

St  Paul 

Stephens                     

56,031 

666,273 

Sweenev 

Syracuse 

Troy 

8,297 

93,109 

Utica  .           

Virginia  group 

123,015 

544,954 

622,712 

955, 739 

749,499 

560,  »>8 

293,651 

417,473 

5,420 

Webb.                

.  3,046 

11,249 

12,357 

18.238 

Wills                           

Yates                       

1 

4,245 

613.620 

.793,052 

2,781,587    2,882,079 

1,275,809 

4,613,766 

B,626.3S4  ,7,809,535 

9.004.890 

HISTORY  OF  LAKE  SUPERIOR  MINING. 


67 


Table  of  Lake  Superior  iron-ore  shipments  from  the  earliest  shipment  to  date — Continued. 

Mesabi  Range— Continued. 
[Gross  tons.] 


Name  of  mine. 

1902. 

1903. 

1904. 

1905. 

1906. 

1907. 

1908. 

1909. 

Total. 

Adams 

1,242,923 

1,109,750 

940, 105 

1,140,984 

1,238,350 

3,294 

103, 2(X) 

9,057 

366,371 

1,136,513 
70, 187 
149,084 

765.. 592 
108, li9 
164, 486 

1,829,372 
107,317 
151,536 

12,585,828 
288,927 
923,851 
207, 650 

45,582 
24,829 

108,847 
23,933 
109,608 

90,435 

912 

153,433 

44,651 

28,439 

241,186 

Albany 

437,521 
31,032 
120,332 

64,860 
51, 143 
35,747 

368,057 

1,731,036 
82  175 

Alberta 

Alexander 

15,073 

60,547 

231, 699 

38,283 

2,143,028 

756,853 

9,121,509 

Bessemer 

80, 303 
647,614 

112,630 
1,092,987 

131,791 
807,374 

78,012 
803, 750 

120,350 
365,781 

227,767 

642,821 

65,514 

14, 212 

1,660,101 

85,505 

623, 127 

807,511 

Bray  ... 

65,514 
269,l>i4 

75,401 
1,376,875 

178,935 

1,501,272 

5,454 

636 

1,460,998 

2,760 

Burt 

1,860,462 

7,859,698 
93,719 
713,048 
241,343 

1,946,993 
152,075 

2,942,375 
16,987 

2,201,854 
636, 176 
678  192 

Canton 

Cass 

50, 155 
168,831 

29,554 
130, 732 

.59,552 
231, 296 
965 
358,091 
1,300 
146,901 

66,961 

379, 156 

1,373 

274,394 

36, 121 

258,793 

6,309 

319,983 

Chisholm 

200,029 

228,386 

4,790 

334,594 

314,697 

4,637 

484,512 

Clark 

350,799 

300,492 

266,873 

Commodore. . . 

65,833 
59,292 

20, 436 
34,043 

249 
30,131 

263, 401 
100,606 
115,373 
162,533 
192,144 

477,203 
172, 326 
227.365 
349, 853 
260,948 

116,069 
77,674 
152,084 
154,868 
115,745 

409, 148 
135,366 
183,470 
159,038 
107,685 

Crosby 

Croxton 

18,594 

100,297 
121,818 
107, 781 

348 

.244,343 

84,530 

130,228 
235,351 

1,075.759 

1,278,034 

319,453 

171 

Cynrus 

106,516 

Diamond 

171 
93, 120 
134,488 

150,220 
207,454 

150,053 
93,616 

149,819 
123,425 

142, 172 
125,724 

158,336 
255,580 

149,185 
147,916 

150,501 

224,202 

82,627 

1,879,357 

6,304 

99,892 

51,393 

1,737,233 

Elba 

1, 668, 853 

Euclid 

82, 627 

Faval 

1,919,172 

1,460,601 

975, 102 
85,280 

1,358,922 
99,785 

1,634,8.53 
41,647 

1,878,812 

4.840 

34.014 

30.921; 

907 

108,610 

100,178 

205,426 

1,439,879 

2.420 

21.511 

8,246 

18,132,550 

Forest 

Fowler 

155  417 

111,085 

92,019 

65,528 

62,884 

244,150 

66, 935 

11,0C,8 

179,468 

1,712,008 
145. 069 

70.210 
281,081 

Genoa 

399,719 

303,700 

2,985  287 

Gilbert 

336,927 
272, 142 

783, 683 
396,591 

1  220  788 

Glen 

23,875 
51,946 

171,705 
18,928 

280, 412 
44,413 

287, 835 
49,227 

279,424 

1,917.410 
164,514 

Grant 

Hanna 

238,873 

""3i6,'783' 

30, 726 

322,604 

238  873 

Hartley 

334, 646 

270.984 

65, 952 

173, 439 

7,339 

16,908 

8,068 

157,366 

2,900.493 

254,329 

99,812 

61,996 

65,462 
248, 246 

390,108 
1,646,523 

418,336 

1,111,146 

8,314 

270  864 

Hawldns 

5,892 
54,289 

107,905 

99, 0.55 

202, 070 

4,990 

238,598 

294,588 
.37.221 

341,319 

975 

95,472 

Hector  (Hale) 

Higgins  No.  2 

35,286 

158, 484 

1,682 
163,020 
2, 926, 083 
151,071 
18.313 
118,529 
31,331 
176,510 

391,157 

400. 907 

Hull 

233,065 

282,592 

1,690.311 

190,971 

84,715 

110,708 

836  043 

Hull-Rust 

3,039,911 
162,510 
10, 477 
13,754 
165,468 
287,431 
7,464 
27,216 

10,557,398 
877  767 

17,562 

50, 215 

61,109 

.   . 

013  317 

Jordon 

147,931 

190,024 

97,474 

185,854 

925,330 

32,352 

6,225 

89, 161 

57,691 

145,989 

796,349 

7,464 

473, 668 

La  Btlle 

70, 753 
766,311 

48.298 
1,226,066 

89,554 
1,415,884 

78,597 

50,466 

56,146 

51,638 

4,963,469 
94  722 

Larkin  (Tesora) 

12,001 
175,670 
138,001 
308.989 
254,308 
367,192 

22,040 
301,522 
149,410 
301.368 
l.W,316 
297,870 

14, 030 
79' 313 
176, 726 
289,490 

46,661 
366,543 
178,110 
653, 162 
6,857 
303,066 

53,335 

79,286 
200,163 

10.591 
279.399 

81,604 

105,170 
3,778 
228,536 
151,952 
153,822 
221 

197,192 
27,207 
352.004 
297,011 
275.777 
16,778 

1,277.745 

Laura . . . 

16,453 
28,784 

768  970 

2,263,496 
858  095 

Leonard 

Lincoln 

87,908 
22,788 

379, 219 

2,144,263 
121  391 

17,706 

1,564.332 

82,065 

137 

113,  .521 

279,463 

1.399 
611.592 
93,072 
30,226 

89,981 

1,561,893 

92,356 

77,690 

109, 086 

1,038,  (H5 
222,640 

1,009.446 
11,675 

706,325 
66,641 

1,011,661 
139,853 

1,274,232 
115,763 

12,531,132 
1,044,325 

Malta 

lOS  053 

107,244 
234,071 

2^0  7(>5 

Miller.. 

118,520 

224,321 

525 

80,330 

119,439 

277,119 

1  133  484 

16,523 
900, 41)3 
557  315 

35,499 

115,886 

121,739 

117,653 

155,541 
92,715 

154,601 
128, 870 

119,164 

210,291 

7,614 

147,621 

1,831,187 

Mohawk 

7,014 

628,899 

7,316,409 

279,396 

Monroe 

13,730 
1,070.937 

60,725 

2,495.0,S9 

188,508 

310,839 

1,809,743 

64,073 

2,563,111 

228,451 

156,809 

2,076, 3,S8 

34,935 

1,973,519 

153,770 

19,172 

621 

71,645 

528,154 

1,571 

206, 098 

160,249 

.35,571 
1,617,772 

49.409 
1,348,714 

33.012 
1,168,855 

Mountain  Iron  (and  Rath  and  Aetna ) . . . 
Myers 

17,198  871 

193. 698 
11.940 
59,389 

914  736 

31  112 

Onondat^a 

30,887 

90  797 

54,884 

50, 204 

235 

66,862 

242,830 

68,RS3 

706,071 

59,029 

496,830 

1,040,265 

190,154 

997,065 

700,140 

1  IGS 

Pearson 

68,683 

209,531 

1,615 

Perkins... 

59,029 
83,548 

Pettit 

17,278 

238.122 

28,972 

52.700 
229,133 

27,088 

140,239 
161,924 

82.757 
33.646 

30,074 
489,718 

57,140 
59,889 

Pillsbury. 

Rust 

272,114 

284,617 

213,355 

227,079 

249,837 

Scranton 

1, 168 
207,990 

193,428 

251,631 

261,501 

241,031 

155,000 

354,780 

026. 169 
23,585 

2  870  890 

Seville 

23,585 

68 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


Table  of  Lake  Superior  iron-ore  shipments  from  the  earliest  shipment  to  date — Continued. 

Mesabl  Range— Continued 
[Gross  tons.] 


Name  of  mine. 

1902. 

1903. 

1904. 

1905. 

1906. 

1907. 

1908. 

1909. 

Total. 

224,520 

48,199 

329,535 
2, .328  605 

Shenango.     .                        

51,712 

213,097 

383,717 

387,093 

401,887 
49,291 

831.099 
256,073 

Sliver 

.305,361 

Sparta  .                                    

227,444 

40,458 

59,692 

27,777 

235 

1,241,197 

15,257 
610,457 

20.510 
430,633 

w 

35, 773 

SDruce  (Cloauet)  .           

543,203 

587,153 
6,148 

589,319 
26,748 

606,295 
61,792 

674,602 

579,903 

5,166,199 
94,688 
137.430 

st.ciair       ':::::;::::::::::::: 

24,230 

113,200 

87,0,'i5 
1,014,582 

367,764 
1.428,614 

454  819 

1,4.34,681 

1,652,021 

1,041.500 
20,984 

1,142.977 
137,207 

516.  770 

182,352 

7,579 

1,030,742 
243,049 

9,984. 191 

583,. 'J92 

7,579 
5,509 

5,509 
256,384 
86,520 

S8, 136 

87,584 

174,309 
146,849 
20, 691 
268,281 
64,820 
5,674 
6,766 
165,604 
17,685 

190,903 
100,730 

61,825 
.304,864 

90,090 

1,015,717 

158,692 

113, 334 

35,267 

174,6.33 
40,283 
20,937 
57, 194 
21.310 

661,329 

853  765 

15.099 
91.496 
156,180 

12,759 

489.824 

103,622 
9,009 

399  877 

Utica                   

120,697 

185,944 

201.480 

113,305 

1,843,450 

60,966 

1,3' .3, 649 

Victoria 

2S9,.525 

Virginia  Rroup     

5,131 

5,866 

5.395 

402,224 

8.218  097 

2*'*6  424 

Webb                     

71,235 

19,610 

369  783 

Williams  (Nortli  Cincinnati)  . 

97  84'' 

Wills                           

12, 158 

4,550 

3,440 
84,614 

5,  .362 
45, 790 

20.148 

Winnifred  

39,179 

81.686 
53.179 

3,415 
265,289 

94,867 

210, 726 

15,453 

61,341 
86, 308 
84, 446 

365. 102 

Yates                              

58,174 

079, 038 

145. 689 

13,342,840 

12,892,542 

12,156,008 

20,158,699 

23,819,029 

27,495,708 

17,257,3.50 

28,176,281 

195,703,424 

Vermilion  Range. 


Name  of  mine. 

1884. 

1885. 

1886. 

1887. 

1888. 

1889. 

1890. 

1891. 

1892. 

1 

54,612 

306,220 
3,144 

336.002 
12,012 

373.969 
3,079 

651,655 
2,651 

Sibley 

62,124 

225,484 

304,396 

394,252 

457.341 

535,318 

532,000 

517,570 

498. 353 

Zenith                        

14  991 

62, 124 

225,484 

304,396 

394,252 

511,953 

844,682 

880,014 

894,618 

1,167,650 

Name  of  mine. 

1893. 

1894. 

1895. 

• 
1896. 

1897. 

1898. 

1899. 

1900. 

1901. 

435.930 

558.050 

605.024 
40.054 

471.545 
149.073 

4.38.365 
207. 103 

715.919 
123. 183 

80?,  359 
339.897 

81.022 

5.169 

457.732 

79.323 

644.801 
460.794 
170.446 
4,670 
325.020 
60.089 

627.379 

678  310 

212,008 

Sibley 

Soudan  (Minnesota) 

370.303 
14.388 

390,463 

432,760 

448.707 
18.765 

592. 196 
40,817 

426,040 

208  284 

Zenith 

60  082 

820,621 

948,513 

1,077,838 

1,088,090 

1,278,481 

1,265,142 

1.771,502 

1.655,820 

1.786.063 

Name  of  mine. 

1902. 

1903. 

1904. 

1905. 

1906. 

1907. 

1908. 

1909. 

Total. 

Chandler                        

645,786 
673,863 
243.9.37 
78,304 
275, 168 
167,205 

460,548 
696.736 
169.616 
113.595 
175. 114 
161.091 

422.162 
505.432 
74.866 
122.783 
70.713 
86,557 

365,739 

663,682 
91,775 
251, 170 
205,002 
109,818 

318,990 

766,853 
106,9.33 
271,496 
146,503 
181,580 

245.684 
830,700 
43.320 
226,8.35 
102,977 
236,751 

50.639 
477,606 

82.521 
127,544 

53,070 

50,264 

9.537  378 

477.226 
83.167 

151.009 
74.862. 

321,951 

6.991.297 

1.359.611 

Siblev 

1.352.575 

Soudan  (Minnesota) 

8  2S1  752 

Zenith 

1.602.672 

2,084,263 

1,676,699 

1,282,513 

1.677,186 

1,792,355 

1,685,267 

841.544 

1.108.215 

29.125.285 

Miscellaneous  (In  WIsconslni. 


Name  of  mine. 

1892. 

1893. 

1894. 

1895. 

1896. 

1897. 

1898. 

1899. 

1900. 

1901. 

Illinois 

Maj^ille 

9,044 

7,925 

10,511 

16,472 

13,144 

10,546 

18, 151 

ig.Tsi 

20.986 

22.400 

9,044 

7,925 

10,  .511 

16,472 

13,144 

10,546 

18,151 

19,731 

20,986 

22.400 

HISTORY  OF  LAKE  SUPERIOR  MINING. 


69 


Table  of  Lake  Superior  iron-ore  shipments  from  the  earliest  shipment  to  date — Continued. 

Miscellaneous  (In  Wisconsin)— Continued. 

[Gross  Ions.] 


Name  of  mine. 

1902. 

1903. 

1904. 

1905. 

1906. 

1907. 

1908. 

1909. 

Total. 

Illinois 

47,922 
19, 558 
26,562 

71,413 
39,978 
20,610 

67, 118 
61,624 
1.5,847 

72,180 
3.966 
19,644 

51,  lOS 

309,741 
158,994 
411,892 

17,913 
18,836 

15,955 

66,804 

23,338 

71,. 541 

23,338 

36,749 

94,042 

132,001 

144,589 

95,790 

122,449 

82,759 

880,627 

Sununaiy. 


Years 
unknown. 

1854. 

1855. 

1856. 

1857. 

1858. 

1859. 

1860. 

1861. 

1862. 

30,000 

3,666 

1,449 

6,343 

25,646 

22,876 

68,832 

114, 401 

49,909 

124, 169 

Grand  total-... 

30,000 

3,000 

1,449 

6,343 

25,646 

22,876 

68,832 

114,401 

49,909 

124, 169 

1863. 

1864. 

1865. 

1866. 

1867. 

1868. 

1869. 

1870. 

1871. 

1872. 

203,055 

243, 127 

186,208 

278,796 

443,567 

491,  454 

617,444 

830,934 

779,607 

893, 169 

203,055 

243,127 

186,208 

278,796 

443,567 

491,454 

617,444 

830,934 

779,607 

893.169 

1 

1873. 

1874. 

1875. 

1876. 

1877. 

1878. 

1879. 

1880. 

1881. 

1882. 

1,158,249 

919,257 

889,477 

1,006,785 

1,010,494 
10,405 

1,023,083 
95,221 

1,130,019 
269,609 

1,384,010 
592,086 

1,579,834 
739,635 

1,829,394 

1, 136, 018 

1,158,249 

919,257 

889,477 

1,006,785 

1,020,899 

1,118,304 

1,399,628 

1,976,096 

2,319,469 

2, 965, 412 

1883. 

1884. 

1885. 

1886. 

1 
1887.      1       1888. 

1889. 

1890. 

1891. 

1892. 

1,022 

1,558,034 

895,634 

1 

119,860 
430,422 
690,435 

1, 

753, 369 
627,380 
880,006 

1,324,878 
1,851,634 
1,193,343 

1,437,096 
1,923,727 
1,191,101 

2,008,394 
2,642,813 
1,796,754 

2,847,810 
2,993,664 
2,282,237 

1,839,574 
2,512,242 
1,824,619 

2,971,991 

1,305,425 
1,047,415 

2,666,856 

2,277,856 

4,245 

62,124 

225,484 

304,396 

394,252 

511,953 

844,682 

880,014 

894,618 

1,167,650 

9,044 

Grand  total  ..    .        

2,352,840 

2,516,814 

2,466,201 

3,565,151 

4, 764, 107 

5,063,877 

7,292,643 

9,003,725 

7,071,053 

9,097,642 

1893. 

1894. 

1895. 

1896. 

1897. 

1898. 

1899. 

1900. 

1901. 

1,329,385 

1,835,893 

1,466,197 

613,020 

820, 621 

7,923 

1,809,468 
2,060,200 
1,137,949 
1,793,052 
948,513 
10,511 

2, 547, 976 
2,097,838 
1,923,798 
2.781,587 
1,077,838 
16,472 

1,799,971 
2,604,221 
1,. 560, 467 
2,882,079 
1,088,090 
13, 144 

2, 258, 236 
2,715,035 
1,937,013 
4, 275,  .809 
1,278,481 
10,546 

2, 498,  461 
3,125,039 
2,522,265 
4,013,700 
1,205,142 
18,151 

2, 795, 866 
3,757,010 
3,301,062 
0,626,384 
1,771,502 
19,731 

2, 875, 295 
3,457,522 
3,261,221 
7,809,535 
1, 655, 820 
20,986 

2, 938, 155 

3,245,346 

3,619,053 

9,004,890 

1,786,063 

Miscellaneous  (in  Wisconsin)... 

22,400 

Grand  total     

6,073,641 

7, 759, 753 

10,445,509 

9,947,972 

12,475,120 

14,042,824 

18,271,535 

19,080,379 

20,615,907 

1902. 

1903. 

1904. 

1905. 

1906. 

1907. 

1908. 

1009. 

Total. 

3,654,929 
3,868,025 
4,612,509 
13,342,840 
2,084,263 
23,338 

2,912,708 
3,040,245 
3,749,567 
12,892,542 
1,676,699 
36, 749 

2,398,287 
2,843,703 
3.074,848 
12,156,008 
1,282,513 
94,042 

3,705,207 
4,215,572 
4,495.451 
20, 158, 699 
1,677,186 

3,643,514 
4,057,187 
5,109,088 
23,819,029 
1,792,355 

3,637,102 
4,388,073 
4,904,728 
27, 495, 708 
1,685,267 

2,699,850 
2,414,632 
2,679,156 
17,257,350 
.841,. 544 

4,088,057 
4,256,172 
4,875,385 
28,176,281 
1,108,215 
82,759 

60,896,457 

Marquette  range        

91,838,558 

71,212,121 

Mesabi  range 

19.5,703,424 

29,125,285 

Miscellaneous  (in  Wisconsin). . . 

132,001 

144, 589 

95,790 

122, 449 

880,627 

Grand  total 

27,585,904 

24, 308, 510 

21,849,401 

34,384,116  1  38.565.762 

42,266.668  !  26.014.987 

42,586.869 

449,656,472 

CHAPTER  III.  HISTORY  OF  GEOLOGIC  WORK  IN  THE  LAKE  SUPERIOR 

REGION. 

GENERAL  STATEMENT. 

The  Lake  Superior  region  is  .among  the  first  in  which  detailed  study  and  mapping  of  tlie  ancient 
crystalline  complex  have  been  extended  over  large^^  areas;  it  has  had  special  attention  })ecause 
of  the  magnitude  of  the  mining  industry  and  the  commercial  importance  in  mining  of  a  correct 
understanding  of  geologic  structure.  Without  the  mines,  expenditure  for  geologic  work  upon 
so  large  a  scale  would  scarcely  have  been  undertaken  in  a  district  so  inaccessible.  The  increase 
of  Ivnowledge  concerning  the  geology  of  the  region  has  closely  followed  the  development  of 
mming. 

The  earlier  geologic  work  in  the  Lake  Superior  region  was  of  a  most  general  nature  and  was 
necessarily  confined  to  the  shores  of  Lake  Superior  and  to  parts  immediately  accessible  from 
canoe  routes  tributary  to  Lake  Superior.  The  great  distances  and  the  difficulties  of  travel  made 
detailed  mapping  impracticable  over  large  areas  in  the  interior.  Numerous  important  observa- 
tions were  made  which  have  subsequently  been  found  to  be  of  value,  but  these  were  in  the  main 
fragmentary.  Detailed  geologic  work  has  been  for  the  most  part  confined  to  the  ore-bearing 
areas  and  was  not  begun  until  these  areas  had  been  located  or  opened  for  mining. 

WORK  OF  INDIVIDUALS. 

On  the  Canadian  shore  of  Lake  Superior  and  in  adjacent  territory  the  geologic  work  has  been 
of  a  somewhat  general  nature  except  in  one  or  two  localities.  This  is  so  largely  because  no 
ore-bearing  districts  have  been  discovered  in  this  part  of  the  region  of  sufficient  commercial 
importance  to  warrant  large  expenditures  for  geologic  work.  The  geologists  who  have  contrib- 
uted most  to  the  loiowledge  of  this  portion  of  the  district  are  Bigsby  (1825,  1852,  1854),  Bayfield 
(1829, 1845),  Logan  (1847, 1852,  1863),  Murray  (1847,  1863),  Macfarlane  (1866,  1868,  1869,  1879), 
Robert  Bell  (1870, 1872-1878,  1883, 1890),  Selwyn  (1873,  1883, 1885, 1890),  G.  M.  Dawson  (1875), 
Lawson  (1886,  1888,  1890,  1891,  1893,  1896),  H.  L.  Smj^th  (1891),  Pumpelly  (1891),  W.  II.  C. 
Smith  (1892,  1893),  Coleman  (1895-1902,  1906,  1907,  1909),  Willmott  (1898,  1901,  1902),  Van 
Hise  (1898,  1900),  Mclnnes  (1899,  1902,  1903),  Parlis  (1898,  1902,  1903),  Clements  (1900), 
Miller  (1903),  W.  N.  Smith  (1905),  Burwash  (1905),  J.  M.  Bell  (1905),  and  Moore  (1907,  1909). 
All  were  in  the  service  of  the  Canadian  government  or  of  the  Canadian  Geological  Survey  except 
Coleman,  Willmott,  J.  M. Bell,  Burwash,  and  Moore,  who  represented  the  Ontario  Bureau  of  Mines, 
and  Pumpelly,  II.  L.  Smyth,  Van  Hise,  Clements,  and  W.  N.  Smith,  American  geologists.  The 
principal  detailed  mapping  has  been  that  in  the  Lake  of  the  Woods  and  Rainy  Lake  district 
by  Lawson  (1886-1888),  that  in  the  Steep  Rock  Lake  region  by  Pumpelly  and  Smyth  (1891), 
and  that  in  the  Michipicoten  iron  district  by  Coleman,  Willmott  (1898),  Burwash  (1905),  and 
J.  y[.  Bell  (1905).  Closely  related  is  the  extremely  important  work  of  Logan  and  Murray  (1863) 
in  the  original  Iluronian  district  east  of  Lake  Superior  and  north  of  Lake  Huron. 

In  the  United  States  portion  of  the  Lake  Superior  region  early  general  observations  were 
made  by  exj)lorers  sent  out  by  the  United  States  Government.  Schoolcraft  visited  the  south 
shore  of  Lake  Superior  and  ascended  St.  Louis  River  (1821,  1854).  Owen  (1847,  1851,  1852) 
visited  particularly  the  west  end  of  Lake  Superior  and  the  upper  Mississippi  and  its  tribu- 
taries. Norwood  (1S52)  ascended  Montreal  and  St.  Louis  rivers.  Wiiittlesey  (1852,  1S76) 
explored  nortliern  Wisconsin  and  northern  Mijinesota.     Whitne}'   (1854,  1856,   1857)  visited 

70 


HISTORY  OF  GEOLOGIC  WOEK  IN  THE  REGION.  71 

nearly  all  parts  of  tho  I^ake  Superior  shore.  Houghton  (lcS4U-lS41)  made  general  observa- 
tions on  the  Lake  Superior  region  as  a  whole. 

However,  much  the  larger  part  of  the  early  geologic  exploration  was  confined  to  the 
regions  now  known  as  the  Marquette  iron  and  Keweenaw  copper  districts,  the  extension  of 
the  Keweenaw  district  into  the  Gogebic  district,  and  adjacent  parts  of  the  Upper  Peninsula. 
The  first  important  detailed  report  on  the  Keweenaw  copper  district  was  that  of  Douglass 
Houghton,  of  the  Michigan  Geological  Survey,  in  1S41,  based  on  work  done  several  years 
before.  This  report  led  directly  to  the  opening  of  the  Keweenaw  copper  district.  He  was 
followed  by  Whitney  (1847-1850),  Foster  (1848,  1850),  Jackson  (1849,  1S50),  and  Agassiz 
(1850,  1867).  Subsequent  geologic  work  on  Keweenaw  Point  of  great  importance  was  that  of 
Brooks  and  Pumpelly  (1872,  187.3),  Marvine  (187.3),  Rominger  (1873),  and  others,  for  the 
Michigan  Geological  Survey.  Field  study  leading  to  the  preparation  of  a  monograph  on  the 
copper-bearing  rocks  of  Lake  Superior  was  begun  by  Irving  prior  to  1880  for  the  Wisconsin 
Geological  Survey  and  completed  in  1882  for  the  United  States  Geological  Survey.  This 
volume  "  has  remained  the  standard  reference  book  on  the  district  to  the  present  time,  though 
contributions  of  much  value  have  been  made  by  Hubbard,  Lane,  Seaman,  and  others. 

The  extension  of  Houghton's  work  in  the  copper  district  and  that  of  Burt,  his  assistant, 
led  directly  to  the  discovery  and  opening  of  the  Manjuette  iron-bearing  district  in  1848.  The 
important  early  geologic  work  in  this  district  was  done  by  Burt  (1850),  Foster  and  Wliitney 
(1851),  Kimball  (1865),  and  Credner  (1869),'  all  in  the  service  of  the  United  States  Government. 
Later  followed  the  important  contributions  of  the  geologists  of  the  Michigan  Geological  Survey — 
Brooks  (1873,  1876),  Wright  (1879,  1880),  Rominger  (1873,  1881),  and  others.  Wadsworth's 
contributions  to  the  geology  of  the  Marquette  and  Keweenaw  districts  (1880,  1881,  1S84,  1890, 
1891)  have  been  the  subject  of  much  controversy. 

After  the  opening  of  the  Keweenaw  and  Marquette  districts  geologic  mapping  began  to 
be  extended  to  the  south  and  west  through  the  Upper  Peninsula  of  Michigan  and  northern 
Wisconsin.  Particularly  noteworthy  are  the  reports  of  the  Michigan  Geological  Survey  on  the 
general  geology  of  the  Upper  Peninsula  of  Michigan,  but  particularly  of  the  Manjuette, 
Menominee,  and  Gogebic  districts,  by  Brooks  (1873,  1876),  Wright  (1879,  1880),  Rominger 
(1881,  1895),  and  Alexander  Winchell  (1888).  The  Menominee  range  in  its  Wisconsin  extension 
was  reported  on  by  Wright  (1880)  and  Brooks  (1880)  for  the  Wisconsm  Survey,  and  Fulton 
(1888).  The  Penokee  district  and  adjacent  territory  in  northern  Wisconsin  was  described  by  the 
geologists  of  the  Wisconsin  Survey —Lapham  (1860),  Whittlesey  (1863),  Irving  (1874,  1877,  1880), 
Sweet  (1876),  Chamberlin  (1878),  and  Wright  (1880).  Early  general  observations  in  northern 
Wisconsm  were  contributed  by  Percival  (1856),  Daniels  (1858),  Lapham  (1860),  Hall  (1861-62), 
Irving  (1872-1874,  1877,  1878,  1880,  1882,  1883),  Murrish  (1873),  Eaton  (1873),  Wright  (1873), 
Chamberlm  (1877,  1878,  1880,  1882,  1883),  Strong  (1880),  Sweet  (1880,  1882),  and  Van  Hise 
(1884). 

The  detailed  geologic  work  by  the  United  States  Geological  Survey  leadmg  up  to  the  prep- 
aration of  the  series  of  monographs  on  the  iron-bearing  districts  of  Michigan  and  Wisconsin 
was  begun  in  the  Gogebic  district  by  R.  D.  Irving  and  C.  R.  Van  Hise  in  1884.  On  the  comple- 
tion of  work  there  detailed  work  was  taken  up  in  the  Marquette  district,  1888  to  1895,  by  Van 
Hise,  Bayley,  Merriam,  Smyth,  and  others,  and  a  monographic  report  ^  was  issued  in  1895; 
similar  work  was  done  in  the  Crystal  Falls  district  from  1893  to  1898  by  Van  Hise,  Bayley, 
Clements,  Smyth,  Merriam,  and  others,  and  a  monograph''  was  issued  in  1899;  and  the  Menomi- 
nee district  was  examined  by  Van  Hise,  Baylej^,  Clements,  Weidman,  and  others,  and  a  mono- 
graph'* was  issued  in  1904.  Since  the  completion  of  the  work  in  the  Menominee  district  in  1900 
the  United  States  Geological  Survey  has  been  devoting  its  attention  to  Mhuaesota,  although  a 
small  amount  of  general  work  has  been  done  in  Michigan  and  Wisconsin.  While  the  United 
States  Geological  Survey  has  been  mapping  the  districts  of  the  Upper  Peninsula,  the  Michigan 
Geological   Survey  has   given  relatively  less  attention  to   this  area  than  it   had    previously, 

.    o  Mon.  U.  S.  Geol.  Survey,  vol.  5, 1SS3.  c  Idem,  vol.  36, 1899,  512  pp.,  53  pis. 

*  Idem,  vol.  28, 1S95,  008  pp.,  35  pis.,  and  atlas.  d  Idem,  vol.  40,  1904, 513  pp.,  43  pis. 


72  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

but  durinf,'  this  poriod  it  lias  issued  important  rp])()rts  on  tho  districts  of  Keweenaw  Point,  Por- 
cupine JMountains,  and  Isle  Royal  l)y  Hubbard,  Lane,  Wri<j;lit,  and  others.  Lane  and  Seaman 
in  1909  and  1910  published  an  interesting  summary  of  their  views  on  Michigan  geologJ^  In 
1909  and  1910  R.  C.  Allen,  successor  to  Mr.  Lane  as  state  geologist,  mapped  and  rcj)orted  on 
the  Iron  River  district  of  Michigan  ami  then  took  up  the  mapping  of  the  region  between  the 
Iron  River  district  and  Lake  Gogebic. 

The  Wisconsin  Geological  Survey,  after  the  completion  of  the  work  of  Irving,  ("hamberlin, 
Wright,  and  others,  was  discontinued  in  1SS3.  The  new  Wisconsin  Geological  and  Natural 
History  Survey,  established  in  1897,  has  been  engaged  continuously  tlu-ough  Weidman  in 
mapping  the  crystalline  rocks  of  north-central  Wisconsin  and  the  outlying  areas,  including  the 
Baraboo  iron  district.  Hobbs  and  Leith  (1907)  mapped  tho  volcanic  rocks  of  Fox  River  in 
central  Wisconsin.  In  1910  W.  O.  Hotchkiss,  for  the  Wisconsin  Geological  and  Natural  History 
Survey,  took  up  the  detailed  mapping  of  the  Florence  iron-bearing  district  of  northeastern 
Wisconsin,  and  F.  T.  Thwaites,  for  the  same  organization,  examined  in  detail  the  Kewee- 
nawan  and  Cambrian  sandstones  on  the  southwestern  shore  of  Lake  Superior,  with  a  view 
of  ascertaining  their  relations. 

In  Minnesota  early  work  of  a  most  general  nature  was  done  by  Owen  (1851,  1852),  School- 
craft (1821,  1854),  Norwood  (1847,  1852),  Eames  (1866),  and  WTiittlesey  (1866,  1876).  The 
Minnesota  shore  and  the  Gunflint  Lake  areas  were  examined  in  detail  by  Irving  and  assistants 
in  1880.  The  Minnesota  Survey  began  its  study  of  the  crystalline  rocks  of  northern  Minnesota 
in  1872  and  continued  it  until  1901.  The  men  engaged  in  this  work  were  N.  H.  Winchell, 
Alexander  N.  Winchell,  H.  V.  Winchell,  U.  S.  Grant,  J.  E.  Spurr,  and  others.  A  number  of 
special  reports  were  issued,  but  the  final  general  account  appeared  in  volumes  4,  5,  and  6  of  the 
Minnesota  Survey,  published,  respectively,  in  1899,  1900,  and  1901.  The  Minnesota  Survey 
was  then  discontinued.  The  work  of  the  United  States  Geological  Survey  in  ^linnesota  was 
begun  in  the  Vermilion  district  m  1896  by  VanHise,  Clements,  Bayley,  and  Leith,  and  a  mono- 
graphic report"  was  issued  in  1903.  Upon  the  completion  of  this  work  in  1899  work  was  taken 
up  in  the  Mesabi  district  by  Leith  under  direction  of  C.  R.  A'an  Hise,  and  the  monograph*  on 
this  district  was  issued  in  1903.  Since  that  time  no  detailed  mapping  has  been  done  in  the 
Minnesota  region  by  the  LTnited  States  Geological  Survey,  but  many  general  observations  have 
been  made.  Geologic  work  in  ^Minnesota  for  commercial  purposes  has  been  done  by  Merriam 
and  Sebenius  in  the  Vermilion  and  Mesabi  districts  and  by  Leith,  Zapfl'e,  and  Adams  in  the 
Cu3Tma  district. 

Detailed  summaries  of  the  work  of  all  the  men  above  mentioned  and  others  will  be  found 
in  the  United  States  Geological  Survey  monographs  on  the  several  Lake  Superior  districts,  and 
in  Bulletin  360  of  the  United  States  Geological  Survey,  on  the  pre-Cambrian  geologj^  of  North 
America.  Only  such  names  and  reports  have  been  mentioned  here  as  seem  necessary  to  a 
general  sketch  of  the  history  of  geologic  knowledge  in  the  region.  A  number  of  the  men 
named  have  contributed,  in  addition  to  the  reports  specifically  mentioned,  valuable  information 
on  the  geology  of  the  Lake  Superior  region  in  general. 

GROWTH  OF  GEOLOGIC  KNOWLEDGE. 

An  attempt  has  been  made  in  Bulletin  360  (cited  above)  to  sum  up  the  salient  features  of 
the  history  of  the  development  of  geologic  knowledge  concerning  the  Lake  Superior  region. 
This  summary  will  not  be  repeated  here.  It  shows  how  the  present  Icnowledge  of  the  district 
has  resulted  from  a  long  series  of  approximations,  in  general  successively  more  adequate  owino 
to  gradual  accumulation  of  facts,  improvement  of  means  of  studying  them,  and  general  advance 
in  knowledge  of  geologic  principles.  Needless  but  perhaps  inevitable  confusion  has  resulted 
locally  from  duplication  of  geologic  terms  by  difi"erent  geologic  observers  anil  from  varying 
inferences  drawn  b}^  different  men  from  the  same  set  of  facts.  It  is  indeed  curious  to  note  how 
differently  truth  is  revealed  to  diflferent  observers.  A  chronologic  series  of  geologic  maps  of 
the  Marciuette  district  shows  how  it  is  possible  in  the  development  of  geologic  knowledge  gradually 

a  UoQ.  U.  S.  Geol.  Survey,  vol.  45,  1903,  4ia  pp.,  13  pis.,  and  atlas.  >>  Idem,  vol.  43,  1903, 316  pp.,  33  pis. 


HISTORY  OF  GEOLOGIC  WORK  IN  THE  REGION.  73 

to  make  closer  approximations  to  actual  conditions.  It  also  illustrates  well  the  fact,  sometimes 
lost  sight  of,  that  a  geologic  map  represents  an  approximation  to  the  truth,  limited  in  its  accu- 
racy and  adequacy  by  the  general  stage  of  advancement  of  the  science,  and  perhaps  falling  short 
of  tliis  limit  if  the  map  maker  does  not  fairly  represent  that  advance.  The  n'aps  published 
with  tliis  monograph  are  closer  approxmaations  to  the  truth  than  the  maps  previously  pub- 
hshed.  These  maps  in  turn  will  be  superseded  by  better  approximations  as  facts  accumulate 
and  geologic  knowledge  advances.  It  is  hoped  that  the  user  of  these  maps  will  measure  them 
by  their  advance  over  preexistmg  maps  rather  than  by  the  distance  they  fall  short  of  the  ideally 
perfect  map. 

In  the  geologic  literature  on  the  Lake  Superior  region  a  progressive  change  may  be  noted 
from  the  fragmentary  descriptions  of  earlier  writers  to  more  elaborate  descriptions  accompanied 
by  attempts  at  stratigraphic  and  structural  classification  and  the  development  of  better  prin- 
ciples for  that  purpose,  and  in  turn  a  change  to  better  understanding  of  the  principles  of  corre- 
lation of  the  rocks,  based  on  better  knowledge  of  these  rocks  and  of  the  conditions  of  the  forma- 
tion of  rocks  of  tliis  kind.  The  work  on  ore  deposits  similarly  began  with  fragmentary  descrip- 
tions, followed  by  fuller  descriptions  and  attempts  at  lithologic  and  structural  classification, 
then  by  hj^^otheses  on  the  origin  of  the  ore,  which  gradually  gave  way  to  accepted  theories 
based  on  qualitative  evidence.  The  present  monograph  is  believed  to  mark  a  further  devel- 
opment in  the  same  direction  by  transferring  the  theories  of  origin  of  the  ore  more  largely  from 
a  qualitative  to  a  quantitative  basis. 

Mention  of  names  in  connection  with  the  general  tendencies  outlined  above  would  lead 
to  endless  detail,  but  the  tendencies  may  be  noted  in  terms  of  years  and  organizations.  Before 
1870  the  geologic  work  was  fragmentary,  descriptive,  and  as  a  whole  unorganized,  though 
work  of  exceptional  merit  was  done  by  individuals.  The  period  from  1870  to  18S0  was 
marked  by  the  better  organized  efforts  of  the  Michigan  and  Wisconsin  geological  surveys,  with 
corresponding  improvements  m  the  organization  of  geologic  knowledge  of  the  parts  of  the  Lake 
Superior  region  studied,  affording  the  first  real  contribution  to  the  stratigraphic  and  structural 
geology  of  the  region.  Then  the  kinds  of  geologic  work  really  began  which  are  now  followed 
in  the  Lake  Superior  region.  In  the  early  eighties  the  United  States  Geological  Survey  took 
up  the  study  of  the  district,  its  first  reports  being  based  largely  on  information  previously 
gathered  by  Irving  and  other  members  of  the  Wisconsin  and  other  State  geological  surveys. 
Since  its  entrance  into  the  region  the  United  States  Geological  Survey  has  studied  the  problem 
more  continuously  than  the  state  surveys,  over  a  larger  area,  and  with  a  uniform  plan,  with 
the  result  that  its  publications  since  the  early  eighties  mark  the  principal  steps  in  the  advance- 
ment of  knowledge  of  the  region.  This  is  said  without  disparagement  of  contemporaneous 
work  by  the  Michigan,  Wisconsui,  Minnesota,  Ontario,  and  Canadian  surveys,  which  have 
issued  reports  on  different  phases  of  the  problem,  but  for  reasons  mentioned  above  these 
reports  for  the  most  part  have  been  more  limited  in  their  scope  than  those  of  the  LTnitetl  States 
Geological  Survey.  In  recent  years  the  Wisconsin  Geological  Survey  has  again  taken  up  the 
mapping  of  the  crystalline  rocks  of  northern  Wisconsin  with  thorouglmess  and  with  good 
results.  The  Michigan  Geological  Survey  also  has  now  taken  up  work  in  the  Upper  Peninsula  of 
Michigan,  on  the  iron-bearing  district  of  Iron  River  and  on  the  copper-bearing  series,  which  is 
rapidly  advancing  our  knowledge.  It  is  to  be  hoped  that  all  local  organizations  will  continue  to 
develop.  Even  though  they  do,  however,  there  will  still  be  need  for  attention  to  the  region  b\' 
the  United  States  Geological  Survey,  because  its  field  of  work  is  broader  and  it  is  in  better 
position  to  take  up  general  correlation  and  structural  problems  common  to  the  district. 

BIBLIOGRAPHY. 

The  following  bibliography  comprises  references  to  literature  on  the  geology  of  the  region 
arranged  first  by  districts  and  then  by  date.  Reports  on  districts  and  mines  that  do  not  refer 
primarily  to  the  geology  are  not  here  included.  Also  no  reference  is  made  in  the  following 
list  to  literature  dealing  with  the  physical  geography  or  with  the  Pleistocene  geology  of  this 
region.     iUl  references  to  these  subjects  will  be  found  as  footnotes  in  Chapters  IV  and  XVI. 


74  GEOLOGY  OF  THE  LAKE  SI^PERIOK  REGION. 

MICHIGAN. 

Third  annual  report  of  the  Geological  Survey  of  Michigan,  by  Douglass  Houghton,  State  of  Michigan,  House 
of  Representatives,  No.  8,  pp.  1-33. 

Fourth  annual  report  of  the  state  geologist,  Douglass  Houghton.  Idem,  No.  27,  184  pp.  See  also  Metalliferous 
veins  of  the  Northern  Peninsula  of  Michigan,  by  Douglass  Houghton.  Am.  Jour.  Sci.,  1st  ser.,  vol.  41,  1841,  pp. 
183-18G. 

Geology  of  Porters  Island  and  Copper  Harbor,  by  John  Locke.  Trans.  Am.  Phil.  Soc,  vol.  9,  1844,  pp.  311-312, 
with  maps. 

Mineralogy  and  geology  of  Lake  Superior,  by  11.  D.  Rogers.  Proc.  Boston  Soc.  Nat.  HLst.,  vol.  2,  1840,  pp. 
124-125. 

Report  of  observations  made  in  the  survey  of  the  Upper  Peninsula  of  Michigan,  by  John  Locke.  Senate  Docs., 
1st  sess.  30th  Cong.,  1847,  vol.  2,  No.  2,  pp.  183-199. 

Report  of  J.  D.  Whitney.     Idem,  pp.  221-230. 

Report  of  J.  D.  Whitney.     Senate  Docs.,  2d  sess.  30th  Cong.,  1848^9,  vol.  2,  No.  2,  pp.  1.54-1,59. 

Report  of  J.  W.  Foster.     Idem,  pp.  159-163. 

The  Lake  Superior  copper  and  iron  district,  by  J.  D.  Whitney.  Proc.  Boston  Soc.  Nat.  Hist.,  vol.  3,  1849,  pp. 
210-212. 

On  the  geological  structure  of  Keweenaw  Point,  by  Charles  T.  Jackson.  Proc.  Am.  Assoc.  Adv.  Sci.,  1849,  2d 
meetmg,  pp.  288-301. 

Report  on  the  geological  and  mineralogical  survey  of  the  mineral  lands  of  the  United  States  in  the  State  of  Michigan, 
by  Charles  T.  Jackson.  Senate  Docs.,  1st  sess.  31st  Cong.,  1849-50,  vol.  3,  No.  1,  pp.  371-935,  with  14  maps.  Contains 
reports  by  Messrs.  Jackson,  Dickenson,  Mclntyre,  Barnes,  Locke,  Foster  and  \Miitney,  Whitney,  Gibbs,  ^\^litney,  jr., 
Hill  and  Foster,  Foster,  Burt,  Hubbard. 

United  States  geological  survey  of  public  lands  in  Michigan;  field  notes,  by  John  Locke.     Idem,  pp.  572-587. 

Synopsis  of  the  explorations  of  the  geological  corps  in  the  Lake  Superior  land  district  in  the  Northern  Peninsula 
of  Michigan,  by  J.  W.  Foster  and  J.  D.  Whitney.     Idem,  pp.  605-626,  with  4  maps. 

Notes  on  the  topography,  soil,  geology,  etc.,  of  the  district  between  Portage  Lake  and  the  Ontonagon,  by  J.  D. 
"WTiitney.     Idem,  pp.  649-666.  ' 

Report  of  J.  D.  "WTiitney.     Idem,  pp.  705-711. 

Report  of  J.  W.  Foster.     Idem,  pp.  766-772. 

Notes  on  the  geology  and  topography  of  the  country  adjacent  to  Lakes  Superior  and  Michigan,  in  the  Chippewa 
land  district,  by  J.  W.  Foster.     Idem,  pp.  773-786. 

To].)(>graphy  and  geology  of  the  survey  with  reference  to  mines  and  minerals  of  a  district  of  township  lines  south 
of  Lake  Superior,  by  William  A.  Burt.     Idem,  pp.  811-832,  with  a  geologic  map  opposite  p.  880. 

General  observations  upon  the  geology  and  topography  of  the  district  south  of  Lake  Superior,  subdi\'ided  in  1845 
under  direction  of  Douglass  Houghton;  deputy  surveyor,  Bela  Hubbard.     Idem,  pp.  833-842. 

Geological  report  of  the  survey  "with  reference  to  mines  and  minerals"  of  a  district  of  township  lines  in  the 
State  of  Michigan,  in  the  year  1846,  and  tabular  statement  of  specimens  collected.  Idem,  pp.  842-S82,  with  a  geologic 
map. 

Report  on  the  geology  and  topography  of  the  Lake  Superior  land  district;  part  1,  copper  lands,  by  J.  W.  Foster 
and  J.  D.  WTiitney.     Executive  Docs.,  1st  sess.  31st  Cong.,  1849-50,  vol.  9,  No.  69,  224  pp.,  with  map. 

Report  on  the  geology  and  topography  of  the  Lake  Superior  land  district;  part  2,  The  iron  region,  by  J.  W.  Foster 
and  J.  D.  Whitney.  Senate  Docs.,  special  sess.  32d  Cong.,  1851,  vol.  3,  No.  4,  406  pp.,  with  maps.  Sec  also  Aperfu 
de  I'ensemble  des  terrains  Siluriens  du  Lac  Sup&ieur,  by  J.  W.  Foster  and  J.  D.  AMiitncy.  Bull.  Soc.  geol.  France, 
vol.  2,  18.50,  pp.  89-100. 

On  the  Azoic  system  as  developed  in  the  Lake  Superior  laud  district,  by  J.  W.  Foster  and  J.  D.  Whitney.  Proc. 
Am.  Assoc.  Adv.  Sci.,  1851,  5th  meeting,  pp.  4-7. 

On  the  age  of  the  sandstone  of  Lake  Superior,  with  a  description  of  the  phenomena  of  the  association  of  igneous 
rocks,  by  J.  W.  Foster  and  J.  D.  Whitney.     Idem,  pp.  22-38. 

Geology,  mineralogy,  and  topography  of  the  lands  around  Lake  Superior,  by  Charles  T.  Jackson.  Senate  Docs., 
1st  sess.  32d  Cong.,  1851-52,  vol.  11,  pp.  232-244. 

Age  of  the  Lake  Superior  sandstone,  by  Charles  T.  Jackson.     Proc.  Boston  Soc.  Nat.  Hist.,  vol.  7,  1860,  pp.  396-398. 

Age  of  the  sandstone,  by  William  B.  Rogers.     Idem,  pp.  394-395. 

First  biennial  rei)ort  of  the  progress  of  the  geological  survey  of  Michigan,  by  Alexander  Winchell.  Lansing,  1861, 
339  pp. 

Some  contributions  to  a  knowledge  of  the  constitution  of  the  copper  ranges  of  Lake  Superior,  by  C.  P.  Williams 
and  J.  F.  Blandy.     Am.  Jour.  Sci.,  2d  ser.,  vol.  34,  1862,  pp.  112-120. 

On  the  iron  ores  of  Marquette,  Michigan,  by  J.  P.  Kimball.     Idem,  vol.  39,  1865,  pp.  290-303. 

On  the  position  of  the  sandstone  of  the  southern  slope  of  a  portion  of  Keweenaw  Point,  Lake  Superior,  by  Alexander 
Agassiz.     Proc.  Boston  Soc.  Nat.  Hist.,  vol.  11,  1867,  pp.  244-246. 

Die  vorsilurischen  Gebilde  der  "Obcm  Halbinsel  von  Michigan"  in  Xord-Amerika,  by  Hermann  Credner. 
Zeitschr.  Deutch.  geol.  Gesell.,  vol.  21,  1869,  pp.  516-554.     See  al.so  Die  Gliederung  der  eozoischen  (vorsilurischen) 


HISTORY  OF  GEOLOGIC  WORK  IN  THE  REGION.  75 

Formationsgruppe  Nord-Amcrikas,  by  Hermann  Credner.  Zeitschr.  pesammtcn  Naturwissenschaften,  vol.  32,  Giebel, 
1868,  pp.  353-405. 

On  the  age  of  the  copper-bearing  rocks  of  Lake  Superior,  by  T.  B.  Brookn  and  R.  Pumpelly.  Am.  Jour.  Sci., 
3d  ser.,  vol.  3,  1872,  pp.  428^32. 

Iron-bearing  rocks,  by  T.  B.  Brooks.    Geol.  Survey  Michigan,  vol.  1,  pt.  1,  1869-1873,  319  pp.,  with  maps. 

Copper-bearing  rocks,  by  R.  Pumpelly.     Idem,  pt.  2,  pp.  1^6„62-94,  with  maps. 

Copper-bearing  rocks,  by  A.  R.  Marvine.     Idem,  pt.  2,  pp.  47-61,  95-140. 

Paleozoic  rocks,  by  C.  Rominger.     Idem,  pt.  3,  102  pp. 

Observations  on  the  Ontonagon  silver-mining  district  and  the  slate  ciuarries  of  Huron  Bay,  by  C.  Rominger. 
Geol.  Survey  Michigan,  vol.  3,  pt.  1,  1876,  pp.  151-166. 

On  the  youngest  Huronian  rocks  south  of  Lake  Superior  and  the  age  of  the  copper-bearing  series,  by  T.  B.  Brooks. 
Am.  Jour.  Sci.,  3d  ser.,  vol.  11,  1876,  pp.  206-211. 

Classified  list  of  rocks  observed  in  the  Huronian  series  south  of  Lake  Superior,  by  T.  B.  Brooks.  Idem,  vol.  12, 
pp.  194-204. 

Metasomatic  development  of  the  copper-bearing  rocks  of  Lake  Superior,  by  Raphael  Pumpelly.  Proc.  Am. 
Acad.  Arts  Sci.,  vol.  13,  1878,  pp.  253-309. 

First  annual  report  of  the  commissioner  of  mineral  statistics  of  the  State  of  Michigan  for  1877-1878,  by  Charles  E. 
Wright.     Marquette,  1879,  229  pp. 

Notes  on  the  iron  and  copper  districts  of  Lake  Superior,  by  M.  E.  Wadsworth.  Bull.  Mus.  Comp.  Zool.  Harvard 
Coll.,  whole  ser.,  vol.  7;  geol.  ser.,  vol.  1,  No.  1,  157  pp.  See  also  On  the  origin  of  the  hon  ores  of  the  Marquette  dis- 
trict, Lake  Superior.  Proc.  Boston  Soc.  Nat.  Hist.,  vol.  20,  1878-1880,  pp.  470-479.  On  the  age  of  the  copper-bearing 
rocks  of  Lake  Superior  (abstract).  Proc.  Am.  Assoc.  Adv.  Sci.,  29th  meeting,  pp.  429-4.30.  On  the  relation  of  the 
"Keweenawan  series"  to  the  Eastern  sandstone  in  the  vicinity  of  Torch  Lake,  Michigan.  Proc.  Boston  Soc.  Nat. 
Hist.,  vol.  23,  1884-1888,  pp.  172-180;  Science,  vol.  1,  1883,  pp.  248-249,  307. 

Upper  Peninsula,  by  C.  Rominger.    Geol.  Survey  Michigan,  vol.  4,  1881,  pp.  1-248,  with  a  geologic  map. 

Geological  report  on  the  Upper  Peninsula  of  Michigan,  exhibiting  the  progress  of  work  from  1881  to  1884,  by 
C.  Rominger.     Geol.  Survey  Michigan,  vol.  5,  pt.  1,  1895,  179  pp. 

On  a  supposed  fossil  from  the  copper-bearing  rocks  of  Lake  Superior,  by  M.  E.  ^^'adsworth.  Proc.  Boston  Soc. 
Nat.  Hist.,  vol.  23,  1884-1888,  pp.  208-212. 

Observations  on  the  junction  l>etween  the  Eastern  sandstone  and  the  Keweenaw  series  on  Keweenaw  Point,  Lake 
Superior,  by  R.  D.  Irving  and  T.  C.  Chamberlin.     Bull.  U.  S.  Geol.  Survey  No.  23,  1885,  124  pp.,  17  pi. 

Mode  of  deposition  of  the  iron  ores  of  the  Menominee  range,  Michigan,  by  John  Fulton.  Trans.  Am.  Inst.  Min. 
Eng.,  vol.  16,  1887,  pp.  525-536. 

Report  of  N.  H.  Winchell.     Sixteenth  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  for  1887,  pp.  13-129. 

Report  of  Alexander  Winchell.     Idem,  pp.  133-391. 

Unpublished  field  notes  made  by  W.  N.  Merriam  in  the  summers  of  1888  and  1889. 

Unpublished  field  notes  made  by  C.  R.  Van  Hise  in  the  summer  of  1890. 

The  greenstone  schist  areas  of  the  Menominee  and  Marquette  regions  of  Michigan,  by  George  Huntington  Williams. 
Bull.  U.  S.  Geol.  Survey  No.  62,  1890,  pp.  31-238,  with  16  pis.  and  maps.  See  also  Some  examples  of  dynamic  meta- 
morphism  of  the  ancient  eruptive  rocks  on  the  south  shore  of  Lake  Superior.  Proc.  Am.  Assoc.  Adv.  Sci.,  36th  meeting, 
1888,  pp.  225-226. 

A  sketch  of  the  geology  of  the  Marquette  and  Keweenawan  district,  by  M.  E.  \\'adsworth.  Along  the  south  shore 
of  Lake  Superior,  by  Julian  Ralph.     1st  ed.,  1890,  pp.  63-82. 

Explanatoi-y  and  historical  note,  by  R.  D.  Irving.     Bull.  U.  S.  Geol.  Survey  No.  62,  1890,  pp.  1-30. 

The  Penokee  iron-bearing  series  of  Michigan  and  Wisconsin,  by  R.  D.  Irving  and  C.  R.  Van  Hise.  Mon.  U.  S. 
Geol.  Survey,  vol.  19, 1892,  534  pp.,  with  plates  and  maps.  See  also  Tenth  Ann.  Rept.  U.  S.  Geol.  Survey,  for  1888-89, 
1890,  pp.  341-507,  with  23  pis.  and  maps. 

A  sketch  of  the  geology  of  the  Marquette  and  Keweenawan  district,  by  M.  E.  \\'adsworth.  Along  the  south  shore 
of  Lake  Superior,  by  Julian  Ralph.     2d  ed.,  1891,  pp.  75-99. 

On  the  relations  of  the  Eastern  sandstone  of  Keweenaw  Point  to  the  Lower  Silurian  limestone,  by  M.  E.  Wadsworth. 
Am.  Jour.  Sci.,  3d  ser.,  vol.  42,  1891,  pp.  170-171  (communicated). 

The  South  Trap  range  of  the  Keweenawan  series,  by  M.  E.  Wadsworth.     Idem,  pp.  417^19. 

Unpublished  field  notes  made  by  Raphael  Pumpelly  and  C.  R.  Van  Hise  in  the  summers  of  1891  and  1892. 

The  Huronian  volcanics  south  of  Lake  Superior,  by  C.  R.  Van  Hise.  Bull.  Geol.  Soc.  America,  vol.  4,  1892, 
pp.  435-436. 

Microscopic  characters  of  rocks  and  minerals,  by  A.  C.  Lane.  Rept.  State  Board  Geol.  Survey  Michigan  for 
1891-92,  Lansing,  1893,  pp.  176-183. 

A  sketch  of  the  geology  of  the  iron,  gold,  and  copper  districts  of  Michigan,  by  M.  E.  Wadsworth.  Idem,  pp.  75-174. 
See  also  Ann.  Repts.  1888-1892,  pp.  38-73. 

Subdivisions  of  the  Azoic  or  Archean  in  northern  Michigan,  by  M.  E.  Wadsworth.  Am.  Jour.  Sci.,  vol.  45,  1893, 
pp.  72-73. 

The  succession  in  the  Marquette  iron  district  of  Michigan,  by  C.  R.  ^'an  Hise.  Bull.  Geol.  Soc.  America,  vol.  5, 
1893,  pp.  5-6. 


76  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

The  geology  of  that  portion  of  the  Menominee  range  east  of  Menominee  River,  by  Nclscm  P.  Ilulst.  Proc.  Lake 
Superior  Min.  Inst.,  March,  1893,  pp.  19-29. 

A  pontact  between  the  Lower  Iluronian  and  the  underlying  graiiile  in  the  Ropublir  trough,  near  Republic,  Mich., 
by  n.  L.  Smyth.    Jour.  Geology,  vol.  1,  Xo.  3,  1893.  pp.  2G8-274. 

Two  new  geological  cross  sections  of  Keweenaw  Point,  by  L.  L.  Hubbard.     Proc.  Lake  Superior  Min.  Inst.,  vol.  2, 

1894,  pp.  79-96. 

Chai-acter  of  folds  in  the  Marquette  iron  district,  by  C.  R.  Van  Hise.     Proc.  Am.  Assoc.  Adv.  Sci.,  42d  meeting, 

1894,  p.  171  (abstract). 

Relations  of  the  Lower  Menominee  and  Lower  Marquette  series  of  Michigan  (preliminary),  by  II.  L.  Smyth.  Am. 
Jour.  Sci.,  3d  ser.,  vol.  47,  1894,  pp.  216-223. 

The  quartzite  tongue  at  Republic,  Mich.,  by  H.  L.  Smyth.    Jour.  Geology,  vol.  2,  1894,  pp.  680-691. 

The  relation  of  the  \ein  at  the  Central  mine,  Keweenaw  Point,  to  the  Kearsarge  conglomerate,  by  L.  L.  Hubbard. 
Proc.  Lake  Superior  Min.  Inst.,  vol.  3,  1895,  pp.  74-83. 

The  Marquette  iron  range  of  Michigan,  by  G.  A.  Xewett.     Idem.  pp.  87-108.     With  geologic  map. 

The  volcanics  of  the  Michigamme  district  of  Michigan  (preliminary),  l)y  J.  Morgan  Clements.  Jour.  Geology, 
vol.  3,  1895,  pp.  802-822. 

A  central  Wisconsin  base-level,  by  C.  R.  Van  Hise.  Science,  new  ser.,  vol.  4,  1896,  pp.  57-59.  See  also  A  northern 
Michigan  base-level.     Idem,  pp.  217-220. 

Organic  markings  in  Lake  Superior  iron  ores,  by  W.  S.  Gresley.  Science,  new  ser.,  vol.  3,  1896,  ])p.  622-623; 
Trans.  Am.  Inst.  Min.  Eng.,  vol.  26,  1897,  pp.  527-534. 

The  Marquette  iron-bearing  district  of  Michigan,  by  C.  R.  Van  Hise  and  W.  S.  Bayley;  with  a  chapter  on  the 
Republic  trough,  by  H.  L.  Smyth.  Men.  U.  S.  Geol.  Survey,  vol.  28,  1897,  608  pp.  With  atlas  of  39  plates.  Pre- 
iminary  report  on  same  district  published  in  Fifteenth  Ann.  Rept.  U.  S.  Geol.  Survey,  1895,  pp.  477-650. 

The  origin  and  mode  of  occurrence  of  the  Lake  Superior  copper  deposits,  by  M.  E.  Wadsworth.  Trans.  Am.  Inst. 
MLn.  Eng.,  vol.  27,  1898,  pp.  669-696. 

Some  dike  features  of  the  Gogebic  iron  range,  by  C.  M.  Ross.     Idem,  pp.  556-563. 

Geological  report  on  Isle  Royale,  Michigan,  by  A.  C.  Lane.  Geol.  Survey  Michigan,  vol.  6,  pt.  1,  1898,  281  pp. 
With  geologic  map. 

Keweenaw  Point,  with  particular  reference  to  the  felsites  and  their  associated  rocks,  by  L.  L.  Hubbard.  Geol. 
Survey  Michigan,  vol.  6,  pt.  2,  1898,  155  pp.     With  plates. 

Unpublished  notes  by  Prof.  A.  E.  Seaman  and  thesis  on  the  Gogebic  district,  by  W.  J.  Sutton,  Michigan  College 

of  Mines. 

The  Crystal  Falls  iron-bearing  district  of  Michigan,  by  J.  Morgan  Clements  and  H.  L.  Smyth,  with  a  chapter  on 
the  Sturgeon  River  tongue,  by  W.  S.  Bayley,  and  an  introduction  by  C.  R.  Van  Hise.  Mon.  V.  S.  Geol.  Survey,  vol. 
36,  1899.     With  geologic  maps. 

Geology  of  the  Mineral  range,  by  A.  E.  Seaman.  First  Ann.  Rept.  Copper-Mining  Industry  of  Lake  Superior, 
1899,  pp.  49-60. 

Note  sur  la  region  cupriffere  de  Textremit^  nordest  de  la  peninsula  de  Keweenaw  (Lac  Superieur),  par  Louis  Duparc. 
Archives  sci.  phys.  et  nat.,  vol.  10,  1900,  p.  21. 

The  Menominee  special  folio,  by  Charles  R.  Van  Hise  and  ^\■.  S.  Bayley.  Geologic  Atlas  U.  S.,  folio  62,  U.  S. 
Geol.  Survey,  1900. 

Unpublished  notes  by  Prof.  A.  E.  Seaman  made  for  Michigan  Geological  Survey,  Michigan  College  of  Mines,  and 
United  States  Cieological  Survey.  See  also  unpublished  maps  prepared  for  Michigan  exhibit  at  St.  Louis  exposition, 
1904. 

Unpublished  thesis  by  W.  O.  Hotchkiss,  Geol.  Dept.  Univ.  Wisconsin,  1903. 

Report  of  special  committee  on  the  Lake  Superior  region  to  Frank  D.  Adams,  Robert  Bell,  C.  Willard  Hayes,  and 
Charles  R.  Van  Hise,  general  committee  on  the  relations  of  the  Canadian  and  the  United  States  geological  sur\-eys, 
1904.  Jour.  Geology,  vol.  13,  1905,  pp.  89-104.  The  special  committee  consisted  of  Frank  D.  Adams,  Robert  Bell, 
C.  K.  Leith,  C.  R.  Van  Hise.  There  were  present  by  invitation  W.  G.  Miller,  A.  C.  Lane,  and  for  parts  of  the  trip 
A.  E.  Seaman,  W.  N.  Merriam,  J.  U.  Sebenius,  and  W.  N.  Smith. 

Maps  of  the  Marquette,  Menominee,  and  Gogebic  districts,  Michigan,  prepared  by  A.  E.  Seaman  for  the  St.  Louis 
exposition,  1904.     Unpublished. 

The  Menominee  ii-on-bearing  district  of  Michigan,  by  W.  S.  Bayley.     Mon.  U.  S.  Geol.  Survey,  vol.  46, 1904,  513  pp. 

The  geology  of  some  of  the  lands  in  the  Upper  Peninsula,  by  R.  S.  Rose.  Proc.  Lake  Superior  Min.  Inst.,  1904, 
pp.  88-102. 

Unpublished  notes  of  field  work  done  in  1905,  by  G.  W.  Corey  and  C.  F.  Bowen. 

Black  River  work,  by  A.  C.  Lane.     Ann.  Rept.  Geol.  Survey  Michigan  for  1904,  1905,  pp.  158-162. 

Report  of  progress  in  the  Porcupines,  by  F.  E.  Wright.  Ann.  Rept.  Geol.  Survey  Michigan  for  1903,  1905,  pp. 
33^4.  Also  Preliminary  geological  map  of  the  Porcupine  Mountains  and  vicinity,  by  F.  E.  Wright  and  A.  C.  Lane. 
Ann.  Rept.  Geol.  Survey  Michigan  for  1908,  1909,  pi.  1. 

The  geology  of  Keweenaw  Point  -a  brief  description,  by  A.  C.  Lane.  Proc.  Lake  Superior  Min.  Inst.,  vol.  12, 
1907,  pp.  81-104. 


HISTORY  OF  GEOLOGIC  WORK  IN  THE  REGION.  77 

Notes  on  the  geological  section  of  Michigan;  part  1,  the  pre-Ordovician,  by  A.  C.  Lane  and  A.  E.  Seaman.  Jour. 
Geology,  vol.  15,  1907,  pp.  680-695.  Also  notes  on  the  geological  section  of  Michigan,  part  2,  from  the  St.  Peter  sand- 
stone up,  by  A.  C.  Lane.     Jour.  Geology,  vol.  18,  1910,  pp.  393^29. 

A  geological  section  from  Bessemer  down  Black  River,  by  W.  C.  Gordon  and  Alfred  C.  Lane.  Ann.  Rept.  Geol. 
Survey  Michigan  for  1906,  1907,  pp.  396-507. 

Unpublished  geologic  maps  of  Menominee  and  Florence  districts,  Michigan  and  Wisconsin,  prepared  for  Oliver 
Iron  Mining  Company  by  W.  N.  Merriam. 

Unpublished  maps  and  report  on  geology  of  Crystal  Falls,  Menominee,  and  Iron  River  districts,  Michigan,  prepared 
during  commercial  surveys,  by  C.  K.  Leith,  R.  C.  Allen,  and  others. 

Report  on  geology  of  Iron  River  district  of  Michigan,  by  R.  C.  Allen.  Michigan  Geo!,  and  Biol.  Survey,  pub.  3, 
1910,  151  pp.,  with  geologic  map. 

The  intrusive  rocks  of  Mount  Bohemia,  Michigan,  by  F.  E.  Wright.  Ann.  Rept.  Michigan  Geol.  Survey  for  1908, 
1909,  pp.  361-397. 

NORTHERN  WISCONSIN. 

Report  of  a  geological  reconnaissance  of  the  Chippewa  land  district  of  Wisconsin,  etc.,  by  David  D.  (.)wen.  Senate 
Docs.,  1st  sess.  30th  Cong.,  1848,  vol.  7,  No.  57,  72  pp. 

Preliminary  report  containing  outlines  of  the  progress  of  the  geological  survey  of  Wisconsin  and  Iowa  up  to  October 
11,  1847,  by  Da\-id  Dale  Owen.     Senate  Docs.,  1st  sess.  30th  Cong.,  1847,  vol.  2,  No.  2,  pp.  160-173. 

Description  of  part  of  Wisconsin  south  of  Lake  Superior,  by  Charles  WTiittlesey.  Report  of  a  geological  survey  of 
Wisconsin,  Iowa,  and  Minnesota,  1852,  pp.  419-470. 

The  Penokee  iron  range,  by  Increase  A.  Lapham.  Trans.  Wisconsin  State  Agr.  Soc,  vol.  5,  1858-59,  pp.  391^00, 
with  map.  See  also  Report  to  the  directors  of  the  Wisconsin  and  Lake  Superior  Mining  and  Smelting  Company,  in 
the  Penokee  iron  range  of  Lake  Superior,  with  reports  and  statistics  showing  its  mineral  wealth  and  prospects,  charter 
and  organization  of  the  Wisconsin  and  Lake  Superior  Mining  and  Smelting  Company,  Milwaukee,  1860,  pp.  22-37. 

Geological  report  of  the  State  of  Wisconsin,  by  James  Hall.  Report  of  the  Superintendent  of  the  Geological  SiuT^ey 
(1861),  exhibiting  the  progress  of  the  work,  52  pp. 

Physical  geography  and  general  geology,  by  James  Hall.  Report  on  the  geological  survey  of  the  State  of  Wisconsin, 
vol.  1,  1862,  pp.  1-72. 

The  Penokee  mineral  range,  Wisconsin,  by  Charles  Whittlesey.  Proc.  Boston  Soc.  Nat.  Hist.,  vol.  9,  1863,  pp 
235-244. 

On  some  points  in  the  geology  of  northern  Wiscon.sin,  by  R.  D.  Irving.  Trans.  WiscoiLsin  Acad.  Sci.,  vol.  2,  1873-74 
pp.  107-119.  See  also  On  the  age  of  the  copper-bearing  rocks  of  Lake  Superior,  and  on  the  westward  continuation  of 
the  Lake  Superior  synclinal.  Am.  Jour.  Sci.,  3d  ser.,  vol.  8,  1874,  pp.  46-56.  Ann.  Rept.  Progress  and  Results  of 
Wisconsin  Geol.  Survey  for  1876,  pp.  17-25.  Report  of  progress  and  results  for  1874.  Geology  of  Wisconsin,  vol.  2, 
pp.  46-49. 

Notes  on  the  geology  of  northern  Wisconsin,  by  E.  T.  Sweet.  Trans.  Wisconsin  Acad.  Sci.,  1875-76,  vol.  3,  pp 
40-55. 

Note  on  the  age  of  the  crystalline  rocks  of  Wisconsin,  by  R.  D.  Irving.  Am.  Jour.  Sci.,  3d  ser.,  vol.  13,  1877,  pp, 
307-309. 

Report  of  progress  and  results  for  the  year  1875,  by  O.  W.  Wight.  Geology  of  Wisconsin,  vol.  2,  1873-1877,  pp, 
67-89. 

Geology  of  central  Wisconsin,  by  R.  D.  Irving.     Idem,  pp.  409-636,  with  2  atlas  maps. 

On  the  geology  of  northern  Wisconsin,  by  R.  D.  Irving.     Ann.  Rept.  Wisconsin  Geol.  Sm-vey  for  1877,  pp.  17-25. 

Report  on  the  eastern  part  of  the  Penokee  range,  by  T.  C.  Chamberlin.     Idem,  pp.  25-29. 

General  geology  of  the  Lake  Superior  region,  by  R.  D.  Irving.  Geology  of  Wisconsin,  vol.  3,  1880,  pp.  1-24.  Geol- 
ogy of  the  eastern  Lake  Superior  district.  Idem,  pp.  51-238,  with  6  atlas  maps.  Mineral  resources  of  Wisconsin. 
Trans.  Am.  Inst.  Min.  Eng.,  vol.  8,  1880,  pp.  478-508,  with  map.  Note  on  the  stratigraphy  of  the  Huronian  series  of 
northern  Wisconsin,  and  on  the  equivalency  of  the  Huronian  of  the  Marquette  and  Penokee  districts.  Am.  Jour.  Sci., 
3d  ser.,  vol.  17,  1879,  pp.  393-398. 

Huronian  series  west  of  Penokee  Gap,  by  C.  E.  Wright.  Geology  of  Wisconsin,  vol.  3,  1880,  pp.  241-301,  with  an 
atlas  map. 

Geology  of  the  western  Lake  Superior  district,  by  E.  T.  Sweet.     Idem,  pp.  303-362,  with  an  atlas  map. 

Geology  of  the  upper  St.  Croix  district,  by  T.  (".  Chamberlin  and  Moses  Strong.  Idem,  pp.  363-428,  with  2  atlas 
maps. 

Geology  of  the  Menominee  region,  by  T.  B.  Brooks.     Idem,  pp.  430-599,  with  3  atlas  maps. 

Geology  of  the  Menominee  iron  region  (economic  resources,  lithology,  and  westerly  and  southerly  extension),  by 
Charles  E.  Wright.     Idem,  pp.  666-734. 

The  quartzitea  of  Barron  and  Chippewa  counties,  by  Moses  Strong,  E.  T.  Sweet,  F.  H.  Brotherton,  and  T.  C. 
Chamberlin.     Geology  of  Wisconsin,  vol.  4,  1873-1879,  pp.  573-581. 

Geology  of  the  upper  Flambeau  Valley,  by  F.  H.  King.     Idem,  pp.  583-615. 

Crystalline  rocks  of  the  Wisconsin  Valley,  by  R.  D.  Irving  and  C.  R.  Van  Hise.     Idem,  pp.  623-714. 


78  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

General  geology  (of  Wisconsin),  by  T.  C.  Chaniberlin.  Geology  of  Wiscontiin,  vol.  1,  1SS3,  pp.  3-:50O,  ■n-ith  an  atlaa 
map. 

Lithologry  of  Wiscon.sin,  by  R.  D.  Irving.     Idem,  pp.  340-361. 

Transition  from  the  copper-bearing  series  to  the  Potsdam,  by  L.  C.  Wooster.  Am.  Jour.  Sci.,  3d  ser.,  vol.  27,  1884, 
pp.  463^65. 

Geology  of  the  St.  Croix  Dalles,  by  C.  P.  lierkey.  Am.  Geologist,  vol.  20,  1897,  pp.  345-383;  vol.  21,  1898,  pp. 
139-155,  270-294. 

Preliminary  report  on  copper-bearing  rocks  in  Douglas  County,  Wis.,  by  U.  S.  Grant.  Hull.  Wisconsin  Geol. 
and  Nat.  Hist.  Survey  No.  (i,  1901. 

The  pre-Potsdam  Peneplain  of  the  pre-Cambrian  of  north-central  Wisconsin,  by  S.  Weidman.  Jour.  Geology, 
vol.  11,  1903,  pp.  289-313. 

Unpublished  thesis  Univ.  Wisconsin,  1905. 

The  geology  of  north-central  W'isconsin,  by  S.  W'eidman.  Bull.  Wisconsin  Cieol.  and  Nat.  Hist.  Survey  No.  16, 
1907.     Summary  fiu-nished  by  author  in  1905. 

MINNESOTA. 

Account  of  a  journey  to  the  Coteau  des  Prairies,  viilh  a  description  of  the  red  pipestone  cpiarry  and  granite  bowlders 
found  there,  by  George  Catlin.     Am.  Jour.  Sci.,  1st  ser.,  vol.  38,  pp.  138-146. 

Report  of  J.  G.  Norwood.     Senate  Docs.,  1st  sess.  30th  Cong.,  1847,  vol.  2,  No.  2,  pp.  73-134. 
Description  of  the  geology  of  middle  and  western  Minnesota,  including  the  country  adjacent  to  the  northwest  and 
part  of  the  southwest  shore  of  Lake  Superior;  illustrated  by  numerous  general  and  local  sections,  woodcuts,  and  a  map, 
by  J.  G.  Norwood.     Report  of  a  geological  survey  of  Wisconsin,  Iowa,  and  Minnesota,  1852,  pp.  209-418. 

Report  of  the  State  geologist  on  the  metalliferous  region  bordering  on  Lake  Superior,  by  Henry  II.  Eames.  St. 
Paul,  1866,  23  pp. 

Geological  reconnaissance  of  the  northern,  middle,  and  other  counties  of  Minnesota,  by  Henry  II.  Eames.  St. 
Paul,  1866,  58  pp. 

Notes  upon  the  geology  of  some  portions  of  Minnesota,  from  St.  Paul  to  the  western  part  of  the  State,  by  James 
Hall.    Trans.  Am.  Philos.  Soc,  new  ser.,  vol.  13,  1869,  pp.  329-340. 

Report  on  the  geological  survey  of  the  State  of  Iowa,  containing  results  of  examinations  and  observations  made 
within  the  years  1866,  1867,  1868,  and  1869,  by  Charles  A.  WTiite.     Des  Moines,  1870,  vol.  1,  391  pp.;  vol.  2,  443  pp. 
First  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  by  N.  H.  Winchell,  1873,  129  pp. 
Thegeology  of  the  Minnesota  Valley,  by  N.  II.  Winchell.     Second  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Minne- 
sota, 1874,  pp.  127-212. 

Ueber  die  krystallinischen  gesteine  von  Minnesota  in  Nord-.\merika,  by  A.  Streng  and  J.  H.  Kloos.  Leonhard's 
Jahrbuch,  1877,  pp.  31,  113,  225.  Translated  by  N.  H.  Winchell  in  Eleventh  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey 
Minnesota,  1883,  pp.  30-85. 

Sixth  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  for  1877,  by  N.  H.  WincheU,  226  pp. 
Sketch  of  the  work  of  the  season  of  1878,  by  N.  H.  Winchell.     Seventh  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey 
Minnesota,  for  1878,  pp.  9-25. 

The  cupriferous  series  at  Duluth,  by  N.  H.  Winchell.  Eighth  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Minnesota, 
for  1879,  pp.  22-26.  ' 

Preliminary  report  on  the  geology  of  central  and  western  Minnesota,  by  Warren  Upham.     Idem,  pp.  70-125. 
Report  of  Prof.  C.  W.  Hall.     Idem,  pp.  126-138. 

Preliminary  list  of  rocks,  by  N.  H.  Winchell.  Ninth  Ann.  Rept.  Geol.  and  Nat.  Hist.  Sur\-ey  Minnesota,  for  1880, 
pp.  10-114. 

The  cupriferous  series  in  Minnesota,  by  N.  H.  Winchell.  Proc.  Am.  Assoc.  Adv.  Sci.,  29th  meeting,  1881,  pp. 
422-425.     See  also  Ninth  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  for  1880,  pp.  38.5-387. 

Preliminary  list  of  rocks,  by  N.  H.  Winchell.  Tenth  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  for  1881, 
pp.  9-122. 

Notes  on  rock  outcrops  in  central  Minnesota,  by  Warren  Upham.  Eleventh  Ann.  Rept.  Geol.  and  Nat.  Hist. 
Survey  Minnesota,  for  1882,  pp.  86-136. 

The  iron  region  of  northern  Minnesota,  by  Albert  H.  Chester.     Idem,  pp.  154-107. 

Note  on  the  age  of  the  rocks  of  the  Mesabi  and  Vermilion  iron  district,  by  N .  H.  Winchell.  Idem,  pp.  168-170.  See 
also  Proc.  Am.  Assoc.  Adv.  Sci.,  1884,  33d  meeting,  pp.  363-379. 

The  geology  of  Minnesota,  by  N.  11.  Winchell  and  Warren  Upham.  Final  Rept.  Geol.  and  Nat.  Hist.  Survey 
Minnesota,  voL  1,  1884,  695  pp.;  vol.  2,  1888,  697  pp. 

Notes  of  a  trip  across  the  Mesabi  range  to  Vermilion  Lake,  by  N.  II.  Winchell.  Thirteenth  Ann.  Rept.  Geol.  and 
Nat.  Hist.  Survey  Minnesota,  for  1884,  pp.  20-24. 

The  crystalline  rocks  of  Minnesota,  by  N.  H.  Winchell.     Idem,  pp.  36-38. 
The  crystalline  rocks  of  the  Northwest,  by  N.  H.  Winchell.     Idem,  i)p.  124-140. 

Report  of  a  trip  on  the  upper  Mississippi  and  to  Vermilion  Lake,  by  Bailey  Willis.  Tenth  Census,  vol.  15,  1886, 
pp  457^67. 


HISTORY  OF  GEOLOGIC  WORK  IN  THE  REGION.  79 

Report  of  geological  observations  made  in  northeastern  Minnesota  during  the  season  of  188G,  by  Alexander 
Winchell.     Fifteenth  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  fur  1886,  pp.  5-207. 

Geological  report  of  N.  H.  Winchell.     Idem,  pp.  209-399,  with  a  map. 

Report  of  N.  H.  Winchell.     Sixteenth  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  for  1887,  pp.  13-129. 

Report  of  Alexander  Winchell.  Idem,  pp.  133-391.  See  also  The  unconformities  of  the  Animikie  in  Minnesota. 
Am.  Geologist,  vol.  1,  1888,  pp.  14-24.  Two  systems  confounded  in  the  Huronian.  Idem,  vol.  3,  1889,  pp.  212-214, 
339-340.  Systematic  results  of  a  field  study  of  the  Archean  rocks  of  the  Northwest.  Proc.  Am.  Assoc.  Adv.  Sci.,  37th 
meeting,  1889,  p.  205.     The  geological  position  of  the  Ogishke  conglomerate.     Idem,  38th  meeting,  1890,  pp.  234-235. 

Report  of  H.  V.  Winchell.  Sixteenth  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  for  1887,  pp.  39-5-462, 
with  map. 

The  distribution  of  the  granites  of  the  Northwestern  States  and  their  general  lithologic  characters,  by  C.  W.  Hall. 
Proc.  Am.  Assoc.  Adv.  Sci.,  37th  meeting,  1889,  pp.  189-190. 

Report  of  N.  H.  Winchell.  Seventeenth  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  for  1888,  pp.  5-74. 
See  also  The  Animikie  black  slates  and  quartzites  and  the  Ogishke  conglomerate  of  Minnesota,  the  equivalent  of  the 
"Original  Huronian."  Am.  Geologist,  vol.  1,  1888,  pp.  11-14.  Methods  of  stratigraphy  in  studying  the  Huronian. 
Idem,  vol.  4,  1889,  342-357. 

Report  of  field  observations  made  during  the  season  of  1888  in  the  iron  regions  of  Minnesota,  by  H.  \'.  \\'inchell. 
Seventeenth  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  for  1888,  pp.  77-145.  See  also  The  diabasic  schists 
containing  the  jaspilite  beds  of  northeastern  Minnesota.     Am.  Geologist,  vol.  3,  1889,  pp.  18-22. 

Report  of  geological  observations  made  in  northeastern  Minnesota  during  the  summer  of  1888,  by  U.  S.  Grant. 
Seventeenth  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  for  1888,  pp.  149-215. 

Conglomerates  inclosed  in  gneissic  terranes,  by  Alexander  Winchell.  Am.  Geologist,  vol.  3,  1889,  pp.  153-165, 
256-262. 

Some  thoughts  on  eruptive  rocks,  with  special  reference  to  those  of  Minnesota,  by  N.  II.  \\'inchell.  Proc.  Am. 
Assoc.  Adv.  Sci.,  37th  meeting,  1888,  pp.  212-221. 

The  Stillwater,  Minn.,  deep  well,  by  A.  D.  Meads.     Am.  Geologist,  vol.  3,  1889,  pp.  341-342. 
^    On  a  possible  chemical  origin  of  the  iron  ores  of  the  Keewatin  in  Minnesota,  by  N.  II.  and  H.  V.  \\'inchell.     Idem, 
vol.  4,  1889,  pp.  291-300,  382-386.     Also  Proc.  Am.  Assoc.  Adv.  Sci.,  38th  meeting,  pp.  235-242. 

Some  results  of  Archean  studies,  by  Alexander  Winchell.     Bull.  Geol.  Soc.  America,  vol.  1,  1890,  pp.  357-394. 

The  Taconic  iron  ores  of  Minnesota  and  of  western  New  England,  by  N.  II.  and  H.  V.  Winchell.  Am.  Geologist, 
vol.  6,  1890,  pp.  263-274. 

Record  of  field  observations  in  1888  and  1889,  by  N.  H.  Winchell.  Eighteenth  Ann.  Rept.  Geol.  and  Nat.  Hist. 
Survey  Minnesota,  for  1889,  pp.  7-47. 

The  iron  ores  of  Minnesota,  by  N.  H.  and  H.  V.  Winchell.  Bull.  Geol.  and  Nat.  Hist.  Survey  Minnesota  No.  6, 
1891,  pp.  430,  with  a  geologic  map. 

Geological  age  of  the  Saganaga  syenite,  by  Horace  V.  Winchell.     Am.  Jour.  Sci.,  3d  ser.,  vol.  41,  1891,  pp.  386-390. 

Notes  on  the  petrography  and  geology  of  the  Akeley  Lake  region,  in  northeastern  Minnesota,  by  W.  S.  Bayley. 
Nineteenth  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Mitmesota,  for  1890,  pp.  193-210. 

The  stratigraphic  position  of  the  Ogishke  conglomerate  of  northeastern  Minnesota,  by  U.  S.  Grant.  Am.  Geologist, 
vol.  10,  1892,  pp.  4-10. 

Paleozoic  formations  of  southeastern  Minnesota,  by  C.  W.  Hall  and  F.  W.  Sardeson.  Bull.  Geol.  Soc.  America, 
vol.  3,  1892,  pp.  331-368. 

The  basic  massive  rocks  of  the  Lake  Superior  region,  by  W.  S.  Bayley.  Jour.  Geology,  vol.  1,  1893,  pp.  433—456, 
587-596,  688-716;  vol.  2,  1894,  pp.  814-825;  vol.  3,  1895,  pp.  1-20. 

The  crystalline  rocks,  by  N.  H.  Winchell.  Twentieth  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  for 
1891,  1893,  pp.  1-28. 

Anorthosites  of  the  Minnesota  shore  of  Lake  Superior,  by  A.  C.  Lawson.  Bull.  Geol.  and  Nat.  Hist.  Survey  Min- 
nesota No.  8,  1893,  pp.  1-23. 

The  geology  of  Kekequabic  Lake,  in  northeastern  Minnesota,  with  special  reference  to  an  augite-soda  granite,  by 
U.  S.  Grant;  thesis  accepted  for  degree  of  Ph.  D.  in  Johns  Hopkins  University,  1893.  Twenty-first  Ann.  Rept.  Geol. 
and  Nat.  Hist.  Survey  Minnesota,  for  1892,  1893,  pp.  5-58.     With  geologic  map  and  plates. 

The  eruptive  and  sedimentary  rocks  on  Pigeon  Point,  Minnesota,  and  their  contact  phenomena,  by  W.  S.  Bayley. 
Bull.  U.  S.  Geol.  Survey  No.  109,  1893,  with  maps  and  plates. 

Field  observations  on  certain  granitic  areas  in  northeastern  Minnesota,  by  V.  S.  Grant.  Twentieth  Ann.  Rept. 
Geol.  and  Nat.  Hist.  Survey  Minnesota,  1893,  pp.  3.5-110. 

Sketch  of  the  coastal  topography  of  the  north  side  of  Lake  Superior,  with  special  reference  to  the  abandoned  strands 
of  Lake  Warren,  by  A.  C.  Lawson.     Idem,  pp.  181-289. 

Actinolite-magnetite  schists  from  the  Mesabi  ir(.in  range,  in  northeastern  Minnesota,  by  W.  S.  Bayley.  Am.  Jour. 
Sci.,  3d  ser.,  vol.  46,  1893,  pp.  176-180. 

The  Mesabi  iron  range,  by  H.  V.  Winchell.  Twentieth  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  for 
1891,  1893,  pp.  111-180. 

Preliminary  report  of  field  work  during  1893  in  northeastern  Minnesota,  by  U.  S.  Grant.  Twenty-second  Ann. 
Rept.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  pt.  4,  1894,  pp.  67-78. 


80  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Notes  on  the  geology  of  Itasca  County,  Minn.,  by  G.  E.  Culver.     Idem,  pt.  8,  1894,  pp.  97-114. 

Preliminary  report  of  field  work  during  1893  in  northeastern  Minnesota,  by  A.  H.  Elftman.  Idem,  pt.  12,  1894, 
pp.  141-180. 

The  stratigraphic  position  of  the  Thompson  slates,  by  J.  E.  Spurr.     Am.. lour.  Sci.,  Sdser.,  vol.48, 1894,  pp.  1.59-1G.5. 

The  iron-bearing  rocks  of  the  Mesiibi  range  in  Minnesota,  by  J.  Edward  Spurr.  Bull.  Geol.  and  Nat.  Hist.  Survey 
Minnesota  No.  10,  1894,  2(i8  pp.,  with  geologic  maps. 

The  origin  of  the  Archean  greenstones,  by  N.  H.  Winchell.  Twenty-third  Ann.  Repl.  Geol.  and  Nat.  Hist.  Survey 
Minnesota,  for  1894,  pt.  2,  1895,  pp.  4-3.5. 

Preliminary  report  on  the  Rainy  Lake  gold  region,  by  II.  V.  ^^'inchell  and  U.  S.  Grant.     Idem,  pp.  36-105. 

The  iron  ranges  of  Minnesota,  by  H.  V.  Winchell.     Proc.  Lake  Superior  Mining  Inst.,  vol.  3,  1895,  pp.  11-32. 

Notes  upon  the  bedded  and  banded  structures  of  the  gabbro  and  upon  an  area  of  troctolyte,  by  A.  H.  Elftman. 
Twenty-third  Ann.  Kept.  (iool.  and  Nat.  Hist.  Survey  Minnesota,  for  1894,  1895,  pt.  12,  pp.  224-230. 

The  geological  structure  of  the  western  part  of  the  Vermilion  range,  Minnesota,  by  H.  L.  Smyth  and  J.  Ralph 
Finlay.     Trans.  Am.  Inst.  Min.  Engineers,  vol.  25,  1895,  pp.  595-605. 

The  Koochiching  granite,  by  Alexander  Winchell.     Am.  Geologist,  vol.  20,  1897,  pp.  293-299. 

Some  new  features  in  the  geology  of  northeastern  Minnesota,  by  N.  H.  Winchell.     Idem,  pp.  41-51. 

The  origin  of  the  Archean  igneous  rocks,  by  N.  H.  Winchell.  Proc.  Am.  Assoc.  Adv.  Sci.,  vol.  47,  1898,  pp.  303- 
304  (abstract);  Am.  Geologist,  vol.  22,  1898,  pp.  299-310. 

Some  resemblances  between  the  Archean  of  Minnesota  and  of  Finland,  by  N.  H.  Winchell.  Am.  Geologist,  vol. 
21,  1898,  pp.  222-229. 

The  significance  of  the  fragmental  eruptive  debris  at  Taylors  Falls,  Minn.,  by  N.  H. Winchell.  Am.  Geologist, 
vol.22,  1898,  pp.  72-78. 

The  oldest  known  rock,  by  N.  II.  Winchell.     Proc.  Am.  Assoc.  Adv.  Sci.,  vol.  47,  1898,  pp.  302-303  (abstract). 

Sketch  of  the  geology  of  the  eastern  end  of  the  Mesabi  iron  range  in  Minnesota,  by  L'.  S.  Grant.  Engineers'  Year 
Book  L'niv.  Minnesota,  1898,  pp.  49-62.     With  sketch  map. 

The  geology  of  Minnesota,  by  N.  H.  Winchell,  V.  S.  Grant,  James  E.  Todd,  Warren  I'pham,  and  H.  V.  Winchell. 
Final  Rept.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  vol.  4,  1899,  pp.  630.  With  31  geologic  plates.  Structural  geology 
of  Minnesota,  by  N.  H.  Winchell,  assisted  by  U.  S.  Grant.  Idem,  vol.  5,  1900,  pp.  1-SO,  972-1000.  Vol.  4  contains 
an  account  of  detailed  field  work  in  northeastern  Minnesota,  with  incidental  discussion  of  general  problems.  The 
area  is  treated  by  counties  and  smaller  arbitrary  geographic  divisions,  in  the  description  of  which  several  men  have 
taken  part.  This  manner  of  treatment  leads  to  repetition  in  the  discussion  of  the  general  geologic  features,  and  in 
many  cases  it  is  extremely  ditBcult  to  correlate  the  facts  recorded  in  the  different  sections.  Vol.  5  contains  an  account 
of  the  general  structural  geology  of  the  State  based  on  the  detailed  work  described  in  vol.  4.  Grant's  views,  as  in- 
dicated in  the  detailed  descriptions  of  special  areas,  in  some  cases  differ  somewhat  widely  from  those  of  Winchell. 

The  gneisses,  gabbro  schists,  and  associated  rocks  of  southwestern  Minnesota,  by  C.  W.  Hall.  Bull.  V .  S.  Geol. 
Survey  No.  157,  1899,  160  pp.     With  geologic  maps. 

Mineralogical  and  petrographic  study  of  the  gabbroid  rocks  of  Minnesota,  and  more  particularly  of  the  plagioclas- 
tites,  by  Alexander  Winchell.  Am.  Geologist,  vol.  26,  1900,  pp.  153-162.  With  geologic  sketch  map  of  northeastern 
Minnesota. 

L^npublished  field  notes,  summer  of  1900,  by  C.  R.  Van  Hise  and  J.  Morgan  Clements. 

Final  Rept.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  vol.  6,  1900-1901.     (N.  H.  Winchell.) 

Keewatin  area  of  eastern  and  central  Minnesota,  by  C.  W.  Hall.  Bull.  Geol.  Soc.  America,  vol.  12,  1901,  pp. 
343-370,  pis.  29-32. 

Keweenawan  area  of  eastern  Minnesota,  by  C.  W.  Hall.     Idem,  pp.  313-342,  pis.  27-28. 

Sketch  of  the  iron  ores  of  Minnesota,  by  N.  H.  Winchell.     Am.  Geologist,  vol.  29,  1902,  pp.  154-162. 

The  Mesabi  iron-bearing  district  of  Minnesota,  by  C.  K.  Leith.     Mon.  U.  S.  Geol.  Survey,  vol.  43,  1903,  316  pp. 

The  Vermilion  iron-bearing  district  of  Minnesota,  by  J.  Morgan  Clements.     Idem,  vol.  45,  1903,  463  pp. 

Some  results  of  the  late  Minnesota  Geological  Survey,  by  N.  H.  Winchell.     Am.  Geologist,  vol.  32, 1903,  pp.  246-253. 

The  geology  of  the  Cuyuna  iron  range,  Minnesota,  by  C.  K.  Leith.     Econ.  Geology,  vol.  2,  1907,  pp.  145-152. 

The  Cuyuna  iron  district  of  Minnesota,  by  Carl  Zapffe.  Unpublished  bachelor's  thesis  L^niv.  Wisconsin,  1907. 
See  also  The  Cuyuna  iron-ore  district  of  Minnesota,  by  Carl  Zapffe.  Supplement  to  the  Brainerd  (Minn.)  Tribune, 
Sept.  2,  1910,  pp.  32-35,  with  map. 

The  iron-ore  deposits  of  the  Ely  trough,  Vermilion  range,  Minnesota,  by  C.  E.  Abbott.  Proc.  Lake  Superior 
Min.  Inst,  (for  1906),  vol.  12,  1907,  pp.  116-142. 

Geological  history  of  the  Redstone  quartzite,  by  Frederick  W.  Sardeson.  Bull.  Geol.  Soc.  America,  vol.  19,  1908, 
pp.  221-242. 

Contribution  to  the  petrography  of  the  Keweenawan  (mth  geologic  ma]3'l,  by  Frank  F.  Grout.  Jour.  Geology,  vol. 
18,  1910,  pp.  633-657. 

The  iron  formation  of  the  Cuyuna  range,  by  F.  S.  Adams.  Econ.  Geology,  vol.  5,  1910,  ])p.  729-740;  vol.  6,  1911, 
pp.  60-70,  156-180. 


HISTORY  OF  GEOLOGIC  WORK  IN  THE  REGION.  81 

ONTARIO. 

Notes  on  the  geography  and  geology  of  Lake  Superior,  by  John  J.  Bigsby.  Quart.  Jour.  Sci.,  Lit.  and  Arts,  voL 
18,  1825,  pp.  1-34,  222-269,  with  map. 

Outlines  of  the  geology  of  Lake  Superior,  by  H.  W.  Bayfield.  Trans.  Lit.  and  IILst.  Soc.  Quebec,  vol.  1,  1829, 
pp.  1^3. 

On  the  junction  of  the  Transition  and  Primary  rocks  of  Canada  and  Labrador,  by  Captain  Bayfield.  Quart.  Jour. 
GeoL  Soc.  London,  voL  1,  1845,  pp.  450-459. 

On  the  geology  and  economic  minerals  of  Lake  Superior,  by  W.  E.  Logan.  Rept.  Prog.  Geol.  Survey  of  Canada 
for  1846-47,  pp.  8-34. 

On  the  geology  of  the  Kaministiquia  and  Michipicoten  rivers,  by  Alexander  Murray.     Idem,  pp.  47-57. 

On  the  age  of  the  copper-bearing  rocks  of  Lakes  Superior  and  Huron,  and  various  facts  relating  to  the  physical 
structure  of  Canada,  by  W.  E.  Logan.  Eept.  Brit.  Assoc.  Adv.  Sci.,  21st  meetmg,  1851,  pp.  59-62,  Trans.;  Am.  Jour. 
Sci.,  2d  ser.,  vol.  14,  1852,  pp.  224-229. 

On  the  geology  of  the  Lake  of  the  Woods,  south  Hudson  Bay,  by  Dr.  J.  J.  Bigsby.  Quart.  Jour.  Geol.  Soc. 
London,  vol.  8,  1852,  pp.  400-406.     With  a  geologic  map  of  the  Lake  of  the  Woods. 

On  the  physical  geography,  geology,  and  commercial  resources  of  Lake  Superior,  by  John  J.  Bigsby.  Edinburgh 
New  Phil.  Jour.,  vol.  53,  1852,  pp.  55-62. 

On  the  geology  of  Ramy  Lake,  south  Hudson  Bay,  by  Dr.  J.  J.  Bigsby.  Quart.  Jour.  Geol.  Soc.  London,  vol.  10, 
1854,  pp.  215-222.     With  a  geologic  map  of  Rainy  Lake. 

On  the  geological  structure  and  mineral  deposits  of  the  promontory  of  Mamainse,  Lake  Superior,  by  John  W.  Dawson. 
Canadian  Naturalist  and  Geologist,  vol.  2,  1857,  pp.  1-12,  with  a  section. 

Report  of  progress  of  the  Geological  Survey  of  Canada  from  its  commencement  to  1863,  by  W.  E.  Logan,  1863, 
983  pp.,  with  an  atlas. 

On  the  Laurentian,  Huronian,  and  upper  copper-bearing  rocks  of  Lake  Superior,  by  Thomas  Macfarlane.  Rept. 
Prog.  Geol.  Survey  Canada,  1863-1866,  pp.  115-164. 

On  the  geological  formations  of  Lake  Superior,  by  Thomas  Macfarlane.  Canadian  Naturalist,  2d  ser.,  vol.  3, 
1806-1868,  pp.  177-202,  241-256. 

On  the  geology  and  silver  ore  of  Woods  Location,  Thunder  Cape,  Lake  Superior,  by  Thomas  Macfarlane.  Cana- 
dian Naturalist,  2d  ser.,  vol.  4,  pp.  37^8,  459^63,  with  a  map. 

On  the  geology  of  the  northwest  coast  of  Lake  Superior  and  the  Nipigon  district,  by  Robert  Bell.  Rept.  Prog. 
Geol.  Survey  Canada,  1866-1869,  pp.  313-364,  with  a  topographic  sketch  map. 

Report  on  the  country  north  of  Lake  Superior,  between  the  Nipigon  and  Michipicoten  rivers,  by  Robert  Bell. 
Idem,  1870-71,  pp.  322-351. 

Report  on  the  country  between  Lake  Superior  and  the  Albany  River,  by  Robert  Bell.  Idem,  1871-72,  pp. 
101-114. 

Notes  of  a  geological  reconnaissance  from  Lake  Superior  to  Fort  Garry,  by  A.  R.  C.  Selwyn.  Idem,  1872-73, 
pp.  8-18. 

On  the  country  between  Lake  Superior  and  Winnipeg,  by  Robert  Bell.     Idem,  pp.  87-111. 

The  geognostical  history  of  the  metals,  by  T.  Sterry  Hunt.  Trans.  Am.  Inst.  Min.  Eng.,  vol.  1,  1873,  pp.  331-345; 
vol.  2,  1874,  pp.  58-59. 

On  the  country  between  Red  River  and  the  South  Saskatchewan,  with  notes  on  the  geology  of  the  region  between 
Lake  Superior  and  Red  River,  by  Robert  Bell.     Rept.  Prog.  Geol.  Survey  Canada,  1873-74,  pp.  66-90. 

Report  on  the  geology  and  resources  of  the  region  in  the  vicinity  of  the  Forty-ninth  parallel,  from  the  Lake  of  the 
Woods  to  the  Rocky  Mountains,  by  George  Mercer  Dawson,  387  pp.,  with  a  geologic  map. 

The  mineral  region  of  Lake  Superior,  by  Robert  Bell.  Canadian  Naturalist  and  Geologist,  2d  ser.,  vol.  7,  1875, 
pp.  49-51. 

On  the  country  west  of  Lakes  Manitoba  and  Winnipegosis,  with  notes  on  the  geology  of  Lake  Winnipeg,  by  Robert 
Bell.     Rept.  Prog.  Geol.  Survey  Canada,  1874-75,  pp.  24-56. 

Report  on  an  exploration  in  1875  between  James  Bay  and  Lakes  Superior  and  Huron,  by  Robert  Bell.  Idem, 
187.5-76,  pp.  294-342. 

Report  on  geological  researches  north  of  Lake  Huron  and  east  of  Lake  Superior,  by  Robert  Bell.  Idem, 
1876-77,  pp.  213-220. 

Remarks  on  Canadian  stratigraphy,  by  Thomas  Macfarlane.     Canadian  Naturalist,  2d  ser.,  vol.  9,  1879,  pp.  91-102. 

Report  on  the  geology  of  the  Lake  of  the  Woods  and  adjacent  country,  by  Robert  Bell.  Rept.  Prog.  Geol.  and 
Nat.  Hist.  Survey  Canada,  1880-1882,  pp.  11-15  c,  with  a  map. 

On  the  geology  of  Lake  Superior,  by  A.  R.  C.  Selwyn.     Trans.  Roy.  Soc.  Canada,  vol.  1,  sec.  4,  1883,  pp.  117-122. 

Age  of  the  rocks  of  the  northern  shore  of  Lake  Superior,  by  A.  R.  C.  Selwyn.  Science,  vol.  1,  1883,  p.  11.  See 
also  The  copper-bearing  rocks  of  Lake  Superior.     Idem,  p.  221. 

Notes  on  observations,  1883,  on  the  geology  of  the  north  shore  of  Lake  Superior,  by  A.  R.  C.  Selwyn.  Trans.  Roy. 
Soc.  Canada,  vol.  2,  sec.  4,  1885,  p.  245. 

47517°— VOL  52—11 6 


82  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Report  on  the  geology  of  the  Lake  of  the  Woods  region,  with  s])ecial  reference  to  the  Keewatin  (Eluronian?)  belt 
of  Archean  rocks,  by  A.  C.  Lawson.  Ann.  Kept.  Geol.  and  Nat.  Ilist.  Survey  Canada  for  1885,  new  ser.,  vol.  1,  pp. 
■5-1.51  cc,  with  a  map. 

Geology  and  lithology  of  Michipicoten  Bay,  by  C.  L.  Herrick,  W.  G.  Tight,  and  11.  L.  Jones.  Bull.  Denison  Univ., 
vol.  2,  188(),  pp.  120-144,  with  3  plates. 

Thecorrelationof  the  Aniraikrie  and  Huronian  rocks  of  Lake  Superior,  by  Peter  McKellar.  Proc.  and  Trans.  Roy. 
Soc.  Canada,  vol.  5,  sec.  4,  1887,  pp.  63-73. 

Report  of  the  geology  of  the  Rainy  Lake  region,  by  A.  C.  Lawson.  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Canada 
for  1887-88,  new  ser.,  vol.  3,  pp.  1-196  f,  with  2  maps  and  8  plates.  See  also  The  Archean  geology  of  the  region  north- 
west of  Lake  Superior.  Etudes  sur  les  schistes  cristallins.  Internat.  Geol.  Cong.,  London,  1888,  pp.  66-88.  Geology 
of  the  Rainy  Lake  region,  with  remarks  on  the  classification  of  the  crystalline  rocks  west  of  Lake  Superior;  prelim- 
inary note.     Am.  Jour.  Sci.,  3d  ser.,  vol.  33,  1877,  pp.  473-480. 

Report  on  mines  and  mining  on  Lake  Superior,  by  B.  D.  Ingall.  Ann,  Rept.  Geol.  and  Nat.  Hist.  Surs'ey  Canada 
for  1887-88,  new  ser.,  vol.  3,  pp.  i-131  n,  \vith  2  maps  and  13  plates. 

Tracks  of  organic  origin  in  rocks  of  the  Animikie  group,  by  A.  R.  C.  Selwyn.  Am.  Jour.  Sci.,  3d  ser.,  vol.  39,  1890, 
pp.  145-147. 

The  internal  relations  and  taxonomy  of  the  Archean  of  central  Canada,  by  Andrew  C.  Lawson.  Bull.  Geol.  Soc. 
America,  voL  1,  1890,  pp.  175-194. 

Geology  of  Ontario,  with  special  reference  to  economic  minerals,  by  Robert  Bell.  Rept.  Roy.  Comm.  on  Min. 
Res.  Ontario,  Toronto,  1890,  pp.  1-70. 

Lake  Superior  stratigraphy,  bj'  Andrew  C.  Lawson.     Am.  Geologist,  vol.  7,  1891,  pp.  320-327. 

The  structural  geology  of  Steep  Rock  Lake,  Ontario,  by  Henry  Lloyd  Smyth.  Am.  Jour.  Sci.,  3d  ser.,  vol.  42, 
1891,  pp.  317-331. 

Report  on  the  geology  of  Hunters  Island  and  adjacent  country,  by  W.  H.  C.  Smith.  Ann.' Rept.  Geol.  Survey 
Canada  for  1890-91,  vol.  5,  pt.  1,  G,  1892,  pp.  5-76. 

The  Archean  rocks  west  of  Lake  Superior,  by  W.  H.  C.  Smith.     Bull.  GeoL  Soc.  America,  vol.  4, 1893,  pp.  333-348. 

The  laccolitic  sills  of  the  northwest  coast  of  Lake  Superior,  by  A.  C.  Lawson.  Bull.  Cieol.  and  Nat.  Hist.  Survey 
Minnesota  No.  8,  1893,  pp.  24-48. 

Multiple  diabase  dike,  by  A.  C.  Lawson.     Am.  Geologist,  vol.  13,  1894,  pp.  293-296. 

Note  on  the  Keweenawan  rocks  of  Grand  Portage  Island,  north  coast  of  Lake  Superior,  by  U.  S.  Grant.  Idem, 
pp.  437-438. 

Gold  in  Ontario;  its  associated  rocks  and  minerals,  by  A.  P.  Coleman.  Fourth  Rept.  Bur.  Mines  Ontario,  for 
1894,  sec.  2,  Toronto,  1895,  pp.  3.5-100,  with  2  geologic  maps  of  parts  of  the  Rainy  River  district. 

The  hinterland  of  Ontario,  by  T.  W.  Ciibson.     Idem,  sec.  3,  pp.  124-125. 

The  new  Ontario,  by  Archibald  Blue.     Fifth  Rept.  Bur.  Mines  Ontario,  for  1895-96,  pp.  193-196. 

A  second  report  on  the  gold  fields  of  western  Ontario,  by  A.  P.  Coleman.     Idem,  sec.  2,  pp.  47-106. 

The  anorthosites  of  the  Rainy  Lake  region,  by  A.  P.  Coleman.  Jour.  Geology,  vol.  4,  1896,  pp.  907-911;  Canadian 
Rec.  Sci.,  vol.  7,  1897,  pp.  230-235. 

Malignite,  a  family  of  basic  plutonic  orthoclase  rocks  rich  in  alkalies  and  lime,  by  Andrew  C.  Lawson.  Bull.  Dept. 
Geology  Univ.  California,  vol.  1,  1896,  pp.  337-362,  pi.  18. 

Third  report  on  the  west  Ontario  gold  region,  by  A.  P.  Coleman.  Rept.  Bur.  Mines  Ontario,  vol.  6,  1897,  pp. 
71-124. 

The  Michipicoten  mining  divL^^ion,  by  A.  B.  Willmott.     Idem,  vol.  7,  1898,  pp.  184-206. 

Geology  of  base  and  meridian  lines  in  the  Rainy  River  district,  by  W.  A.  Parks.  Idem,  pp.  161-183,  with 
geologic  map. 

Clastic  Huronian  rocks  of  western  Ontario,  by  A.  P.  Coleman.  Idem,  pp.  151-160;  Bull.  Geol.  Soc.  America, 
vol.  9,  1898,  pp.  223-238. 

Unpublished  field  notes  by  C.  R.  Van  Hise,  1898. 

The  geology  of  the  area  covered  by  the  Seine  River  and  Lake  Shebandowan  map  sheets,  comprising  portions 
of  Rainy  River  and  Thunder  Bay  districts,  Ontario,  by  Wm.  Mclnnes.  Ann.  Rept.  Geol.  Survey  Canada,  vol.  10, 
pt.  H,  1899,  pp.  13-51,  with  geologic  map. 

Copper  regions  of  the  upper  lakes,  by  A.  P.  Colejnan.     Rept.  Bm-.  Mines  Ontario,  vol.  8,  pt.  2,  1899,  pp.  121-174. 

Copper  and  iron  regions  of  Ontario,  by  A.  P.  Coleman.     Idem,  vol.  9,  1900,  pp.  143-191. 

Upper  and  lower  Huronian  in  Ontario,  by  A.  P.  Coleman.     Bull.  Geol.  Soc.  America,  vol.  11,  1900,  pp.  107-114. 

Unpublished  field  notes  by  C.  R.  A'an  Hise  and  J.  Morgan  Clements,  summer  of  1900. 

The  iron  belt  on  Lake  Nipigon,  by  J.  W.  Bain.     Rept.  Biu-.  Mines  Ontario,  vol.  10,  1901,  pp.  212-214. 

Iron  ranges  of  the  lower  Huronian,  by  A.  P.  Coleman.     Idem,  pp.  181-212. 

The  Michipicoten  Huronian  area,  by  A.  B.  Willmott.     Am.  Geologist,  vol.  28,  1901,  pp.  14-19. 

The  Michipicoten  iron  range,  by  A.  P.  Coleman  and  A.  B.  Willmott.  Univ.  Toronto  studies,  geol.  ser..  No.  2, 
1902,  47  pp.     See  also  Rept.  Bur.  Mines  Ontario,  1902,  pp.  152-185. 

Rock  basins  of  Helen  mine,  Michipicoten,  Canada,  by  A.  P.  Coleman.  Bull.  Geol.  Soc.  America,  vol.  13, 1902,  pp. 
293-304. 


HISTORY  OF  GEOLOGIC  WORK  IN  THE  REGION.  83 

Nepheline  and  other  syenites  near  Port  Coldwell,  Ontario,  by  A.  P.  Coleman.  Am.  Jour.  Sci.,  4th  ser.,  vol.  14, 
1902,  pp.  147-155.     See  also  Rept.  Bur.  Mines  Ontario,  1902,  pp.  208-213. 

Region  southeast  of  Lac  Seul,  by  William  Mclnnes.     Summary  Rept.  Geol.  Survey  Canada  for  1901-2,  pp.  87-93. 

The  country  west  of  Nipigon  Lake  and  River,  by  Alfred  W.  G.  Wilson.     Idem,  pp.  94-103. 

The  country  east  of  Nipigon  Lake  and  River,  by  W.  A.  Parks.     Idem,  pp.  103-107. 

Iron  ranges  of  northwestern  Ontario,  by  A.  P.  Coleman.     Rept.  Bur.  Mines  Ontario,  1902,  pp.  128-151. 

Iron  ranges  of  northern  Ontario,  by  W.  G.  Miller.     Idem,  1903,  pp.  304-317. 

Region  lying  northeast  of  Lake  Nipigon,  by  W.  A.  Parks.  Summary  Rept.  (ieol.  SlU'vey  Canada  for  1902-3,  pp. 
211-220. 

Region  on  the  northwest  side  of  Lake  Nipigon,  by  William  Mclnnes.     Idem,  pp.  206-211. 

Nepheline  syenite  in  western  Ontario,  by  W.  G.  Miller.     Am.  Geologist,  vol.  32,  1903,  pp.  182-185. 

Genesis  of  the  Animikie  iron  range,  Ontario,  by  F.  Hille.    Jour.  Canadian  Min.  Inst.,  vol.  6,  1904,  pp.  245-287. 

The  Animikie  or  Loon  Lake  iron-bearing  district,  by  W.  N.  Smith  (in  charge  of  a  party  consisting  of  A.  W.  Lewis, 
J.  U.  Warner,  G.  W.  Crane,  and  R.  C.  Allen).     Min.  World,  vol.  22,  1905,  pp.  206-208,  with  geologic  map. 

Iron  ranges  of  Michipicoten  West,  by  J.  M.  Bell.  Rept.  Bur.  Mines  Ontario,  vol.  14,  1905,  pt.  1,  pp.  278-355, 
with  geologic  map.  See  also  The  possible  granitization  of  acidic  lower  Iluronian  schists  on  the  north  shore  of  Lake 
Superior.     Jour.  Geology,  vol,  14,  1906,  pp.  233-242. 

The  geology  of  Michipicoten  Island,  by  E.  N.  Burwash.  Univ.  Toronto  studies,  geol.  ser..  No.  3,  Toronto,  1905, 
with  map. 

Pre-Cambrian  nomenclature,  by  A.  P.  Coleman.    Jour.  Geology,  vol.  14,  1906,  pp.  60-64. 

The  Animikie  iron  range,  by  L.  P.  Silver.     Rept.  Bur.  Mines  Ontario,  vol.  15,  1906,  pt.  1,  pp.  156-172. 

Iron  ranges  east  of  Lake  Nipigon,  by  A.  P.  Coleman.  Sixteenth  Ann.  Rept.  Bur.  Mines  Ontario,  1907,  pt.  1, 
pp.  105-135. 

Iron  ranges  eaat'of  Lake  Nipigon,  the  ranges  around  Lake  Windebegokan,  by  E.  S.  Moore.     Idem,  pp.  136-148. 

Iron  ranges  of  Nipigon  district,  by  A.  P.  Coleman.  Eighteenth  Ann.  Rept.  Bur.  Mines  Ontario,  1909,  pt.  1, 
pp.  141-153. 

Iron  range  north  of  Round  Lake,  by  E.  S.  Moore.     Idem,  pp.  154-162. 

Geology  of  Onaman  iron  range  area,  by  E.  S.  Moore.     Idem,  pp.  196-253. 

The  quartz  diabases  of  the  Nipissing  district,  Ontario,  by  W.  H.  Collins.  Econ.  Geology,  vol  5,  1910,  pp. 
538-550. 

Diabase  and  granophyre  of  the  Gowganda  Lake  district,  Ontario,  by  Norman  L.  Bowen.  Jour.  Geology,  vol.  18, 
1910,  pp.  658-674. 

LAKE   SUPERIOR   REGION  (GENERAL). 

Narrative  journal  of  travels  through  the  northwestern  regions  of  the  United  States,  extending  from  Detroit  through 
the  great  chain  of  American  lakes  to  the  sources  of  the  Mississippi  River,  by  Henry  R.  Schoolcraft.  Albany,  1821, 
419  pp.,  with  map. 

Report  of  Walter  Cunningham,  late  mineral  agent  on  Lake  Superior,  January  8,  1845.  Senate  Docs.,  2d  sess.  28th 
Cong.,  1844-^5,  vol.  7,  No.  98,  5  pp. 

Mineral  report,  by  George  N.  Sanders.     Idem,  No.  117,  pp.  3-9. 

Report  of  J.  B.  Campbell.     Idem,  vol.  11,  No.  175,  pp.  4-8. 

Report  of  George  N.  Sanders.     Idem,  pp.  8-14. 

Report  of  A.  B.  Gray.     Idem,  pp.  15-22. 

Report  of  A.  B.  Gray  on  mineral  lands  of  Lake  Superior.  Executive  Docs.,  1st  sess.  29th  Cong.,  1845^6,  vol.  7, 
No.  211,  23  pp.,  with  map. 

On  the  origin  of  the  actual  outlines  of  Lake  Superior  (discussion),  by  William  B.  Rogers.  Proc.  Am.  Assoc.  Adv. 
Sci.,  1st  meeting,  1848,  pp.  79-80. 

The  outlines  of  Lake  Superior,  by  Louis  Agassiz.  Lake  Superior;  its  physical  character,  vegetation,  and  animals 
compared  with  those  of  other  and  similar  regions,  by  Louis  Agassiz  and  J.  Elliot  Cabot,  pp.  417-426.  See  also  Proc. 
Am.  Assoc.  Adv.  Sci.,  1st  meeting,  1848,  p.  79. 

Abstract  of  an  introduction  to  the  final  report  of  the  geological  siu'veys  made  in  Wisconsin,  Iowa,  and  Minnesota, 
in  the  years  1847,  1848,  1849,  and  1850,  containing  a  synopsis  of  the  geological  featm-es  of  the  country,  by  Da%'id  D. 
Owen.     Proc.  Am.  Assoc.  Adv.  Sci.,  vol.  5,  1851,  pp.  119-131. 

On  the  age,  character,  and  true  geological  position  of  the  Lake  Superior  red  sandstone  formation,  by  Da^dd  D. 
Owen.     Report  of  a  geological  survey  of  Wisconsin,  Iowa,  and  Minnesota,  1852,  pp.  187-193. 

Report  of  a  geological  survey  of  Wisconsin,  Iowa,  and  Minnesota,  and,  incidentally,  of  a  portion  of  Nebraska 
Territory,  made  under  instructions  from  the  United  States  Treasury  Department,  by  David  D.  Owen.    1852,  638  pp. 

A  geological  map  of  the  United  States  and  the  British  Provinces  of  North  America,  with  an  explanatory  text, 
geological  sections,  etc,  by  Jules  Marcou,  Boston,  1853,  92  pp.  See  also  Reponse  a  la  lettre  de  MM.  Foster  et  Whit- 
ney sur  le  Lac  Superieur.     Bull.  Soc.  g^ol.  France,  2d  ser.,  vol.  8,  1851,  pp.  101105.- 

The  metallic  wealth  of  the  United  States,  by  J.  D.  Wliitney.     Philadelphia,  1854,  510  pp. 

Observations  on  the  geology  and  mineralogy  of  the  region  embracing  the  sources  of  the  Mississippi  River,  and  the 
Great  Lake  basins,  during  the  expedition  of  1820,  by  Henry  R.  Schoolcraft.     Summary  narrative  of  an  exploratory 


84  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

expedition  to  the  sources  of  tlie  Mississippi  River  in  1820,  resumed  and  rompleted  by  the  discovery  of  its  orijjin  in 
Itasca  Lalce  in  1832.     Philadelphia,  1854,  pp.  303-362. 

Remarks  on  some  points  connected  with  the  geology  of  the  north  shore  of  Lake  Superior,  by  J.  D.  Whitney.  Proc. 
Am.  Assoc.  Adv.  Sci.,  vol.  9,  1856,  pp.  204-209. 

On  the  occurrence  of  the  ores  of  iron  in  the  Azoic  sy.stem,  by  J.  D.  Whitney.     Idem,  pp.  209-216. 

Remarks  on  the  Iluronian  and  Laurentian  systems  of  the  Canada  Geological  Survey,  by  J.  D.  \\1iitney.  Am. 
Jour.  Sci.,  2d  ser.,  vol.  23,  1857,  pp.  305-314. 

Physical  geology  of  Lake  Superior,  by  Charles  ^\liittlesey.  Proc.  Am.  Assoc.  Adv.  Sci.,  vol.  24,  1876,  pt.  2,  pp. 
60-72,  mth  map. 

The  copper-bearing  rocks  of  Lake  Superior,  by  R.  D.  Irving.  Mon.  U.  S.  Geol.  Survey,  vol.  5,  1883,  464  pp. 
15  1.,  29  pis.  and  maps.  See  also  Third  Ann.  Rept.  U.  S.  Geol.  Survey,  1883,  pp.  89-188,  15  pis.  and  maps;  Science, 
vol.  1,  1883,  pp.  140,  359,  422;  Am.  Jour.  Sci.,  3d  ser.,  vol.  28,  1884,  p.  462;  vol.  29,  1885,  pp.  67-68,  2-58-259,  339-340. 

The  copper-bearing  series  of  Lake  Superior,  by  T.  C.  Chamberlin.     Science,  vol.  1,  1883,  pp.  453—1.55. 

On  secondary  enlargements  of  mineral  fragments  in  certain  rocks,  by  R.  D.  Irving  and  C.  R.  Van  Ilise.  Bull. 
U.  S.  Geol.  Survey  No.  8,  1884,  56  pp.,  6  pis. 

Di\'isibility  of  the  Archean  in  the  Northwest,  by  R.  D.  Irving.     Am.  Jour.  Sci.,  3d  ser.,  vol.  29,  1885,  pp.  237-249. 

Preliminary  paper  on  an  investigation  of  the  Archean  formations  of  the  Northwestern  States,  by  R.  D.  Irving. 
Fifth  Ann.  Rept.  U.  S.  Geol.  Survey,  1885,  pp.  175-242,  10  pis. 

Origin  of  the  ferruginous  schists  and  iron  ores  of  the  Lake  Superior  region,  by  R.  D.  Irving.  Am.  Jour.  Sci.,  3d 
ser.,  vol.  32,  1886,  pp.  255-272. 

Is  there  a  Iluronian  group?  by  R.  D.  Irving.     Am.  Jour.  Sci.,  3d  ser.,  vol.  34,  1887,  pp.  204-216,  249-263,  365-374. 

A  great  Primordial  quartzite,  by  N.  H.  Winchell.  Am.  Geologist,  vol.  1,  1888,  pp.  173-178.  See  also  Seventeenth 
Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  for  1888,  pp.  25-56. 

On  the  classification  of  the  early  Cambrian  and  pre-Cambrian  formations,  by  R.  D.  Irving.  Seventh  Ann.  Rept. 
U.  S.  Geol.  Survey,  1888,  pp.  365-454,  with  22  pis.  and  maps. 

The  iron  ores  of  the  Penokee-Gogebic  series  of  Michigan  and  Wisconsin,  by  C.  R.  Van  Ilise.  Am.  Jour.  Sci.,  3d 
ser.,  vol.  37,  1889,  pp.  32^8,  with  plate. 

An  attempt  to  harmonize  some  apparently  conflicting  views  of  Lake  Superior  stratigraphy,  by  C.  R.  Van  Hise. 
Idem,  vol.  41,  1891,  pp.  117-137. 

The  Norian  rocks  of  Canada,  by  A.  C.  Lawson.     Science,  vol.  21,  1893,  pp.  281-282. 

The  Norian  of  the  Northwest,  by  N.  H.  Winchell.  Bull.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  No.  8,  1893, 
pp.  iii-xxii. 

An  historical  sketch  of  the  Lake  Superior  region  to  Cambrian  time,  by  C.  R.  Van  Hise.  Jour.  Cieology,  vol.  1,  1893, 
pp.  113-128,  with  geologic  map. 

■  Crucial  points  in  the  geology  of  the  Lake  Superior  region,  by  N.  11.  Winchell.  Am.  Geologist,  vol.  15,  1895,  pp. 
153-162,  229-234,  295-304,  356-363;  vol.  16,  1895,  pp.  12-20,  75-«6,  150-162,  269-274,  331-337.  See  also  Compt.  Rend. 
Congrfes  gfol.  intemat.,  6th  sess.  (1894),  1897,  pp.  273-308. 

Pre-Cambrian  fossiliferous  formations,  by  Charles  D.  Walcott.     Bull.  Geol.  Soc.  America,  vol.  10, 1899,  pp.  199-244. 

The  iron-ore  deposits  of  the  Lake  Superior  region,  by  C.  R.  Van  Hise,  assisted  in  Mesabi  and  Vermilion  sections  by 
C.  K.  Leith  and  J.  Morgaii  Clements,  respectively.  Twenty-first  Ann.  Rept.  U.  S.  Geol.  Survey,  pt.  3,  1901,  pp.  305- 
434,  with  geologic  maps. 

Geological  work  in  the  Lake  Superior  region,  by  C.  R.  Van  Hise.  Proc.  Lake  Superior  Min.  Inst.,  vol.  7,  1902, 
pp.  62-69. 

The  original  source  of  the  Lake  Superior  iron  ores,  by  J.  E.  Spurr.     Am.  Geology,  vol.  19,  1902,  pp.  335-349. 

A  comparison  of  the  origin  and  development  of  the  iron  ores  of  the  Mesabi  and  Gogebic  iron  ranges,  by  C.  K.  Leith. 
Proc.  Lake  Superior  Min.  Inst.,  vol.  7,  1902,  pp.  75-81. 

The  Eparchean  interval ;  a  criticism  of  the  use  of  the  term  Algonkian,  byAndrew  C.  Lawson.  Bull.  Dept.  Geology 
Univ.  California,  vol.  3,  1902,  pp.  51-62. 

The  Iluronian  question,  by  A.  P.  Coleman.     Am.  Geology,  vol.  29,  1902,  pp.  325-334. 

The  nomenclature  of  the  Lake  Superior  formations,  by  A.  B.  Willmott.    Jour.  Geology,  vol.  10,  1902,  pp.  67-76. 

Report  of  the  special  committee  for  the  Lake  Superior  region,  by  C.  R.  Van  Hise  and  others.  Jour.  Geologj-. 
vol.  13,  1905,  pp.  89-104;  Rept.  Ontario  Bur.  Mines,  vol.  14,  pt.  1,  1905,  pp.  269-277;  Rept.  Geol.  Survey  Michigan 
for  1904,  1905,  pp.  133-143. 

Report  of  the  special  committee  for  the  Lake  Superior  region,  personal  comments,  by  A.  C.  Lane.  Ann.  Repl. 
Geol.  Survey  Michigan  for  1904,  1905,  pp.  143-153.  See  also  Comment  on  the  report  of  the  special  committee  on  the 
Lake  Superior  region,  Jour.  Geology,  vol.  13,  1905,  pp.  457-461. 

A  summary  of  Lake  Superior  geology  with  special  reference  to  recent  studies  of  the  iron-bearing  series,  by  C.  K. 
Leith.     Trans.  Am.  Inst.  Min.  Eng.,  vol.  36,  1906,  jip.  101-153,  with  geologic  map. 

The  movement  of  Lake  Superior  iron  ores  in  1909,  with  a  map  showing  distribution  of  ores,  by  John  Birkinbine. 
Advance  chapter  from  Mineral  Resources  U.  S.  for  1909,  U.  S.  Geol.  Survey,  1910,  7  l)p. 

\n  Algonkian  basin  in  Hudson  Bay — a  comparison  with  the  Lake  Superior  basin,  by  C.  K.  Leith.  Ecou.  Geol- 
ogy, vol.  5,  1910,  pp.  227-240. 


CHAPTER  IV.  PHYSICAL  GEOGRAPHY  OF  THE  LAKE  SUPERIOR 

REGION. 


By  Lawkkxce  Martin. 


TOPOGRAPHIC  PROVINCES. 

The  Lake  Superior  region  as  described  in  this  report  inchides  three  topos^rapliic  provinces 
(fig.  5) — (1)  the  Lake  Superior  highlands,  a  peneplain  with  hilly  upland  and  lowland  subdi- 
visions; (2)  a  series  of  lowland  plains  surrounding  the  peneplain  on  the  east,  south,  and  west; 
and  (3)  the  deep  basin  of  Lake  Superior  embraced  between  parts  of  the  liighland  and  the  low- 
land. These  three  topographic  provinces  are  in  various  stages  of  development  and  preservation, 
depending  on  the  underlying  rock  structure,  the  process  by  which  they  are  being  modified,  and 
the  length  of  their  period  of  development.  The  first  consists  essentially  of  Archean  and  Algon- 
kian  rocks;  the  second  of  Cambrian  and  other  early  Paleozoic  rocks  and  of  Cretaceous  rocks; 
the  third  is  a  present  seat  of  rock  deposition,  and  probably  includes  rocks  of  all  ages  represented 
in  the  other  provinces,  in  addition  to  the  glacial  drift  of  the  Quaternary,  which  also  partly  man- 
tles the  rocks  in  the  first  province  and  almost  completely  buries  those  of  the  second. 

The  peneplain  liighland  was  worn  down  from  former  lofty  mountains."  Diastropliism  (warp- 
ing, folding,  and  faulting)  has  notably  modified  the  peneplain,  tilting  its  borders  and  introducing 
the  deep  basin  of  Lake  Superior.  (See  PI.  II.)  Subsequent  deposition  of  early  Paleozoic  and 
Cretaceous  rocks  in  the  Lake  Superior  basin  and  about  the  margin  of  the  peneplam  (see  fig.  5) 
has  been  followed  by  the  exhuming  of  fossil  topography  and  the  production  of  a  belted  plain  with 
alternate  uplands  and  lowlands  in  the  region  of  horizontal  and  gently  tilted  post-Algonkian 
rocks.  Continental  glaciation  has  slightly  modified  the  relief  and  completely  altered  the  soil  and 
drainage  of  the  region  (Chapter  XVI,  pp.  427-459). 

THE  LAKE  SUPERIOR  HIGHLANDS. 
.  TOPOGRAPHIC  DEVELOPMENT. 

The  highlands  about  Lake  Sujierior  fall  into  two  classes — (1)  those  underlain  by  coarse- 
grained homogeneous  rocks,  chiefly  igneous,  of  both  Archean  and  Algonkian  age,  and  (2)  those 
underlain  by  banded  (both  areally  and  structurally)  alternating  weak  and  resistant  tilted  rocks, 
chiefly  sediments  and  lavas  of  Algonkian  age.  The  areas  of  homogeneous  igneous  rocks  still 
preserve  plateaus  or  liigh  plains  of  slight  relief,  diversified  only  by  monadnocks  and  by  some 
valleys  of  greater  than  normal  depth;  the  areas  including  belts  of  sediments  have  narrow  pla- 
teaus, monoclinal  ridges,  and  mesas  isolated  among  broader  .intermediate  lowlands. 

It  is  possible  that  the  whole  highland  area  was  reduced  to  a  peneplain,  now  represented  by  the 
plateau  surfaces,  the  crests  of  some  of  the  higher  monoclinal  ridges,  and  the  tabular  surfaces 
of  tlie  higher  mesas,  none  of  the  adjacent  lowland  areas  having  been  down-warped  or  down- 
faulted  or  excavated  when  the  peneplain  was  most  nearly  perfected. 

c  Van  Hise,  C.  R .,  Science,  new ser.,  vol .  4, 1896, pp.  57-59  and  217-220;  Weidman,  Samuel,  Jour.  Geology,  vol.  11, 1903,  pp.  289-313;  Wilson,  A.  W.  G., 
Jour.  Geology,  vol.  11, 1903,  pp.  015-667;  Weidman,  Samuel,  Bull.  Wisconsin  Geol.  and  Nat.  Hist.  Survey  No.  16, 1907,  pp.  592-603  and  385-395. 

85 


86 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


Diastropliism  during  post-Algonldan  time,  by  changing  the  altitude  of  tlie  peneplain  with 
reference  to  base-level,  enabled  demidiitioti  to  reattack  this  peneplain.  Stream  erosion  was 
renewed  actively  along  the  fault  escarpments,  jjossibly  being  delayed  in  areas  that  had  been 
submerged  and  buried  Ijy  Paleozoic  sediments  (p.  116).  This  renewal  of  cutting  was  weak  or 
not  yet  active  at  all  in  regions  remote  from  the  escarpments  (here  also  possibly  being  delayed 
in  the  buried  and  protected  parts),  but  was  strongest  in  the  areas  of  banded  Algonkiau  rocks, 
especially  those  near  the  steeper  slopes.  In  these  areas  of  banded  rocks  the  remnants  of  the 
original  peneplain  surface  are  small  and  scattered,  being  largest  where  the  vertical  beds  resisted 
erosion  best,  smaller  wliere  gentle  tilting  made  development  of  monoclinal  ridges  and  interme- 
diate valleys  possible,  and  of  least  extent  where  horizontal  beds  allowed  the  opening  of  broad 
lowlands  with  only  isolated  mesas,  as  in  the  Thunder  Bay  region,  or  with  protruding  reexposed 
knobs,  like  the  Baraboo  range  of  Wisconsin  and  knobs  north  and  east  of  it  (figs.  5.3,  54,  pp.  359, 
360).  The  lowlands  developed  at  several  points  may  be  incipient  stages  of  a  peneplain  of  a 
later  generation,  developed  with  respect  to  a  much  lower  base-level. 

The  older  penejilain  surface  is  found  at  various  altitudes,  some  of  which  are  shown  in  the  fol- 
lowing table; 

Altitude  of  different  parts  of  the  Lake  Superior  hir/hlands. 


Localitv. 


Average 
height 

above  sea 
level.a 


Feet. 

Southeast  of  Michipicoten 1.500—1,000 

Near  Mic-hipkoten 'l. 2(»^1, 400 

Northwest  of  Michipicoten ,*1, 200— 1,400 

Near  Heron  Bay «!.10O-l,.3a0 

North  of  Lake  Superior *    900—1,050 

West  of  Lalte  Nipigon «1. 2.50— 1,500 

Thunder  Bay  and  Hunters  Island  region *1, 400— 1, 700 

Rainy  Lake  and  Lake  of  the  Woods  region *1.200— 1, 400 

Gunfiint  Lake ,  1,800-2,000 

VermiUon  district I  1.  IM>— 1 .  700 

Mesabi  district j  1,400—1.500 

Gabbro  plateau 1,400—1,700 

Northern  Wisconsin *I, 400— 1,500 

Keweenaw  Point About  1..350 

Marquette  dLstrict 1,400-1,600 

Crystal  Fails  district '  1, 400— 1, 600 

Menominee  district 1  1 .  200 — 1 ,  400 

North-central  Wisconsin '  1,  .300— 1 ,  500 

Edge  of  Potsdam  sandstone *About  1, 000 


Highest 
hill. 


Feet. 


1,700 
2,120 


Lowest 
valley. 


Feet. 


±  1,100 


1,700 
2  232 
i!910 
1,920 
2, 320 
1,900 
1,409 
*1,950 
1,900 
1,370 
1.940 


1,072 
1,547 
1.300 
1.400 
1.400 
1,400 


±  i.mo 

1,120 

800 

1,100 


«  Altitudes  marlced  with  an  asterisk  are  accurate  approximations  based  upon  railway  grades,  etc.     All  other  altitudes  are  averaged  from 
accurate  topographic  maps. 

It  will  be  noted  (figs.  4  and  5)  that  the  general  peneplain  surface  lies  between  1,000  and 
1,700  feet,  though  it  is  a  trifle  low^er  locally,  and  rises  in  monadnocks  to  exceptional  heights  of 
a  little  more  than  2,300  feet.  The  maximum  relief  of  the  peneplain  proper  (excluding  the  basin 
of  Lake  Superior)  is  less  than  1,450  feet  (900  to  2,320),  and  these  extremes  are  many  miles  apart. 
The  maximum  local  relief  of  any  part  of  the  peneplain  at  the  time  of  its  greatest  perfection  may 
be  quite  safely  placed  between  400  and  500  feet,  and  the  average  relief  would  be  much  less, 
perhaps  100  to  200  feet. 

The  present  differences  of  elevation  in  the  peneplain  remnants  might  be  explainetl  as 
inherited,  for  the  writer  does  not  conceive  of  peneplains  as  approacliing  at  all  closely  to  a  plane 
or  perfectly  base-leveled  surface.  Possibly  the  peneplain  in  the  Lake  Superior  region  when 
most  nearly  perfect  stood  at  levels  perhaps  corresponding  to  present  elevations  of  1,400  feet 
in  central  Wisconsin,  1,350  feet  on  Keweenaw  Point,  1,600  feet  in  northeastern  Minnesota,  and 
1,400  feet  northeast  of  Lake  Superior  in  Canada,  etc.  Because  there  was  upon  the  well- 
developed  peneplain  a  series  of  old  streams  whose  valleys  laj-  at  lower  levels  than  the  low 
intermediate  ridges  and  at  slighth'  different  levels  with  reference  to  one  another,  the  surface 
beveled  back  smoothly  up  the  stream  courses  antl  the  Unes  <lividing  parallel  drainage  systems. 
As  we  do  not  know  where  these  ancient  trunk  streams  were,  we  must  regartl  the  various  preserved 
peneplain  fragments  merely  as  parts  of  a  lowland  worn  down  where  mountains  hat!  been;  and 


PHYSICAL  GEOGRAPHY  OF  THE  REGION. 


87 


it  is  quite  unnecessary  to  assume  warping  to  account  for  their  discrepancies  of  level,  as  has 
been  clone  with  regard  to  numerous  penei)hiins,  though  warping  in  this  region  is  indicated  on 
other  grounds. 

The  chief  evidence  of  diastrophic  modification  qf  the  levels  of  the  penepla,in  is  the  rift 
or  graben  faulting  indicated  by  displacements  and  by  the  great  escarpments  and  their  drainage 
coniHtions.  (See  p.  113.)  One  such  modification  of  the  peneplain  took  place  when  portions 
of  it  on  the  site  of  the  west  half  of  the  present  Lake  Superior  were  down  faulted. 

We  have  excellent  evidence  that  the  peneplain  has  been  modified  by  warping.  There  are 
three  suggestive  contlitions:  (1)  In  Wisconsin  the  peneplain  dips  down  under  the  Paleozoic 
cover,"  being  1,000  feet  above  sea  level  at  Grand  Rapids  and  500  feet  at  Kilbourne,  or  385 


75  100  125  150  MILES 


Elevation  above  sea  level 


^ 


E3 


5B0-I000ft.  1000-1700  ft.         Above  1700  ft.       Mississippi-St.Lav^rence- 

Hudson  Bay  divides 

Figure  4. — Generalized  topographic  map  of  the  Lake  Superior  region. 

feet  below  the  surface,  one  of  its  monadnocks  rising  through  the  Cambrian  sandstone  in  the 
Baraboo  range,  wliile  at  Madison  its  surface  hes  70  feet  above  sea  level,  or  810  feet  below  the 
present  surface;  (2)  the  gradients  of  the  peneplain  surface,  especially  in  central  Wisconsin, 
are  greater  than  would  be  normal  in  aged  rivers  on  a  peneplain;  (3)  the  Paleozoic  rocks  are  in 
such  positions  as  almost  to  prove  warping,  for  a  broad  north-south  post-Cambrian  anticline 
is  recognized  in  Wisconsin.  All  these  suggestive  conditions  are  corroborated  by  the  well- 
estabhshed  fact,  to  be  described  in  the  chapter  on  the  Pleistocene,  that  tilting  of  the  originally 
horizontal  shore  Unes  of  former  glacial  lakes  definiteh-  proves  shght  recent  warping  of  the 
region.  The  fact  that  such  warping  has  been  and  is  still  taking  place  is  ailecpiate  ground  for 
sajing  that  the  peneplain  remnants  are  not  at  their  original  levels. 


n  Weidman,  Samuel,  Jour.  Geology,  vol.  U,  1903,  pp.  300-307;  Bull.  Wisconsin  Geol.  and  Nat.  Uist.  Survey  No.  IC,  1907,  pp.  393-394. 


88 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


The  peneplain  might  be  conceived  to  represent  facets  of  one  or  more  earlier  peneplains, 
but  tliis  does  not  seem  likelj''  unless  the  main  peneplain  is  Cretaceous  and  parts  of  it  represent 
preserved  facets  of  a  late  .ygonldan  or  early  Cambrian  peneplain.  Earher  possible  peneplain 
levels — ^in  the  Huronian,  for  example — would  have  been  warped  or  folded  by  pre-Algonkian 
deformation  from  their  original  nearly  horizontal  position  to  almost  any  conceivable  angle. 
The  several  great  unconformities  of  the  region  tloubtless  represent  peneplain  stages,  and  the 
very  fine  material  deposited  after  certain  unconformities  also  suggests  a  low  gradient  of  rivers 
and  a  lack  of  coarse  sediments — conditions  characteristic  of  a  nearly  base-leveleil  region.  Some 
of  these  unconformities,  now  exposed  by  denudation,  reach  the  surface  at  low  angles,  but  it 
does  not  follow  that  a  renmant  of  a  lower  Huronian  peneplain  is  anj-where  visible.     In  v\e\v 


Peneplain  Monoclinal  ridges  Monadnocks  Mesas 

Figure  5. — The  topographic  provinces  of  the  Lake  Superior  region,  with  some  subdivisions  of  the  peneplain. 

of  the  tremendous  pre-Cambrian  base-leveling,  any  such  surface,  in  the  writer's  opinion,  should 
be  regarded  as  either  still  buried  or  else  long  ago  eroded  away,  unless  definite  evidence  to  the 
contrary  can  be  produced.  If  the  peneplain  is  not  Cretaceous  but  a  dissected  late  Algonkian 
or  earl}'  Cambrian  peneplain,  it  seems  hardly  likely  that  any  facets  of  its  surface  represent 
earlier  base-leveHng. 

The  age  of  the  Lake  Superior  peneplain,  where  studied  in  parts  of  the  area,  has  been  tenta- 
tively suggested  by  Van  Hise  to  be  Cretaceous."  Weidman  dates  the  Wisconsin  i)art  ()f  it  as 
pre-Potsdam,*  apparently  recognizing  it  beneath  the  first  Paleozoic  rocks  (Potsdam  or  Ipper 
Cambrian)  in  Wisconsin.  The  LaiU'cntian  peneplain  described  by  A.  W.  G.  Wilson  ■■  docs  not 
include  the  Keweenawan  areas  of  northeastern  Minnesota,  Isle  Royal,  northern  Wisconsin, 
and  Keweenaw  Point,  and  therefore  represents  for  our  area  merelj^  the  possibility  of  the  several 

a  Science,  new  ser.,  vol.  -1,  1S9U,  pp.  59  and  220:  Twenty-first  .\nn.  Kept.  U.  S.  Geol.  Survey,  pt.  3, 1S.S9-1900,  pp.  333-336. 
t  Jour.  Oeology,  vol.  U,  1903,  p.  310;  Bull.  Wisc-onsin  Geol.  and  Nat.  Hist.  Siurey  No.  16,  p.  388. 
c  Jour.  Geology,  vol.  11, 1903,  pp.  615-669. 


U.  S.  GEOLOGICAL  SURVEY 


MONOGRAPH   Lll     PL.  Ill 


A.     PRE-CAMBRIAN     PENEPLAIN   IN     ONTARIO,     NEAR     MICHIPICOTEN. 

See  page  89. 


Jl.     JASPER     PEAK,     NEAR     TOWER,     MINN. 
A  monadnock  rising  above  tlie  even  upland  of  the  Pre-Cambi  ian  peneplain.     See  page  90. 


PHYSICAL  GEOGRAPHY  OF  THE  REGION. 


89 


pre-Keweenawan  (Huronian  and  Archean)  peneplains.  The  more  specific  fixirlg  of  tlie  age  of 
tlie  whole  Lake  Superior  peneplain  depends  largely  on  the  age  of  certain  escarpments  and  of 
certain  faults  and  on  the  overlap  of  certain  sediments;  the  trend  of  the  evidence  (see  dis- 
cussion of  basin  of  Lake  Superior)  suggests  an  early  origin  of  the  peneplain,  perhaps  late  AJgon- 
kian  or  early  Cambrian.  It  is  not  conclusively  estabUshed  that  a  later  peneplain,  perhaps 
Cretaceous,  was  developed  in  the  area,  though  the  regions  of  low  relief  close  to  or  witliin  the 
basin  of  Lake  Superior  ma\'  be  Cretaceous — as,  for  example,  the  lowlands  of  central  jVIiimesota 
and  eastern  upper  ^licliigan. 

THE   BROAD   UPLANDS. 

POSITION,  BELIEF,  AND  SKY  LINE. 

The  Lake  Superior  highlands  form  a  broad  upland  cut  by  valley's  and  tliversified  by  monad- 
nocks  and  other  ridges.  The  upland  is  made  up  cliiefly  of  the  Aix-hean,  but  most  of  its  ridges 
and  monadnocks  are  composed  of  rocks  of  Algonkian  age.  This  is  so  because  the  Archean 
rocks  in  tliis  region  are  chiefly  granites,  greenstones,  and  other  coarse-grained  rocks,  together 
with  scliists  and  gneisses,  most  of  which  are  homogeneous  over  broad  areas  in  their  resistance 
to  weathering  and  erosion  and  at  present  generally  still  preserve  the  peneplain  developed  upon 
them;  whereas  in  the  Algonkian  areas,  because  of  folding  and  faulting,  the  Huronian  scliists, 
gneisses,  quartzites,  etc.,  anti  the  Keweenawan  lavas  usuall^^  present  homogeneous  resistance  to 
weathering  and  erosion,  not  over  broad  areas  but  in  narrow  hnear  belts,  so  that  the  former  pene- 
plain is  dissected  in  these  regions  to  hilly  uplands  and  lowlands  with  notable  ridge  topography. 
There  are  some  exceptions,  however,  in  the  iUgonkian — for  example,  where  the  homogeneous, 
coarse-grained  Duluth  gabbro  of  the  Keweenawan  northeast  of  Duluth  and  the  similar  granite 
of  northern  Wisconsin,  which  is  possibty  lower  Huronian  rather  than  Ai-chean,  form  broad 
uplands. 

These  broad  pre-Cambrian  uplands  stand  above  the'adjacent  relatively  lower  plains  of  the 
Paleozoic  and  Cretaceous  and  above  the  deep  basm  of  Lake  Superior  at  an  average  height  of 
about  1,350  feet  above  sea  level  (fig.  4).  Their  local  relief  is  slight.  The  following  elevations 
in  representative  areas  are  taken  from  topographic  maps: 

Northeast-southwest  section  along  the  Vermilion  iron  range  in  northeastern  Minnesota. 


Hilltops 

Valley  bottoms 

East-west  section,  west  of  Marquette,  Mich. 

Hilltops 

Valley  bottoms 


Feet. 
1.1)20 
1,380 


Feet. 

i,eso 

1,300 


Feet. 
1,800 
1,480 


Feet. 
2,120 
1,760 


Feet. 
1,700 
1,550 

Feet. 
1,750 
1,550 

Feet. 
1,800 
1,500 

Feet. 
1,550 
1,350 


East-west  section  in  north-central  Wisconsin. 


Hilltops 

Valley  bottoms. 


Feet. 
1,340 
1,200 


Feet. 
1,412 
1,200 


Feet. 
1,440 
1,100 


Feet. 
1,460 
1,300 


It  will  be  seen  that  the  average  local  relief  here  is  about  240  feet.  A  few  hills  rise  slightly 
above  the  general  level  and  many  valleys  are  cut  sliglitly  below  it,  but  from  an  emmence  an 
observer  views  a  region  of  slight  relief  with  an  even  sky  line.     (See  PI.  Ill,  A.) 

RELATION  OF  ORIGINAL  AND  PRESENT  TOPOGRAPHY. 


It  is  of  interest  now  to  compare  the  present  surface,  which  bevels  indifferently  across 
structural  lines,  with  the  surface  which  must  have  existed  when  most  of  the  Archean  and 
Algonkian  rocks  received  their  present  texture  and  structure.     The  granite  and  similar  rocks 


90  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

could  have  been  made  coarse  grained  only  by  cooling  under  a  hea\-j-  mantle  of  overlying  rock. 
(See  lig.  11,  p.  116,  and  cress  sections  on  PI.  X\'I1,  in  jwcket.)  Evidently  the  surface  when 
the  granite  was  intruded  here  was  far  higher  than  the  present  surface.  The  greenstones,  some 
of  which  cooled  at  the  surface,  arc  truncated  in  such  positions  that  the  original  folds,  if  restored, 
would  extend  lugh  above  the  present  surface  and  deep  into  the  earth  (figs.  7, .p.  101;  35,  p.  253). 
Some  of  the  gneisses  and  schists  contain  ciystals  and  show  structures  such  as  slaty  cleavage 
and  schistosity  which  could  have  been  produced  only  under  a  heavj'  load  of  overlying  rock. 
Restoration  of  the  missing  parts  of  the  folds,  as  revealed  by  study  of  the  structure,  shows  that 
all  the  gneisses  and  schists  are  parts  of  the  arcliitectural  scheme  of  an  edifice  entirely  different 
from  the  present  Lake  Superior  i-egion.  (See  fig.  54,  p.  360,  and  structure  sections  on  Phs.  I, 
VIII,  XVI,  and  XVII,  in  pocket.)  In  all  other  sorts  of  plains  besides  peneplams  the  strata 
normally  lie  nearly  horizontal,  or  nearly  parallel  to  the  surface  of  the  plain.  In  tlie  Lake 
Su])erior  region  the  strata  almost  nowhere  coincide  in  position  with  the  surface,  the  dips  at 
many  places  being  almost  vertical.  The  texture,  the  position,  and  the  relations  of  the  rocks 
are  such  as  are  found  in  existing  mountainous  regions.  Evidently  this  peneplain  was  anciently 
a  region  of  lofty  mountains." 

MONADNOCKS. 

In  some  parts  of  the  region  knobs  or  monadnocks  (fig.  5)  rise  conspicuously  above  the 
penejilain  surface.  None  of  them  is  of  great  area  or  of  great  height.  In  fact,  many  of  them 
would  not  be  noticeable  if  it  were  not  for  the  evenness  of  the  general  upland  surface  of  the 
region.  Of  these  monadnocks,  Jasper  Peak  (1,710  feet),  near  Tower,  Minn.,  is  a  good  example 
(PI.  Ill,  B)  and  will  be  described  as  typical  of  the  class.  Other  monadnocks  are  Minnesota 
Hill,  at  Soudan,  Minn.;  the  2,'230-foot  peak  among  the  Misquah  Hills  in  Cook  County,  Minn., 
the  highest  in  the  Lake  Superior  region;  Eagle  Mountain  and  Brule  Mountain,  in  the  same 
region;  Tiptop  Mountain  (2,122  feet),  northwest  of  the  Michipicoten  district,  probably  the 
liighest  in  Ontario;  Hematite  Mountain  (1,700  feet),  at  the  Helen  mine  in  the  Michipicoten 
district;  the  Porcupine  Mountains  and  parts  of  the  Huron  Mountains  in  western  upper  Michi- 
gan; and  Rib  Hill  (PI.  IV,  A)  (1,942  feet),  Hardwood  Hill,  the  Mosinee  Hills,  and  Powers  Bluff 
in  northern  Wisconsin. 

Jasper  Peak  is  an  oval  eminence  about  one-half  mile  long  from  northeast  to  southwest 
and  three-eighths  of  a  mile  in  the  shorter  dimension.  It  rises  nearly  500  feet  above  the  valleys 
on  eitlier  side  but  only  350  to  400  feet  above  the  general  upland  of  the  region.  It  stands  up 
as  a  monadnock  because  the  jasper  and  ferruginous  chert  of  wliich  it  is  made  are  more  resistant 
to  denudation  than  the  adjacent  rocks.  Other  resistant  rocks  to  which  monadnocks  of  the 
Lake  Superior  region  are  due  are  the  Archean  gneiss  in  Tiptop  Mountain,  ferruginous  chert  and 
iron-bearing  formation  in  Hemlock  Mountam  in  the  Michipicoten  district,  and  Huronian  quartz- 
ite  in  Rib  Hill,  Wis.*  Various  other  resistant  Huronian  and  Keweenawan  formations  stand 
up  as  monoelinal  or  other  ridges.  The  long  ridges  of  this  character  that  rise  liigh  enough  above 
their  surroundmgs  to  be  called  monadnocks  include  the  Giants  Range  of  ^Imnesota,  the  Penokee 
Range  of  Wisconsiji,  and  others  which  will  be  specifically  described  later. 

VALLEYS  IN  THE  PENEPLAIN. 

There  are,  of  course,  general  inequalities  in  the  peneplain,  hut  there  are  also  valleys  cut 
100  to  400  feet  below  the  general  level,  which  may  be  interpreteil  as  evidence  of  slight  u|)lift 
after  the  completion  of  the  base-leveling  that  produced  the  peneplain.  In  general  these  vallej's 
are  fairly  broad  and  mature,  and  most  of  them  are  most  widely  opened  along  the  areas  of  the 
weaker  rocks.  The  original  consequent  drainage  of  this  region  was  modified  as  the  mountain- 
ous area  was  worn  down,  and  the  streams  on  the  belts  of  weaker  rocks  naturaU_y  wore  their  val- 
leys lower,  received  more  w^ater,  and  captured  tributaries  from  the  more  slowly  ej-oding  streams 

o  A  different  opinion  has  been  advanced  by  A.  C.  Lawson  (Geol.  and  Nat.  Hist.  Survey  Canada,  vol.  1,  new  ser.,  1885,  p.  23CC). 
6  Van  Hise,  C.  R.,  Science,  new  ser.,  vol.  4, 1896,  p.  58;  Weidman,  Samuel,  Jour.  Geology,  vol,  11, 1903,  p.  297 


2q 


PHYSICAL  GEOGRAPHY  OF  THE  REGION.  91 

on  the  durable  rocks.  The  stream  systems  are  now  subsequent  rather  than  consequent — that  Is, 
they  are  adjustetl  to  the  weaker  structures — crossmg  tlie  ridges  of  resistant  strata  in  narrow 
transverse  courses  and  flowing  in  greater  depressions  along  the  longitudinal  belts  of  weak  rock. 
Most  of  the  streams  of  the  pre-Cambrian  upland  are  in  this  adjusted  condition;  but  Weidman 
has  discussed  evidences  of  a  lack  of  adjustment  of  the  stream  courses  in  northern  Wisconsin, 
the  rivers  being  superposed  indifferently  upon  weak  and  resistant  beds  because  of  original 
courses  consequent  upon  the  dip  of  unconformably  overlying  Paleozoic  sediments. 

SOIL  AND  GLACIAL  TOPOGRAPHY. 

A  striking  feature  in  the  uplands  is  the  absence  nearly  everywhere  of  any  local  or  residual 
soil,  such  as  would  be  derived  fi-om  the  weathering  and  decay  of  the  various  strata  during  the 
very  long  time  necessary  to  reduce  this  from  a  mountainous  region  to  a  peneplain  of  sHght 
relief.  In  the  driftless  portion  of  the  region  the  ledges  are  deeply  covered  with  residual  soil 
derived  from  their  decay.  Elsewhere  the  soil  is  of  a  cliflFerent  kind  from  the  underlying  rock, 
with  which  it  forms  a  sharp  contact.  It  shows  almost  no  sign  of  decay.  It  is  not  a  residual 
but  a  transported  soil,  produced  through  erosion  and  deposition  by  the  great  continental  ice 
sheet.  This  ice  sheet  removed  the  residual  soil,  brought  a  new  and  less  fertile  soil  or  left  the 
ledges  bare,  displaced  stream  courses  from  the  zones  of  weaker  rock,  producing  many  of  the 
existing  waterfalls  and  rapids,  clogged  the  longitudinal  subsequent  valleys  so  as  to  form  one 
class  of  lakes, °  deepened  some  of  the  valleys  so  as  to  form  lakes  of  another  type,  and  produced 
numerous  other  effects,  which  are  described  in  Chapter  XVI  (pj).  427-459).  It  is  owing  chiefly 
to  this  glacial  invasion  that  the  region  dift'ers  from  the  normal  peneplain  type  in  minor  topog- 
raphy, in  drainage,  and  in  soils. 

DESCRIPTION  OF  DISTRICTS  IN  DETAIL. 

The  following  description  of  the  upland  topography  in  the  several  districts  is  designed  to 
exhibit  its  variations  in  accordance  with  the  character  of  the  constituent  rocks.  A  geographic 
order  has  been  atlopted,  starting  with  the  part  of  the  peneplain  at  the  west  end  of  Lake  Supe- 
rior, north  of  Dulutli,  continiung  around  north  of  Lake  Superior  in  Ontario,  and  thence  pi'o- 
ceeding  to  the  districts  south  of  Lake  Superior  in  upper  Michigan  and  Wisconsin. 

GABBRO    PLATEAU. 

The  area  in  northeastern  Minnesota  underlain  by  the  various  Keweenawan  gabbros,  por- 
phyries, etc.,  is  a  broad  upland  or  plateau  and  forms  a  typical  well-developed  part  of  the  pene- 
plain. The  rocks  of  the  gabbro  jjlateau  are  prevailingly  coarse  grained  and  homogeneous 
and  hence  furnish  a  notable  exception  to  the  hnear  topography  commonly  developed  in  the 
Algonkian.  Locally  they  form  ridges  grading  into  the  monoclinal  ridge  or  sawtooth  country 
to  the  east  and  north,  which  is  mostly  composed  of  granite  or  felsite  or  diabase  rather  than 
gabbro.     The  gabbro  plateau  surface  is  thus  described  by  Grant  r** 

In  general  the  surface  of  this  area  is  in  the  nature  of  an  undulating  plain.  Many  small  elevations  occur,  but  few 
which  rise  to  a  hundred  feet  above  the  surrounding  country.  *  *  *  While  the  surface  is  in  general  one  of  low 
relief,  the  minor  irregularities  are  pronounced.  Steep  rock  hills  are  common,  and  srnall  vertical  escarpments  10  tc 
20  feet  in  height  are  of  frequent  occurrence.  Some  of  the  water  bodies,  none  of  which  are  deep,  stretch  through  con- 
siderable areas.  *  *  *  The  general  plainlike  character  of  the  gabbro-covered  area  can  be  ascribed  to  weathering, 
erosion,  and  glaciation,  acting  upon  a  surface  composed  of  a  single  rock  mass  (the  gabbro)  uniform  in  constitution, 
grain,  and  resistance  to  disintegrating  agents. 

Clements''  refers  to  it  as  a  peneplained  upland  with  minor  irregularities  due  to  joints, 
composition,  etc.,  with  irregular  shallow  lakes.  He  speaks  of  it  as  "reduced  almost  to  base- 
level."     In  places  the  gabbro  forms  a  more  hilly  topography  .<* 

a  Clements,  J.  M.,  The  Vermilion  iron-bearing  district  of  Minnesota:  Mon.  U.  S.  Geol.  Survey,  Tol.  45,  1903,  pp.  43-46. 
6  Grant,  U.  S.,  Final  Kept.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  vol.  4,  pp.  434-435,  482,  492. 
c  Op.  cit.,  pp.  37-38,  399-400. 
*  d  Grant,  U.  S.,  op.  cit.,  pp.  399,  420,  462. 


92  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

In  St.  Louis  tounty,  nortli  and  nort.lioast  of  Duhith,  the  plutoau  topotjraphy  is  mild" 
(PI.  V,  A),  being  largely  obscured  b}'  the  glacial  (h'ift.  X.  11.  Wincheil  has  referred  to  this 
plateau  topography  ''  as  "very  monotonous  and  nearly  flat,"  with  the  gabbro  "rising  in  iri'egular 
rocky  domes  about  10  to  30  feet  above  the  sunounding  country."  Grant  speaks  of  it  m  Lake 
and  Cook  counties,  Minn.,"  as  a  "broad  undulating  plateau,"  with  a  "surface  which  is  still 
rough  but  has  no  marked  elevations." 

The  areas  of  resistant  red  rock  (fclsites,  porphyries,  syenites,  and  granites)  form  monad- 
nocks  and  higher  ridges,  among  these  bemg  the  Misquah  Hills,  one  of  whose  summits,  reaching 
a  height  of  about  2,2.30  feet,  is  the  highest  point  in  Minnesota  and  the  highest  in  the  Lake 
Superior  region.  Other  monadnocks  of  the  resistant  "red  rock,"  including  Eagle  Mountam, 
rise  to  2,100  or  2,200  feet.  Certain  anorthosites  also  form  resistant  knobs,  such  as  Carlton 
Peak,  which  rises  927  feet  above  Lake  Superior;  it  has  been  described  by  A.  C.  Lawson.'' 

The  diabases  generally  form  monoclinal  ridges  of  the  type  described  on  page  99;  they 
do  not  properly  form  a  part  of  the  topcjgraphic  subprovince  here  discussed  (the  broad  uplands) 
but  are  located  upon  its  margins. 

The  whole  plateau  is  deeply  covered  with  glacial  deposits,  which  conceal  the  ledges  and 
the  preglacial  topography  to  some  extent  and  have  disarranged  the  drainage  so  that  there 
are  abundant  lakes,  swamps,  and  muskegs. 

The  east  border  of  the  plateau  is  the  steep  escarpment  which  descends  abruptly  to  Lake 
Superior  (PI.  V,  A).  The  west  boundary  is  obscured  by  glacial  deposits,  so  that  the  topo- 
graphic relationship  of  the  Keweenawan  of  the  plateau  and  the  upper  Huronian  slates  south 
of  the  Giants  Range  is  obscure.  The  north  boundary  of  the  gabbro  plateau,  as  described  by 
Clements «  and  by  Leith,  is  a  "conspicuous  northward-facing  escarpment  overlooking  the 
low-lying  area  of  Virginia  slate  and  iron  formation  immediately  to  the  north.  To  this  the' 
name  'Mesabi  Range'/  was  first  applied."^  In  places  the  gabbro  overlies  the  granite  arid 
there  is  no  intermediate  lowland. 

ST.    LOUIS    PLAIN. 

West  of  the  gabbro  plateau,  in  the  region  drained  by  St.  Louis  River  and  its  tributaries, 
the  homogeneous  upper  Huronian  slates  form  a  broad  plain  at  a  lower  level,  extending  north 
to  the  Giants  Range  anil  in  most  places  deeply  covered  by  glacial  drift  but  still  retaining  the 
even  penejilain  topography. 

VERMILION    DISTRICT.* 

In  the  Vermihon  district,  which  is  separated  from  the  gabbro  plateau  and  the  slate  plateau 
of  upper  .St.  Louis  River  by  a  great  linear  monadnock  called  the  Giants  Range,  the  peneplain 
topography  is  also  well  developed.  Here,  however,  the  even-featured  surface  bevels  across 
Archean  and  Huronian  rocks  rather  than  Keweenawan  gabbros.  The  truncated  folds  of  con- 
glomerates, slate,  iron  formation,  etc.,  and  the  exposed  masses  of  greenstone,  granite,  etc., 
indicate,  however,  that  tliis  was  originally  the  heart  of  a  loftj^  mountain  region.  (See  structure 
profiles.  Pis.  I,  XXVI,  in  pocket.)  The  peneplain  seems  to  have  been  base-leveled  and  subse- 
quently slightly  uplifted,  so  that  the  streams  have  incised  valleys,  between  which  flat  uplands 
and  ridges  rise  to  the  peneplain  level.  The  present  topography  has  been  describeil  by  C.  K. 
Van  Hise'  as  follows: 

a  See  topographic  map  of  Duluth  quadrangle,  U.  S.  Geol.  Survey,  and  maps  of  St.  Louis,  Cook,  and  Lake  counties  In  atlas  accompanying 
Final  Rept.  (led.  and  Nat.  Hist.  Survey  Minnesota. 

i>  Final  Rcpt.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  vol.  4,  pp.  212,  2G5. 

c  Idem,  pp.  207,  317. 

<t  Bull.  Geol.  and  Nat.  Hist.  Survey  Minnesota  No.  S,  1893,  pp.  18-19. 

'  The  Vermilion  iron-bearing  district  of  Minnesota;  Mon.  U.  S.  Geol.  Survey,  vol.  45,  I9a3.  pp.  .399-400. 

/  The  name  "Giants  Range"  is  generally  applied  to  the  high  ridge  area  underlain  by  the  Huronian  granite,  though  there  is  a  tendency  to 
extend  the  name  eastward  to  the  somewhat  disconnected  peaks  (Misquah  Hills,  etc.,)  underlain  by  the  "red  rock"  (a  Keweenawan  granite).  See 
footnote  on  p.  103. 

0  The  Mesabi  iron-bearing  district  of  Minnesota,  Mon.  U.  S.  Geol.  Survey,  vol.  43, 1903,  p.  182. 

A  A  brief  statement  about  the  topography  is  included  in  each  of  the  chapters  on  the  iron-producing  districts. 

'  Manuscript  notes. 


PHYSICAL  GEOGRAPHY  OF  THE  REGION.  93 

The  ridges  correspond  in  direction  with  the  greater  extent  of  the  district.  Tliey  are  parallel  with  the  major  structure 
and  also  with  the  secondary  structures,  such  as  cleavage  and  schistosity.  The  trend  of  these  ridges  is  therefore  about 
N.  70°  E.,  although  locally  they  vary  much  from  this  direction. 

These  ridges  vary  in  altitude  somewhat  rapidly  in  the  direction  of  their  trend,  and  a  single  ridge  is  usually  no 
longer  than  a  fraction  of  a  mile  to  a  few  miles,  ordinarOy  1  or  2  miles.  The  slopes  parallel  with  the  trend  of  the  ridges 
are  comparatively  gentle.  The  slopes  transverse  to  the  course  of  the  ridges  are  steep  between  the  ridges,  the  valleys 
being  deep  and  many  of  them  narrow.  Also  the  cross  section  of  a  single  ridge  may  be  complex,  so  that  it  consists  of  a 
series  of  minor  ridges  between  which  are  minor  valleys.  Though  the  major  features  of  the  region  are  undoubtedly 
preglacial,  the  action  of  the  ice  has  been  very  important,  so  that  the  hills  and  bluffs  are  now  round-topped,  the  slopes 
steep,  and  the  valleys  flat-bottomed  and  U-shaped.  This  form  is,  however,  subordinatel;^  due  to  filling  rather  than 
to  erosion. 

Many  of  these  valleys  are  occupied  by  lakes.  The  greater  number  of  these  lakes  are  almost  exactly  parallel  to 
the  trend  of  the  ridges  and  are  generally  several  times  as  long  as  broad.  This  is  true  not  only  of  the  main  body  of 
each  lake  but  also  of  its  arms.  Characteristic  lakes  of  this  class  are  Long  Lake,  Fall  Lake,  Moose  Lake,  New  Found 
Lake,  and  Knife  Lake. 

Where  the  structure  of  the  district  locally  is  not  linear,  as  in  the  granites,  the  lakes  also  lack  the  linear  character. 
As  illustrating  this  may  be  cited  Snowbank  Lake,  Gabimichigama  Lake,  Gull  Lake,  and  Lake  Saganaga. 

From  high  knobs  or  recently  burned  areas  may  be  had  the  best  views  of  the  topography.  From  a  point  like  Jasper 
Peak,  or  Disappointment  Mountain,  or  one  of  the  high  ridges  in  the  neighborhood  of  Gunflint,  an  observer  sees  in 
the  foreground  the  linear  ridges,  rcjugh  and  partly  covered  by  trees  in  various  stages  of  growth,  in  the  valley  at  his 
feet  a  lake,  and  along  the  range,  if  the  point  of  view  is  advantageous,  many  lakes.  From  Jasper  Peak  he  may  follow 
nearly  all  the  bays  and  arms  of  the  largest  and  most  complex  of  the  lakes  of  the  district,  Vermilion  Lake  [PI.  VI]. 
However,  if  the  observer  ignores  the  immediate  surroundings  and  looks  farther  away,  he  gets  an  idea  of  the  more  ancient 
topography  of  the  region.  Southward  from  a  high  point  in  the  western  part  of  the  range  his  view  extends  over  a  number 
of  ridges  and  valleys,  and  as  a  horizon  line  he  sees  the  Giants  Range  north  of  the  Mesabi  district.  This  range  in  the 
western  part  of  the  district  is  composed  of  the  Giants  Range  granite  and  in  the  eastern  part  of  the  district  of  the  Ke- 
weenawan  gabbro.     To  the  north  his  range  of  vision  is  limited  by  the  granitic  hills  of  Basswood  Lake. 

From  the  various  points  of  view  he  learns  that,  though  the  Vermilion  district  has  numerous  hUls  and  bluffs  not 
inferior  in  altitude  to  the  areas  north  and  south  of  the  district,  on  the  whole  it  is  an  area  in  which  erosion  has  played 
an  important  role,  the  valleys  being  wider  and  deeper  and  containing  lakes  in  especial  abundance. 

Ignoring  all  these  minor  irregularities,  he  is  astonished  at  the  apparent  horizontality  of  the  sky  line.  A  few  points, 
however,  project  above  this  sky  line — for  instance,  Jasper  Peak  [PL  III,  B]. 

This  impressive  feature  of  the  topography  suggests  very  strongly  that  this  region  was  at  some  time  in  the  distant 
past  nearly  base-leveled;  that  the  high  projecting  points  were  not  reduced  to  this  level;  that  since  that  ancient  time 
a  new  cycle  of  erosion  has  far  advanced.  Into  this  base-leveled  plain  the  present  topography  of  the  Vermilion  district 
has  been  incised.  It  almost  surely  was  mainly  accomplished  by  river  erosion  in  preglacial  times.  However,  the 
glacial  erosion  has  been  exceedingly  vigorous  here.  It  was  preeminently  an  area  of  glacial  erosion  and  not  of  deposition. 
The  hills  and  bluffs  are  almost  devoid  of  glacial  debris;  even  the  valleys  contain  comparatively  little  as  compared 
with  moraine  areas  of  Wisconsin  and  Minnesota.  The  present  forms  of  topography  are  not  typical  river-sculpture 
forms;  they  are  rather  such  forms  considerably  modified  by  glacial  sculpture  and  glacial  deposition. 

J.  M.  Clements  reviews  the  relation  between  topography  and  structure  in  this  district,  empha- 
sizing many  points  bj^  local  examples."  He  refers  to  the  whole  region  **  as  "  characterized  by 
ridges  trending  N.  70°-80°  E.,  with  intervening  valleys,  the  larger  ones  usually  occupied  by 
streams  or  lakes.  In  tliis  area  the  topography  is  rugged  but  the  range  of  altitude  is  not  very 
great." 

Clements''  has  described  in  detail  the  topography  characteristic  of  the  various  Ai-chean 
formations  in  the  Vermilion  district  as  follows: 

Ely  gi-eenstone:  Prominent  east- west  hills  and  ridges,  or  broad,  low,  rounded  knobs  and  ridges. 

Soudan  formation  (iron  bearing):  No  great  effect  upon  topography  usually,  though  locally  very  important,  as  where 
its  jaspers  form  prominent  peaks,  such  as  Jasper  Peak,  Lee  and  Tower  hills,  and  other  notable  knobs  and  ridges,  some 
of  them  monadnocks. 

Granites  of  Vermilion  Lake:  Usually  occupies  hill  crests  or ' '  occm-s  in  rounded  or  oval  hills  higher  than  those  occupied 
by  the  surrounding  rocks." 

Granites  of  Trout,  Bumtside,  and  Basswood  lakes:  "Does  not  seem  to  affect  the  topography  very  materially." 
Topography  rough  in  detail  but  with  no  notable  relief.  Irregular  or  rounded  lakes  contrast  strikingly  with  linear  lakes 
of  sedimentary  areas.     Has  small  area,  pcssibly  base-leveled  in  second  cycle. 

Granites  between  Moose  Lake  and  Kawishiwi  River:  Exposures  numerous  in  oval  mass. 

Granites  of  Saganaga  Lake:  LTnemphatic  topography,  with  low  rounded  hills  rising  to  same  level  and  suggesting 
rather  complete  peneplaining. 

a  The  Vermilion  iron-bearing  district  of  Minnesota:  Mon.  U.  S.  Geol.  Survey,  vol.  45, 1903,  pp.  431-436. 

b  Idem,  pp.  19,  36-37. 

c  Idem,  pp.  134-135,  175,  248,  259,  264,  266. 


94  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

The  several  Iluronian  intrusivcs  of  smaller  area  also  produce  a  typical  peneplain  topojr- 
raphy  in  the  somewhat  isolated  highlands  of  the  plain  of  Iluronian  slate  at  the  edge  of  the 
Keweenawan  gabbro.     Their  several  efTects  are  thus  described  bv  Clements:" 

Giants  Range  granite:  Low  rounded  to  oval  hills,  perhaps  a  topograi)hic  continuation,  though  lower,  of  the  Giants 
Range  north  of  the  Mesabi  iron  range.     lias  small  area,  possibly  base-leveled  in  second  cycle. 

Granite  of  Snowbank  Lake:  Rounded  topography  characteris.tic  of  glaciated  granite.  Stands  lower  topographically 
than  might  be  expected  from  its  position  in  center  of  a  structural  anticline. 

Cacaquabic  granite:  Occupies  prominent  position,  with  fairly  high  hills. 

Acid  dikes:  Locally  form  knolls  and  ridges. 

The  Huronian  beds  are  nearly  always  associated  with  depressions  or  witii  minor  ridges 
and  the  slates  form  a  linear  lowland  with  local  ridges  and  knobs.  Clements''  has  described 
the  topography  developed  upon  the  lower  Huronian  sediments  as  follows: 

Conglomerates  and  slates  of  Vermilion  Lake  area:  Depressions  underlying  lakes,  swamps,  etc.,  or  generally  low  land 
between  ridges  of  Archean  gi-eenstone  and  iron  formation,  trending  north-northeast  and  south-southwest. 

Conglomerate,  iron-bearing  Agawa  formation,  and  Knife  Lake  slate  of  Knife  Lake  area:  Much  rougher  topography 
than  in  \'ermilion  Lake  area  to  west.  Relief  of  400  feet  or  600  feet  including  depth  of  lakes.  Normally  conglomerates 
form  ridges  and  slate  forms  depressions.  Ridges  and  valleys,  all  on  comparatively  small  scale,  trend  east-northeast  and 
west-southwest.     Locally  siliceous  slates  form  sheer  clirfs  of  100  feet. 

RAINY    LAKE    AND    LAKE    OF    THE    WOODS    DISTRICT. 

The  Rainy  Lake  and  Lake  of  the  Woods  region,  north  of  the  international  boundary,  is  under- 
lain chiefly  by  the  .Ajchean,  but  has  subordinate  linear  areas  of  not  clearly  separated  Huronian. 
A.  C.  Lawson  has  characterized  the  topography  near  the  Lake  of  the  Woods  as  one  which, 
"although  extremely  hummocky  or  mammiUated  in  its  surface  aspects,  presents  extraordinarily 
little  variation  in  level.  There  are  no  great  valleys  or  high  hills.  The  whole  country  is  prac- 
tically a  plateau  of  very  moderate  elevation  above  the  sea  for  so  inland  a  region."  <■  He  describes 
the  Rainy  Lake  region  as  "remarkably  flat  and  devoid  of  prominent  elevations,  although  the 
surface  in  detail  is  extremely  uneven  and  hummocky  or  mammiUated."'^ 

He  regards  the  region  as  probably  never  having  been  mountainous,  giving  as  his  reasons 
the  lack  of  proof  that  phcations  in  general  make  mountains  and  the  absence  of  immense  valleys 
or  gorges.  His  report  was  WTitten,  however,  long  before  the  idea  was  developed  that  the  con- 
dition of  moderate  relief  in  a  peneplain  belongs  to  a  later  state  of  denudation  than  that  of  the 
mountains,  deep  gorges,  etc. 

Lawson "  shows  how-  the  variously  folded  weak  and  resistant  beds  form  minor  ridges  and 
valleys  on  the  land,  or  peninsulas  and  bays  and  islands  where  lake  waters  -s^-rap  around  an 
irregular  series  of  valleys  and  hillocks  produced  by  subaerial  erosion  previous  to  the  formation 
of  the  lakes.  Near  Rainy  Lake  elevations  average  only  100  to  200  feet,  though  certain  excep- 
tional ridges  and  knobs,  which  seem  to  be  monadnocks  held  up  on  resistant  schists,  gneisses, 
and  granites,  rise  300  to  500  feet  above  lake  level,  the  highest  being  Kishkutena  Ridge,  approxi- 
mately 1,700  feet  above  sea  level  and  visible  for  long  distances.  The  lakes  average  less  than 
50  feet  deep,  the  greatest  depth  found  being  165  feet. 

HUNTERS    ISLAND    AND    THUNDER    BAT    REGION. 

East  of  the  regions  described  by  Lawson  lies  the  region  of  Hunters  Island  and  Thunder 
Bar.  W.  H.  C.  Smith  '  and  William  Mclnnes  »  have  described  the  topography  as  belonging 
to  the  same  sorts  and  having  the  same  relationships  as  that  to  the  w-est.     The  greenstones, 


o  The  Vermilion  iron-l)earing  district  of  Minnesota:  Mon.  \J.  S.  Geol.  Survey,  vol.  45,  1903,  pp.  354,  3C1,  364,  370. 
>>  Idem,  pp.  36,  278,  299. 

c  Geology  ol  the  Lake  of  the  Woods  region:  .\nn.  Kepi,  r.eol.  and  Nat.  Hist.  Survey  Canada  for  1885,  new  ser.,  vol.  1,  1886,  Rept.  CC,  p.  22. 
i  Geology  of  the  Rainy  Lake  region:  .\nn.  Rept.  (Seol.  and  Nat.  Hisl.  Survey  Canada  for  1887-88,  vol.  3,  new  ser.,  1889,  Rept.  F,  p.  10. 
t  Op.  cit.,  vol.  1,  Rept.  CC,  pp.  15-25  and  2f.-2.S;  vol.  3.  Rept.  F,  pp.  10-20. 

/  Geology  of  Hunters  Island  and  adjacent  country:  Ann.  Rept.  Geol.  Survey  Canada  for  1890-91,  new  ser..  vol.  5, 1893,  Rept.  O,  pp.  9-11. 
a  Geology  of  the  area  covered  by  the  Seine  Uiver  and  Lake  Shebandowan  map  sheets:  Ann.  RepL  Geol.  Survey  Canada  for  1S97,  new  ser., 
vol.  10, 1899,  Rept.  U,  pp.  0-10. 


PHYSICAL  GEOGRAPHY  OF  THE  REGION.  95 

jaspilites,  and  iron  formation  and  certain  schists  form  ridges  rising  at  most  300  feet  above  tlio 
neigliboring  lakes,  whose  greatest  depth  is  280  feet;  the  other  Arclaean  rocks  are  "characterized 
by  low,  rouniled  hills,  with  softened  outlines." 

REGION    NOKTII    OF    LAKE    SUPERIOR. 

W.  H.  Collins  "  describes  the  region  between  Nipigon  Ba}^  and  Heron  Bay  and  northward 
to  the  Height  of  Land  as  "a  peneplain  of  rounded  hills  of  crystaUine  rocks  300  to  400  feet  high, 
terminating  abruptly  along  the  south,  "  and  with  steeply  descending  streams  affording  excellent 
water  power. 

Collins ''  also  describes  the  Archean  area  north  of  the  Canadian  Pacific  Railway  and  west 
of  Lake  Nipigon  as  possessing  "a  surface  of  low  relief  and  moderate  altitude."  Water  levels 
vary  from  1,149  to  1,382  feet.  Few  hills  reach  250  feet  in  height.  The  sky  line  is  exceedingly 
even.  The  area  also  possesses  the  linear  topography  of  the  Algonkian  in  places  and  the  mesa 
topography  of  the  Keweenawan  near  Lake  Nipigon  and  to  the  west. 

REGION    NORTHEAST    OF    LAKE    SUPERIOR. 

J.  M.  Bell "  has  characterized  the  region  north  of  Lake  Superior  and  west  of  the  Michipicoten 
district  to  Heron  Bay  as  hilly,  with  greater  ranges  of  relief  than  elsewhere  in  the  Laurentian 
peneplain,  witli  valleys  opened  on  weak  rocks,  ridges  formed  on  resistant  beds,  and  with  monad- 
nocks  rising  above  the  general  peneplain  level  on  the  site  of  the  still  more  resistant  beds. 

MICHIPICOTEN    DISTRICT. 

The  part  of  the  peneplain  that  includes  the  Michipicoten  district  has  been  described  as 
follows:'' 

The  topography  is  of  the  rugged  character  usual  on  the  north  shore  of  Lake  Superior,  and  Hematite  Mountain,  the 
highest  point,  rises  1,100  feet  above  the  lake  within  a  distance  of  7  miles.  In  general  the  hills  form  steep  ridges  with  a 
direction  of  about  70°  east  of  north,  corresponding  to  the  strike  of  the  schists,  and  traveling  is  difficult  across  the  line  of 
strike.  *  *  *  From  the  summit  of  Hematite  Mountain,  which  is  situated  about  in  the  middle  of  the  region  and 
rises  200  feet  above  any  of  its  neighbors,  there  is  presented  more  than  the  usual  variety  of  surface,  including  long  ridges 
of  Huronian  schist,  rounded  hills  of  eruptives,  which  sometimes  rise  like  islands  out  of  lacustrine  plains,  stretches  of 
the  hummocky  surface  so  common  in  glaciated  Archean  districts,  lake  basins,  rock  rimmed  or  bordered  with  muskeg, 
rivers  with  lakelike  stretches  of  dead  water,  tumultuous  rapids  over  morainic  bowlders  and  falls  over  rocky  descents, 
and,  finally,  the  splendid  promontories  of  the  shore  of  Lake  Superior.  *  *  *  The  intimate  dependence  of  the 
topography  on  the  geological  history  of  the  country  is  well  brought  out  in  the  Michipicoten  region, where  the  folding  of 
the  schists  has  determined  the  direction  and  steepness  of  the  main  ranges  of  hills;  while  bosses  and  irregular  masses  of 
eruptives  give  rise  to  less  uniform  hills  associated  with  the  ridges  or  standing  isolated.  The  basis  of  the  topography  is 
to  be  found  in  the  pre-Cambrian  arrangement  and  the  varying  power  of  resistance  to  weathering  and  erosion  shown  by 
the  different  rocks;  so  that  the  prominent  features  may  be  of  very  ancient  date,  even  Paleozoic. 

REGION    NORTH    OF    SAULT    STE.  MARIE. 

In  the  upland  north  of  Sault  Ste.  Marie  and  east  of  Michipicoten  relief  of  as  much  as  100  to 
200  feet  is  common.  Nearer  the  lake  and  southeast  of  Michipicoten  there  are  several  very 
deep  valleys,  notably  those  of  Agawa,  Montreal,  Batchawana,  Chippewa,  and  Goulais  rivers. 
Owing  to  the  considerable  rehef ,  some  very  liigh  and  expensive  trestles  will'  be  required  where  the 
Algoma  Central  and  Hudson  Bay  Railway  is  to  cross  the  first  three  rivers  mentioned;  and 
the  building  of  the  railway  heyond  Pangissin  has  been  hindered  by  the  necessity  of  high  steel 
bridges,  though  the  railway  is  graded  all  the  way  to  the  Michipicoten  district.  Such  expense  in 
railroad  building  in  the  Lake  Superior  region  away  from  the  lake  shore  is  distinctly  excep- 
tional and  indicates  the  high  degree. of  the  local  relief. 

tt  Summary  Rept.  Geol.  Survey  Canada  for  1905-0,  pp.  80-81. 
i>  Idem,  p.  103. 

••Rept.  Bur.  Mines  Ontario,  vol.  14,  1305,  pt.  1,  pp.  281-299. 

d  Coleman,  \.  P.,  and  Willinott,  A.  B.,  The  Michipicoten  iron  ranges:  Univ.  Toronto  studies,  Geol.  ser.,  1902,  pp.  4-6;  also  Eleventh  Rept. 
Bur.  Mmes  Ontario,  1902,  pp.  153-154. 


96  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

The  areas  between  these  deep  valleys  are  broad  aiul  relatively  Hat  or  round  topped,  and 
some  of  the  hills  "present  steep  slopes  toward  the  valleys  and  often  dropoff  in  impassable  cliffs 
100  feet  or  more  in  height.  None  of  the  hills  rise  much  over  1,000  feet  above  Lake  .Superior, 
but  many  reach  900  feet ""  (1,500  to  1,600  feet  above  sea  level).  The  surface  bevels  indifferently 
across  variously  durable  structures  of  gneiss,  schist,  and  granite  in  a  characteristic  peneplain  sur- 
face, with  the  usual  nionadnocks.  The  deep  valleys  resemble  those  of  the  north  shore  of  Lake 
Superior,  which  arc  crossed  near  their  moutlis  by  expensive  britlges  and  trestles  of  tiie  Canadian 
Pacific  Railway ;  in  both  regions  they  are  deep  cut  because  of  the  low  adjacent  base-level  of  Lake 
Superior. 

MARQUETTE    DISTRICT.'' 

North  of  ^larquette  the  granite  area  forms  a  monadnock  group  known  as  tlie  Huron  Moun- 
tains, rising  about  1,200  to  1,350  feet  above  the  lake.*^  The  elevations  were  thus  described  by 
Foster  and  Whitney: 

They  do  not  range  in  continuous  chains,  but  exist  in  groups  radiating  from  a  common  center,  presenting  a  series  of 
knobs  rising  one  above  another  until  the  summit  level  is  attained.  Their  outline  is  rounded  or  waving,  their  slope 
gradual.     The  scenery  is  tame  and  uninteresting. 

C.  A.  Davis  "*  writes  with  regard  to  the  same  region: 

The  hills  are  only  150  or  200  feet  above  the  valleys,  hence  the  general  level  is  relatively  high  and  the  district  is  a 
plateau,  or  high  peneplain,  rather  than  mountainous. 

The  granite  of  the  Archean  south  of  Marquette  was  early  described  by  Brooks  as  having  an 
irregular  topograph j',  with  low  knobs,  ridges,  and  cliffs.*  Rominger  contrasts  the  area  south  of 
Marquette,^  where  the  granites  occupy  lower  levels  than  the  Huronian,  with  the  northern  granite 
outcrops,  wliich  "occupy  the  highest  elevations  and  constitute  the  most  conspicuous  ridges." 
The  topography  (see  topographic  map  and  structure  profiles,  PI.  XVII,  in  pocket)  characteristic 
of  the  Archean  formations  in  this  district  has  been  described  in  greater  detail  by  C.  R.  Van 
Hise  and  W.  S.  Bayley  s  as  follows: 

Northern  complex: 

Mona  schists:  Minor  rugged  hills,  strongly  glaciated. 

Kitchi  schists:  Rugged  hills  similar  to  those  of  Mona  schist. 

Gneissoid  granites:  Rounded  knobs,  invariably  smoothed  by  glaciition. 

Hornblende  syenite:  Exactly  like  that  of  granite. 
Southern  complex:  Knobs,  as  in  northern  granite  areas. 

MENOMINEE    DISTRICT.* 

W.  S.  Bayley  '  has  described  the  topography  associated  witli  the  various  Archean  rock 
series  in  the  Menominee  district  (PI.  XXVI,  in  pocket)  as  follows: 

Quinnesec  schist  (southern  area):  Rough  and  broken,  forming  deep  gorges,  with  many  ridges  and  elongated  hills. 
Quinnesec  schist  (western  area):  Without  distinctive  peculiaiities  except  small  rugged  knobs. 
Granites,  gneisses,  and  schists  of  northern  comijlcx:  Irregular  rugged  knolls,  intensely  glaciated. 

CRYSTAL    FALLS    DISTRICT..' 

The  topography  characteristic  of  the  Archean  in  the  Crj'stal  Falls  district  (PI.  XXII,  in 
pocket)  has  been  described  by  J.  M.  Clements,  H.  L.  Smyth,  and  W.  S.  Bayley  *  as  follows: 

Granite:  Small  rounded  isolated  knobs,  chiefly  obscured  by  glacial  drift  (gaps  in  granite  range  where  resistant 
greenstone  dikes  cross) . 

a  Coleman,  A.  P..  Rept.  Bur.  Mines  Ontario,  vol.  15,  pt.  1,  1906,  pp.  175-177. 

b  For  topography  of  Marquette  and  adjacent  districts  see  also  the  chapters  on  these  di-stricts. 

r  Report  on  the  geology  and  topography  of  the  Lake  Superior  land  district,  ISoO,  pt.  1,  p.  34. 

It  ..>,nn.  Rept.  Geol.  Survey  Michigan,  1900,  p.  2(i0. 

e  Brooks,  T.  B.,  Geol.  Survey  Michigan,  vol.  1,  1873,  pp.  72-73. 

/  Rominger,  Carl,  Geol.  Survey  .Vlidiigan,  vol.  i,  ISSl,  p.  1.3. 

e  The  Marquette  iron-hearing  district  of  Michigan:  Mon.  V.  S.  Geol.  Survey,  vol.  28, 1895,  pp.  152.  102, 170, 170, 191. 

ft  See  also  chapter  on  Menominee  district,  where  topography  is  discussed. 

(  The  Menominee  iron-hearing  district  of  Michigan:  Mon.  U.  S.  Geol.  Survey,  vol.  46,  1904,  pp.  132,  159, 16^. 

>  See  also  chapter  on  Crystal  Falls  district,  where  topography  is  discussed. 

t  The  Crystal  Falls  Iron-bearing  district  of  Michigan:  Mon.  U.  S.  Geol.  Survey,  vol.  30,  1899  (western,  p.  38;  eastern,  pp.  329, 386.  428,  and  463). 


PHYSICAL  GEOGRAPHY  OF  THE  REGION.  97 

Archean  crystallines:  Mammillated  with  rocky  knobs  separated  by  bowl-like  depressions,  the  hummocks  and 
bowls  being  generally  elongated  east  and  west. 

Granites,  gneisses,  schists,  and  amphibolites  of  Felch  Mountain  district:  Characteristic  rough  topography  with 
east-west  elongated  hummocks  and  bowls.  A  topographic  depression  always  exists  along  the  contact  of  the  Archean 
and  Algonkian,  usually  holding  a  swamp  or  stream. 

Gneissoid  granites  and  various  schists  of  Sturgeon  River  tongue:  Scattered  bare  knolls. 

West  of  the  Crystal  Falls,  Menominee,  and  Marquette  districts  (fig.  43,  p.  292;  PL  XXIV,  in 
pocket)  there  is  a  general  plain  produced  by  erosion  upon  the  homogeneous  slates,  in  places 
deeply  cut  by  streams  antl  partly  obscured  by  the  glacial  drift.  Through  both  slates  and  drift 
certain  knobs  of  resistant  greenstone,  etc.,  project  as  eminences. 

KEWEENAW    POINT. 

On  Keweenaw  Point  the  highland  peninsula,  generally  referred  to  in  atlases  and  maps  as 
the  Copper  Range,  has  rocks  vertical  or  very  highly  inclined.  Erosion  has  thus  far  been  unable 
to  significantly  alter  the  plateau  "  or  peneplain  ''  which  was  developed  on  these  inclined  beds  in 
the  period  of  base-leveling.  This  is  the  case  on  the  part  of  Keweenaw  Point  (fig.  59,  p.  422)  that 
extends  southward  from  Gratiot  River  to  Portage  Lake,  where  the  ridges  of  the  eastern  tip  of 
the  point,  as  described  by  Irving,  merge  into  "one  broad  swell"  or  "a  broad  central  ridge" 
which  extends  west  as  far  as  the  Porcupine  Movmtams,  beyond  which  it  resumes  its  continuity 
to  the  neighborhood  of  Bad  River,  Wisconsm.  Upon  this  long,  narrow  plateau  relief  is  not 
wanting,  small  monadnocks  rising  above  the  general  level,  which  other^vise  bevels  indifferently 
across  the  various  weak  and  resistant  beds.  This  plateau  surface  is  also  diversified  between 
Porcupine  Mountams  and  Bad  River  by  "rounded  ridges  and  knobs  with  cliffs  facmg  indiffer- 
ently in  all  directions."  It  is  still,  however,  essentially  a  peneplain,  the  valleys  cut  in  it  not 
havmg  notably  dissected  its  surface  into  distinctive  forms  like  monoclinal  ridges  or  mesas. 
To  the  northeast,  at  the  tip  of  Keweenaw  Pomt,  there  are  monoclinal  ridges  and  longitudinal 
valleys,  replacing  the  former  peneplain  surface,  above  whose  level  monadnocks  like  Mounts 
Houghton  and  Bohemia  still  rise,  the  former  owing  its  emmence  to  a  resistant  red  felsite.'' 
In  the  plateau  region,  where  the  dips  have  prevented  equally  rapid  dissection,  the  peneplain 
surface  remains.  It  is  marginally  cut  by  deep  gorges,  to  be  sure,  but  these  valleys  are  of  mod- 
erate area  and  are  not  separated  by  monoclinal  ridges  or  by  mesas,  such  as  occur  where  the 
dips  are  below  30°  or  nearly  horizontal  respectively.  Minor  monadnocks  rise  everywhere  above 
the  partty  dissected  peneplain. 

The  moderate  elevation  on  the  south  shore  of  Lake  Superior  known  as  the  Porcupine 
Mountams  "^  forms  a  monachiock  area  rising  600  to  1,421  feet  above  the  lake  and  averaging 
1,800  feet  above  sea  level.  The  highest  pomt  is  2,023  feet.  These  mountains  owe  their  relief 
to  the  resistant  quality  of  a  body  of  quartz  porphyries  and  felsites  here  faulted  up  against  the 
adjacent  weaker  beds  on  the  south  and  exposed  by  denudation.  That  they  form  a  group  of 
monadnocks  was  fii'st  noted  by  Van  Hise.'' 

NORTHERN    WISCONSIN. 

R.  D.  Irving  ^  in  1878  briefly  described  the  topography  of  the  Archean  area  south  of  the 
Penokee-Gogebic  range  as  the  "elevated  interior"  or  "interior  table-land,"  with  a  gently 
undulatmg  surface,  few  ledges,  low  granite  domes,  and  abundant  glacial  lakes  and  swamps. 

In  their  report  on  the  Penokee-Gogebic  district  Irving  and  Van  Hise  f  have  not  specifically 
described  the  topography  associated  with  the  north  edge  of  the  peneplain  within  that  district, 
but  the  Archean  gneisses  and  schists  there  may  be  mferred  to  have  characteristic  knobby  topog- 
raphy (PI.  XVI,  p.  226). 

a  Brooks,  T.  B.,  Geol.  Survey  Michigan,  vol.  1, 1873,  pp.  69-70;  Irving,  R.  D.,  Mon.  U.  S.  Geol.  Survey,  vol.  5, 1883,  pp.  164-166, 186. 

h  Van  Hise,  C.  R.,  Science,  new  ser.,  vol.  4,  1896,  p.  217. 

c  Irving,  R.  D.,  Mon.  U.  S.  Geol.  Survey,  vol.  S,  1883,  pp.  181-182. 

i  Idem,  pp.  206-225,  and  geologic  section  3,  pi.  20.    Also  Wright,  F.  E.,  Ann.  Rept.  Geol.  Survey  Michigan,  1903,  pp.  35-44. 

«  The  geology  of  the  eastern  Lake  Superior  district:  Geology  of  Wisconsin,  vol.  3,  1S7.V1S79,  pp.  61-1)2,  pi.  11. 

/  The  Penokee  iron-bearing  series  of  Michigan  and  Wisconsin:  Mon.  U.  S.  Geol.  Survey,  vol.  19,  1892,  p.  104. 

47517°— VOL  52—11 7 


98  GEOLOGY  OF  THE  LAKE  SUPEKlOli  liEGION. 

CENTRAL    WISCONSIN. 

R.  D.  Irving  "■  wrote  as  follows  regarding  the  topography  of  central  Wisconsin: 

The  region  of  crystiilliiu'  rocks  (Archeau  and  lluronian)  of  north-rcntral  Wisconsin,  descending  gradnally  south- 
ward, has  a  gently  undulating  surface,  which  is,  however,  often  broken  in  minor  detail  by  low.  abrupt  ridges  with 
outcropping  tilted  rock  ledges. 

Weifhnan'' has  described  the  topography  associated  with  the  Archoaii  formation  in  north- 
central  Wisconsin  (Pis.  IV,  A,  p.  90;  XXXI,  i\.,  p.  436)  as  follows: 

The  basal  group  (gneiss  and  schists)  forms  a  gently  sloping  plain,  with  low  crystalline  ledges  sometimes  thinly 
covered  by  sandstone,  sometimes  by  glacial  drift,  but  generally  exposed  in  the  ri\-er  beds. 

The  Hiironian  granite  and  syenite  form  the  princij)al  undiversified  peneplain  here. 

-   NORTHEASTERN    WISCONSIN. 

With  regard  to  the  topography  in  northeastern  Wisconsin,  T.  C.  Chamberlin  '^  says: 

The  Archean  surface  is  very  irregular,  and  here  and  there  knobs  rise  through  the  superincumbent  formations, 
giving  rise  to  isolated  hills  of  quartzite,  porphyry,  and  granite  in  the  midst  of  the  areas  of  lower  rocks. 

He  infers  that  these  knobs  are  protruding  through  the  Paleozoic  sediments,  not  intrusive 
in  them. 

LINEAR  MONADNOCKS  AND  OTHER  RIDGES. 

GENERAL  DESCRIPTION. 

Besides  the  smaller  monadnocks  which  rise  above  the  broad  uplands  of  the  peneplain,  there 
are  numerous  Imear  monadnocks  and  elongated  ridges  below  the  peneplain  level,  which  are 
related  to  the  formations  that  outcrop  in  narrow  bands,  notablj'  the  Algonkian  formations 
but  to  some  extent  also  the  Archean.  A  few  linear  monadnocks  also  rise  above  the  level  of 
the  peneplain. 

Where  the  rocks  are  gently  inclined  erosion  has  been  able  to  attack  them  more  success- 
fully than  in  the  areas  of  steeper  dips,  and  has  developed  the  monoclinal  ridge  (PI.  IV,  B), 
which  has  its  gentler  slope  following  the  dip  of  the  beds  and  its  steep  escarpment  on  the  opposite 
side.     Part  of  these  monoclinal  ridges  are  monadnocks,  but  a  number  are  not. 

In  the  Keweenawan  rocks  of  the  Lake  Superior  region  these  monoclinal  ridges  are  best 
developed  in  northeastern  Mimiesota,  on  Isle  Koj^al,  and  at  the  end  of  Keweenaw  Point; 
among  the  lluronian  rocks  they  are  well  developed  in  northern  ilinnesota  and  southern  Ontario, 
near  Gunflint  Lake,  m  the  Penokee  Range,  in  the  Giants  Range,  and  in  all  the  iron  districts, 
and  as  monadnocks  m  the  peneplain  (fig.  5). 

The  origin  of  these  monoclinal  ridges  as  specialized  forms  due  to  differential  erosion  (fig.  6) 
upon  weak  and  resistant  strata  has  not  been  agreed  to  by  all  the  workers  in  the  Lake  Superior 
region.  N.  H.  WinchelH  ascribed  the  Sawteeth  Mountains  of  the  Minnesota  coast  to  faulting 
and  has  been  followed  by  A.  C.  Lawson,*^  who  ascribes  the  monoclinal  ridges  of  the  Animikie 
in  southern  Ontario  and  northern  Minnesota  to  faulting,  and  by  A.  H.  Elftmann.-'  Irving,*'  on 
the  other  hand,  points  out  that  the  topography  "is  just  such  as  is  found  in  every  region  of 
flat-dipping  hard  rocks,  and  especially  where  softer  layers  are  interleaved,  as  ill  this  case." 
He  also  cites  numerous  monoclinal  ridges  of  similar  type  in  equivalent  nonfaulted  rocks  on 
eastern  Keweenaw  Point,  in  northern  Wisconsin,  and  elsewhere,  where  the  sawtooth  shape  is 
well  developed.     U.  S.  Grant ''  writes: 

The  numerous  northward-facing  cliffs  suggest  the  iiroljability  of  a  series  of  compai'ati\'cly  recent  east  and  west 
fault  lines,  along  the  north  sides  of  which  the  strata  are  depressed.  *  *  *  The  evidence  of  profound  faulting  in 
these  strata,  aside  from  the  evidence  of  topography,  is  small.     It  seems  that  the  present  siu'face  configuration  could 

<■  Geology  ot  Wisconsin,  vol.  2,  1873-1877,  pp.  453,  462. 

i>  Bull.  Geol.  and  Nat.  Hist.  Survey  Wisconsin  No.  IC,  1907,  p.  10. 

c  Geology  of  Wisconsin,  vol.  2,  1S73-1S77,  p.  248. 

d  ScTcnlh  .\nn.  Kept.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  1878,  p.  12. 

c  Bull.  Geol.  and  Nat.  Hist.  Survey  Minnesota  No.  8,  IS'13,  p.  33;  Twentieth  .\nn.  Rept.  Geol.  and  Nat.  Hist.  Sun-cy,  Minnesota,  1S91,  p.  192. 

/  Am.  Geologist,  vol.  21,  1898,  p.  183. 

»  Mon.  U.  S.  Geol.  Survey,  vol.  5,  18&3,  pp.  142-143. 

ft  Final  Hept.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  vol.  4.  1899,  pp.  483,  485. 


PHYSICAL  GEOGRAPPIY  OF  THE  REGION. 


99 


have  been  brought  about  by  eroaioii  acting  on  gently  inclined  strata  of  different  degrees 
and  fissile  Animikie  slates  being  more  susceptible  to  disintegration  and  erosion  than  the 

Grant  subsequently  proved  absence  of  faulting  in  one  of  the 
"supposetl  fault  scarps"  to  the  satisfaction  of  a  number  of  accom- 
panj'ing  geologists,  including  N.  H.  Winchell  ami  A.  II.  Elftmann, 
two  of  the  advocates  of  the  fault  origin  of  these  monoclinal  ridges. 

As  major  faulting  has  never  been  proved  to  be  associated  with 
the  scarps  of  the  monoclinal  ridges,  as  their  origin  by  differential 
erosion  in  nonfaulted  strata  has  been  rejjeatedly  shown,  and  as  they 
are  associated  only  with  marked  cross  faults — for  instance,  on  Isle 
Royal  and  north  of  Thunder  Bay — the  fault  hypothesis  for  the  mono- 
clinal ridges  (sawteeth)  is  regarded  as  not  warranted.  Indeed,  in  the 
Vermilion  monograph  J.  M.  Clements,"  who  discusses  this  type  of 
topography,  does  not  even  mention  the  possibility  of  faulting. 

As  the  strike  of  the  Algonkian  rocks  is  generalh'  northeast  and 
southwest,  the  trend  of  the  monoclinal  ridges  and  of  the  subsec[uent 
valleys  between  is  in  the  same  direction,  the  longitudinal  valleys  that 
extend  parallel  to  the  strike  of  tlie  rocks  being  usually  broatl  and 
persistent,  whereas  the  transverse  valleys  extendmg  across  the  strike 
of  the  rocks  are  narrow  and  irregularly  arranged. 

T^Tiere  these  ridges  and  vallej^s  are  partly  submerged  the  resulting 
bays  are  extreme!}'  long,  straight,  and  persistent,  and  the  peninsulas 
and  islands  are  in  long  parallel  hnes,  as  on  the  coast  of  Isle  Royal. 
Glaciation,  acting  upon  tliis  monoclinal-ridge  topography,  has  pro- 
duced one  striking  series  of  lakes  in  northeastern  ilinnesota;  these,  as 
well  as  similar  lakes  in  other  parts  of  the  region,  are  due  to  glacial 
clogging  of  the  subsecjuent  axial  valleys  between  the  monoclinal  ridges. 

KEWEENAW  AN  MONOCLINAL  BIDGES. 
GENERAL    STATEMENT. 

In  northeastern  Minnesota,  on  Isle  Royal,  on  the  end  of  Ke- 
weenaw Point,  and  in  northern  Michigan  and  northern  Wisconsin,  the 
monoclinal-ridge  type  of  topography  is  so  well  developed  that  the 
name  Sawteeth  Mountains*  has  been  given  to  these  ridges  on 
account  of  their  resemblance  to  the  jagged  teeth  of  a  saw  when  seen 
in  profile.  The  same  name  is  also  applied  to  the  Huronian  mono- 
clinal ridges  near  Gunflint  Lake  and  northward  in  Ontario.  (See  fig. 
5,  p.  88.) 

NORTHEASTERN    MINNESOTA. 

Ridges  of  this  sort  in  Minnesota,  near  Grand  Marais,  with  back 
slopes  of  5°  to  10°  and  steep  escarpments,  are  described  by  Irvmg"  as 
forms  due  to  differential  erosion  on  weak  and  resistant  beds. 

ISLE    ROYAL    AND    MICHIPICOTEN    ISLAND. 


of  resistance, 
diabase  sills. 


the  111  in-bedded 


^•Q 


^ 


t 
^ 


La 


;\?. 


i^\  s. 


«S' 


The  monoclinal  ridges  on  Isle  Royal  (PI.  IV,  B)  are  described  by 
Lane."*     No  other  information  concerning  the  relation  of  the  geology  to  the  minor  topography 
of  Michipicoten  Island  has  been  obtained  by  the  writer. 

a  Mon.  U.  S.  Geol.  Survey,  vol.  45, 190.3,  pp.  400-401. 

i>  Irving,  E.  D.,  The  copper-bearing  rocks  of  Lalie  Superior:  Mon.  U.  S.  Geol.  Survey,  vol,  o,  1SS3,  fig.  1,  p.  142;  also  flgs.  16,  26,  and  29,  on 
pp.  297,  32.1,  and  320. 

cidem,  pp.  141-143.  ''  Lane,  .V.  C,  Geol.  Survey  Micliigan,  vol.  0, 1S93-1S97,  pp.  1S0-1S3. 


100  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

KEWEENAW    POINT    AND    NORTHERN    MICHIGAN    AND    WISCONSIN. 

The  parallel  monoclinal  ridges  and  intervening  valleys  near  the  enil  of  Keweenaw  Point 
(fig.  6)  were  early  described  by  Marvine"  and  later  in  some  detail  by  Irving,''  who  associated 
the  various  valleys  and  "parallel  ridges  with  cliffy  southern  and  flat  northern  faces"  with 
specific  gently  dipping  Kewcenawan  beds — the  valleys  with  weak  amygdaloids  and  easily  decom- 
posable diabases,  the  ridges  with  resistant  melaphyres,  coarse  diabases,  and  bowlder  conglom- 
erates— and  showed  the  topogiaphy  associated  with  them  in  various  profiles.'^  In  regard  to 
the  east  part  of  Keweenaw  Point,  Irving'*  emphasizes  the  relation  of  dip  to  topography: 

Where  the  dip  flattens  the  structure  comes  out  finely  in  a  series  of  bold  ridges.  Toward  Portage  Lake,  however, 
the  dip  becomes  as  high  as  50°  or  more  and  the  several  ridges  merge  into  one  broad  swell.  This  holds  until  the 
Porcupine  Mountains  are  reached,  where,  although  the  dip  angle  is  as  high  as  30°,  the  .structure  is  most  beautifully 
illustrated  in  the  outer  ridge. «  This  ridge  rises  from  the  lake  shore  somewhat  more  gradually  than  the  dip  to  a  height 
of  over  1,000  feet  and  then  drops  off  in  a  bold  escarpment  of  400  feet  into  the  valley  of  Carp  Lake. 

This  cliff/  extends  nearly  continuously  across  T.  51  N.,  R.  43  W.,  a  distance  of  over  6  miles.  The  crown  of  the 
cliff  is  from  800  to  1,000  feet  above  Lake  Superior  and  from  400  to  600  feet  above  the  valley  of  Carp  Lake.  The  base 
of  the  cliff  is  marked  by  a  long  slope  of  fragments  fallen  from  the  diabase  and  amygdaloid  which  forms  its  upper  por- 
tions, but  through  the  greater  part  of  its  length  there  is  a  perpendicular  face  of  about  400  feet  above  the  talus. 

Farther  west  again,  as  far  as  Bad  River,ff  the  dips  are  high,  often  reaching  90°,  and  the  harder  rocks  constitute 
merely  rounded  ridges  and  knobs  with  the  cliffs  facing  indifferently  in  all  directions.  Beyond  Bad  River  and  all 
across  Wisconsin  to  the  St.  CroLx  the  dips  flatten  once  more,  and  the  "sawtooth"  shape  in  the  ridges  is  everywhere 
well  marked.'' 

This  is  notably  true  throughout  Douglas  County,  Wis.'' 

U.  S.  Grant  •'  refers  briefly  to  the  surface  features  characteristic  of  the  Keweenawan  in 
Douglas  County,  Wis.,  where  four  belts  of  different  topography  are  produced,  vaiying  with 
the  part  of  the  Keweenawan  exposed,  the  dip,  and  the  glacial  overburden.  The  more  resistant 
portions  of  the  Keweenawan  form  two  main  ranges  in  northern  Wisconsin  because  of  the  s^-n- 
clinal  structure  there.  T.  C.  Chamberlin,  writing  as  editor  of  the  notes  of  the  late  Moses  Strong, 
in  reviewing  the  surface  features  of  northwestern  Wisconsin  *  says  that  the  linear  topography 
referred  to  and  represented  in  ])rofiles  shows  sjtlendid  Keweenawan  monoclinal  ridges. 

KEWEENAWAN  MESAS. 

On  the  north  shore  of  Lake  Superior  the  tabular-mesa  topograjihy  (fig.  .5,  p.  8S)  is  develo]>ed 
in  places  where  the  Algonkian  beds  lie  practically  horizontal  and  weaker  strata  underlie  more 
resistant  beds,  so  that  erosion  has  been  able  to  open  lowlands  on  the  weak  rocks  and  leave  iso- 
lated highlands  or  ridges.  Three  great  valleys  have  been  opened  up  in  the  weaker  beds  in  the 
upper  Iluronian  (Animikie  group)  and  the  Keweenawan,  and  two  great  mesa  ridges  have 
been  left  between  these  valleys.  The  waters  of  Lake  Superior  have  subsequently  risen  to  such 
a  level  that  they  occupy  the  floors  of  these  valleys  and  form  Thunder,  Black,  and  Xipigon 
bays  (PI.  II,  p.  86).  Thunder  Cape,  the  narrow  end  of  one  of  the  peninsulas,  is  a  cliaracteristic 
bit  of  mesa  topography,  its  flat  top  rising  1,350  feet  above  the  level  of  the  lake.  Pie  Island  is 
another  mesa  of  the  same  kind  which  erosion  has  isolated  completely,  the  lake  waters  covering 
the  valley  bottoms  surrounding  it,  and  Mount  McKay,'  south  of  Mount  William,  is  a  similar 

a  Marvine,  A.  R.,  Geol.  Survey  Michigan,  vol.  1, 1S73,  pt.  2,  p.  95. 

6  Mon.  U.  S.  Geol.  Survey,  vol.  5, 1883,  pp.  164-106. 

c  Idem,  fig.  2  on  p.  ITS,  and  pi.  18. 

d  Idem,  pp.  142-143. 

c  Described  by  Foster  and  Whitney,  pt.  1,  1850,  p.  35.  Shown  well  in  topographic  map  by  Michigan  fleol.  Survey,  .Vnn.  Kept,  for  1905,  fig. 
3,  p.  15.  Just  east  oi  the  I'onupine  Mount.iins,  in  the  ISIack  River  region,  between  Bessemer  and  Lake  Superior,  the  topography  o(  the  Kewee- 
nawan in  a  typical  strip  from  the  I'enokee  Range  to  Lake  Superior  is  described  by  W.  C.  Gordon,  who  has  prepared  an  excellent  topographic  map 
(Geol.  Survey  Michigan,  Ann.  Rept.  for  1906,  pp.  40S-109,  420;  pi.  32). 

/  Mon.  U.  S.  Geol.  Survey,  vol.  5, 1883,  p.  218. 

0  Idem,  p.  143. 

»  See  also  Irving,  R.  D.,  Geology  of  Wisconsin,  vol.  3, 187.3-1879,  pp.  (12,  67-(iS. 

•  Sweet,  E.  T.,  Geology  of  Wisconsin,  vol.  3, 1S73-IS79,  pp.  310-329. 

}  Grant,  U.  S.,  Bull.  Geol.  and  Xat.  Hist.  Survey  Wisconsin  No.  0, 1901,  pp.  6-8. 

tocology  of  the  upper  St.  Croi.\  district:  Geology  of  Wisconsin,  vot.  3,  1873-1879,  pp.  367-381. 

iMon.  U.  S.  Geol.  Survey,  vol.  5, 1883,  p.  374. 


PHYSICAL  GEOGRAPHY  OF  THE  REGION.  101 

mesa  perhaps  small  enough  to  be  called  a  butte,  rising  980  feet  above  Lake  Superior,  and  isolated 
in  the  broad,  unsubmerged  valley  of  Kaministikwia  River.  William  Mclnnes  "■  refers  to  the 
area  of  flat-lj'ing  Animikie  rocks  near  Thunder  Bay  as  showing  "table-topped  hills,  and  escarp- 
ments with  perpendicular  faces  and  sharply  angular  outlines." 

A.  C.  Lawson,  who  ascribed  the  escarpment  of  the  monoclinal  ridges  near  Gunflint  Lake  to 
faultmg,  has  also  indicated  his  belief  that  the  east  side  of  Thunder  Bay,  which  "presents  a  very 
bold  and  remarkably  straight  cliff  several  hundred  feet  high  composed  of  Keweenawan  sandstone 
resting  on  Animikie  slate,  both  flat-bedded  and  in  apparent  unconformity,  *  *  *  ig  prob- 
ably originally  and  genetically  a  fault  scarp."''  The  writer  feels  inclined  to  ascribe  this  escarp- 
ment to  subaerial  denudation,  partly  (1)  because  of  the  insufficient  evidence  of  larger  faulting 
here,"^  as  pointed  out  in  the  discussion  of  the  cliffs  of  the  monoclinal  ridges  (p.  99),  partly  (2) 
because  denudation  m  the  region  is  producing  just  such  escarpments  wliere  rei^istant  horizontal 
strata  overlie  weaker  beds,  and  partly  (3)  because  a  fault  scarp  m  this  location  could  not  pos- 
sibly have  retained  its  present  position  and  form  since  the  latest  possible  date  of  formation 
unless  it  were  protected  by  some  lately  removed  mantle,  as  the  larger  possible  fault  scarps  of 
the  northwest  coast  of  Lake  Superior  and  the  southeast  side  of  Keweenaw  Point  seem  to  have 
been.  (See  pp.  112-116.)  The  chief  reason  for  doubting  the  fault  origin  of  the  east  boundary  of 
Thunder  Bay  is  that  such  an  origin  would  imply  the  fault  origm  of  the  boundaries  of  all  the 
mesas  in  this  district  which  have  escarpments  that  are  very  similar  topographically  and  geo- 
logically. Because  of  the  great  complexity  of  block  faulting  that  would  isolate  Thunder  Cape 
and  the  adjacent  peninsulas,  as  well  as  Pie  Island  and  Mount  McKay,  etc.,  and  the  total  absence 
of  evidence  of  such  faulting,  it  seems  far  more  reasonable  to  ascribe  these  forms  to  the  well- 
established  cycle  of  forms  resulting  from  normal  subaerial  denudation. 

North  of  Lake  Superior,  in  Ontario,  near  Lake  Nipigon  and  to  the  east  and  west,  there 
seems  to  be  a  great  many  more  mesas  and  valleys  of  exactly  this  kind,"*  all  in  an  area  underlain 
by  Keweenawan  rocks  or  by  upper  Huronian  (Animikie)  slates  and  Logan  sills,  as  along  the 
Canadian  Pacific  Railway  east  of  Port  Arthur  and  especially  beyond  Nipigon.  A.  C.  Lawson « 
writes : 

It  is  to  the  presence  of  these  trap  sheets  (the  Logan  sills)  that  the  bold  and  picturesque  topography  of  Thunder 
Cape,  Mount  McKay,  Pie  Island,  Nipigon  Bay,  and  the  many  sheer-walled  mesas  and  tilted  blocks  of  the  region  is 
due. 

All  these  mesas  apparentl}'  have  their  present  form  because  erosion  has  had  more  power 
to  open  up  broad  valleys  in  a  region  where  the  rocks  lie  practically  horizontal  than  in  adjacent 
regions  where  the  rocks  are  more  highly  inclined. 

Three  topographic  types  are  well  represented  in  the  Keweenawan  division  of  the  Algonkian, 
where  they  seem  to  form  a  distinctly  graded  series  (fig.  7)  ratlier  directh'  associated  with  the 

Monoclinal 
Peneplain    with    monadnocks  ^  I^i^lf?  Mesas 


Figure  7. — Hypothetical  cross  section  showing  relation  of  secondary  lowlands,  mesas,  monocllDal  ridges,  etc.,  to  peneplain. 

dip  of  the  constituent  beds./  In  exactly  the  same  length  of  time  precisely  the  same  erosional 
agencies  have  been  able  to  produce  almost  no  effect  upon  the  vertical  and  highly  inclined  beds 
(merely  cutting  gorges  in  the  peneplain),  to  develop  longitudinal  valleys  between  monoclinal 

a  Geology  of  the  area  covered  by  the  Seine  River  and  Lake  Shebandowan  map-sheets,  comprising  portions  of  theRainy  River  and  Thunder 
Bay  districts  of  Ontario:  Geol.  Survey  Canada,  new  ser.,  vol.  10,  1S97,  Rept.  H,  p.  6. 

b  Twentieth  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  1S91,  pp.  265-266. 

<^  Mmor  faults  extending  in  another  direction  with  smaller  possible  fault  escarpments  are  described  by  R.  C.  Allen  in  an  unpublished  thesis 
(1905)  of  the  University  of  Wisconsin.  Allen  also  appeals  to  faulting  to  explain  the  Mackenzie  Valley  (PI.  XIII),  the  depression  which  nearly 
connects  Thunder  and  Black  bays  and  is  followed  by  the  Canadian  Pacific  Railway:  the  writer  believes  it  to  be  due  to  normal  denudation. 

d  Collins,  W.  H.,  Summary  Rept.,  Geol.  Survey  Canada,  1906,  pp.  103,  105;  Coleman,  A.  P.,  Rept.  Bur.  Mines,  Ontario,  vol.  16,  pt.  1, 1907,  pp. 
107,  110. 

«  The  laccolithic  sills  of  the  northwest  coast  of  Lake  Superior:  Bull,  Geol.  and  Nat.  Hist.  Survey  Minnesota  No.  S,  1893,  pp.  24,  43. 

/Mon.  U.  S.  Geol.  Survey,  vol.  5,  1S,S3,  pp.  142-143,  166. 


102  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

ridges  on  the  gently  inclined  beds,  and  to  advance  the  region  where  the  l)e<ls  are  horizontal  to  a 
maturity  of  form  witli  l)roa(l  viiilovs  and  small  isolated  uplands  (mesas). 

HURONIAN  MONOCLINAL  RIDGES  AND  VALLEYS. 

Monoclinal  ridges,  however,  are  not  confined  to  the  Keweenawan  series  of  the  Algonkian. 
In  a  number  of  localities  in  the  Tluronian,  diabases,  quartzites,  and  other  strata  with  a  mo<h^r- 
atcly  gentle  dij)  have  developed  monoclinal  ridges  which  have  tlie  usual  uns\-mmetrical  profile 
with  a  gentle  slope  and  a  steep  scarp  face.     (See  fig.  5,  p.  88.) 

GUNFLINT    LAKE    DISTRICT. 

Near  Gunflint  Lake  and  in  adjacent  regions  of  Minnesota  monoclinal-ridge  topography, 
described  by  U.  S.  Grant,"  is  developed  in  the  upper  Humnian.  Tlie  slates  of  the  Animikie 
group  are  intruded  by  the  Keweenawan  Logan  sills,  which  are  now  gently  tilted  and  exposed, 
forming  the  crests  of  monoclinal  ridges  (fig.  24)  whose  scarp  faces  show  the  weaker  slates. 
These  consist  of  "long  jiarallel  ridges  running  approximately  east  and  west,  with  sharp  mural 
escarpments  on  the  north  sides  of  the  ridges  and  on  the  south  gentle  slopes."  This  topog- 
raphy is  also  described  by  J.  M.  Clements,''  in  effect  as  follows: 

Upper  Huronian  (.Vnimikie  group): 

Gunflint  formation:  Lower  slope  of  escarpments  in  monoclinal  ridges. 

Rove  slate:  Weak  lower  slope  of  monoclinal  ridges,  in  many  places  talus-covered. 
Keweenawan  Logan  sills:  Usually  cap  monoclinal  ridges,  at  many  points  forming  perpendicular  cliffs,  above 
gentler  talus-covered  slopes  of  Gimflint  formation  or  Rove  slate. 

In  this  region,  as  in  many  parts  of  the  Huronian  where  notably  long  and  narrow  monoclinal 
ridges  are  formed,  the  drainage,  whatever  it  may  have  been  initially,  has  become  so  thoroughly 
adjusted  to  the  topography  that  the  streams  flow  in  longitudinal  (subsequent)  courses  along  the 
strike  of  the  weaker  rocks,  generally  with  rather  broad  valleys.  In  nearly  all  places  where  the 
streams  cross  the  ridges  of  more  resistant  rocks  they  are  in  much  steeper-sided  valleys. 

PENOKEE    RANGE. "^ 

In  the  Penokee-Gogebic  iron  range  the  lulls  show  tliis  topographic  quality  most  distinctly. 
(See  structure  profile,  PI.  XVI,  p.  226.)  The  Penokee  Range  consists  of  a  series  of  hills,  in  the 
north  slopes  of  which  are  iron  mines.  North  of  this  range  there  is  a  broad  longitudinal  valley. 
The  range  is  not  made  up  of  one  continuous  ridge  but  rather  of  a  series  of  disconnected  Hnear 
liighlands  (PI.  XVI)  cut  through  by  narrow  gaps  wliich  cross  the  strike  of  the  more  resistant 
beds,  including  the  iron  formation,  and  are  therefore  not  so  ■wide  as  the  subsequent  valley,  wliich 
follows  the  strike  of  the  weaker  slates.  The  northern  boundary  of  the  valley  is  a  range  of 
Keweenawan  hills  with  well-developed  monocUnal  ridges,  which  are  also  cut  through  by  narrow 
transverse  valleys  that  continue  northward  toward  Lake  Superior.  The  narrow  transverse 
valleys  are  probably  consequent  upon  the  original  slope.  The  broad  longitudinal  valley  is  a 
consequent  lowland,  though  not  yet  drained  by  any  single  trunk  stream. 

South  of  tins  principal  longitudinal  valley  another  lowland  seems  to  be  developing  in 
places;  it  stands  at  a  rather  lugher  level  than  the  northern  one  and  is  less  continuous  from  end  to 
end,  because  interrupted  by  low  ridges,  wliich  extend  back  from  many  of  the  higher  liills  in  the 
Penokee  Range  proper.  It  is  a  subsequent  lowland  in  process  of  formation  between  the  resist- 
ant granite  and  the  resistant  rocks  of  the  Penokee  Range.  South  of  tliis  incipient  valley  rises 
the  northein  edge  of  the  Archean  peneplain,  with  rather  ragged  liills  of  granite,  Mluch  in  many 
places  reach  directly  to  the  foot  of  the  Penokee  Range  without  any  intervening  lowland. 

R.  D.  Ining"*  first  described  the  topography  of  the  Penokee  Range,  "the  ridge  or  mountain 
belt,"  as  well  as  that  of  the  Copper  Range  to  the  north.     The  former  rises  to  about  1,.500  to 

a  Final  Kept.  Oeol.  and  Nat.  Hist.  Sun-ey  Minnesota,  vol.  4,  1899,  pp.  4S2-4S3,  492,  49fi. 

ft  yermilicn  iron-liciring  di.strict  of  Minnesota:  Mon.  IT.  S.  Oeol.  Survey,  vol.  45.  1903,  pp.  .I.*.  370,  391-392,  399-401. 

cSec  also  the  lirief  statement  aljoiit  topography  in  the  chapters  on  the  Penokee-Gogebic  district. 

d  Geology  of  Wisconsin,  vol.  3,  1S73-1ST9,  pp.  02-70,  101-103. 


PHYSICAL  GEOGRAPHY  OF  THE  REGION.  103 

l.SOO  feet,  100  to  300  feet  above  the  lower  land  to  the  south,  with  a  less  abrupt  north  slope, 
varying  from  a  ridge  a  few  rods  wide  to  a  broad  swell  of  a  mile.  It  is  a  continuous  ridge  for  ncarl v 
50  miles  from  the  northern  half  of  sec.  24,  T.  44  N.,  R.  4  W.,  Wisconsin,  eastward  beyond  Sunday 
Lake  in  Michigan."  Beside  this  there  are  detached  ridges  with  the  same  ahgnment  to  the 
west  and  to  the  east. 

In  places  the  ridge  rises  from  100  to  300  feet  above  the  elevated  swampy  area  south  of  it  and  from  100  to  600  feet 
above  the  lower  area  north.  In  its  more  western  portions  this  range  is  wide  and  has  a  rather  narrow  serrated  crest, 
while  eastward  from  Tylers  Fork  it  becomes  more  and  more  of  a  gentle  swell  until  a  point  west  of  Sunday  Lake  is 
reached,  where  there  is  again  a  broader  ridge.  In  much  of  this  distance  the  ridge  forms  the  most  prominent  feature 
of  the  topography  of  the  country,  being  visible  from  the  waters  of  Lake  Superior  in  the  vicinity  of  the  Apostle  Islands 
as  a  blue  line  against  the  horizon. 

At  Penokee  Gap,  where  there  is  a  notable  fault,  there  is  also  a  marked  offset  in  the  crest 
of  the  range"  (PI.  XVI). 

Irving  and  Van  Hise  "^  have  described  the  detailed  topography  associated  with  the  various 
Algonkian  rocks  in  the  Penokee-Gogebic  district  in  effect  as  follows,  also  reviewing  in  a  paragraph 
the  relationship  of  the  various  formations  to  the  crest,  slopes,  etc.,  of  the  ridge  in  various 
locaHties  and  showing  the  topography  by  three  detailed  topographic  maps.**  (These  contours 
are  used  in  PI.  XVI  of  this  report.) 

Cherty  limestone:  In  one  place  forming  a  bluff  200  feet  high  and  half  a  niilo  long. 
Quartz  slate  member:  Conspicuous  outcrops  forming  the  base  or  capping  the  Penokee  Range. 
Iron-bearing  member:  Shares  with  quartz  slate  member  in  forming  crest  of  conspicuous  Penokee  Range,  100  to 
600  feet  high. 

Upper  slate  member:  Forms  great  east-west  valley  between  Penokee  Range  and  Keweenawan  ridges. 
Fragmental  rocks  south  of  greenstone  conglomerate:  Quartzite  outcrops  in  bold  exposures. 
The  greenstones:  Form  a  conspicuous  east-west  ridge  500  feet  high. 

GIANTS    RANGE. « 

The  Giants  Range  1  (see  PI.  VTII,  in  pocket ;  fig.  5,  p.  88)  is  one  of  the  most  striking  features 
in  the  topography  of  the  Lake'  Superior  region.  It  is  a  long,  narrow  range  extending  east-north- 
east and  west-southwest  in  northern  Minnesota  for  80  to  100  miles,  conspicuous  becau.se  it  rises 
above  low,  flat  country  on  either  side.  It  rises  400  to  500  feet  above  the  adjacent  country 
near  the  east  end,  the  greatest  height  above  sea  level  being  about  1,900  feet.  West  of  the 
Duluth  and  Iron  Range  Railroad  the  range  gradually  decreases  in  height  toward  the  southwest, 
and  near  Grand  Rapids,  Minn.,  where  it  crosses  Mississippi  River,  its  height  above  the  adja- 
cent country  is  relatively  small.  Beyond  Pokegama  Lake  the  Giants  Range  loses  its  individu- 
ality and  is  completely  buried  beneath  glacial  drift,  grading  into  the  general  level  of  the  country 
at  1,400  feet.  It  is  not  a  continuous  range  but  is  "made  up  of  a  great  number  of  small  hill 
ranges,  having  in  general  the  trend  of  the  main  range  to  which  they  belong."^  The  west  part 
is  low  and  the  divide  at  some  places  is  on  the  quartzite'*  instead  of  the  granite.  There  are  many 
gentle  bends  in  the  crest  and  one  market!  bend  where  tlie  i-ange  extends  southward  6  miles,  at 
Virginia  and  Eveleth  in  the  "Horn." 

The  crest  of  the  range  is  in  places  broad  and  flat,  m  others  comparatively  narrow  and  sharp.  The  southern  slope 
is  very  gentle;  the  northern  slope  is  somewhat  less  so.  At  frequent  intervals  both  crests  and  slopes  are  notched  by 
drainage  channels. « 

a  Irving,  R.  D.,  and  Van  Hise,  C.  R.,  The  Penokee  iron-bearing  series  of  nortliem  Wisconsin  and  Micliigan:  Mon.  U.  S.  Geol.  Sur\'ey,  vol.  19, 
1892,  p.  18S. 

6  Irving,  R.  D.,  Geology  of  Wisconsin,  vol.  3,  1873-1879,  pp.  103-104. 

cMon.  U.  S.  Geol.  Survey,  vol.  19,  1892,  pp.  145,  188-189,  301,  361,  308,  374,  387,  and  410. 

il Idem,  Pis.  VII,  IX,  and  XI. 

«  See  also  the  brief  description  of  the  topography  in  the  chapter  on  the  Mesabi  district. 

/  Final  Rept.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  vol.  4,  Pis.  LXX-LXXXI;  also  Mon.  U.  S.  Geol.  Survey,  vol.  45,  1S03,  pp.  35-36;  also 
Mon.  U.S.  Geol.  Survey,  vol.  43,  1903,  p.  21.  The  Giants  (or  Mesabi)  Range  is  called  Missabay  Heights  in  many  atlases  and  geographies  in  .\merica 
and  Europe.    See  footnote  on  p.  02. 

3  Clements,  J.  M.,  The  Vermilion  iron-bearing  district  of  Minnesota:  Mon.  U.  S.  Geol.  Survey,  vol.  45,  1903,  p.  30. 

ftSpurr,  J.  E.,  The  iron-bearing  rocks  of  the  Mesabi  range:  Bull.  Geol.  and  Nat.  Hist.  Survey  Minnesota  No.  10,  1894,  p.  13. 

•  Leith,  C.  K.,  The  Mesabi  iron-bearing  district  of  Minnesota:  Mon.  U.  s.  Geol.  Survey,  vol.  43,  1903,  p.  21. 


104  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Several  stream  valleys  cut  com])letcly  across  tiie  Giants  Range  in  deep,  narrow  gorges  or 
water  gaps,  that  of  the  Mississipjii  being  relatively  sliallow.  Of  the  deeper  water  gajis,  that 
occupied  by  Wine  Lake  and  Embarrass  River  is  the  most  prominent.  Another  transverse 
valley,  not  occupied  by  a  stream,  is  traversed  by  the  Duluth  and  Iron  Range  Railroad;  tliis 
col  or  wind  gaj)  is  100  feet  above  the  adjacent  country  and  2()()  feet  below  the  crest  of  tlie  Giants 
Range.  There  are  a  number  of  similar  wind  gaps,  each  of  wliich  was  doubtless  formed  Vjy  a 
stream  that  abandoned  its  course  while  the  land  surface  to  tlie  north  stood  at  the  level  of  the 
gap,  having  been  captured  by  an  adjacent  stream  that  continued  to  cut  its  water  gap  down. 
The  water  gaps  which  cross  the  range  are  much  deeper  than  the  wind  gaps,  that  of  Embarrass 
River  (Wine  Lake)  being  cut  down  to  an  elevation  of  1  ,.380  feet,  or  more  than  400  feet  below 
the  crest  of  the  range;  soundings  in  the  lake  and  diamond-drill  records  to  the  south  show  the 
valley  to  be  many  feet  deeper. 

C.  K.  Leith"  has  described  several  of  the  transverse  valleys  as  deep,  steep-sided  gorges, 
cut  or  deepened  by  glacial  waters  when  a  glacial  lake  to  the  north  overflowed  southward  across 
the  Giants  Range.  Some  of  the  gorges  are  liigh  up  in  the  range  and  are  no  longer  occupied 
by  streams.  One  such  gorge  40  feet  deej)  is  sho^vn  on  a  topographic  map  (fig.  5)  in  Monograph 
43;  that  crossed  by  the  railway  is  well  shown  on  the  general  topograpliic  and  geologic  map  of 
the  Mesabi  district  (PI.  VIII) .  It  seems  possible  that  these  gaps  or  cols  were  already  in  exist- 
ence when  the  glacial  streams  found  and  modified  them  in  the  manner  described  by  Leith.  The 
lower  wind  gaps,  especially  tliat  followed  by  the  Duluth  and  Iron  Range  Railroad,  suggest  a 
preglacial  origin;  no  question  is  raised,  however,  of  their  occupation  and  modification  by  run- 
ning water  when  the  marginal  glacial  lakes  referretl  to  existed. 

The  rock  underlying  the  Giants  Range  itself  is  cliiefly  lower  Huronian  granite,  but  Kewee- 
nawan  granite  and  Archean  igneous  rocks  are  also  represented.  The  topographic  anomah'  of  an 
exceedingly  long,  narrow  range  (figs.  4  and  5)  owing  its  prominence  to  the  resistant  quahtics  of 
granite  is  so  great  that  it  seems  to  require  a  word  of  especial  explanation.  It  is  common  for 
a  quartzite  or  other  sedimentary  rock  to  form  a  long,  narrow  range  of  just  tliis  kind.  It  is 
usual  for  the  protruding  edge  of  a  dike  or  a  sill  of  sufficient  resistance  to  form  just  such  a  long, 
narrow  eminence  as  this.  Granite,  however,  is  not  normally  intruded  in  the  form  of  lUkes  and 
sills,  and  we  must  therefore  account  for  this  occurrence  by  some  selective  process  of  folcUng, 
faulting,  or  erosion. 

Three  hypotheses  accordingly  present  themselves.  The  first  is  that  of  folding.  Under  this 
hypothesis  it  might  be  conceived  that  the  Giants  Range  since  its  intrusion  has  participated  in 
the  folding  of  a  long  east-northeast  to  west-southwest  antichne.  Such  a  deformation  might 
result  in  the  production  of  a  long,  naiTow  ridge.  In  the  front  ranges  of  the  Rocky  Mountains 
granites  outcrop  in  long,  narrow  bands,  none  of  which,  however,  is  so  narrow  in  proportion  to 
length  as  the  Giants  Range.  Moreover,  there  is  no  evidence  in  the  Mesabi  region  of  any  such 
movement,  though  N.  H.  Winchell  has  conceived **  that  the  Giants  Range  was  uphfted  in  a 
contemporaneous  isostatic  adjustment  with  the  extrusion  of  tlie  gabbro,  and  R.  D.  Irving 
implies  tliis  sort  of  origin  in  his  diagram  ■"  of  the  relationship  of  tlie  Huronian  on  ojiposite  sides 
of  the  Giants  Range.  The  imphed  equivalence  of  highly  folded  and  nearly  horizontal  rocks  on 
opposite  sides  of  the  range  has  since  been  disproved  by  the  weU-established  unconformity 
between  these  two  series. 

The  second  hypothesis  supposes  that  faulting  has  occurred  along  a  fine  parallel  to  the 
Giants  Range  and  that  the  granite  appears  in  its  present  position^as  the  edge  of  a  larger  faulted 
granite  block  wliich  is  exposed  only  along  tlie  narrow  width  because  other  rocks  overlie  the 
granite  elsewhere.  Tliis  In'pothesis  has  little  more  support  than  the  first,  and  it  seems  jirobable 
from  otlier  e\'idence  in  the  region  that  no  great  fault  movement  such  as  would  form  the  Giants 
Range  occurred  since  the  upper  Huronian,  though  the  Duluth  escarpment  of  Keweenawan  galibro 
suggests  such  a  faulting. 

a  Mon.  U.  S.  Geol.  Survey,  vol.  43,  1903,  pp.  193-194,  199. 

6  Twentieth  Ann.  Rcpt.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  1S91,  pp.  lJO-121. 
'  Mon.  U.  S.  Cieol.  Survey,  vol.  3, 1SS3,  fig.  34,  p.  399. 


PHYSICAL  GEOGRAPHY  OF  THE  REGION.  105 

The  third  hypothesis  supposes  that  the  granite  is  the  outcropping  edge  of  a  great  intruded 
granite  mass,  exposed  for  a  great  distance  east  and  west  by  erosion  upon  the  granite  as  a  retreat- 
ing escarpment,  and  revealed  for  only  a  narrow  width  because  it  is,  or  has  recently  been,  capped 
and  protected  by  a  resistant  bed  of  quartzite.  In  other  words,  the  Giiints  Range  may  be 
regarded  as  a  monoclinal  ridge  exactly  similar  in  most  respects  to  the  other  monocUnal  ridges 
of  the  Algonkian,  but  with  an  immensely  greater  length  and  a  rather  marked  relief  above  the 
adjacent  region  because  of  the  resistant  powers  of  the  granite,  giving  the  Giants  Range  its 
present  topographic  prominence. 

The  Giants  Range  is  the  largest  monadnock  in  the  Lake  Superior  region.  It  is  such  a  barrier 
to  trafiic  that  travel  across  it  is  limited  to  the  valleys  (PI.  VIII,  in  pocket).  Rather  curiously, 
the  railway,  in  order  to  cross  this  range,  selected  not  a  water  gap  but  a  wind  gap,  because  the 
glacier  and  stream  erosion  in  the  lower  part  of  the  adjacent  water  gap  (the  valley  of  Embarrass 
River  and  Wine  Lake)  has  so  deepened  it  and  so  steepened  its  sides  that  it  is  not  a  convenient 
pass  for  either  a  highway  or  a  railroad. 

Upon  the  south  slopes  of  the  Giants  Range — that  is,  in  the  Mesabi  iron  range — the  minor 
monoclinal-ridge  topography  is  exceedingly  well  developed  in  one  or  two  places,  and  i,t  is  sus- 
pected that  even  better  development  would  be  apparent  in  places  were  it  not  for  the  thick  drift 
mantle  nearly  everj'where  obscurmg  the  preglacial  topography.  Northwest  of  Hibbing,  for  ■ 
example,  the  Pokegama  cjuartzite  stands  up  as  a  distinct  monoclinal  ridge,  with  a  lowland  to 
the  north  between  the  cjuartzite  escarpment  and  the  granite  and  a  gentler  slope  southward 
toward  the  iron  mines.  The  topographic  map  shows  this  relationship  in  many  places  not 
visited  by  the  writer.  Rarely  in  other  parts  of  the  Mesabi  iron  range  the  rpiartzite  seems  to 
be  weaker  than  the  iron-bearing  Biwabik  formation,  forming  a  lowland  between  the  older  rocks 
and  an  iron-formation  ridge.  In  the  1  ,S20-foot  hiU  between  Virginia  and  Eveleth,  on  the  west 
side  of  the  Horn  (PI.  VIII),  there  is  a  quartzite  lowland  of  this  sort  with  an  escarpment  and 
monoclinal  ridge  of  a  resistant  part  of  the  iron  formation,  though  the  quartzite  rises  up  to  form 
the  base  of  tliis  escai-pment.  There  are  other  iron-formation  ridges  on  the  east  side  of  the  Horn 
and  elsewhere. 

MARQUETTE    DISTRICT. 

In  the  Marquette  district  also  linear  topography  is  developed  (PI.  XVII,  in  pocket)  in 
the  area  underlain  by  the  Algonkian  rocks,  though  before  detailed  studies  were  made  it  was  said 
to  have  a  notabl}"  hilly  surface  without  obvious  systematic  relation  to  the  structure."  The 
relief  was  said  by  Rominger  to  be  comparatively  slight,  50  to  100  feet  and  rarely  200  feet,** 
though  the  recent  topographic  maps  show  greater  extremes  and  an  average  relief  of  200  to  400 
feet.  Upon  the  basis  of  the  more  detailed  work  the  topography  characteristic  of  the  several 
Algonkian  formations  in  the  Marquette  district  has  been  described  by  Van  Hise  and  Bayley," 
detailed  studies  revealing  a  most  faithful  correspondence  of  the  low  hills  and  vallej^s  to  resistant 
and  weak  beds.  Van  Hise  and  Bayley<*  characterize  the  region  as  worn  dowai  from  mountains 
but  now  "merely  bluffy,"  with  maximum  elevations  of  400  feet  or  less,  the  valleys  and  ridges 
being  due  to  difl'erential  erosion  of  weak  and  resistant  beds.  The  following  classification  of  the 
topography  and  rock  formations  gives  the  substance  of  their  descriptions: 

Mesnard  quartzite:  Prominent  ranges  with  minor  sharp  ravines  and  steep  ridges.  , 

Kona  dolomite:  Steep  hills  with  vertical  ragged  cliff.s. 

Wewe  slate:  Forms  valleys,  except  in  a  few  places. 

Ajibik  quartzite:  Bdid  ridges  with  precipitous  bluffs  and  steep  ravines.     Some  ridges  200  feet  high. 

Sianio  slate:  Prevailingly  forms  valleys. 

Negaunee  formation:  Forms  valleys,  except  locally. 


"  Brooks,  T.  B.,  Geol.  Survey  Michigan,  vol.  1, 1873,  pp.  70-72. 
i>  Rominger,  Carl,  Geol.  Sur\-ey  Michigan,  vol.  4, 1881,  pp.  1-3. 

'  The  Marquette  iron-bearing  district  of  Michigan:  Mon.  U.  S.  Geol.  Survey,  vol.  28,  1895,  pp.  222,  241,  257,  283-284,314,  331-.332,  410,  417, 
444-445,  461,  4,88-189,  499,  572-573:  also  in  Fifteenth  .\nn.  Rept.  U.  S.  Geol.  Survey,  1895. 
<i  Fifteenth  Ann.  Rept.  U.  S.  Geol.  Survey,  1895,  pp.  G44-U45. 


106  GEOLOGY  OF  THE  LAKE  SUPEKIOK  REGION. 

Ishpeming  formation: 

Goodrich  quartzite:  In  some  places  forms  prominent  range. 

Bijiki  schist:  Conspicuous  ridges  and  headlands  in  Lake  Michigamme. 
Michigammc  formation:  Ubually  lowlands,  except  locally. 
Clarksburg  formation:  Roimdcd  knobs  and  narrow  ridges. 
Pre-Clarksburg  greenstones:  Prominent  irregular  knobs  or  long  narrow  ridges. 

In  the  Republic  trough  the  topograpliy  of  the  Archcan  ui)lan{ls,  as  briefly  described  by 
H.  L.  Smyth,"  sliovvs  characteristic  granite  knobs,  rounded  and  gluciati-d ;  Michigamme  River 
flows  througl;  the  lower  land  undcriam  b}'  the  bedded  rocks  (Algonkian),  and  the  various  fjuartz- 
ites,  mica  schists,  ferruginous  schists,  and  igneous  intrusives  form  the  usual  elevations  and 
depressions,  nowhere  rising  to  the  height  of  the  granite  uplands  on  the  east  and  west  except  in 
Republic  Mountam. 

So  many  of  the  ridges  are  related  to  resistant  beds  and  so  many  valleys  to  weak  beds  that 
the  character  of  the  rocks  maybe  predicted  with  some  assurance  from  the  general  form  of  to]iog- 
raplij' .  Local  variations,  however,  make  it  impossible  always  to  feel  sure,  for  example,  tliat  the 
same  weak  slate  which  in  one  place  forms  a  valle}'^  will  also  be  found  in  the  lowland  in  an  adja- 
cent locality.  An  exception  of  this  kind  is  found  in  the  Siamo  slate  of  the  middle  Huronian, 
whose  outcrop  east  of  Teal  Lake,  near  Negaunee,  is  marked  by  a  very  distinct  east-west  trending 
valley,  which  is  followed  farther  east  by  Carp  River.  Directly  south  and  west  of  Teal  Lake, 
however,  some  more  resistant  members  of  the  Siamo  slate  are  found  in  the  Siamo  Hills  and  adja- 
cent ridges,  and  for  the  next  10  or  12  miles  westward  the  Siamo  slate  locally  forms  ridges,  though 
more  commonly  found  in  the  valleys. 

Just  south  of  Negaunee  and  east  of  Ishpeming  there  is  a  series  of  rather  abrupt  knobs  which 
are  not  exactly  of  the  class  characteristic  of  this  region.  A  number  of  diorite  and  diabase  masses 
intruded  in  the  middle  Huronian  iron  formation  have  been  so  resistant  to  erosion  that  many  of 
them  form  knobs  which  rise  above  the  adjacent  valleys.  At  near-by  points,  however,  the  iron 
formation  itself  is  so  resistant  that  it  stands  up  as  a  distinct  knob  or  ridge. 

Li  this  region  the  glacial  deposits  have  not  masked  the  preglacial  topography  to  a  great 
degree,  because  the  region  seems  to  have  had  rather  marked  rehef  in  preglacial  times,  somewhat 
in  contrast  with  the  Crystal  Falls  district.  Immediately  adjacent  to  the  ilarquette  district  on 
the  east  and  south  are  lowlands  where  the  lack  of  relief  in  glacial  time  is  indirectly  reflected  by 
the  masldng  of  the  bed-rock  topography  almost  entirely  by  glacial  deposits. 

MENOMINEE    DISTRICT. 

The  topography  of  the  Menominee  district  has  been  described  by  Bayley,''  who  speaks  of 
the  area  as  forming  two  plains  (PI.  XXVI,  in  pocket),  one  in  the  bottoms  of  the  present  valleys, 
the  other  on  the  level  of  the  tops  of  the  hills.  The  effect  of  erosion  on  the  Huronian  beds  of  the 
Algonkian  system  has  been  to  produce  a  series  of  east-\\est  trending  valleys  and  ridges  which 
correspond  very  closely  to  the  weak  and  resistant  members  of  the  Huronian  series.  There  is 
clear  evidence  that  the  ridges  and  valleys  in  the  Menominee  district  east  of  Iron  Mountain, 
Mich.,  were  formed  in  ])re-Cambrian  time  and  that  they  have  been  preserved  since  that  time 
because  of  their  burial  beneath  the  Cambrian  sandstone,  which  served  as  a  protectmg  nuintie. 
Erosion  has  recently  removed  most  of  these  Cambrian  overburdens  and  has  reexposed  the  pre- 
Cambrian  topography.  To-day  the  hills  rise  appro.ximately  300  to  400  feet  above  the  valley 
bottoms.  They  are  a  little  higher  than  they  were  before  the  Upper  Cambrian  sandstone  was 
deposited,  because  a  cap  of  sandstone  still  surmounts  most  of  the  hilltops.  From  tlu'  valleys 
it  has  been  largelj-  removed.  The  old  cliffs  and  bluffs  against  which  the  Cambrian  sandstone 
was  deposited  are  still  exposed  in  the  valley  slopes;  the  drainage  was  in  preglacial  time  almost 
as  well  adjusted  to  the  weak  and  resistant  rocks  as  it  had  been  before  the  Cambrian  trans- 
gression, though  Bayley  has  supposed  the  topography  then  to  have  been  sharper  and  more 
rugged.  Glaciation  has,  however,  somewhat  modified  the  stream  courses,  and  the  perfect 
adjustment  of  preglacial  time  is  lacking,  as  the  gorges  and  w'aterfalls  suggest; 

a  Mon.  U.  S.  Oeol.  Survey,  vol.  2.8, 1895,  p.  520. 

I>  Idem,  vol.  40, 1904,  pp.  125-129;  Menominee  special  folio  (No.  82),  Geol.  .\tlas  V.  S.,  U.  S.  Geol.  Survey,  1900. 


PHYSICAL  GEOGRAPHY  OF  THE  REGION.  107 

Bayley"  has  described  the  toj^ography  associated  with  the  various  Algonkian  rock  for- 
mations in  the  Menominee  district  in  effect  as  follows : 

Sturgeon  quartzite:  Great  bare  regular  bluffs  with  smooth  tops  and  almost  precipitous  sides. 

Randville  dolomite  (northern  belt):  Valleys  and  other  depressions. 

Randville  dolomite  (central  belt):  Usually  insignificant,  forming  bases  of  hills  and  rarely  little  plateaus  with  small 
escarpments. 

Randville  dolomite  (southern  belt):  Conspicuous  irregular  cliffs  or  blul'fs. 

Vulcan  iron  formation:  Either  inconspicuous  in  valleys  or  clinging  to  slopes  of  dolomite.  Ledges  rarely 
prominent. 

Hanbury  slate:  Entirely  confined  to  low  ground,  forming  njinor  protrusions  only  where  the  slate  is  locally  harder. 

CRYSTAL    FALLS    DISTRICT. 

In  the  Crystal  Falls  district,  whose  physiograjih}'  lias  been  described  by  J.  M.  Clements 
and  H.  L.  Smyth,''  the  adjustment  of  ridges  and  valleys  to  resistant  and  weaker  structures  has 
been  somewhat  similarly  developed  (PI.  XXII,  in  pocket),  except  that  here  the  ridges  and  valleys 
are  arranged  in  a  less  simple  and  orderly  way.  The  average  relief  is  200  feet  in  the  western  part. 
The  Cambrian  has  been  almost  entirely  removed.  Furthermore,  this  region  seems  to  have 
been,  even  in  pre-Cambrian  time,  one  of  less  relief  than  the  Menominee  region;  certamly  it 
was  a  region  of  very  slight  relief  (called  by  Clements  "an  approximate  peneplain")  when  the 
continental  glacier  overrode  it,  and  as  a  result  the  glacial  deposits  are  far  more  prominent  and 
have  more  thoroughly  obscured  the  preglacial  topography  than  in  any  other  iron  district  in  the 
Lake  Superior  region  except  the  Mesabi. 

The  topography  characteristic  of  the  several  Algonkian  formations  in  the  Crystal  Falls  dis- 
trict has  been  described  by  J.  M.  Clements,  H.  L.  Smyth,  and  W.  S.  Bayley,*^  in  effect  as  follows: 

Western  part. 

Rand\dlle  dolomite:  No  marked  effect  on  topography  or  drainage  (in  depressions). 

Mansfield  slate:  Marked  depressions,  followed  by  Michigamme  River. 

Hemlock  formation:  Exceedingly  irregular  topography;  tuffs  forming  valleys;  lava  flows  or  intrusives  forming 
higher  ground,  and  resistant  tuffs  forming  high  hills. 

Bone  Lake  crystalline  schists:  Apparently  forms  knobs,  but  usually  covered  by  glacial  drift. 

Upper  Huronian:  In  many  places  covered  by  glacial  drift  or  by  Cambrian  sandstone.  Shales  form  valleys  and 
softly  rounded  hills.     Graywackes  and  cherty  rocks  form  more  striking  topography. 

Eastern  part — Felch  MounUiin. 

Sturgeon  quartzite:  Linear  ridges,  usually  lower  than  those  in  the  Archean,  though  locally  lower  than  dolomite. 
Randville  dolomite:  Low,  steep-sided  knolls,  occasionally  linear  ridges. 
Mansfield  schist:  No  depressions;  occasionally  steep-sided  valleys. 

Groveland  formation:  Moderately  resistant,  forming  elevations  such  as  Felch  Mountain  and  Groveland  Hill, 
100  feet  high. 

Upper  Huronian  mica  schists  and  quartzites:  Lowlands  and  low  flat-topped  ridges. 

Eastern  part — Michigamme  Mountain  and  Fence  River: 

Sturgeon  formation:  Apparently  here  weak  and  forming  lowlands;  Randville  dolomite  underlying  swamp. 

Mansfield  formation:  Indistinguishable  topographically  in  gently  rolling  plain  of  dolomite  (miniature  ridges). 

Hemlock  formation:  Rough  topographical  details,  with  abrupt  ridges  and  narrow  ravines  (in  some  parts  till 
covered). 

Groveland  formation:  No  topographic  prominence  except  in  Michigamme  Mountain;  in  Fence  River  area  topog- 
raphy less  important  than  that  of  glacial  drift. 

NORTH-CENTRAL    WISCONSIN. 

Weidman''  has  described  the  topography  associated  with  the  various  Algonkian  formations- 
in  north-central  Wisconsin  (see  Pis.  IV,  A,  p.  90;  XXXI,  A,  p.  436)  in  effect  as  follows: 

a  Mon.  U.  S.  Geol.  Survey,  vol.  46.  1904,  pp.  177,  200,  291,  402. 

b  Idem,  vol.  36,  1,S99.  pp.  29-36,  331-335. 

cidem,  vol.  36,  1899  (western,  pp.  .50-51,  54,  73,  148.  155.  187-190;    eastern,  pp.  331,  398,  406.  411,  415,  423,  4.30,  431,  438,  440.  446.  471-473). 

d  Bull.  Geol.  and  Nat.  Hist.  Survey  Wisconsin  No.  16,  1907,  pp.  42,  55,  62,  82,  88,  91, 100,  112,  118,  177,  358,  306,  371. 


108  GEOLOGY  OF  THE  LAIvE  SUPERIOR  REGION. 

Lower  sedimentary  series  (lower  Buroniant). 

Rib  Hill  quartzite:  Bold  knobs  forming  the  highest  land  in  the  region  in  monadnocks,  and  jirominent  because 
surrounding  weaker  granite  and  syenite  are  base-leveled. 

Wausau  graywacke:  Not  prominent,  forming  very  few  low  exposures. 

Hamburg  slate:  Not  forming  valleys  lower  than  adjacent  more  resistant  formations  because  of  lack  of  dissection 
of  perfected  pene])lain. 

Powers  lUuff  (luartzite:  Forms  notable  prominence  300  to  400  feet  below  sunoundings;  smaller  ridges. 

Quartzite  at  Rudolph:  Low  ridges  and  knobs. 

Juration  City  quartzite:  No  notable  topography. 

Igneous  intrusive  formations  (rhyolite  series). 

Wausau  area:  Absence  of  sharply  rugged  topography,  though  low  ledges  project  slightly  through  younger  formations. 
Rhyolite  schists  of  Eau  Claire  River:  Forms  striking  cliffs  in  dells  of  Eau  Claire  River,  due  to  joints. 
Rhyolite  schists  of  Pine  River:  Marked  gorge,  a  mile  long,  IKO  feet  deep,  known  as  dells  of  Pine  River,  with  sharp 
tributary  gorges  related  to  joints. 

Upper  sedimentary  series  {middle  Huronianf). 

Marshall  Hill  graywacke:  Steep  slopes  and  ledges.  i^ 

Arpin  quartzite:  Low  sloping  land;  less  resistant  than  Powers  Bluff  quartzite  and  more  resistant  than  adjacent 
granite. 

North  Mound  quartzite:  Prominent  mound  rising  above  surrounding  Cambrian  lowland. 

NORTHWESTERN    WISCONSIN. 

The  Iluronian  quartzites  of  Barron  and  Chippewa  counties,  Wis.,°  form  notably  prominent 
monochnal  ridges  rising  as  much  as  300  feet  above  the  adjacent  plain  ant!  ha\nng  gentle  dip 
slopes  and  steep  escarpment  faces  with  talus  at  the  base. 

THE  LOWLAND  PLAINS. 

AREA. 

The  lowland  region  of  horizontal  or  gently  folded  post-Algonkian  rocks  (figs.  4  and  5,  pp.  87, 
88,  Pis.  I,  in  pocket ;  II,  p.  86)  includes  cliiefly  rocks  of  Cambrian  and  other  early  Paleozoic  age  so 
generally  buried  beneath  glacial  deposits  that  ledges  are  comparatively  rare  tliroughout  the 
area  and  the  preglacial  topography  is  partly  or  wholly  masked.  A  small  area  of  drift-covered 
Cretaceous,  also  flat  lying,  is  found  in  northern  Mimiesota. 

The  lowland  is  made  up  of  narrow  areas  on  the  south  shore  of  Lake  Superior,  a  broad  belted 
plain  in  Micliigan,  Llinnesota,  and  Wisconsin,  and  another  plain  in  Miimesota.  As  the  map 
(PI.  I)  indicates,  there  is  a  narrow  strip  at  the  west  end  of  Lake  Superior,  on  the  south  shore, 
and  a  narrow  strip  fringing  the  shore  from  L'Anse  to  Marquette.  Besides  tliis  rather  small 
Httoral  zone,  a  considerable  area  now  buried  by  the  waters  of  Lake  Superior  is,  without  much 
doubt,  covered  by  horizontal  Paleozoic  rocks. 

These  early  Paleozoic  rocks  cover  all  of  the  Upper  Peninsula  of  I\Iichigan  east  of  Marquette 
and  overlap  the  highland  country  of  northern  Wisconsin  and  upper  ilichigan,  including  the 
Archean  and  Algonkian  areas,  in  a  great  semicircle  which  extends  southwestward  into  Wiscon- 
sin to  the  vicinity  of  Grand  Rapids  and  thence  northwestward  through  Chippewa  Falls,  etc., 
to  the  region  where  the  Paleozoic  overlaps  the  Keweenawan  of  northern  Wisconsin  and  sends 
a  narrow  tongue  northeastward  to  join  the  horizontal  Cambrian  of  the  head  of  Lake  Superior 
at  Duluth.     Very  small  patches  are  found  on  the  north  shore  of  Lake  Superior. 

CHARACTER  AND  STRUCTURE. 

These  early  Paleozoic  rocks  consist  chiefly  of  Upper  Cambrian  sandstone  overlain  in 
places  by  a  conformable  or  nearly  conformable  series  which  extends  upward  to  the  Silurian  in 
Wisconsin  and  to  Devonian  and  Carboniferous  in  lower  Michigan.     North  of  the  Archean  and 

o  Geology  of  Wisconsin,  vol.  4, 1873-1879,  pp.  575-581. 


PHYSICAL  GEOGRAPHY  OF  THE  REGION.  109 

Algonkian  of  upper  Wisconsin  and  IVIicliigan  tliis  Cambrian  sandstone  (Lake  Superior  sand- 
stone) lies  essentiall}'  horizontal  and  is  probably  preserved  because  it  is  downfaulted.  In 
upper  and  lower  iliclugan,  in  Wisconsin,  and  in  Minnesota,  however,  there  is  evidence  that  the 
sedimentary  rocks  have  been  thrown  into  a  series  of  broad  folds — a  synclinal  basin  in  Michigan 
and  a  broad  anticline  in  south-central  Wisconsin.  The  Cretaceous  in  northern  Minnesota  is 
essentially  horizontal. 

DENUDATION. 

Earth  movements  have  left  some  areas  of  Paleozoic  rocks  higher  than  others,  and  as  a  result 
of  the  elevation  and  inclination  of  these  beds  eroding  agencies  have  removed  them  entirely 
from  some  areas,  the  boimdaries  of  wliich  have  a  direct  relation  to  the  broatl  folding.  The 
upper  beds  of  the  Paleozoic  are  almost  entirely  absent  in  northern  and  central  Wisconsin  and 
northwestern  Micliigan  (fig.  1 1,  p.  116),  from  wliich  it  is  inferred  that,  though  they  were  once  present 
over  the  whole  of  tliis  area,  they  have  since  been  removed  by  the  active  erosion  which  has  taken 
place  in  tliis  elevated  region.  As  an  evidence  of  the  former  greater  distribution  of  the  Paleozoic 
sediments  we  may  refer  to  the  isolated  horizontal  Cambrian  beds  that  cap  the  ridges  in  the 
Menominee  district  east  of  Iron  Mountain,  Mich.  (PI.  XXVI,  in  pocket),  and  various  outliers  of 
Cambrian  age,  wliich  form  mounds  rising  above  the  general  peneplain  level  in  Portage,  Wood, 
and  Clark  counties.  Wis.,"  far  north  of  the  area  of  Cambrian  rocks.  Quite  in  contrast  to  these 
mounds  of  the  border  zone  between  the  Paleozoic  and  pre-Cambrian  in  Wisconsin  are  the  knobs 
of  the  older  rocks  wliich  project  through  the  thin  Paleozoic  edge.  The  knobs  are  inliers;  the 
mounds  are  outhers.  Chamberhn ''  refers  briefly  to  such  knobs  that  protrude  through  the 
Cambrian  in  northeastern  Wisconsin.  Tlie  Baraboo  quartzite  ridges  and  those  at  Necedah, 
Waterloo,  etc.  (figs.  53,  54,  and  55,  pp.  359,  360,  364),  are  features  of  the  same  sort.  Because 
of  their  conspicuous  positions  as  monadnocks  on  the  pre-Cambrian  peneplain  they  have  been  the 
first  of  the  older  rocks  to  emerge  when  the  Paleozoic  sediments  which  formerly  covered  their 
tops  were  eroded. 

THE  BELTED  PLAIN. 

The  distribution  of  the  Paleozoic  sediments  in  a  broad  semicircle  on  the  south  flank  of  the 
Archean  peneplain  is  to  be  explained,  therefore,  as  a  result  of  erosion  after  unequal  upUft."^ 
The  lowest  bed,  the  Cambrian  sandstone,  is  distributed  in  a  curAang  lowland  belt  around  the 
Archean  (PI.  I,  in  pocket),  with  outhers  scattered  far  back  upon  the  Archean  surface,  and  the 
overhnng  Paleozoic  formations  are  distributed  in  parallel  curving  belts,  the  more  resistant  beds 
standing  up  as  highlands,  the  weaker  beds  being  worn  down  into  lowlands.  A  hnear  series  of 
iTunor  liiglilands  underlain  by  the  "  Lower  Magnesian "  limestone  stretches  southwestward  in 
Micliigan  and  eastern  Wisconsin  (PL  I),  and  thence  northwestward  in  central  and  western  Wis- 
consin. South  and  east  of  this  is  a  broad  valley  wliich  has  been  eroded  upon  the  weaker  members 
of  the  Ordovician,  especially  the  Upper  Ordovician  (Cincinnatian)  shales  ami  parts  of  the 
Galena  and  Trenton  limestones.  The  waters  of  Green  Bay  have  filled  part  of  this  great  lowlantl 
valley,  wliich  extends  southward,  inclucUng  the  broad,  shallow  depression  containing  Lake  Win- 
nebago (PI.  II,  p.  86).  East  of  tliis  valley  there  is  a  long,  low  monochnal  ridge,  wliich  was 
produced  by  the  effects  of  erosion  on  the  resistant  eastward-dipping  Niagara  limestone,  and 
which  has  a  steep  scarp  face  on  the  northwest  side  and  a  gently  dipping  back  slope  toward 
Lake  Michigan,  diversified  by  minor  monochnal  ridges  due  to  weak  and  resistant  members  of 
the  Niagara.  It  is  overlain  by  glacial  and  lake  deposits.  It  forms  an  upland  ridge  (fig.  5, 
p.  88)  east  of  Lake  Winnebago  and  extends  north  in  the  Door  Peninsula,  Washington  and 
adjacent  islands  of  Wisconsin,  and  the  Garden  Peninsula  of  upper  Mchigan;  the  scarp  con- 
tinues first  northeast,  then  south  as  the  Niagara  escarpment  of  Georgian  Bay,  southern  Ontario, 
and  northern  New  York.  East  of  this  ridge  is  the  lowland  of  weak  rock  in  wliich  Lake  Michigan 
ies  and  the  upland  of  the  northern  part  of  lower  Michigan. 

o  Weidman,  Samuel,  Bull.  Geol.  and  Nat.  Hist.  Survey  Wisconsin  No.  16, 1907,  pp.  400,  405-407. 
i>  Geology  of  Wisconsin,  vol.  2,  1873-1877,  p.  248. 
cidem,  vol.  1, 1873-1879,  pp.  24S-252. 


110  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

The  topography  in  the  part  of  western  Wisconsui  inchidcd  in  this  report  is  (Icscribeil  by 
Moses  Strong,"  that  in  central  Wisconsin  by  R.  D.  Irving,*  and  that  in  eastern  Wisconsin  by 
T.  C.  ChaniberUn.'^     Tlip  physiography  of  Wisconsin  as  a  whoK>  is  briefly  treated  by  G.  L.  Collie.'' 

Russell  <^  has  shown  that  in  the  greater  part  of  the  northern  peninsula  of  Micliigan  the 
wearing  dowm  of  the  gently  inchned  Paleozoic  rocks  has  resulted  in  belts  of  upland  and  lowland 
of  a  sufficient  degree  of  rehef  to  be  apparent  beneath  the  glacial  deposits.  The  topograpliy  of 
this  region  was  described  previously  in  a  more  general  way  by  Douglass  Houghton  ■'^  and  by 
Brooks. 3 

The  portion  of  the  southern  peninsula  of  Michigan  here  mapped  as  within  the  Lake  Superior 
region  has  been  described  by  Rominger ''  anil  by  Lane.' 

The  arrangement  of  the  gently  inclined  Paleozoic  rocks  in  curving  zones  has  led  W.  M. 
Davis  to  describe  Wisconsin  as  an  ancient  coastal  plain,  referring  to  the  peneplained  Archean 
area  of  northern  Wisconsin  as  an  oldland,  the  area  underlain  by  Cambrian  sandstone  as  an 
inner  lowland,  with  a  first  and  a  second  cuesta  (monoclinal  ridge)  extending  around  its  margin 
along  the  outcrop  of  the  "  Lower  Magnesian"  and  the  Niagara  limestones  respectively.-'  Objec- 
tion has  been  raised  to  the  use  of  the  term  "ancient  coastal  plain"  on  the  ground  that  the 
upland  area  of  northern  Wisconsin  is  not  known  to  be  the  old  land  from  wliich  the  local  Paleo- 
zoic sediments  were  derived.  Though  it  is  hence  not  permissible  to  classify  Wisconsin  as  an 
ancient  coastal  plain,  there  is  good  warrant  for  describing  these  parts  of  Wisconsin  and  Michigan 
as  a  belted  plain  (fig.  5,  p.  88 ;  fig.  1 1 ,  p.  116)  with  upland  and  lowland  zones  sj^stematically  related 
to  the  weak  and  resistant  rocks. 

THE  MINNESOTA   LOWLANDS. 

In  the  western  part  of  the  Lake  Superior  region,  extending  into  the  vallevs  of  Red  River 
of  the  North  and  Mississippi  River,  is  a  great  lowland  region,  which  seems  to  have  been  reduced 
to  a  peneplain  in  Mesozoic  time,  perhaps  in  the  Cretaceous.*  The  Cretaceous  peneplain  extends 
into  the  Lake  Superior  region  from  the  west  and  southwest  and  Cretaceous  sediments  overlap 
all  the  westward  extension  of  the  Giants  Range.  Just  what  this  distribution  of  the  Cretaceous 
may  mean  can  not  be  said  at  present;  but  it  seems  probable  either  that  sedimentation  did  not 
take  place  in  the  Lake  Superior  basin  during  the  Cretaceous  or  else  that  wliile  the  Cretaceous 
base-levehng  was  going  on  over  a  great  part  of  the  United  States  the  great  mass  of  Paleozoic 
and  perhaps  later  sediments  were  being  removed  from  the  basin  of  Lake  Superior  and  the 
adjacent  Mglilands,  perhaps  uncovering  the  several  great  escarpments  presently  to  be  described 
and  producing  the  several  lowland  belts  adjacent  to  Lake  Superior  and  the  Paleozoic  areas  to 
the  south. 

THE  BASIN  OF  LAKE  SUPERIOR. 

GENERAL   CHARACTER   AND   ORIGIN. 

The  basin  (PI.  II)  wliich  contains  the  largest  of  the  North  American  lakes  probably  includes 
parts  of  every  system  of  rocks  known  to  be  in  the  region,  from  the  Archean  to  the  Recent. 
It  is  not  known  whether  Paleozoic  or  Keweenawan  rocks  occupy  the  greater  part  of  the  basin. 

The  Lake  Superior  basin  is  e.xcejjtional  in  that  it  is  nearly  surrounded  by  liiglilands.  Going 
back  from  Lake  Superior  in  any  direction  except  the  southeast,  one  soon  comes  to  an  escarp- 
ment, as  at  Diduth  or  on  the  south  shore,  above  wliich  is  a  distinct  upland  wliicli  overlooks 
the  lake  basin.  In  some  places  this  escarpment  overlooks  the  waters  of  the  lake  directly  (PI.  V) : 
in  others  it  is  some  distance  back  (PI.  II  and  figs.  4  and  5).  Moreover,  this  escarpment  (400  to 
800  feet  in  height)  at  many  points  descends  into  very  d<>op  water  (500  to  900  feet),  so  that  the 

o  Ooology  of  Wisconsin,  vol.  4,  187;i-1879,  pp.  7-37.  s  Idem,  vol.  1, 1S73,  pp.  68-09. 

i>  Idem.  vol.  2,  1873-1877,  pp.  453-153,  S3!,  548.  *  Idem,  vol.  3,  lS7o,  pp.  1-20. 

eldem,  pp.  97-100.  <  Watcr-Supplj- Paper  U.S.  Geol.  Survey  No.  30, 1S99,  pp.  57-.58,  9i>-91. 

d  Bull.  Am.  Bur.  Oeography,  vol.  2,  19;)1,  pp.  270-287.  i  Davis,  W.  M.,  Physical  geography,  1S9S,  pp.  13(>-137,  flg.  S5. 

c  Ann.  Kept.  Geol.  Survey  Michigan  for  1904,  pp.  52.^).'  *  Leith,  C.  K.,  Kcon.  Geology,  vol.  2,  1907,  p.  149. 

/  Geol.  Survey  -Michigan,  vol.  2, 1873,  p.  241. 


PHYSICAL  GEOGRAPHY  OF  THE  REGION.  Ill 

whole  height  of  the  suiroundinp;  rim  is  not  everywhere  apparent.  Some  of  the  other  Great 
Lakes  have  siicli  a  bounchiry  on  one  sitle,  hut  none  is  so  nearly  walled  in  as  Lake  Supciior. 

As  the  submerged  contours  (PI.  II)  show,  this  basin  has  a  depth  of  almost  1,000  feet,  the 
deepest  sounding  being  163  fathoms,  or  978  feet,  near  latitude  87°  W.,  longitude  47°  45'  N.,  or 
nearly  400  feet  below  sea  level,  without  considering  the  possible  filling  of  recent  lake  silts  or 
glacial  deposits.  There  is  a  notable  depression  between  the  pre-Cambrian  of  northern  Wis- 
consin and  the  pre-Cambrian  of  Minnesota  and  Canada.  This  depression  consists  of  a  long, 
narrow  trough  trending  northeast  and  southwest  and  limited  on  the  north  by  the  great  escarp- 
ment wliich  extends  from  Duluth  northeastward  to  the  mouth  of  Nipigon  Bay,  a  distance  of 
250  miles.  This  trough  is  25  to  70  miles  TOde.  Its  southern  boundary  is  Keweenaw  Point  and 
the  Michigan  and  Wisconsin  shore;  at  Oronto  Bay,  east  of  Ashland,  there  is  an  angular  offset 
in  passing  the  Apostle  Islands,  diminisliing  the  width  of  the  lake  by  half.  Thence  the  wall  of 
the  depression  goes  on  parallel  to  and  near  the  Wisconsin  shore,  the  fault  line  converging  west- 
ward toward  the  Duluth  escarpment  fault  line,  probably  meeting  it  west  of  the  head  of  the 
lakes  in  Minnesota. 

From  th^  mouth  of  Nipigon  Bay  the  border  of  the  Lake  Superior  depression  extends 
southeastward  to  Sault  wSte.  Marie  as  a  high  wall  or  escarpment  of  xmknowni  origin.  Here  it  is 
not  a  straight  line  but  has  great  embayments  and  salients.  On  the  south  shore  a  fault  escarp- 
ment extends  southward  on  the  east  side  of  Keweenaw  Point.  The  liighland  border  thence 
trends  irregularly  southeastward  to  the  vicinity  of  Marcjuette,  beyond  which  it  extends  south 
and  a  httle  west  of  south  into  Wisconsin.  The  area  between  Marquette  and  Sault  Ste.  Marie 
on  the  south  shore  is  lowland. 

The  North  American  Great  Lakes  are  situated  in  pairs  on  either  side  of  an  escarpment 
which  faces  the  boundary  between  the  resistant  pre-Cambrian  and  the  relatively  weak  Paleozoic 
rocks.  In  this  respect  they  resemble  the  great  lakes  of  the  pre-Cambrian  area  of  northwestern 
Europe.  An  escarpment  thus  situated  and  formed  is  called  by  Suess  a  glint  line.  Lake 
Superior,  however,  should  not  be  included  among  the  glint  lakes,  where  it  is  classified  by 
Suess,"  together  with  Lake  Ontario,  Georgian  Bay,  Lake  Winnipeg,  etc.  The  southeastern 
part  of  Lake  Superior  might  be  considered  a  glint  lake  because  it  has  one  early  Paleozoic  and 
one  Archean  shore,  as  was  pointed  out  by  Agassiz,**  if  it  were  not  known  on  other-  evidence  to 
be  chiefly  a  structural  basin. 

In  the  origin  of  its  basin,  also.  Lake  Superior  is  exceptional.  The  other  great  lakes,  four 
to  the  east  in  the  United  States  and  four  to  the  north  in  Canada,  lie  in  lowland  areas  where 
differential  erosion  acting  upon  alternate  weak-  and  resistant  beds  would  produce  basins  if 
aided  by  glacial  erosion,  glacial  clogging,  etc.,  though  some  of  the  basins  are  possibly  also  in 
part  structural.  Lake  Michigan,  for  example,  lies  between  the  broad,  anticlinal,  southward- 
pitcliing  fold  of  central  Wisconsin  and  the  basin-like  syncline  of  central  Micliigan,  its  location 
suggesting  a  partly  structural  basin,  as  does  also  the  knowm  warping  in  the  basins  of  the  other 
great  lakes,  though  the  structural  feature  is  certainly  of  minor  importance.  The  correspondence 
of  the  Lake  ^Michigan  lowland  with  a  belt  of  weak  strata  (Silurian  and  Devonian),  perhaps 
somewhat  deepened  by  glacial  erosion,"^  is  probably  of  principal  importance. 

The  reason  for  the  present  depression. of  the  Lake  Superior  basin  is  somewhat  doubtful, 
the  earliest  explanations  being  regarded  as  inadequate  to  account  for  certain  features  of  it. 
The  fact  that  it  is  a  synchne  (see  structure  section,  PI.  I,  in  pocket),  first  pointed  out  by  Foster 
and  Whitney  "^  and  amplified  by  Irving,"  has  never  been  called  in  doubt,  for  there  is  ample 
proof  of  it.  But  for  so  old  a  structural  basin  to  remain  unfilled  f  and  for  it  to  retain  abrupt 
boundaries  which  bear  all  the  characteristics  of  youth  are  departures  from  the  normal  con- 
dition wluch  require  special  explanation. 

»  Su3ss,  Ediiard,  The  face  of  the  earth  (Das  Antlitz  der  Erde),  translated  by  H.  B.  C.  and  W.  J.  SoUas,  vol.  2,  O.xford,  190G,  p.  65. 
6  Lake  Superior,  etc.,  1S50,  p.  420. 

«  Chamberlin,  T.  C,  Geology  of  Wisconsin,  vol.  1,  1S73-1879,  pp.  253-259. 
i  Report  on  the  Lake  Superior  land  district,  pt.  1,  1850,  p.  109. 
c  Mon.  U.  S.  Geol.  Survey,  vol.  5,  l.SS.^,  pp.  410-418. 

/  Barrel!  f.Jour.  Geology,  vol.  14,  laoti,  p.  .33.5)  has  computed  that  it  would  take  Mississippi  River  only  60,000  years  to  completely  fill  Lake 
Superior  if  it  flowed  into  that  water  body  with  its  present  volume  and  load. 


112  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

The  hypothesis  that  the  present  Lake  Superior  basin  exists  because  of  a  geosyncline,  as 
first  stated,  needs  to  be  modified,  therefore,  bj'  consideration  of  the  possibihty  of  graben  or 
rift  fauhing.  The  amphfication  of  this  revised  hypothesis  and  its  verification  in  detail  remain 
for  future  work.     The  possibility,  however,  seems  worth  outlining  here. 

It  is  thought  reasonable  to  suppose  that  after  the  late  Algonkian  deformation,  whose  struc- 
tural warping  produced  or  redeepened  the  major  sjTichne,  the  basin  was  filled  to  a  considerable 
extent  by  lavas  and  by  sediments  overlj-ing  the  Keweenawan  flows.  Between  the  close  of  tliis 
period  of  deposition  and  the  beginning  of  the  Upper  Cambrian  a  great  period  of  denudation 
produced  the  pre-Cambrian  peneplain,  whose  surface  of  low  relief  beveled  across  the  weak  and 
resistant  members  of  the  Archean  and  Algonkian,  the  syncUnal  basin  perhaps  being  filled  with 
the  material  worn  away  in  making  the  peneplain  or  perhaps  ])eing  replaced  bj'  part  of  the 
peneplain  surface.  At  some  subsequent  date,  probalily  also  pre-Cambrian,  faulting  took  place, 
producing  the  great  escarpment  which  extends  northeastward  from  Duluth  and  smaller  nearly 
parallel  escarpments  on  the  south  shore  of  the  lake.  These  two  fault  fines  bound  what  is 
perhaps  a  great  graben  or  rift,  which  forms  the  rectangular  body  of  northern  and  western  Lake 
Superior  (fig.  8).  The  evidence  of  the  fault  origin  of  these  escarpments  may  be  gathered  from 
a  detailed  consideration  of  their  characteristics. 


PENEPLAIN  _.^  gsCARgMENT 


CAMBRIAN 
\  LAKE  SUPERIOR  GRABLN 


Sea  level 


UPPER     HURONIAN  MEWEENAWAN  KEWEENAWAN 

(ANiMmi£    group) 


FiGUKE  8.— Graben  or  rift  valley  of  western  Lake  Superior.tshowing  escarpment  on  either  side  and  ^J^neplain  above. 

DESCRIPTION   OF  ESCARPMENTS. 

DULUTH  ESCARPMENT. 

Rising  steeply  above  the  waters  of  Lake  Superior  for  about  600  to  800  feet  at  Duluth  and 
with  diminishing  height  toward  the  northeast  is  the  Duluth  escarpment  (PI.  II,  p.  86).  It  has  a 
slope  at  Duluth  of  450  to  1,000  feet  to  the  mile,  and  the  steeply  ascending  face  is  1^  to  2  miles 
wide  (PI.  V,  A).  Above  rises  the  fairly  level-topped  gabbro  plateau,  wluch  extends  north- 
ward as  part  of  the  peneplain.  The  escarpment,  wlxich  bounds  tliis  plateau  on  the  southeast, 
is  remarkably  simple  in  its  outhne,  with  none  of  the  irregularity  which  characterizes  slopes 
long  eroded  by  streams.  Tliis  simplicity  of  outline  is  shared  by  the  gently  curved  escarpment 
of  Keweenaw  Point  and  by  that  of  northern  Wisconsin,  both  of  which  are  kno\vn  to  foUow  fault 
fines.  Lawson  has  suggested  that  the  Duluth  escarpment  also  foUows  a  fault  line."  We  have 
then  to  account  for  its  fresh  and  uneroded  form,  for  it  is  quite  inconceivable  that  a  fault  scarp 
could  have  been  produced,  as  tliis  may  have  been,  in  pre-Paleozoic  or  verj'  early  Paleozoic  time 
and  not  have  been  more  largely  altered  by  weathering  and  stream  erosion. 

The  streams  of  the  Duluth  escarpment  descend  very  steeply  to  Lake  Superior;  few  of 
them  head  more  than  4  or  5  miles  from  Lake  Superior  (PL  II),  the  greatest  distance  being  12 
to  14  miles,  in  contrast  with  lengths  of  30  to  75  miles  on  the  north  and  northeast  shores  of  Lake 
Superior.  Many  of  them  have  as  steep  an  average  grade  as  150  to  250  feet  to  the  mile  (PI.  V,  ^-1), 
the  general  average  being  80  to  160  feet  to  the  mile.  No  one  of  these  rather  tumultuous  streams 
has  cut  a  significantly  deep  valley  in  the  face  of  the  escarpment  and  most  of  them  have  only 
cut  short  gorges  with  small  rapids  and  waterfalls. 

Quite  in  contrast  with  these  steep-graded,  rapidly  falling  streams  of  the  escarpment  are 
the  leisurely  flowing  streams  of  the  plateau  surface  above.  The  Cloquet,  the  upper  St.  Louis, 
and  various  other  rivers  have  an  average  slope  of  about  8  or  10  feet  to  the  nnle.  It  is  well 
established  that  a  rapidly  flowing  stream  with  a  steep  grade  is  able  to  deepen  its  vaUey  rapidly 
and  to  extend  its  headwater  area  so  that  it  encroaches  upon  the  area  drained  by  an  adjacent 


a  Twentieth  Ann.  Rept.  Ocol.  and  Nat.  Hist.  Survey  Minnesota,  1891,  p.  192. 


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PHYSICAL  GEOGRAPHY  OF  THE  REGION. 


113 


FiGUEE  i 


-The  drainage  of  the  St.  Louis  and  Mississippi  headwaters  before  the  stream  captures 
along  the  Duluth  escarpment. 


leisurely  flowing  stream  (fig.  9),  capturing  and  diverting  the  latter  or  some  portion  of  its  head- 
waters. Stream  captures  or  piracies,  as  they  are  called,  of  tliis  Icind  are  common.  We  should 
expect,  then,  that  in  the  course  of  stream  development  for  a  great  length  of  time  several  of  the 
swiftly  flowing  streams  of  the  escarpment  would  have  extended  their  headwaters  back  to  the 
region  drained  by  the  leisurely  flowing  streams  of  the  plateau  surface  and  captured  part  or 
all  of  these  drainage  systems. 
The  fact  that  many  of  the 
large  streams  have  not  done 
so  is  evidence  of  their  youth. 
The  largest  stream  in  the 
region,  however,  seems  to 
have  already  done  just  what 
would  be  expected  (fig.  10), 
and  it  is  natural  that  the 
largest  stream  should  be  able 
to  do  tliis  first.  St.  Louis 
River,  cutting  back  at  a  point 
near  the  end  of  the  escarp- 
ment where  it  is  rather  low, 
has  been  able  to  extend  its 
headwater  region  northwest- 
ward until  it  has  captured  the 
southwestward-flowing  Clo- 
quet  and  the  southwestward-flowing  stream  that  forms  the  present  headwaters  of  the  St. 
Louis  itself.  These  captured  streams  had  been  a  part  of  the  leisurely  drainage  system  of 
the  plateau  surface,  and,  it  seems  certain,  were  withm  the  Mississippi  basin  (Pis.  I  and  II). 
Indeed,  a  large  valley  extending  southwestward  from  the  town  of  Floodwood,  where  the 
St.  Louis  now  turns  abruptly  to  the  southeast,  indicates  that  this  is  probably  the  latest 
elbow  of  capture  at  which  the  piratical  St.  Louis  has  been  able  to  divert  to  the  Lake  Supe- 

rior-St.  Lawrence  drainage 
system  a  large  headwater 
tributary  of  Mississippi 
River,  as  it  had  previously 
diverted  the  Clociuet,  an- 
other ^Mississippi  head- 
water, or  possibly  one  of 
the  St.  Croix. 

A  study  of  similar  fault 
scarps  acted  upon  by 
stream  erosion  in  other 
parts  of  the  world  indi- 
cates that  this  fault  scarp 
has  not  been  acted  upon 
by  erosional  agencies  for 
a  great  length  of  time. 
If  it  had  been  so  eroded 
for  a  long  period,  we  should 
find  it  deeply  cut  by  valleys  with  outlymg  knobs  on  the  lower  slopes,  like  the  erosion  escarji- 
ment  at  ^larciuette  (PL  V,  B),  and  with  stream  captures  at  the  upper  shoulder,  where  the 
escarpment  meets  the  plateau  top. 

Comparison  of  this  escarpment  with  the  ec[ually  abnipt  escai'pments  on  the  north  shore  of 
Lake  Superior  from  Thunder  Bay  to  Sault  Ste.  Marie  emphasizes  the  freshness  of  the  Duluth 
escarpment;  there  is  a  striking  contrast  in  stream  and  vallej^  distribution.     The  north-shore 

47517°— VOL  52— 11 8 


Figure  10. — The  drainage  of  the  St.  Louis  and  Mississippi  headwaters  at  present,  after  stream 

captures  and  diversions. 


114  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

escarpment  has  much  lonj^er  streams  flowing  directly  to  the  lake  from  tiie  north,  with  deep 
valleys  everywhere  cut  to  lake  level.  It  is  a  mucii-breached  wall;  the  Duluth  escarpment  is  an 
unbroken  barrier.  Tlie  drainage  of  the  former  proclaims  greater  length  of  time  for  stream 
dissection  in  the  same  language  by  which  the  drainage  of  the  hitter  aimounces  youth. 

It  seems  possible  that  erosion  by  the  Lake  Superior  lobe  of  tlui  Labrador  ice  sheet  might 
have  so  smoothed  the  face  of  this  escarpment  and  steepened  and  intensified  it  that  topography 
of  the  kind  suggested  wouhl  be  destroyed  or  that  longer  streams  draining  to  Lake  Superior 
would  be  diverted  by  the  ice  barrier  and  acquire  new  courses.  Such  modification  may  have 
taken  place  to  a  slight  degree,  but  even  if  the  maximum  of  glacial  erosion  is  assumed  the  lack 
of  stream  diversions  is  c(uite  unexplained,  as  is  also  the  resemblance  to  the  acknowledged  fault 
scarp  on  the  east  side  of  Keweenaw  Point. 

Along  the  line  by  which  this  escarpment  can  be  discriminated  as  a  form  initially  produced 
by  faulting  rather  than  by  glacial  erosion  a  scrutiny  of  the  submerged  continuation  of  the 
same  escarpment  reveals  several  significant  facts.  Fortunately  the  detailed  soundings  made 
by  the  Corps  of  Engineers  of  the  United  States  Army  in  charting  the  Great  Lakes  give  us  detailed 
information  (PI.  II)  concerning  the  escarpment  below  present  lake  level.  First,  it  continues 
to  descend  at  as  steep  or  steeper  angles  than  on  the  land,  a  depth  of  400  to  600  feet  being  found 
within  2  to  3  miles  from  any  part  of  the  shore.  The  escarpment,  therefore,  is  not  merely  400 
to  600  feet  but  1,000  to  1,200  feet  in  height.  Second,  it  extends  directly  across  the  moutlis 
of  the  several  large  bays  (Thunder,  Black,  and  Nipigon)  at  the  north  end  of  the  lake,  where  the 
escarpment  feature  in  the  unsubmerged  land  surface  is  interrupted  by  these  broad  valleys, 
partly  drowned  beneath  the  present  lake  level.  These  are  therefore  hanging  valleys,  entermg 
the  lake  basin  or  the  linear  depression  to  which  they  are  tributary  at  levels  400  to  600  feet " 
above  its  bottom.  (See  PI.  II.)  This  submerged  hanging  valley  condition  might  be  explained 
either  by  glacial  erosion  or  by  faulting. 

The  facts  in  favor  of  glacial  erosion  are  (a)  known  ice  flow  along  this  coast  and  parallel  to 
it;  (h)  probably  accentuated  erosive  ability  in  this  portion  of  the  Lake  Superior  basin,  where 
more  rapid  movement  would  result  from  the  constriction  of  the  ice  between  Isle  Royal  and 
the  mainland;  (c)  the  known  ability  of  glaciers  of  no  greater  thickness  and  less  width  to  erode 
so  deeply  that  main  valleys  receive  discordant  tributaries  (hanging  vallej-s)  as  much  as  500 
to  1,000  feet  above,  as  in  Alaska,  the  Swiss  Alps,  Scotland,  Norway,  New  Zealand,  etc. 

Points  in  favor  of  faulting  are  the  following:  (a)  The  straightness  of  the  escarpment; 
(h)  the  continuation  below  lake  level  of  a  topograpliic  feature  whose  drainage  and  other  land 
phenomena  are  inexplicable  by  glacial  erosion  alone;  (c)  the  uniform  level  at  which  the  sub- 
merged hanging  valleys  stand  (Thunder  Bay  22  to  23  fathoms,  Black  Bay  22  fathoms,  Xipigon 
Strait  20  to  21  fathoms).  Such  uniformity  is  unusual  in  glacially  eroded  hanging  valleys, 
where  the  size  of  the  glaciers  in  tributary  valle^^s,  their  width,  tliickness,  and  eroding  power, 
produce  hanging  valleys  at  diverse  levels.  Glaciers  of  the  unecjual  sizes  denoted  by  these 
bays  would  surely  have  done  so.  (d)  The  varying  age,  character,  and  resistance  of  the  rocks 
beveled  across  by  this  supposed  fault  (Cambrian  sandstones,  Keweenawan  lavas  and  sediments, 
upper  Huronian  intrusives  and  slates,  and  older  rocks). 

The  escarpment  therefore  seems  to  have  features  inexplicable  b\'  glacial  erosion  alone, 
but  none  that  do  not  fit  the  hypothesis  of  glacial  erosion  motlifying  a  faulted  form.  The 
exceptional  depth  of  water  just  opposite  the  mouth  of  Thunder  Bay  (156  fathoms),  making  this 
point  936  feet  deep,  or  more  than  300  feet  below  sea  level,  and  the  second  deepest  place  in  tlio 
lake,  can  be  readily  explained  by  glacial  scooping  at  just  this  point,  for  such  irregularity  in  the 
bottoms  of  glacially  eroded  channels  like  the  Norwegian  and  Alaskan  fiords  are  not  uncommon. 

The  writer  accordingly  feels  that  there  is  a  reasonable  possibility  that  the  northwest  shore 
of  Lake  Superior  from  a  point  west  of  Duluth  to  St.  Ignace  at  the  north,  with  its  direct  but 
broadly-curving  course,  represents  the  position  of  a  fault  line.  This  fault  scarp,  with  1 ,000  feet 
or  more  of  throw,  may  either  be  very  recent,  though  several  considerations  lead  to  the  belief 

o  Bottom  of  Thunder  Bay,  22  fathoms  or  132  feet;  depth  of  trough  opposite  mouth,  US  fathoms  or  0T8  feet. 


PHYSICAL  GEOGRAPHY  OF  THE  REGION.  115 

that  this  is  not  so,  or  else  it  may  have  been  faulted  long  ago  and  then  buried  and  protected  so 
that  erosion  has  only  recently  begun  to  attack  it.  Accordingly  it  may  owe  the  preservation 
of  its  southwesterly  portion  (Minnesota  shore)  to  protection  by  Cambrian  or  later  sediments 
and  the  dissection  of  its  northeasterly  part  (Ontario  shore)  to  the  earlier  removal  of  such  a 
protecting  Cambrian  mantle.  Glaciation  is  believed  to  have  modified  this  escarpment  in  its 
minor  features  only,  as  in  changing  a  more  precipitous  slope  to  the  present  flaring  wall  and  in 
locally  deepening  the  depression  at  its  base. 

KEWEENAW  ESCAKPMENT. 

The  escarpment  of  the  east  side  of  the  Keweenaw  Point  "■  very  closely  resembles  the  Duhith 
escarpment  in  form  and  condition  of  erosion  though  not  so  high  nor  so  steep  (PI.  II).  A  north- 
east-southwest trendmg  escarpment  borders  the  east  side  of  "an  elongated  promontory,*  not 
greatly  dissected  by  erosion  nor  deeply  undulate  nor  serrate  in  its  crest  line,"  whose  flat  top 
has  been  formed  by  the  base-levehng  '^  of  a  series  of  steeply  dipping  Keweenawan  beds  and  whose 
western  and  northwestern  sides  slope  more  gradually  to  the  level  of  Lake  Superior;  the  east 
side  slopes  steeply  to  the  open  lake  near  the  tip  and  is  elsewhere  separated  from  the  lake  by  the 
low-lying  flat  portion  imderlain  by  the  Cambrian  sandstone  (PI.  XXVIII,  p.  380). 

This  escarpment  differs,  however,  fi-om  the  Duluth  gabbro  escarpment  in  one  important 
respect.  It  is  cut  entirely  through  by  stream  valleys  in  at  least  two  places.  It  is  believed 
that  the  great  transverse  valley  of  Portage  Lake  (PI.  XXX,  B,  p.  434)  and  the  valley  of  Ontonagon 
River  were  formed  before  the  present  Lake  Superior  existed,  by  streams  which  were  superposed 
on  this  long,  narrow  peninsula  through  a  mantle  of  Cambrian  (Lake  Superior)  sandstone,  whose 
remnants  are  still  preserved  high  ujjon  the  fault  scarp  near  the  highest  part  of  Keweenaw 
Pomt.''  Irving  and  Chamberlin,"  after  careful  consideration  of  the  many  earlier  hypotheses, 
reach  the  conclusion  that  the  Keweenaw  Point  scarp  is  a  pre-Potsdam  fault  modified  by  wave 
work,  buried,  and  slightly  refaulted  in  post-Potsdam  or  post-Cambrian  time.     (See  fig.  75,  p.  574.) 

ESCARPMENT  OF  NORTHERN  WISCONSIN  (SUPERIOR  ESCARPMENT). 

The  escarpment  wliich  forms  the  boundary  of  the  northern  highlands  of  Wisconsin  f  and 
overlooks  the  basm  of  Lake  Superior  from  a  point  west  of  Duluth  eastward  to  the  Apostle 
Islands  is  a  lower  and  more  gently  sloping  scarp  (PL  II).  It  has  the  characteristics  of  the 
other  two  escarpments  in  being  without  topographic  outliers  and  in  having  short,  steeply 
sloping  stream  courses  which  have  not  extended  headward  much  beyond  the  shoulder  of  the 
escarpment. 

Chamberlin?  concludes  that  this  escarpment  of  Bajrtield  and  Douglas  counties,  Wis.,  is  a 
pre-Potsdam  fault  scarp,  and  Grant  ^  has  supported  this  conclusion  but  makes  its  age  post- 
Potsdam.  Like  the  Duluth  and  Keweenaw  escarpments,  it  seems  to  have  been  protected  so  that 
its  dissection  has  been  somewhat  postponed  Its  youth  is  therefore  not  so  anomalous  as  W.  M. 
Davis  has  suggested.  * 

ISLE  ROYAIi   ESCARPMENT. 

On  the  north  side  of  Isle  Royal  there  is  a  submerged  escarpment  of  400  to  500  feet,  suggest- 
ing a  parallel  fault  here  (PL  II) ,  wliich  Irving  and  Chamberlin '  conceived  of  as  possibly  a  contin- 
uation of  the  fault  of  Bayfield  and  Douglas  counties  on  the  south  shore.     There  is  no  continua- 

0  Ir^dng,  R.  D.,  and  Chamberlin,  T.  C,  Observations  on  the  junction  between  the  Eastern  sandstone  and  the  Keweenaw  series  on  Keweenaw 
Point:  Bull.  U.  S.  Geol.  Survey  No.  23, 1885,  pp.  12,  98-119. 

6  Idem,  p.  103. 

<■  Van  Hise,  C.  R.,  Science,  new  ser.,  vol.  4, 1896,  pp.  217-220. 
i  Bull.  U.  S.  Geol.  Survey  No.  23, 1885,  pp.  109-110. 
t  Idem,  p.  119. 

f  Chamberlin,  T.  C,  Geology  of  Wisconsin,  vol.  1,  1SS3,  pp.  105-100.    Grant,  U.  S.,  Bull.  Geol.  and  Nat.  Hist.  Survey  Wisconsin  No.  6, 
I90I,  p.  0. 

9  Geology  of  Wisconsin,  vol.  1, 1883,  p.  105. 

»  Bull.  Geol.  and  Nat.  Hist.  Survey  Wisconsin  No.  fi,  1901,  pp.  17-20. 

*  Science,  new  ser.,  vol.  15,  1902,  p.  234. 

1  Bull.  U.  S.  Geo!.  Survey  No.  23,  1885,  p.  111. 


116  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

tion  of  this  steep  slope  northeast  or  southwest  of  Isle  Royal,  wliich  stands  on  a  high  base  with 
steep  descents  on  all  sides  of  it,  especially  the  northwest  and  southeast.  If  the  channel  north- 
west of  Isle  Royal  is  ascribed  to  block  faulting,  the  island  itself  must  be  regarded  as  a  land 
mass  that  stands  as  a  horst  above  the  deep  surrounding  basin  because  of  failure  to  be  faulted 

down. 

Isle  Royal  and  Keweenaw  Point  accordingly  have  certain  features  in  commcjn  aside  from 
familiar  fact  that  the  Keweenawan  rocks  in  Isle  Royal  dip  soutlieast  and  those  at  Keweenaw 
Point  dip  northwest.  The  slopes  facing  each  other  seem  to  be  dip  slopes,  but  of  the  sides  facing 
awiiy  fiom  each  other  that  of  Keweenaw  Point  is  known  to  be  a  fault  Une,  and  that  of  Isle 
Royal  may  possibly  be  a  smaller  one.  This  structural  feature,  then,  would  be  a  great  synchnal 
trough  between  Isle  Royal  and  Keweenaw  Point,  with  downfaultmg  on  each  side. 

Massing  of  the  contours  in  other  parts  of  the  lake  (PI.  II)  suggests  submerged  escarpments 
east  of  this  trough,  but  there  is  not  enough  information  for  detailed  discussion. 

AGE  OF  ESCARPMENTS. 

For  all  these  subparallel  escarpments  grouped  about  the  west  end  of  Lake  Superior  the 
hypothesis  is  advanced  that  they  have  been  formed  by  faulting.  Their  later  liistory  may 
have  accorded  with  one  of  two  hypotheses.  One  supposes  that  they  are  old  escarpments 
(pre-Cambrian)  sHghtly  modified  by  stream  erosion  and  in  places  possibly  developed  mto  sea 
chffs  and  then  buried  beneath  Paleozoic  sediments.  Durmg  the  ensumg  long  period  of  denuda^ 
tion  the  escarpments  themselves  were  protected  from  erosion  by  the  overlyuig  sediments. 
They  were  gradually  uncovered  and  are  now  just  in  the  begiiming  of  a  cycle  of  erosion,  wliich 
was  postponed  until  their  rather  recent  disinterment.  The  alternative  hypothesis  that  these 
are  much  more  recent  fault  scarps  (post-Cretaceous  or  pre-Pleistocene)  is  supported  by  the 
evidence  of  slight  post-Cambrian  movement  along  two  of  these  scarps  (along  which  tliere  was 
surely  much  greater  pre-Cambrian  f  aultmg)  and  by  the  evidence  of  post-Cretaceous  and  of  post- 
Pleistocene  faulting  in  other  parts  of  the  area.  The  question  of  the  date  of  this  faulting  is  a 
large  one,  involving  the  determination  of  the  age  of  the  great  peneplam  of  the  area  and  the 
age  of  the  present  Lake  Superior  basin. 

BEARING  OF  ESCARPMENTS  ON  AGE  OF  PENEPLAIN. 

There  are  three  fields  for  attacking  the  problem  of  the  age  of  the  peneplain  in  the  Lake 
Superior  region.  The  first  is  m  northern  Wisconsm,  where  the  truncated  siu-face  of  the  pre- 
Cambrian  now  dips  down  imder  the  Paleozoic.     The  conditions  here  are  shown  m  figure  IL 


BELTED    PLAIN 
''^^'^,^'.'l;n..  ,  .  CL      -.  - -—       A '-°r^L^!l.°f°'?.   -.—„.„-_-    .... 


HUPONIAN  SERIES 


FiGUEE  11.— Structure  profile  in  northern  Wisconsin,  showing  the  south  edge  of  the  peneplain  on  the  pre-Cambrian  rocks  and  the  northern  part  of 

the  belted  plain  of  the  Paleozoic. 

Weidman  has  demonstrated  that  h-c  is  a  buried  pre-Potsdam  peneplain  and  mferred  that  a-h  is 
its  exhumed  equivalent.  Van  Ilise  previously  referred  to  h-d  as  a  Cretaceous  i>eneplaui  and  to 
a-h  as  its  equivalent.  So  far  as  the  writer  can  see,  evidence  for  decidmg  conclusively  between 
these  two  hypotheses  is  not  present,  though  the  Paleozoic  outliers  on  the  peneplain  suggest  that 
it  is  pre-Potsdam  rather  than  CYctaceous. 

The  second  field  of  attack  is  in  the  region  to  the  west,  in  Minnesota  (PI. XIV,  p.  212).  'Ilere  the 
Cretaceous  overlaps  the  peneplain.  Numerous  diamond-drill  holes  tlirough  tlie  glacial  drift  on 
the  Cuyuna  range  show  the  Cretaceous  as  a  thin  mantle  on  the  peneplam  of  pre-Cambrian  rocks. 
Elsewhere  the  drift  covers  it  deeply,  but  on  the  border  of  the  Giants  Range  monadnock,  in  the 
]\Iesabi  iron  range,  Cretaceous  outliers  are  found  m  valleys  and  on  ridge  slopes  (PI.  MIT,  in  pocket). 
These  are  marme  Upper  Cretaceous,  so  the  peneplam  might  perfectly  well  be  either  pre-Cambrian 


PHYSICAL  GEOGRAPHY  OF  THE  REGION.  117 

or  early  Cretaceous  in  age.  If  the  Cretaceous  cau  be  found  in  valleys  in  the  peneplain  as  well 
as  in  valleys  on  the  slopes  of  its  monadnocks,  the  probability  of  pre-Cambrian  age  will  be 
strengthened . 

The  thirtl  and  most  jiromising  field  for  investigation  is  in  tlie  fault  scarps  themselves.  The 
escarpments  were  clearly  made  after  the  great  peneplain  was  developed,  for  the  nearly  base- 
leveled  upland  areas  now  extend  neatly  up  to  the  edges  of  these  steep  slopes  (fig.  8,  p.  112)  and  could 
not  have  done  so  when  the  peneplain  was  formed.  The  two  latest  periods  of  great  base-leveling 
in  the  area  are  thought  to  be  pre-Cambrian  (pre-Potsdam)  and  Cretaceous.  The  known  periods 
of  faultmg  are  pre-Cambrian,  post-Cambrian,  post-Cretaceous,  and  post-Pleistocene.  The  Lake 
Superior  basin  was  surely  here  in  pre-Pleistocene  time,  so  the  post-Pleistocene  may  be  elimmated 
as  a  period  of  major  faulting.  The  choice  seems  to  lie  between  (a)  regarding  the  peneplain  as 
due  to  Cretaceous  base-leveling  and  the  escarpments  as  due  to  post-Cretaceous  faulting,  to 
which  there  are  certain  objections,  and  (b)  regarding  the  peneplam  as  an  exhumed  slightly 
dissected  pre-Cambrian  surface  and  the  escarpments  as  due  to  pre-Camorian  faulting.  The 
assimaption  of  protection  by  Paleozoic  sediments  is  necessary  in  order  to  explain  the  relatively 
fresh  fault-scarp  forms,'  and  from  this  assumption  naturally  follows  the  hypothesis  of  the  clear- 
ing out  of  the  basin  and  exhumation  of  the  escarpments  iluring  th,e  Cretaceous  base-leveling  and 
the  glacial  period,  all  the  later  faulting  beuig  considered  of  slight  amomit.  There  are  objections 
to  this  hypothesis  also,  but  in  the  mind  of  the  writer  they  are  of  less  weight. 


CHAPTER  V.   THE  VERMILION  IRON  DISTRICT  OF  MINNESOTA." 

LOCATION,  AREA,  AND  GENERAL  GEOLOGIC  SUCCESSION. 

'Ihe  Vcnnilion  iron-bearing  district  lies  in  northeastern  Minnesota,  in  St.  Louis,  Lai<e,  and 
Cook  counties  (Pi.  VI).  The  district  extends  about  N.  70°  E.  from  near  the  west  end  of  \'er- 
niilion  Lake,  in  west  longitude  92°  30',  to  the  vicinity  of  Gunflint  Lake  on  tiic  international 
boundary,  longitude  90°  45',  and  lies  between  47°  45'  and  4S°  15'  north  latitude.  The  district 
is  for  the  most  part  5  to  10  miles  broad  but  locally  as  much  as  12  or  15  miles,  and  at  the  east- 
ern end  it  is  divided  into  two  narrow  belts  by  the  granite  of  Saganaga  Lake.  The  length  of 
the  district  is  about  100  miles. 

The  jjrotkictive  iron-bearing  rocks  are  bounded  on  the  north  In'  the  granite  of  Basswood 
Lake,  on  the  east  by  the  granite  of  Saganaga  Lake  and  the  Animikie  group,  and  on  the  south 
in  turn  from  east  to  west  by  the  Keweenawan  Duluth  gabbro,  lower  Iluronian  granite,  and 
Archean  granite.  On  the  west  the  iron-bearing  and  other  formations  disappear  under  the  Pleis- 
tocene. Part  of  the  eastern  half  of  the  Vermilion  range  extends  north  of  the  international 
boundary  into  Hunters  Island.  The  rocks  of  the  eastern  extension  of  the  north  arm  of  tiie 
VermiHon  range  are  known  locally  as  the  Hunters  Island  iron-bearing  series. 

The  stratigraphic  succession  in  the  Vermilion  district  is  as  follows,  in  descending  order: 

Quaternary  system: 

Pleistocene  series Drift. 

Unconformity. 
Algonkian  system: 

Keweenawan  series Duluth  pabbro  and  Logan  sills. 

Unconformity. 

Huronian  series: 

Upper   Huronian  (Animikie  group)  * J         „■         ' 

[Guniiint  formation  (iron  beanng). 

Unconformity. 

Intrusive    rocks:  Granites,    granite    por- 
phyries, dolerites,  and  lamprophyres. 
Knife  Lake  slate. 
Agawa  formation  (iron  bearing). 
Ogishke  conglomerate. 
Unconformity. 
Archean  system : 

Laurentian  series Granite  of  Basswood  Lake  and  other  intru- 
sive rocks. 
{Soudan  formation  (iron  bearing). 
Ely  greenstone,   an   ellipsoidally   parted 
basic  igneous  and  largely  volcanic  rock. 

This  chapter  is  primarily  concerned  with  the  Archean  and  the  lower-middle  Huronian, 
■which  really  constitute  the  rocks  of  the  Vermilion  district.  The  higher  rocks  will  be  mentioned 
only  so  far  as  it  is  necessary  to  do  so  in  order  to  give  a  satisfactory  treatment  of  tlie  lower  rocks. 
The  Animikie  group,  which  occurs  at  the  east  end  of  the  district,  and  the  Keweenawan  series, 
which  borders  a  large  part  of  the  southern  portion  of  the  district,  will  be  treated  in  Chapters 
VIII  and  XV. 


Lower-middle  Huronian. 


a  For  a  further  detailed  description  of  the  geology  of  this  dlstriot,  see  Clements,  J.  M. ,  The  Vermilion  iron-bearing  district  of  Minnesota:  Mon. 
U.  S.  Geoi.  Survey,  vol.  45,  1903,  and  references  there  given. 
'Confined  to  eas;  end  of  district. 

118 


MONOGRAPH  Lll     PLATE  VI 


VERMILION  IRON  DISTRICT.  119 

TOPOGRAPHY. 

The  topography  of  the  district  may  be  defined  briefly  as  characterized  by  hiK^-ir  l)lufl^y 
ridges,  in  the  depressions  between  whicli  are  numerous  hnear  lakes,  the  whole  constituting  a 
relatively  even  peneplain  with  a  few  monadnocks.  The  general  physiography  is  discussed  in 
Chapter  IV. 

The  position  of  the  ridges  and  valleys  is  determined  by  the  character  of  the  rocks.  The 
more  resistant  rocks  form  the  ridges,  the  less  resistant  the  valleys.  On  the  whole  the  most 
resistant  rock  of  the  region  is  the  Ely  greenstone,  and  this  constitutes  a  greater  proportion  of 
the  bluffs  of  the  district  than  any  other  formation. 

Next  in  importance  to  the  greenstone  as  a  bluff-makmg  formation  is  the  iron-bearing 
Soudan  formation.  This  independently  constitutes  a  number  of  high  bluffs,  and  conjointly 
with  the  greenstone  helps  to  make  many  others. 

The  depressions,  especially  those  containing  lakes,  are  mamly  engraved  in  the  Knife  Lake 
slate.  This  is  true  of  most  of  the  important  lakes  of  the  district,  such  as  Vermilion  Lake,  Long 
Lake,  Fall  Lake,  Moose  Lake,  Ogishke  Lake.  However,  some  of  the  lakes,  especially  those  that 
are  roundish,  are  in  other  formations,  notably  the  granite,  which  in  this  district  seems  to  be 
not  much  more  resistant  than  the  slate.  Important  lakes  of  this  class  are  Wliite  Iron  Lake, 
Basswood  Lake,  Snowbank  Lake,  and  Saganaga  Lake. 

The  Ogishke  conglomerate  is  intermediate  in  resisting  power  between  the  slates  and  green- 
stones. In  places,  therefore,  it  occupies  the  valley,  as  at  ^"erniilion  Lake,  and  in  places  makes 
considerable  bluffs,  as  in  the  eastern  part  of  the  district;  but  more  commonly  the  conglomerate 
is  found  on  the  slopes,  because  it  lies  structurally  between  the  harder  greenstones  and  the  softer 
slates. 

ARCHEAN   SYSTEM. 

The  Archean  is  represented  by  both  the  Keewatin  series  and  the  Laurentian  series.  The 
Keewatin  comprises  the  Ely  greenstone  and  the  Soudan  formation.  The  Laurentian  includes 
granites,  porphyry,  and  associated  acidic  rocks. 

KEEWATIN  SERIES. 
ELY  GREENSTONE. 

DISTRIBUTION. 

The  Ely  greenstone  is  the  most  conspicuous  and  extensive  formation  of  the  district.  From 
Vermilion  Lake  to  the  central  part  of  the  district  it  occupies  the  larger  part  of  the  area  between 
the  granites  to  the  north  and  south.     In  the  eastern  half  of  the  district  it  is  less  extensive. 

The  formation  is  conspicuous  not  only  because  of  its  areal  extent,  but  because  of  its  topo- 
graphic importance.  In  general  its  rocks  are  resistant,  and  many  of  the  high  knobs  of  the  dis- 
trict are  composed  of  them — for  example,  those  about  Tower  and  Ely.  They  form  Disappoint- 
ment Mountain,  near  Disappointment  Lake,  one  of  the  most  prominent  features  of  the  district. 
They  compose  the  great  promontory  of  Knife  Lake,  in  sec.  21,  T.  65  N.,  R.  7  W.,  so  conspicuous 
a  feature  along  the  mternational  boundary.  In  fact,  most  of  the  high  knobs  to  be  seen  from 
almost  any  commanding  point  of  view  between  the  northern  and  southern  granites  are  composed 
of  the  Ely  greenstone.  Such  knobs  are  consj^icuous  even  where  the  areas  of  the  greenstone  are 
subordinate — for  example,  the  high  bare  headland  above  Moose  Lake. 

A  few  of  the  important  bluffs  are  due  to  the  resistant  quality  of  the  Ely  and  Soudan  for- 
mations together — for  instance,  Soudan  and  Lee  hills,  near  Tower,  and  a  number  of  the  promi- 
nent bluffs  of  Hunters  Island,  along  the  north  side  of  Otter  Track  Lake,  and  elsewhere. 

APPEARANCE    AND    STRUCTURE. 

The  Ely  greenstone  has  as  its  dominant  color  various  tones  of  green.  It  comprises  green- 
stones, tuffs,  and  slates,  but  the  latter  two  varieties  of  rock  are  very  subordinate.  The  domi- 
nant rocks  of  the  formation  are  called  greenstone  rather  than  a  jietrographic  name  because 


120  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

niiiny  of  them  have  been  so  modified  by  metamorphism  that  in  the  field  it  is  often  impossible 
to  determine  their  character  or  to  discriminate  between  the  difFerent  i)hases.  This  alteration 
is  no  more  than  one  would  expect  from  their  great  age.  For  the  most  part  the  changes  are 
dominantly  metasomatic  rather  than  dynamic,  so  that  the  massive  rocks  still  retain  their  original 
structures  and  textures,  though  their  mineral  composition  is  now  largely  or  wholly  changed. 

Clements's  petrograpliic  study  of  these  greenstones  shows  that  they  correspond  to  inter- 
mediate aadesites  and  basic  basalts.  The  massive  exposures  of  this  greenstone  very  commonly 
show  one  or  more  of  the  three  structures — the  amygdaloidal,  spheruhtic,  and  elhpsoidal.  Xot 
only  are  these  macroscopic  structures  common,  but  textures  such  as  ophitic,  poikilitic,  and 
porphyritic  often  may  be  seen.  The  rocks  vary  greatly  in  their  fineness  of  grain  from  aphaiiitic 
to  coarse  gramed. 

Of  the  structures  mentioned  as  characteristic  of  the  rocks  the  most  common  is  tlie  amyg- 
daloidal, tliis  structure  usually  being  found  in  the  fuier-grained  varieties.  It  is  especially 
noticeable  on  the  weathered  surface. 

The  greenstones  not  uncommonly  show  true  spheruhtic  structures,  but  these  are  not  by 
any  means  so  common  as  the  amygdaloidal  structure.  This  structure,  though  very  rare  in  basic 
rocks,  is  exliibited  m  this  ancient  formation  in  as  great  perfection  as  in  modern  acidic  rocks. 

The  third  structure,  tlie  ellipsoidal,  is  the  most  distinctive  one  of  the  formation.  Almost 
any  large  mass  of  the  Ely  greenstone  encountered  between  Tower  and  Gunflint  Lake  will  exliibit 
this  structure.  The  rock,  observed  at  a  distance,  seems  to  be  mainly  composed  of  a  mass  of 
elhpsoids  of  rock,  var\-ing  from  a  few  inches  to  several  feet  in  diameter  (PI.  VII).  Ordinarily, 
however,  the  elhpsoids  range  from  6  inches  to  3  feet  in  diameter,  and  perhaps  most  commonly 
they  are  between  1  and  2  feet  in  diameter.  These  ellipsoids  are  set  in  a  matrix  of  material  not 
greatly  different  from  the  ellipsoids  themselves  but  usually  of  slightly  different  color  and  text- 
ure. In  many  ])laces  they  have  undergone  peripheral  alteration,  so  that  they  exhibit  a  zonal 
arrangement. 

If  the  ellipsoids  are  examined  somewhat  more  closely,  many  of  them  are  found  to  be  amj'g- 
daloidal;  moreover,  in  many  of  the  spheroids  the  amygdules  are  more  abundant  near  the  border 
than  m  the  interior,  and  not  uncommonly  all  the  ellipsoids  of  an  exposure  are  more  amygdaloidal 
on  the  same  side.  The  origin  of  these  elhpsoidal  rocks  is  discussed  by  Clements  in  the  mono- 
graph on  the  Vermilion  district  and  by  the  authors  on  pages  510-512  of  this  monograph. 

Within  short  distances  the  greenstones  vary  from  fuie  to  coarse  textures  and  from  varieties 
which  exliibit  the  structures  mentioned  to  others  in  which  they  are  absent.  In  many  i)laces 
these  phases  alternate  at  short  mtcrvals. 

Everj^  gradation  maj^  be  found  from  the  undeformed  elhpsoids  to  a  schist.  In  the  transition 
the  elhpsoids  become  flatter  and  flatter,  until  finaUy  the  representative  of  each  is  f£  lenticular 
area  perhaps  many  times  as  long  as  it  is  broad.  Since  the  exterior  of  tlie  ellipsoids,  as  has  already 
been  explamed,  usually  has  a  diirerent  color  from  the  core  and  a  somewhat  different  texture,  an 
extremely  flattened  ellipsoid  has  tliree  bands.  The  occurrence  of  this  phenomenon  in  the  manj' 
ellipsoids  transforms  the  greenstone  to  a  fissile  banfled  schist  which  has  a  ver}*  marked  sedi- 
mentary appearance.  Indeed,  m  dealing  Adtli  the  extremely  altered  phases  it  is  difficult  to 
believe  that  the  rock  is  not  a  sediment  rather  than  a  metamorphosed  lava. 

In  many  places,  without  reference  to  the  ellipsoidal  structure,  the  greenstones  are  scliistose. 
However,  this  schistosity  is  not  nearly  so  common  as  in  the  Marquette  and  ^Icneminee  districts. 
In  consequence  of  the  relative  lack  of  schistosity,  the  original  cliaracters  of  the  iVi-chean  green- 
stone are  better  exhibited  in  this  district  tlian  in  any  other  on  the  iVmerican  side  of  the  boundary. 
It  is  not  unreasonable  to  suppose  that  it  may  be  possible  by  further  detailed  mapping  to  work 
out  the  succession  of  flows  for  the  Ely  greenstone. 

MINERAL   CONSTITUENTS. 

A  microscopical  study  of  the  greenstones  shows  that  the  original  minerals  are  largely 
altered.  The  following  origuial  constituents  are  disclosed:  IIornl)lcn(le,  augite,  plagiodase, 
quartz,  titaniferous  magnetite,  and  apatite.  The  original  hornblende  is  the  common  l)rown 
variety.     The  augite  varies  from  yellow  to  yellowish  green  and  possesses  its  normal  cliaracters. 


U.  S.  GEOLOGICAL  SURVEY 


MONOGRAPH    Lll     PL.  VII 


.1.     ELLIPSOIDAL    PARTING    IN     ELY    GREENSTONE. 

After  Clements.     See  page  120. 


Ji.     ELLIPSOIDALLY    PARTED    ELY    GREENSTONE,    SHOWING    SPHERULITIC    DEVELOPMENT. 

After  Clements.     See  page  120. 


VERMILION  IRON  DISTRICT.  121 

The  feldspar  is  generally  so  much  decomposed  that  one  can  not  determine  its  exact  characters. 
It  is  presumed  to  be  a  labradorite.  There  is  very  little  quartz,  but  some  was  found  in  micro- 
pegmatitic  intergrowth  with  the  feldspar  and  is  presumed  to  be  a  primary  constituent.  It  may 
fill  irregular  interstices  between  the  other  minerals  as  primary  quartz  representing  the  last 
product  of  the  crystallization  of  the  rock. 

The  secondary  constituents  are  calcite,  common  green  hornblende,  actinolite,  biotite, 
clilorite,  sericite,  epidote,  zoisite,  sphene,  rutile,  feldspar,  quartz,  pyrite,  and  hematite.  The 
feldspar  has  usually  altered  to  a  mass  of  sericite,  kaolin  (?),  feldspar,  and  quartz.  In  some 
places  it  is  completely  saussuritized.  There  were  observed  occasional  irregular  but  in  general 
rounded  serpentinous  areas,  which  strongly  suggest  aggregates  of  olivine  individuals  in  wliich 
the  olivine  possesses  no  definite  crystallographic  outline.  Locallj'  the  rock  is  largely  replaced 
by  calcite.  The  abundance  of  secondary  calcite  is  one  of  the  conspicuous  features  of  the 
formation. 

CLASTIC   BOCKS. 

At  a  very  few  localities  associated  with  the  greenstones  are  small  masses  of  tuffaceous- 
looking  rocks  wliich  are  believed  to  have  been  interbedded  volcanic  elastics.  Locally  these 
tuffaceous  rocks  grade  into  fine-grained  volcanic  ash,  and  in  some  places  this  passes  into  a  well- 
banded  slavy  rock,  the  material  of  which  was  doubtless  arranged  by  water.  It  is  probable  that 
by  far  the  greater  amount,  if  not  all,  of  the  material  for  the  slate  has  been  derived  from  other 
parts  of  the  Archean  Ely  greenstone.  Parts  of  the  iron-bearing  Soudan  formation  have  similar 
relations  to  the  Ely  greenstone.     (See  pp.  126-128.) 

ACIDIC    PLOWS. 

Interbedded  and  conformable  with  the  ellipsoidal  basalts  are  frequently  to  be  observed 
intermediate  and  acidic  flows  with  surface  textures,  in  many  places  closely  associated  with 
thin  layers  of  the  Soudan  formation.  These  acidic  flows  have  been  connected  with  a  dike  of 
quartz  porphyry  similar  to  the  porphyry  cutting  the  Ely  ellipsoidal  flows,  as  in  sees.  13  and 
14,  T.  62  N.,  R.  13  W.  (See  fig.  13,  p.  123.)  These  flows  seem  to  be  later,  more  acidic  phases  of 
extrusion  than  the  Ely  basalts  and  undoubtedly  have  a  close  relation  to  the  acidic  intrusive 
rocks  discussed  under  later  headings. 

INTRUSIVE   KOCKS. 

The  Ely  greenstone  is  intruded  by  the  great  batholithic  area  of  Archean  granite  of  Basswood 
Lake  on  the  north  and  by  Ai'chean,  Huronian,  and  Keweenawan  granites  on  the  south.  There 
is  a  considerable  zone,  varying  from  less  than  half  a  mile  to  1 J  or  even  2  miles  in  extent,  adjacent 
to  these  intrusive  masses,  in  which  profound  metamorphism  has  taken  place  in  consequence 
of  the  intrusions.  The  amount  of  metamorphism  is  least  at  a  distance  from  the  granite  and 
gradually  becomes  more  intense  as  the  distance  lessens. 

The  fu'st  of  the  changes  that  are  noted  m  passmg  from  the  greenstone  toward  the  granite  area 
is  that  the  greenstone  becomes  more  schistose  and  crystalline;  also  there  is  a  large  development 
of  hornblende.  Thus  the  rock  becomes  a  hornblende  schist.  With  approach  to  the  granite 
the  hornblende  scliist  becomes  better  and  better  developed  until  it  is  a  coarsely  crystalline 
typical  hornblende  schist.  The  schist  may  be  injected  parallel  to  the  schistosity,  so  that  there 
is  produced  a  banded  gneiss,  a  part  of  the  layers  of  which  consist  mainly  of  the  modified  green- 
stone in  the  form  of  hornblende  cchists  and  the  other  part  of  the  grumite.  Both  parts  are 
igneous  rocks,  the  more  basic  parts  being  dominantly  profoundly  metamorphosed  lava,  the  more 
acid  parts  mainly  an  intrusive  rock.  Witliin  the  breadth  of  a  hand  specimen  there  may  be 
a  dozen  or  more  alternations  of  this  schist  and  granite.  In  many  places  where  the  granite 
can  not  be  distinguished  as  clear-cut  parallel  layers  in  the  schist  granitic  minerals  are  found 
along  the  lamin^E,  so  that  the  rock  has  abundant  feldspar.  There  are  all  transitions  from  the 
little-altered  greenstone  to  the  hornblende  schist,  and  from  this  kuid  of  rock  to  rocks  in  which 
feldspathic  minerals  are  developed  along  the  laminje,  and  from  tliis  variet\'  to  rocks  in  which 
the  granite  is  clearly  injected  in  parallel  layers,  thus  producing  a  gneiss. 


122  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

No  better  instance  is  known  to  us  of  the  production  of  schists  and  gneisses  the  different 
parts  of  which  are  of  different  origins  and  ages.  The  background  of  the  schist  or  gneiss  is  an 
ancient  basic  or  intcrmodiato  Lava;  another  portion  is  a  deop-soatod  acidic  uitrusivc  rock.  By 
combinadoii  of  dynamic  and  contact  action  tlie  profoundly  metamorphosed  rock  has  been 
producetl. 

A  microscopic  study  sliows  that  the  schists  and  gneisses  contain  tlie  following  constituents 
in  varying  jjroportions:  Common  green  iiornblcndc,  actinolitc,  biotite,  muscovite,  cidorite, 
epidote,  calcite,  sphene,  quartz,  feldspar,  pyrite,  and  magnetite.  The  mica  is  present  in  ver\' 
small  quantity  and  is  invariably  associated  with  amphibolc. 

The  more  metamorphosed  rocks  not  onl\-  contain  minute  granitic  injections  but  also  are 
cut  by  many  large  and  small  granite  dikes,  whicli  may  run  ])arallel  to  the  scliisto.se  structures 
or  traverse  them  at  any  angle. 

Also  within  the  uitrusive  rocks  are  fragments  of  the  Ely  greenstone,  ranging  from  small 
to  great.     These  are  usually  profoundly  metamorphosed  and  some  of  them  arc  partly  absorbed. 

The  cliaracter  of  the  contact  metamorphism  may  be  particularly  well  seen  on  the  islands 
and  mainland  along  the  northern  part  of  Vermilion  Lake  and  m  the  area  between  Ely  and 
White  Iron  Lake.  The  relations  illustrated  between  the  granite  and  the  greenstone  are  identical 
with  those  which  have  been  described  by  Lawson  with  reference  to  the  Keewatin  and  Lau- 
rentian  of  the  Rainy  Lake  and  Lake  of  the  Woods  district. 

The  Ely  greenstone  where  intruded  by  the  gabbro,  at  the  south  side  of  the  east  end  of  the 
district,  has  been  metamorphosed  into  a  spotted  hornblendic  rock  with  less  schistosity  than 
the  rock  along  the  granite  contacts. 

EXTENSION    OP   ELY   GREENSTONE    BEYOND   DISTRICT. 

It  has  already  been  noted  that  the  Ely  greenstone  extends  to  the  northeast  into  Hunters 
Island.  This  formation  has  a  very  wide  extent  in  that  district  and  the  Rainy  Lake  and  Lake  of 
the  Woods  region;  in  fact,  it  is  the  most  characteristic  rock  of  the  Keewatin  of  the  Lake  Superior 
geologic  province.  It  is  therefore  clear  that  this  volcanic  formation  is  regional  rather  than 
local. 

SOTTDAN  FORMATION. 

DISTRIBUTION. 

The  chief  exposures  of  the  iron-bearing  Soudan  formation  occur  between  Tower  on  the 
west  and  a  few  miles  east  of  Ely  on  the  east,  a  distance  of  less  than  30  miles.  Numerous  smaller 
exposures  of  the  formation  are  found  within  the  area  of  the  Ely  greenstone  for  12  or  15  miles 
farther  east,  and  large  exposures  are  also  known  to  exist  in  the  eastern  part  of  the  district,  in 
the  vicinity  of  Emerald  Lake.  A  few  of  the  more  important  localities  in  wliich  the  formation 
may  be  well  studied  are  Tower,  Lee,  and  Soudan  hills  and  Jasper  Peak.  The  Soudan  formation 
is  confined  to  the  area  of  the  Ely  greenstone  and  its  border.  Even  the  belts  mapped  as  Sou(hui 
formation  consist  of  bands  of  the  iron-bearing  formation  interbedded  or  at  least  interlaminated 
with  small  quantities  of  clastic  roclvs  and  associated  with  large  quantities  of  the  Ely  greenstone 
and  later  intrusive  roclvs.  From  tlie  large  belts  more  than  half  a  mile  wide,  dominantly  com- 
posed of  the  Soudan  formation,  to  very  narrow  stringers  or  patches  in  the  El\'  greenstone  there 
are  all  variations.  Though  here  and  there  the  large  areas  are  well  exposed,  on  the  whole  the 
formation  is  relatively  soft  as  compared  with  the  Ely  gi-eenstone,  and  therefore  it  usually  forms 
valleys.     This  is  true  even  of  the  belt  at  Ely,  which  has  been  so  great  a  producer  of  iron  ore. 

Westward  and  southwestward  from  Lake  Vermilion,  beyond  the  limits  of  the  Vermilion 
map  (Pi.  VI),  Keewatin,  Laurentian,  and  Iluronian  formations  have  been  traced  for  a  consid- 
erable distance.  An  iron-bearing  formation,  correlated  with  the  Soudan,  forms  a  consitlcrable 
belt  extending  from  Tps.  60  and  61  N.,  R.  22  W.,  southwestward  to  T.  5S  N.,  R.  27  W.  It 
is  sparsely  exposed  and  is  known  principally  by  its  disturbance  of  the  magnetic  field.  A  small 
amount  of  exploration  has  been  done  on  this  belt.  For  the  most  part  this  iron  formation 
seems  to  be  lean  and  unpromising. 


VEKMILION  IRON  DISTRICT. 


123 


DEFORMATION. 


The  folding  of  the  Soudan  formation  is  of  the  most  complicated  character.  The  major 
folds  extend  parallel  to  the  trend  of  tlio  range.  The  pressure  has  been  so  great  as  to  give  at 
many  jalaces  monoclmal  dips  entirely  across  the  formation.  For  instance,  at  the  section  near 
Tower  the  dips  are  almost  uniformly  to  the  north,  the  angles  running  as  low  as  50°.  However, 
at  many  places  on  Tower,  Lee,  and  Soudan  hills  the  dips  are  nearly  vertical,  and  at  one  place 
on  Lee  Hill,  on  the  south  side,  they  are  steep  to  the  south. 

The  cross  folding  of  the  district  has  been  only  less  severe  than  the  major  folding.  The 
pitches  of  the  folds  are  ordinarily  steep,  from  50°  to  60°,  and  at  many  places  are  vertical  or 
even  overturned. 

Both  the  longitudinal  and  the  cross  folds  are  composite — that  is,  folds  of  the  second  order 
are  superposed  upon  the  major  folds  in  each  direction,  and  upon  tliese  folds  are  folds  of  the 


FiGUBE  12. — Diagram  to  illustrate  folding  of  "drag"  type,  common  in  the  Vermilion  and  other  ranges.     Note  the  facts  that  folding  tends  to 
multiply  the  thickness  by  3  and  that  folding  of  adjacent  beds  may  not  be  marked. 

tliird  order,  and  so  on  down  to  minute  plications.  The  pressure  has  been  so  great  as  to  produce 
all  varieties  of  minor  folds,  including  isoclinal  and  fan-shaped.  Moreover,  these  varieties  of 
folds  may  be  almost  equally  well  seen  in  a  ground  plan  or  in  a  vertical  cross  section.  They  are 
beautifully  shown  at  various  places  about  Tower  and  Ely,  but  perhaps  the  most  extraordinary 
complex  folding  to  be  seen  is  that  at  the  west  end  of  the  large  island  in  the  east  part  of  Emerald 
Lake.    A  common  t_ype  of  fold  is  a  drag  fold  (illustrated  in  fig.  12),  by  which  the  formation 


m 


mm 


TT 


1 


1 


-'-■.;-J*liN 


>1 


mm 


lit 


lik 


Pi?   o 


S  11 


Basalt  extrusives      Porphyry  intrusives 
and  extrusives 


Jasper 
FiGtJEE  13.— Section  across  jasper  belt  in  sees.  13  and  14,  T.  03  N.,  R.  13  W.,  Vermilion  iron  range.  Minnesota,    Scale,  1  inch=about  85  feet. 


becomes  locally  buckled  along  an  axis  lying  in  any  direction  in  the  plane  of  bedding.  This 
type  of  folding,  while  leaving  great  local  complexity",  does  not  destroy  the  general  attitude  or 
trend  of  the  bed.  It  is  frequently  possible,  where  these  folds  are  present,  to  work  out  the  general 
trend  of  the  formation  and  its  top  and  bottom — as,  for  instance,  in  sees.  13  and  14,  T.  62  N., 
R.  13  W.,  Minnesota  (see  fig.  13) — and  for  other  areas  it  will  be  possible  by  close  detailed  surveys 
to  work  out  the  stratigraphy  of  the  Keewatin  series. 


124  GEOLOOY  OF  THE  LAKE  SUPERIOR  REGION. 

'rii(>  folding,  notwithstanding  tlio  extraordinarily  l)rittle  character  of  the  rock,  was  accom- 
plished without  major  fracture.  Frequently  Sr  solid  belt  of  jasper  may  be  seen  bent  back 
upon  itself  within  its  own  radius  with  no  sign  of  fracture.  The  deformation,  therefore,  was  in 
the  zone  of  rock  flowage,  and  no  bettor  instance  is  knowTi  to  us  of  this  kind  of  earth  movement. 
Though  the  folding  is  so  complex  as  to  give  isoclinal  or  fan-shaped  folds,  ordinarily  the  turns 
are  round  rather  than  acute,  as  they  commonly  are  in  the  Menominee  district. 

Folding  without  brecciation  is  tlie  rule,  but  in  some  places  the  Soudan  formation  has 
been  brecciated  in  an  extraordinary  manner.  It  is  broken  tlarough  and  through  by  cracks 
and  crevices,  along  which  minor  faulting  has  taken  place.  In  some  places  the  grinding  of  the 
fractured  fragments  over  one  another  has  been  so  marked  as  to  give  them  a  well-rounded  char- 
acter, and  such  a  rock  resembles  a  conglomerate,  though  it  is  really  autoclastic.  This  local 
brecciation  of  the  Soudan  formation  has  been  favorable  to  the  deposition  of  the  ores,  and  it 
may  be  suggested  that  the  general  absence  of  the  brecciation  is  the  partial  explanation,  at 
least,  of  the  very  irregular  tlistribution  and  scarcity  of  the  ore  bodies. 

The  VermiUon  district  affords  excellent  illustrations  of  complex  folds,  or  folding  in  two 
directions  at  right  angles,  and  the  formation  which  best  exliibits  tliis  folding  is  the  Soudan. 
This  is  because  the  banding  of  the  formation  is  very  marked,  so  that  the  position  of  bedding  is 
readily  determined,  and  also  because  for  the  most  part  the  rock  does  not  take  on  schistosity. 
Schistose  structure  is  absent  partly  because  the  minerals  of  the  rocks  are  not  adapted  to  a 
parallel  arrangement.  Furthermore,  the  Soudan  rocks  are  frec[uently  found  in  contact  wdth. 
the  Ely  greenstone,  and  the  contacts  give  the  pitches  of  the  cross  folds. 

The  remarkalile  complex  folding  partly  explains  the  distribution  of  the  Soudan  formation 
with  reference  to  the  Ely  greenstone.  As  upon  the  major  folds  are  superposed  secondary  and 
tertiary  folds,  numerous  patches  of  the  jasper  are  naturally  found  in  the  greenstone.  More- 
over, because  of  the  cross  folding  these  patches  may  be  very  narrow  at  one  place,  widen  out 
within  a  very  short  distance  so  as  to  make  a  thick  formation,  and  again  become  narrow. 
Wlien  the  extraordinary  complexity  of  this  folding  is  understood  it  is  only  necessary  to 
premise  an  erosion  extending  to  different  depths  in  the  Soudan  formation  before  the  lower 
Huronian  was  deposited  in  order  to  see  how  in  the  greenstone  there  may  be  patches  of  jasper 
ranging  from  a  few  feet  in  wiilth  and  length  to  the  dimensions  of  great  continuous  formation 
about  Tower  and  Ely.  But  folding  is  not  the  only  cause  of  the  present  relations,  as  is  shown  on 
page  126. 

LITHOLOGY. 

The  iron-bearing  Soudan  formation  comprises  two  classes  of  rocks.  To  all  the  varieties 
of  the  first  the  miners  apply  the  name  "jasper,"  although  only  a  portion  of  it  falls  strictly  under 
this  designation.  This  is  the  dominant  variety  of  the  rock.  Locally  interstratified  with  the 
"jasper"  or  under  it  is  an  argillaceous  variety,  which  is  mainly  slaty  but  in  some  places  is 
conglomeratic. 

The  "jaspery"  phase  of  the  Soudan  formation  consists  of  interlaminated  bands  of  finely 
crystalline  quartz,  iron  oxides,  and  various  mixtures  of  the  two.  With  these  preponderating 
minerals  are  various  subordinate  constituents,  among  which  amphibole  is  the  most  abundant, 
including  actinolite,  cummingtonite,  and  griinerite.  Pyrite  is  also  present  in  many  places.  The 
alternate  bands  of  material  of  different  color,  combined  with  the  complicated  fracturing'  and 
brecciation  of  the  formation,  make  it  a  striking  rock  which  alwaj's  attracts  the  attention  of 
the  traveler,  even  if  he  is  not  accustomed  to  closely  noticing  rocks.  The  bands  of  material 
of  different  color  vary  from  a  fraction  of  an  inch  to  several  inches  across.  The  quartzose 
bands  havfiT  various  colors — nearly  pure  white,  gray,  red  of  various  hues,  including  brilliant 
red,  and  black.  The  diirerence  in  the  color  is  chiefly  caused  by  the  contained  iron.  Hematite, 
if  in  sufficiently  fine  particles,  gives  the  brilliant  red  colors;  magnetite  and  hematite  in  larger 
particles  give  the  grays  and  blacks. 

Between  the  bands  dominantly  (|uartzose  ai-e  usually  liands  nuiiniy  composed  of  non  oxide 
Tliis  iron  oxide  may  be  either   hematite   or  magnetite  or  various  intermixtures  of  the  two- 
Occasionally  also  some  limonite  is  present. 


VERMILION  IRON  DISTRICT.  125 

The  chief  varieties  of  the  "jasper"  are  (1)  the  cherty  variety,  (2)  the  black-handed 
variety,  (3)  the  red-banded  variety,  and  (4)  the  white-banded  variety.  With  these  are 
subordinate  masses  of  (5)  the  carbonated  variety  and  (0)  the  ore  boilies. 

1.  The  cherty  variety  is  characterized  by  the  presence  of  a  predominating  amount  of 
gray  cliert,  the  iron  oxide  being  subordinate.  The  rock  is  there  a  sliglitly  ferruginous  well- 
banded  chert. 

2.  The  black-banded  form  of  the  Soudan  formtvtion  has  dark-gray  or  black  chert  bands 
interlaminated  with  black  iron-oxide  bands.  Tiie  iron  oxide  is  commonly  in  large  part  mag- 
netite. Usually  associated  with  this  magnetite  are  some  of  the  amphibole  minerals  already 
mentioned. 

3.  In  the  red-banded  kind  the  quartzose  layers  are  stained  with  innumerable  minute 
flakes  of  hematite,  which  give  the  rock  a  red  color,  in  many  places  a  brilliant  red.  The  iron 
oxide  between  the  red  bands  is  ordinarily  hematite,  usually  specular  hematite.  With  this  hema- 
tite may  be  some  magnetite.  This  red-banded  variety  is  a  well-known  jasper  of  the  Lake 
Superior  region,  to  which  Wadsworth  has  applied  the  name  jaspilite. 

4.  In  the  white-banded  kind  the  quartzose  bands  contain  comparatively  little  iron  oxide. 
The  iron-oxide  bands  between  the  layers  of  chert  are  generally  hematite,  but  this  hematite 
differs  in  many  places  from  that  of  the  jaspilite  bands  in  that  it  is  of  the  red  or  brown  variety. 
With  it  also,  in  many  places,  there  is  a  certain  amount  of  limonite. 

5.  The  banded  carbonate  variety,  while  subordinate  in  quantity,  is  important  in  refer- 
ence to  the  genesis  of  the  formation.  It  is  a  gray-banded  rock,  the  light-colored  layers  of  which 
consist  largely  of  siderite.  Between  this  sideritic  rock  and  the  ordinary  forms  there  are  aU 
stages  of  gradation. 

6.  The  positions  of  the  iron-ore  bodies  will  be  fully  discussed  later.  In  the  iron  ores  the 
silica  is  very  subordinate,  the  place  of  the  quartzose  bands  being  taken  by  iron  oxide.  The 
iron  ore  is  dominantly  hematite. 

At  the  contact  of  the  Soudan  formatioii  and  Ely  greenstone  the  cherty  variety  of  rock  is 
very  common  indeed.  In  many  places  the  rock  at  this  horizon  is  much  brecciated  and  com- 
monly has  a  conglomeratic  appearance,  which,  however^  is  believed  to  be  due  to  movement 
rather  than  to  deposition  as  a  conglomerate.  Ordinarily  this  cherty  variety  of  the  formation 
is  not  more  than  a  few  feet  thick.  Resting  upon  the  cherty  zone  in  many  places  is  the  black- 
banded  kind.  Ordinarily  at  the  top  of  the  formation  is  the  red-banded  rock,  jasper,  or  the 
white-banded  kind. 

The  succession  given  above  prevails  in  many  places  where  the  formation  is  now  thick. 
Where  the  formation  is  thin  the  red  and  white  banded  rocks  extend  from  the  top  to  the  bottom, 
and  as  at  many  places  the  formation  is  rather  thin  it  may  be  said  that  the  entire  Soudan  forma- 
tion for  much  of  that  district  consists  of  these  kinds  of  rocks,  the  cherty  variety  and  the  black- 
banded  variety  not  appearing. 

The  sideritic  rock  is  notably  local  in  its  occurrence.  It  is  generally  found  close  to  the  over- 
lying upper  Huronian  rocks. 

The  slaty  phase  of  the  Soudan  formation  differs  from  the  ordinary  phases  in  having  between 
the  silica  and  iron-oxide  bands  so  large  an  amount  of  argillaceous  material  as  to  make  laminae 
of  slate.  In  some  places  a  slaty  cleavage  has  developed  in  the  clayey  layers  but  does  not  pass 
through  the  iron-oxide  bands,  and  this  may  be  so  even  where  the  bands  of  slate  are  not  more 
than  one-fourth  inch  across.  Locally  the  slate  may  be  in  a  belt  several  feet  thick  without  inter- 
stratified  jaspery  material.  In  some  places  this  slate  is  graphitic.  At  a  few  places  at  the  bot- 
tom of  the  Soudan  formation  the  slate  passes  down  into  a  fme-grained  conglomerate  or  into  a 
tuff.  A  microscopic  examination  of  the  argillaceous  varieties  of  the  slates  shows  these  sedi- 
ments to  be  made  up  of  chlorite,  actinolite,  epidote,  sericito,  sphene,  quartz,  carbonaceous 
material  (graphite),  and  some  iron  oxides,  in  various  proportions.  The  graphitic  slates  consist 
essentially  of  graphite  and  quartz  in  exceedingly  fine  grains  and  in  some  specimens  in  very 
small  quantity. 


126  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

The  conglomeratic  phases  of  the  formation,  when  studied  under  tlie  microscope,  are  founri 
to  be  substantially  identical  with  the  tuffs  of  the  Ely  greenstone.  Thej^  now  consist  largely  of 
actinblite,  clilorite,  cpidote,  and  cjuartz. 

ORIGIN. 

From  the  foregoing  facts  it  is  clear  that  the  Soudan  is  a  sedimentary  formation,  mainly  of 
nonclastic  character.  This  would  ])crhaps  be  evident  from  the  well-bedded  character  of  the 
formation  and  especially  from  the  iron  carbonate.  Also,  as  already  indicated  by  the  descrip- 
tion of  the  different  rock  varieties,  certain  phases  of  the  formation  have  argillaceous  bands 
between  the  iron-oxide  bands,  which  are  not  uncommonly  graphitic.  Finally,  it  contains  local 
conglomerates. 

There  is  reason  for  believing  that  many  varieties  of  rock  in  the  Soudan  formation  are 
derived  from  siliceous  iron-bearing  carbonate,  precisely  as  similar  rocks  are  derived  from  this 
material  in  other  districts  of  the  Lake  Superior  region.  The  analogy  between  the  vSoudan 
formation  and  the  Negaunee  formation  of  the  Marquette  district  is  especially  close.  Substan- 
tially every  variety  of  rock  which  is  found  in  one  district  may  be  found  in  the  other.  A  variety 
may  be  somewhat  more  prevalent,  however,  in  one  district  than  in  the  other;  for  instance,  the 
amphibole  minerals  are  less  abundant  in  the  Soudan  formation  than  in  the  Xegaunee  formation. 
In  the  absence  of  local  specific  evidence  of  the  original  character  of  the  iron-bearing  rocks  in 
the  Vermihon  district  it  is  probably  not  safe  to  put  too  much  stress  on  the  similarities  with. 
other  districts  where  the  original  character  of  the  rock  is  certainly  knowTi.  One  must  admit 
the  distinct  possibility  that  the  iron-bearing  sediments  may  have  been  originally  deposited  sub- 
stantially as  banded  chert  and  iron  oxide  of  the  jasper  type. 

RELATIONS  OF  ELY  GREENSTONE  AND  SOUDAN  FORMATION. 

The  main  mass  of  the  Soudan  formation  seems  to  be  above  the  Ely  greenstone.  In  certain 
places  it  is  Itnown  to  be  in  pitching  troughs  formed  by  folding,  the  greenstone  forming  the 
walls  and  bottom,  as,  for  instance,  at  Ely  and  Soudan. 

Some  of  the  jasper  belts  of  the  Vermihon  district  are  clearly  interbedded  with  successive 
basalt  extrusives.  Such  beds,  but  a  few  feet  thick,  may  be  traced  for  hundreds  of  yards  with 
uniform  widths,  even  contacts,  and  lack  of  folding.  Wlien  tiie  adjacent  igneous  rocks  arc 
examined  closely  it  is  found  that  the  sedimentary  bands  lie  parallel  to  the  tops  and  bottoms 
of  separate  flows,  as  marked  by  amygdaloidal  and  other  surface  textures,  without  intervening 
fragmental  sediments.  This  is  well  illustrated  in  sees.  13  and  14,  T.  62  \.,  R.  13  W.,  Mimiesota. 
(See  fig.  13,  p.  123.) 

Many  of  the  jasper  bands  are  associated  even  more  closely  with  intrusive  and  extrusive 
porphj^ries  than  with  the  greenstones.  (See  p.  128.)  These  porphyries  are  found  to  be 
closely  related  to  the  extrusive  basalts  but  on  the  whole  to  follow  them  and  to  be  associated 
with  their  later  phases  of  extrusion.  This  association  of  the  iron  with  the  later  acidic  phase 
of  extrusion  is  also  seen  in  the  Woman  River  district  of  Ontario.  Its  significance  is  discussed 
on  page  513. 

The  most  common  contact  between  the  Ely  greenstone  and  the  Soudan  formation  is 
perfectly  sharp — indeed,  knifelike  in  its  sharpness.  The  rocks  are  as  sharply  separated  from 
each  other  as  if  the  Soudan  formation  were  intersected  by  the  greenstone  by  intrusion,  and 
doubtless  this  is,  at  least  in  a  few  places,  the  true  significance  of  the  relations.  Contacts  of 
the  kind  mentioned  may  be  seen  at  many  places  in  both  the  west  and  the  east  end  of  the 
district.  They  are  especially  clear  and  numerous  in  liimters  Island  and  at  Jasper  Lake,  Birch 
Lake,  and  Emerald  Lake.  At  each  of  these  lakes,  almost  at  every  large  outcrop  of  Soudan 
material,  somewhere  along  the  base  of  the  formation  the  contact  may  be  found. 

The  kind  of  contact  next  most  common  to  that  just  described  is  that  in  which  a  brecciated 
rock  occurs  between  the  iron-bearing  Soudan  formation  and  the  Ely  greenstone.  This  breccia 
ordinarily  is  not  more  than  a  few  feet  wide.  In  some  places  it  mvolves  only  the  greenstone, 
elsewhere  the  Soudan  formation  only,  in  still  other  places  both.     Thus  a  conglomerate-like 


VERMILION  IRON  DISTRICT.  127 

rock  may  show  fragments  and  matrix  mainly  of  greenstone  or  almost  wholly  of  Soudan 
formation,  or  the  two  intermingled.  In  the  last  case  the  greenstone  is  more  likely  to  be  the 
matrix  and  the  Soudan  rock  to  constitute  the  fragments.  A  breccia  of  the  greenstone  class  is 
well  seen  on  an  island  near  the  west  end  of  Otter  Track  Lake.  The  brecciated  Soudan  forma- 
tion is  well  exhibited  in  belts  of  Soudan  rock  north  of  Robinson  Lake,  in  sec.  7,  T.  62  N.,  R.  13  W. 
A  breccia  composed  of  greenstone  and  Soudan  material  is  seen  at  various  places  on  Lee  Hill. 
Here  is  a  green  schist  matrix  containing  numerous  fragments  of  red  jasper,  each  exhibiting  its 
banding,  which  lies  in  diverse  directions.  Some  of  these  fragments  are  well  rounded;  others 
are  subangular;  many  others  have  angular  rhomboidal  forms,  such  as  are  produced  by  shear- 
ing stresses.  However,  these  fragments  are  not  more,  angular  than  those  in  a  basal  conglom- 
erate at  many  localities. 

The  c^uestion  may  be  asked  whether  the  breccias  were  conglomerates  before  they  were 
breccias.  At  present  their  dominant  structure  is  doubtless  that  of  a  dynamic  breccia,  but  it 
is  also  possible  that  some  of  them  at  least  were  originallj^  conglomerates  and  were  subse- 
quently brecciated.  This  question,  early  asked,  is  still  unanswered.  Probably  certain  of  the 
rocks  referred  to  are  wholly  breccias,  being  produced  by  readjustment  along  the  contact  of 
the  two  formations  during  orogenic  movements.  A  sharp  contact  of  the  first  class  might,  by 
close  folding  and  adjustment  between  the  formations,  produce  a  contact  of  the  second  class 
by  brecciation  and  rounding  of  the  fragments,  thus  forming  a  pseudoconglomerate. 

At  contacts  of  a  third  kind  is  a  rock  which  seems  to  be  a  metamorphosed  mechanical 
sediment.  As  a  rule,  this  rock  varies  from  a  few  inches  to  several  feet  in  thickness.  It  consists 
of  alternating  laj'ers  of  green  schist  or  slate  and  light-colored,  strongly  siliceous,  graywacke-like 
material.  These  alternations  of  schist  and  graj^wacke  naturally  give  a  remarkably  sedimentary 
appearance;  in  fact,  it  seems  as  if  the  banding  could  have'  been  produced  in  no  other  way. 
The  two  localities  which  best  exhibit  these  materials  are  a  neck  of  land  between  two  small 
lakes  about  a  mile  north  of  Moose  Lake  and  one  place  on  Lee  Hill.  At  the  first  locality 
alternating  bands  of  slate  and  graywacke  rest  against  perfectly  typical  ellipsoidal  greenstone, 
and  interstratified  with  these  slates  and  graywackes  are  narrow  bands  of  jasper.  These  alter- 
nations are  overlain  by  a  broader  belt  of  jasper.  The  probable  interpretation  of  the  phenomena 
seen  here  is  that  a  few  feet  of  mechanical  sediments  were  deposited  upon  the  Ely  greenstone 
before  the  deposition  of  the  nonclastic  material  of  the  Soudan  formation.  Moreover,  it  seems 
that  there  were  alternations  between  the  condition  of  mechanical  deposition  and  the  peculiar 
condition  of  chemical  or  organic  deposition  of  the  Soudan  formation. 

The  relations  at  Lee  Hill  are  substantially  the  same,  except  that  at  this  place  the  folding 
is  so  close  that  a  cross  cleavage  cuts  through  the  finer-grained  sediments,  and  on  account  of 
tlus  close  folding  and  the  secondary  cleavage  the  phenomenon  is  more  difficult  to  certainly 
interpret.  However,  the  slate  and  graywacke  appear  to  plunge  under  the  jasper  of  the  Soudan 
formation,  and  the  explanation  is  with  little  doubt  the  same  as  for  the  contact  noi-th  of  Moose 
Lake. 

A  contact  of  a  fourth  kind  is  marked  by  a  thin  belt  of  greenstone  conglomerate.  The 
best  localities  at  which  this  is  seen  are  north  of  Robinson  Lake  and  at  the  pits  of  the  Lee  mine. 
At  the  first  locality,  at  the  west  end  of  the  belt  of  Soudan  formation,  the  ellipsoidal  greenstone 
is  overlain  by  a  layer  a  few  feet  thick  of  greenstone  conglomerate,  which  passes  up  into  gray- 
wacke. The  pebbles  of  this  greenstone  conglomerate  are  flattened,  and  it  could  not  be  said 
positively  that  the  rock  is  not  a  tuff  rather  than  a  conglomerate. 

Finally,  the  Soudan  and  Ely  formations  may  be  separated  by  a  thin  layer  of  graphitic 
black  slate,  well  shown  on  the  southwest  .side  of  vSoudan  Hill. 

From  the  fact  that  the  greater  masses  of  the  Ely  greenstone  were  deposited  before  the 
larger  masses  of  the  Soudan  formation  it  is  believed  that  the  great  volcanic  period  of  the  Ely 
greenstone  had  practically  ceased  before  Soudan  time.  However,  the  extremely  intricate 
relations  and  apparent  interstratification  of  the  minor  masses  of  the  Soudan  formation  ■with 
the  Ely  greenstone  and  the  fact  that  both  the  Ely  and  Soudan  formations  locally  contain 
interstratified  fragmental  material  lead  to  the  belief  that  volcanic* activity  had  not  entirely 


128  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

died  out  in  lUl  parts  ot  tlie  district  at  the  time  of  tiic  deposition  of  the  earliest  Soudan  rocks. 
In  consequence  tiiere  are  interlaminations  of  rocks  essentially  belonginj^  to  t  lie  Ely  with  rocks 
essentially  belonging  to  the  Soudan. 

Wliat  were  the  physical  contlitions  which  peiinitted  the  deposition  of  the  nonmechanical 
Soudan  formation  upon  the  Ely  greenstone  with  so  insignificant  an  amount  of  intci-vening 
mechanical  sediment  and  erosion  surfaces  ?  If  the  Ely  greenstone  was  subaerial,  it  is  dilhcult 
to  understand  how  tliis  material  could  have  got  below  the  water  without  the  deposition  of  a 
greater  thickness  of  mechanical  sediments  than  exists  in  the  Veraiilion  district.  We  know 
that  such  lavas  are  very  rough  in  their  surface  expression  and  vary  greatly  in  thickness,  and 
therefore  in  altitude.  It  is  impossible  to  believe  that  the  sea  could  advance  over  such  an  area 
without  the  production  somewhere  of  mechanical  sediments  of  considerable  thickness.  The 
answer  to  this  question  seems  to  be  that  the  eruptions  of  the  Ely  greenstone  were  submarine. 
The  ellipsoidal  textures  are  regarded  as  evidence  of  submarine  flows,  for  reasons  given  on 
pages  510-512.  The  lack  of  erosion  surfaces  in  the  flows  and  the  absence  of  fragmental  mate- 
rial at  the  base  of  the  formation  itself  are  evidence  of  such  an  origin.  If  these  lavas  issuing 
from  the  interior  of  the  earth  were  spread  out  below  the  surface  of  the  water,  after  the  period 
of  volcanism  had  ceased  and  conditions  became  quiescent  nonmechanical  sediments  of  the 
iron-bearing  formation  might  at  once  be  deposited,  provided  the  conditions  were  proper.  The 
conditions  of  sedimentation  are  further  discussed  in  the  chapter  on  the  origin  of  the  iron  ores. 

LAURENTIAN  SERIES. 

PORPHYRY. 

Intrusive  into  the  Ely  greenstone  and  Soudan  formation  are  various  Archean  felsites  and 
porphyries  in  dikes  and  bosses.  These  are  exceptionally  well  seen  in  the  Vermilion  Lake  area, 
especialty  at  Stuntz  Bay.  As  already  noted,  these  intrusives  may  be  in  part  connected  with 
acidic  flows  interbedded  with  some  of  the  later  flows  of  basalt  in  tlie  Ely  greenstone.  (See 
p.  126.) 

Petrographically  the  porphyry  comprises  rhyolite  porphyry,  feldspathic  porphyry,  micro- 
granite,  granite,  microgranite  porphyry,  and  granite  porphyry.  In  places  these  rocks  have 
been  metamorphosed  into  sericite  schists  and  chlorite  schists.  There  is  no  doubt  that  these 
rocks  are  older  than  the  lower  Huronian,  because  they  yield  fragments  t©  the  Ogishke  conglom- 
erate, but  at  various  places  their  relations  to  the  conglomerate  are  extremely  intricate.  (See 
p.  131.)  The  folding  has  formed  breccias  and  pseudoconglomerates  from  the  felsites  and 
porphyries,  which  when  very  much  mashed  have  been  sometimes  confused  with  the  true 
Ogishke  conglomerate. 

GRANITE   OF   BASSWOOD    LAKE. 

The  granite  of  Basswood  Lake  extends  as  a  great  continuous  formation  north  of  the  Ely 
greenstone  and  the  Huronian  rocks  from  the  western  to  the  eastern  end  of  the  district,  where  it 
is  locally  known  as  the  "Saganaga  Lake  granite."  Lakes  are  rather  numerous  in  this  great 
granitic  area,  but  they  are  not  so  numerous  nor  so  regularly  ordered  as  those  in  the  Ely  and 
Soudan  formations.  On  the  whole  the  granite  area  is  one  of  highlands  and  divides  between 
the  waters  running  north  and  south. 

Petrographically  the  granite  varies  from  hornblende  and  mica  granite  to  syenite.  Struc- 
turally it  varies  from  massive  granite  through  schistose  granite  to  gneiss.  Texturally  it  includes 
granites  and  granite  porphyries.  The  mineral  constituents  arc  green  hornblende,  ])iotite, 
orthoclase,  quartz,  and  plagioclase,  with  accessory  sphene,  zircon,  and  iron  oxide.  In  many 
places  these  minerals  have  been  very  much  altered,  so  that  their  places  are  taken  largely  l)y 
secondar}^  minerals,  of  which  chlorite  is  the  most  prominent  and  ejjidote,  sericite.  and  secondary 
feldspar  come  next.  There  is  a  variation  in  the  mineral  character,  hornblende  being  virtually 
absent  in  some  specimens  and  abundant  in  others.  No  specimens  were  found  in  which  quartz 
was  not  present,  but  the  amount  is  small  in  some  of  them. 


VERMILION  IRON  DISTRICT.  129 

The  granite  is  intrusive  into  the  Elj^  and  Soudan  formations.  The  field  relations  are  most 
complex  but  are  practically  the  same  in  all  parts  of  the  district — that  is,  the  phenomena  to 
be  seen  in  passing  from  the  other  Ai'chean  formations  to  the  granite  are  substantially  the  same 
whether  the  traverse  be  made  at  Vermilion  Lake,  at  Bumtside  Lake,  at  Basswood  Lake,  or  at 
any  other  point. 

In  apjM-oach  to  the  granite  from  the  Ely  greenstone  side  little  stringers  of  quartz  first 
appear  in  the  greenstone,  then  sparse  veins  of  feldspar,  then  clean-cut  dikes  of  granite,  usually 
of  small  size.  With  closer  approach  these  increase  in  number  and  size  until  they  constitute  a 
plexus  of  granite  dikes  in  the  greenstone.  Still  farther  north  the  greenstone  and  granite  may 
be  found  in  such  confused  and  intricate  relations  as  to  make  it  difficult  to  say  which  is  the 
more  abundant.  Here  great  knobs  of  granite  as  well  as  dikes  occur  in  the  greenstone  masses. 
In  the  granite  knobs  are  included  fragments  of  the  greenstone,  large  and  small,  in  many  places 
in  great  numbers.  Farther  north  the  granite  becomes  dominant  and  finally  altogether  excludes 
continuous  masses  of  greenstone.  If  any  greenstone  is  found  it  will  be  only  in  the  form  of 
included  masses.  In  brief,  the  relations  are  like  those,  so  clearly  described  by  Lawson,  between 
the  batholiths  of  granite  and  the  contiguous  greenstones  of  Rainy  Lake  and  Lake  of  the  Woods. 

The  granite  has  been  spoken  of  as  if  its  intrusion  were  a  single  episode.  This  is  not  sup- 
posed to  be  true.  On  the  contrary,  the  relations  of  the  different  granites  to  one  another  and  to 
the  greenstones  are  very  intricate,  hence  it  is  thought  that  various  intnisions  were  separated 
by  long  intervals  of  time,  that  many  of  the  intrusions  were  of  themselves  complex  and  long 
continued,  and  that,  in  fact,  this  igneous  period  was  a  complex  and  long-continued  one. 

ALGONKIAN   SYSTEM. 

HURONIAN  SERIES. 

LOWER-MIDDLE    HITRONIAN. 
GENERAL   STATEMENT. 

The  inferior  series  of  Huronian  rocks  occupies  the  general  position  of  the  lower  and  middle 
Iluronian  of  the  south  shore.  It  will  be  called  lower-middle  Huronian,  with  the  understanding 
that  it  may  include  either  or  both  lower  Huronian  and  n)id(lle  Iluronian. 

The  lower-middle  Iluronian  consists  of  four  divisions — (1)  a  lower  division,  predominantly 
conglomeratic,  which  is  most  typically  developed  near  Ogishke  Muncie  Lake  and  is  known  as 
the  Ogishke  conglomerate;  (2)  a  division  represented  only  in  the  eastern  portion  of  the  district, 
consisting  of  iron-bearing  rocks  and  known  as  the  Agawa  formation;  (3)  a  division  which  is 
predominantly  a  slate  formation  and  which  is  called  the  Knife  Lake  slate  because  it  is  well 
developed  and  splendidly  exposed  on  and  near  Knife  Lake;  and  (4)  intrusive  rocks. 

OGISHKE   CONGLOMERATE. 
DI.STRIBUTION. 

The  Ogishke  conglomerate  extends  from  the  western  end  of  the  district  to  the  east  end, 
though  it  varies  greatlj'  in  thickness.  In  places  it  is  a  great  formation;  in  other  places  it  is 
nearly  absent  or  is  so  thin  that  it  can  not  be  represented  on  the  maps  without  a  gross  exaggeration. 

The  localities  at  which  the  conglomerate  can  be  best  studied,  beginning  at  the  west,  are  ( 1) 
southeastern  Vermilion  Lake  and  especially  Stuntz  Bay  and  vicinity;  (2)  Moose,  Snowbank, 
and  Disappointment  lakes  and  vicinity;  (3)  Ogishke  Lake  and  the  extensions  of  the.  belt  there 
to  the  southeast,  northeast,  and  west. 

DEFORMATION. 

The  Ogislike  conglomerate  is  infolded  in  an  extremely  intricate  manner  with  the  luulcr- 

lying  formations.     This  infolding  is  almost  if  not  quite  as  complex  as  the  infolding  of  the  Soudan 

formation  and  the  Ely  greenstone  already  described.     Owing  to  isoclinal  folding  and  cross 

folding  with  steep  pitches,  a  rock  surface  cutting  diagonally  across  the  plane  of  contact  shows 

47517°— VOL  52—11 9 


130  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

tho  most  extraordinarily  irregular  distribution  of  the  Ogishke  and  the  underlying  formations. 
Because  of  this  it  was  supposed  by  a  number  of  the  early  geologists  that  the  Ely  greenstone 
and  the  porphyry  of  Stuntz  Bay  were  intrusive  into  tlie  Ogishke  conglomerate. 

UTHOLOOY. 

In  general  all  the  belts  of  conglomerates  arc  coarser  below  and  become  finer  toward  higher 
horizons.  This  statement  is,  however,  only  true  as  an  average.  There  are  places  where  the 
conglomerate  is  somewhat  fine  at  the  bottom,  is  coarser  above  for  a  certain  thickness,  and 
thence  becomes  finer  upward. 

The  character  of  the  Ogishke  conglomerate  depends  largely  on  the  nature  of  the  underlying 
formations.  These  formations,  as  already  noted,  are  the  Ely  greenstone,  the*  Laurentian  granite 
of  Basswood  Lake,  the  Soudan  formation,  and  the  Laurentian  porphyry  of  Stuntz  Bay.  \Miere 
the  conglomerate  rests  on  one  of  these  formations  the  material  comjiosing  it  is  mainly  derived 
from  that  formation.  There  are  four  special  varieties  of  the  Ogislike  conglomerate — (1)  green- 
stone conglomerate,  (2)  granite  conglomerate,  (3)  porphyry  conglomerate,  (4)  chert  and  jasper 
conglomerate.  The  common  kind  of  Ogishke  conglomerate  (5)  represents  combinations  of  the 
special  phases. 

Greenstone  conglomerate. — The  Ogishke  is  a  greenstone  conglomerate  at  those  localities 
where  the  conglomerate  rests  upon  the  Ely  greenstone  and  other  lower  formations  are  not  atlja- 
cent.  One  of  the  localities  which  exhibit  this  greenstone  conglomerate  in  its  typical  character 
is  the  south  side  of  Ogishke  Lake  and,  peripheral  to  the  Ely  greenstone  massifs,  to  the  east 
on  Frog  Rock  Lake.  The  rock  is  also  found  in  equally  good  development  on  Hunters  Island, 
at  the  southwest  of  Lake  Saganaga. 

At  these  localities  the  greenstone  conglomerate  consists  for  the  most  part  of  very  well 
rounded  fragments  of  the  Ely  greenstone  set  in  a  matrix  derived  from  the  same  source.  These 
fragments  are  ordinarily  of  a  size  to  make  pebble  conglomerates,  but  at  some  places  many  of 
them  are  so  large  as  to  constitute  bowlder  conglomerates.  Between  the  bowlders  and  pebbles 
are  smaller  fragments  of  the  same  material,  and  between  these  is  a  finer  matrLx  derived  from  the 
same  source.  In  most  places  upon  the  weathered  surface  the  conglomerate  character  of  tliis  rock 
is  e\'ident,  but  on  a  freslily  broken  surface  tlie  matrix  and  pebbles  are  so  similar  that  the  rock 
seems  to  be  a  continuous  mass  of  greenstone.  The  conglomerate  character  is  especially  diificult 
to  discover  in  the  unbroken  forests,  wliere  the  rocks  are  covered  witli  moss  and  otlier  vegetation. 
The  debris,  being  derived  from  the  Ely  greenstone,  consists  of  all  tlie  varieties  of  rocks  shown  by 
that  formation.  There  are,  accordingly,  fragments  of  dense,  massive  greenstone,  of  amygdaloidal 
greenstone,  of  various  kinds  of  ellipsoidal  greenstone,  etc.  These  rocks  grade  locally  into  rocks 
that  may  be  tuft's.  In  certain  places  the  conglomerate  is  discriminated  from  the  tuff  only  by 
finding  that  the  rock  occupies  a  definite  stratigrapliic  zone  at  the  base  of  the  lower  Huronian 
sediments.     Locally  discrimination  is  still  impossible. 

Granite  conglomerate. — The  granite  conglomerate  occurs  along  the  west  border  of  Lake 
Saganaga.  At  the  west  side  of  the  south  arm  of  Cache  Bay  is  a  great  bowlder  conglomerate 
the  fragments  of  which  are  directly  derived  from  the  granite.  The  matrLx  also  came  almost 
wholly  from  tliis  source.  The  exact  contact  of  the  conglomerate  and  granite  may  be  seen. 
Tha  bowlders  and  pebbles  of  the  granite  conglomerate  are  well  rounded,  and  in  every  respect 
tliis  conglomerate  bears  the  same  relations  to  the  granite  that  the  greenstone  conglomerate  does 
to  the  Ely  greenstone. 

The  granite  conglomerate  is  associated  with  a  peculiar  variety  of  rock,  which  may  be  called 
recomposed  granite.  It  appears  tliat  when  the  Ogislike  formation  was  laid  down  the  granite 
only  locally  jnelded  coarse  debris.  For  the  most  part  it  yielded  the  separate  individual  minerals 
of  the  coarse  gi-anite — that  is,  feldspar,  cjuartz,  etc.  As  a  result  a  clastic  formation  was  laid  down 
upon  the  granite,  the  particles  of  which  were  the  individual  minerals  of  the  granite.  Further- 
more, these  particles  were  but  little  waterworn.     The  result  is  that  when  they  were  recemeuted 


VERMILION  IRON  DISTRICT.  131 

a  rock  was  produced  which  closely  resembles  the  gi-anite.  This  resemblance  is,  indeed,  so  close 
that  the  rock  was  first  mistaken  by  a  number  of  geologists  for  the  granite. 

This  rock  is  exposed  along  the  west  side  of  Cache  Bay,  at  Swamp  Lake,  at  the  west  side  of 
West  Seagull  Lake,  and  at  intervening  points.  For  much  of  tMs  distance  this  peculiar  forma- 
tion has  a  breadth  of  nearly  half  a  mile. 

Porphyry  conglomerate. — The  porphyry  conglomerate  is  confmed  mainly  to  the  area 
about  Stuntz  Bay,  the  debris  being  derived  from  the  Laurentian  porphjrry.  In  the  past  it  has 
been  known  as  the  "Stuntz"  conglomerate.  In  places  there  is  a  coarse  bowlder  conglomerate, 
in  other  places  a  fine  conglomerate,  and  in  still  other  places  a  graywacke  composed  of  the 
individual  minerals  of  the  porphyry,  so  that  the  rock  closely  resembles  the  original  porphyry. 
Furthermore,  so  similar  are  the  bowlders  and  the  matrix  that  the  conglomerate  itself  has  been 
confused  with  the  brecciated  porphyry. 

Chert  and  jasper  conglomerate. — The  chert  and  jasper  conglomerate  is  found  where  the 
underlyuig  formation  is  the  Soudan.  Tliis  conglomerate  is,  however,  not  anywhere  known  to 
be  solely  composed  of  the  Soudan  material.  In  tliis  respect  this  variety  of  rock  cUffers  from  the 
varieties  already  described.  Locally,  however,  the  conglomerate  is  predominantly  composed 
of  material  derived  from  the  u-on-bearing  formation.  This  variety  of  rock  may  be  seen  on  Lee 
Hill,  just  north  of  Tower,  on  the  Burnt  Forties  southeast  of  Vermilion  Lake,  and  at  other 
localities. 

Common  OgishJce  roclc. — The  varieties  of  the  Ogishke  conglomerate  heretofore  described, 
each  consisting  largely  of  material  from  a  single  source,  are,  on  the  whole,  rather  exceptional, 
though  the  greenstone  conglomerate  and  the  porphyry  conglomerate  occupy  considerable  areas. 
It  is  natural  to  suppose  that  the  Ogishke  would  have  material  derived  from  more  than  one  of  the 
previously  existing  formations,  and  ordmarily  it  has.  Thus  the  normal  Ogishke  conglomerate 
consists  of  interrmxtures  in  various  proportions  of  the  materials  derived  from  the  Ely  and 
Soudan  formations,  the  granite  of  Basswood  Lake,  and  the  Laurentian  porphyry,  or  two  or  more 
of  them.  Hence  there  is  every  gradation  between  the  average  form  of  the  Ogishke  conglomerate 
and  the  special  forms  wliich  have  been  described.  Witliin  the  Ogishke  conglomerate,  in  addition 
to  the  common  fragments  already  enumerated,  there  are  occasional  unciuestionable  slate  frag- 
ments. These  are  seen  at  various  places,  but  are  especially  abundant  south  of  Moose  Lake.  It 
is  believed  that  the  source  of  the  fragments  of  tliis  kind  is  the  slate  and  graywacke  of  the  Ely  and 
Soudan  formations. 

METAIIORPHISM. 

The  Ogishke  conglomerate  varies  greatly  in  its  metamorphism.  In  general  the  processes 
of  the  change  have  been  mainly  those  of  metasomatism  and  cementation,  but  locally  the  con- 
glomerate is  recrystallized  and  schistose.  These  phases  are  especially  likely  to  be  adjacent  to 
the  massive  granite,  greenstone,  or  other  rock  against  wliich  they  rest.  Wliere  the  process  has 
gone  to  an  extreme  it  is  difficult  to  place  the  exact  cUviding  line  between  the  original  and 
recomposed  formations.  The  difficulty  is  particularly  likely  to  occur  in  reference  to  the  green- 
stone conglomerate  and  the  Ely  greenstone. 

The  extreme  phase  of  the  metamorphism  of  the  Ogishke  conglomerate  results  from  the  intru- 
sion of  igneous  rocks,  and  especially  the  Huronian  Snowbank  granite  and  the  Keweenawan 
Dulutli  gabbro.  Adjacent  to  these  intrusives  the  conglomerate  is  a  conglomerate  schist  or 
gneiss,  the  matrix  of  wlucli  is  usually  mica  schist  where  the  Huronian  is  of  an  acidic  Ivind  or 
ampliibole  scliist  where  it  is  of  a  basic  kind. 

The  conglomerate  scliist  adjacent  to  the  gabbro  may  be  found  from  points  east  of  Fay  Lake 
to  Lake  Gabimiclugami.  The  conglomerate  scliist  near  Snowbank  Lake  and  Disappointment 
Lake  has  suffered  the  metamorphosing  effect  of  the  Snowbank  gi-anite  and  the  Duluth  gabbro. 
The_  changes  in  the  conglomerate  are  analogous  to  those  wliich  have  taken  place  in  the  Knife 
Lake  slate,  which  is  in  a  similar  position  with  reference  to  the  granite.     (See  pp.  133-135.) 


132  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


RELATIONS   TO   ADJACENT   FORMATIONS. 


The  Ogishkc  conglomerate,  as  the  foregoing  description  plainly  shows,  is  unconformable 
with  the  iiiulcrh'ing  formations.  It  may  safely  be  inferred  tiiat  this  unconformity  is  one  of 
great  magnitude.  TJie  evidence  is  of  two  kinds — the  ciiaracter  of  the  detritus  and  the 
structural  relations. 

The  detritus  mcludes  every  variety  of  each  of  the  formations  of  the  Archean,  including  the 
many  phases  of  the  Ely  and  Soudan  formations  and  the  granite  of  Basswood  Lake.  Jn  order 
to  produce  these  many  varieties,  the  Archean  went  tlu-ough  a  long  and  complex  history  of 
folding,  intrusions,  metamorphism,  and  erosion. 

As  to  the  structural  relations,  the  Ogishke  conglomerate  is  here  in  contact  with  one  of  the 
underlying  formations,  there  with  another.  It  is  therefore  clear  that  after  the  Archean  complex 
was  produced  it  underwent  deep  erosion  before  the  deposition  of  the  Ogishke  conglomerate,  for 
some  of  the  formations  constituting  the  Archean  were  produced  at  great  depth. 

Upward  the  Ogishke  conglomerate  grades  into  finer  and  finer  material  and  passes  con- 
formably into  the  Agawa  formation  or  the  Knife  Lake  slate. 


THICKNESS. 


The  tluclcness  of  the  Ogishke  conglomerate  varies  greatly.  It  is  nowhere  possible  to 
make  accurate  measurements,  o\ving  to  the  general  absence  of  bedding  and  to  the  clo.se  folding, 
but  it  is  certain  that  the  formation  has  a  considerable  thickness,  certainly  several  hundred  feet, 
and  perhaps  in  some  places  more  than  1,000,  possibly  2,000.  From  this  maximum  thickness 
the  foi-mation  varies  to  a  thickness  of  only  a  few  feet  or  less,  and  is  absent  in  places. 


AGAWA  FORMATION. 


In  the  eastern  part  of  the  district,  above  the  Ogishke  conglomerate,  or,  where  that  forma- 
tion is  absent,  beneath  the  Knife  Lake  slate,  is  an  non-bearmg  formation  called  the  Agawa. 
On  the  American  side  of  the  international  boundary  this  formation  is  so  thin  that  it  can  not 
be  regarded  as  continuous.  On  the  Canadian  side  of  the  boundary,  especially  at  That  Mans, 
Agawa,  This  Mans,  and  Other  Mans  lakes,  the  formation  ranges  up  to  50  feet  in  thickness  and 
has  all  the  characteristic  rocks  of  the  other  iron-bearing  formations  of  the  Lake  Superior  region, 
includmg  ferruginous  caibonate,  ferruginous  slate,  ferruginous  chert,  jasper,  and  iron  oxides. 
Interlaminated  wath  the  ferruginous  varieties  are  belts  of  slate.  Thus  the  iion-l)earing  forma- 
tion is  both  small  and  impure.  There  is  every  reason  to  suppose  that  the  origin  of  this  iron- 
bearing  formation  is  similar  to  that  of  the  other  Lake  Superior  iron-bearing  formations. 

The  Agawa  formation,  so  far  as  at  present  knowTi,has  no  economic  importance,  but  it  may 
have  a  geologic  significance,  considering  that  it  is  in  the  lower-middle  Huronian.  The  only 
iron  foiTnation  at  this  horizon  in  other  parts  of  the  Lake  Sujierior  region  is  the  Negaunee,  and 
so  correlation  would  l:)e  suggested  with  that  formation.  The  bearing  of  this  suggestion  on  the 
position  of  the  group  to  which  the  Agawa  belongs  is  pointed  out  elsewhere  (pp.  603-604). 


KNIFE   LAKE   SLATE. 
GENERAL   STATEMENT. 


The  Knife  Lake  slate  was  so  named  because  it  occurs  in  its  typical  character  at  Knife 
Lake.  Nearly  all  the  long  arms  of  that  lake  lie  withm  the  slates,  and  by  far  the  greater 
number  of  the  many  islands  and  headlands  are  composed  of  them. 

The  slates  are  found  in  two  great  areas,  one  in  the  western  part  of  the  district  and  the 
other  in  the  central  and  eastern  parts.  The  western  area  extends  from  the  east  end  of  Vemiilion 
Lake  westward  to  parts  where  the  rocks  are  covered  by  the  Pleistocene.  It  occu])ios  much  of 
the  shore  and  many  of  the  islands  of  Vermilion  Lake.  The  eastern  area  begins  west  of  Long 
Lake  and  extends  eastward,  becoming  gradually  broader,  and  m  the  eastern  part  of  the  district 
is  the  most  extensive  formation  there  found. 


VERMILION  IRON  DISTRICT.  133 


LITHOLOGY. 


The  Knife  Lake  slate  comprises  the  following  main  varieties: 

1.  Argillaceous  slates. 

2.  Cherty  slates. 

3.  Graywacke  slates  and  graywackes. 

4.  Conglomerates. 

5.  Tuffaceous  slates. 

6.  Micaceous  (and,  less  commonly,  amphibolitic)  schists  and  gneisses. 

7.  Graj'  granular  rocks. 

There  ai'e  also  all  gradations  between  these  varieties.  The  materials  of  different  coarse- 
ness are  in  many  places  finely  interlaminated,  so  that  it  is  easy  to  ascertain  strikes  and  dips. 

The  argillaceous  slates  vary  in  color  from  gray  to  black.  They  are  usually  very  dense, 
break  with  a  smooth,  conchoidal  fracture,  and  have  a  perfect  cleavage,  which  in  a  general  way 
commonly  follows  the  trend  of  the  district  but  whose  direction  varies  much  locally,  depending 
on  the  surrounding  rocks,  the  folding,  and  other  factors. 

The  chertj^  slates  differ  from  the  argillaceous  slates  in  that  they  contain  an  unusual  amount 
of  finely  crystalline  quartz.  In  many  places  this  quartz  is  the  dominant  constituent.  Between 
the  beds  of  very  siliceous  slate  in  many  places  there  are  also  pure  bands  of  chert.  These  cherty 
bantls  m  most  places  appear  to  be  secondary  segregations.  In  many  places  the  amount  of  the 
fhiely  crystallme  quartz  in  the  separate  cherty  bands  and  in  the  main  mass  of  the  slate  is  so 
great  as  to  suggest  that  the  deposits  of  fine  mud  had  mingled  with  it  silica  of  organic  or  chemical 
origm.     Conchoidal  fractures  are  especially  characteristic  of  the  cherty  slates. 

The  argillaceous  slates  and  cherty  slates  pass  into  varieties  which  may  be  called  graywacke 
slate  and  graywacke.  These  differ  but  little  from  the  finer-grained  slates  except  that  cleavage 
is  less  likely  to  be  developed  in  them.  Cleavage  is  usually  present  in  the  graywacke  slates  but 
not  in  the  graywackes. 

Not  uncommonly  the  graywackes  pass  into  conglomerates.  The  fragments  found  in  the 
conglomerate  comprise  all  the  varieties  of  material  found  in  the  Ogishke  conglomerate.  These, 
it  may  be  recalled,  are  the  many  phases  of  material  derived  from  the  Archean.  Indeed,  there 
is  no  essential  difference  between  these  conglomerate  bands  antl  the  Ogishke  conglomerate, 
except  that  the  conglomerate  bands  of  the  Knife  Lake  slate  are  ordinarily  fine  grained  and 
are  subordinate  in  quantity  to  the  slates. 

During  Knife  Lake  time  there  was  volcanic  action,  and  close  to  the  volcanoes,  as  at 
Lake  Kekekabic,  ash  and  larger  fragments  produced  by  explosive  volcanic  action  are  mingled 
with  the  other  materials  of  the  Knife  Lake  slate.  These  volcanic  materials  constitute  the 
tuffaceous  slates.  Between  the  tuft's  and  the  conglomerates  antl  slates  there  are  all  gradation 
varieties.  Indeed,  microscopic  examinations  show  that  the  ashy  products  of  the  volcanoes 
were  widely  distributed  and  are  important  constituents  of  the  varieties  of  the  formation  already 
described — the  argillaceous  and  cherty  slates  and  graywackes. 

The  mica  slates,  mica  schists,  and  mica  gneisses  are  confuied  to  areas  adjacent  to  subse- 
quent intrusive  rocks.  The  most  important  areas  are  south  of  Tower,  along  Kawisliiwi  River, 
adjacent  to  Snowbank,  Disappointment,  and  Kekekabic  lakes,  and  adjacent  to  the  Kewee- 
nawan  gabbro. 

At  Snowbank  Lake  and  near  it  the  granite  has  been  intruded  into  the  slates  in  a  most  com- 
plex fashion,  and  here  next  to  the  granite  the  Knife  Lake  slate  is  represented  by  mica  schists. 
Between  the  mica  schists  and  the  ordinary  slates  there  are  gradations  through  mica  slates. 
Here  the  granite  is  found  in  numerous  great  dikes  intersecting  the  Knife  Lake  slate.  Moreover, 
in  many  places  the  granite  injections  have  followed  the  banding  of  the  slate  so  as  to  give  close 
parallel  mjections.  In  some  places  there  are  %vithin  a  single  hand  specimen  several  bands  of 
granite.  Also  bands  are  found  intermediate  in  character  between  the  well-recognized  granite 
and  the  slate.  There  is  no  doul)t  that  these  bands  ai'e  due  to  granitization.  Wlrere  the  injection 
is  of  the  most  complex  kind  the  rock  is  a  mica  gneiss,  the  darker-colored  bands  of  which  are 


134  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

largely  the  extremely  metamorphosed  granite.  However,  some  material  in  the  black  bands 
has  doubtloss  been  doriviMl  fiom  the  granite  and  some  material  in  the  light  bands  has  been 
derived  from  the  slate. 

The  scliists  and  gneisses  are  especially  well  exposed  on  the  north  side  of  Snowbank  Lake. 
South  of  Tower,  adjacent  to  the  granite,  and  especially  at  localities  near  the  Duluth  and  Iron 
Range  liailroad,  the  alterations  are  essentiaUy  the  same  as  at  Snowbank  Lake,  excei)t  that  the 
amphibole  schists  are  more  prominent.  Also  the  alteration  phenomena  at  Kekekabic  Lake 
are  in  the  same  direction  as  at  Snowbank  Lake,  but  the  processes  have  not  gone  so  far. 

At  Kawisliiwi  River  southwest  of  Snowbank,  and  at  Disappoiatment  and  Gabimichi- 
gami  lakes,  the  great  gabbro  mass  of  the  Keweenawan  has  profoundly  affected  the  character  of 
the  Knife  Lake  slate  and  has  produced  a  peculiar  gray  granular  rock  wluch  the  Miimesota 
geologists  have  called  "muscovado."  These  rocks  differ  from  the  slates  and  schists  about 
Snowbank  Lake  in  being  almost  massive.  They  are  particularly  well  seen  at  Disappointment 
Lake.  Between  the  schists  north  of  Snowbank  Lake  and  the  granular  rocks  of  Disai)pointment 
Lake  there  are  gradations.  These  granular  metamorphic  rocks  adjacent  to  the  gabbro  are 
regarded  by  Grant  as  the  result  of  contact  metamorpliism  of  the  Knife  Lake  slate.  They 
recrvstallized  under  deep-seated  static  conditions  at  high  temperature  and  probably  influenced 
by  abundant  moisture.  The  difference  between  them  and  the  scliists  and  gneisses  of  Snowbank 
Lake  shows  how  important  a  part  orogenic  movement  probably  had  in  the  production  of  the 
structures  of  the  latter  rocks.  The  schists  and  gneisses  of  the  Knife  Lake  slate  are  the  joint 
product  of  djTiamic  and  contact  action.  The  granular  rocks  wliich  are  adjacent  to  both  tlie 
Snowbank  granite  and  to  the  gabbro  have  doubtless  undergone  two  periods  of  metamorphism, 
the  earlier  one  at  the  time  of  the  introduction  of  the  Huronian  Snowbank  granite  and  a  later  one 
by  the  Keweenawan  gabbro.  At  the  earlier  time  doubtless  schists  and  gneisses  were  produced 
under  dynamic  conditions  which  at  the  earlier  time  were  transformed  to  granular  rocks  under 
static  conditions. 

MICROSCOPIC   CHARACTER. 

Clements's  microscopic  study  shows  that  the  rocks  of  the  Knife  Lake  slate,  iacluding 
argillaceous  and  cherty  slates,  graywacke  slates,  graywackes,  conglomerates,  and  tuffs,  have  as 
recognizable  primary  constituents  feldspar,  quartz,  brown  mica,  wliite  to  green  and  violent-brown 
pyi-oxene,  and  greeuish-browai  hornblende.  The  clastic  mineral  grains  very  commonly  have 
been  extensively  altered,  and  from  these  have  been  produced  the  following  secondary  namerals, 
which,  in  some  places  where  the  rocks  are  completely  recrystalhzed,  are  the  sole  constituents: 
Chlorite,  epidote,  sericite,  actinolite,  massive  dark-bro\\-n  and  green  hornblende,  quartz,  calcite, 
and  pyrite.  The  minerals  between  the  grains  in  the  coarser  sediments  are  sericite,  chlorite, 
epidote,  quartz,  and  feldspar.  These  are  believe<rto  have  been  produced  from  the  recrvstal- 
lization  of  the  fiine  detrital  material  originally  lying  between  the  larger  grains. 

The  miuerals  constituting  the  mica  slates,  mica  scliists,  and  mica  gneisses,  recrystalhzed 
under  the  influence  of  the  granite  intrusion,  are  usually  biotite  and  locally  some  muscovite, 
hornblende,  actinolite,  quartz,  feldspar,  epidote,  and  garnet. 

The  granular  rocks  metamorphosed  by  the  gabbro  are  mica,  hornblende,  and  p^Toxene 
feldspar  rocks  containing  Httle  quartz.  The  mica  (chiefly  biotite,  but  with  some  muscovite) 
and  honiblenile  together  predominate  over  the  feldspar,  and  the  mica  is  usually  more  abun- 
dant than  the  hornblende.  With  these  chief  constituents  there  occur  considerable  amounts  of 
hypcrsthene,  light-green  pyroxene,  olivine  (?),  and  magnetite,  and  with  these  suborilinate 
amounts  of  titanite,  epidote,  garnet,  and  chlorite.  Exceptionally  in  these  gabbro  contact  rocks 
the  hypersthene  is  the  jiredominant  constituent,  when  it  is  usually  associated  ^\•ith  considerable 
mica  and  magnetite.  In  general  we  may  say  that  the  production  of  miuerals  rich  m  magnesium 
and  iron  is  characteristic  of  the  gabbro  contact. 


VERMILION  IRON  DISTRICT.  135 


DEFORMATION. 


The  Iviiife  Lake  slate  lias  undergone  the  same  orogenic  movements  as  the  Ogishke  con- 
glomerate. The  slates  have  tiierefore  been  folded  in  a  composite  and  com[)lex  fashion.  For 
the  most  part  it  is  cUfhcult  to  make  out  in  detail  the  structure  of  the  slates,  but  enough  has 
been  done  to  show  that  the  foldmg  is  exceedingly  complex.  Superimposed  upon  folds  of  the 
first  order  are  those  of  the  second  order;  on  these  there  are  those  of  tiie  third  order,  and  so 
on  indefinitely.  The  relations  of  the  Knife  Lake  slate  to  the  Ogishke  conglomerate  and  to  the 
Ely  greenstone  disclose  in  a  general  way  the  character  of  the  major  folds. 

Usually  the  slates  are  in  synclmes  between  anticlines  composed  of  the  Ely  and  Ogishke 
formations  or  one  of  them.  As  the  formation  is  relatively  nonresistant,  many  of  the  lakes  are 
in  the  centers  of  these  synclines.  Such  synclines  are  occupied  by  the  following  linear  lakes  or 
groups  of  lakes:  Vermilion  Lake;  Long  and  Fall  lakes;  Pme,  Moose,  New  Found,  Sucker, 
Birch,  and  Carp  lakes;  That  Mans  Lake,  Agawa  Lake,  Tliis  Mans  Lake,  and  No  Mans  Lake; 
Knife  Lake  and  its  two  principal  arms;  Kekekabic  and  Ogishke  lakes.  Not  uncommonly  the 
synclines  of  slate  are  broken  up  into  two  or  more  minor  folds  by  sul)ordinate  anticlines,  which 
may  be  marked  by  the  appearance  at  the  surface  of  the  Ogishke  conglomerate. 

RELATION   TO    ADJACENT   FORMATIONS. 

The  Knife  Lake  slate  in  the  eastern  part  of  the  district  reposes  on  the  Ogishke  conglomerate 
or  the  Agawa  formation.  For  the  western  part  of  the  district  it  lies  on  the  Ogishke  conglom- 
erate. In  both  places  the  transition  to  the  EJiife  Lake  slate  is  conformable.  The  Knife  Lake 
slate  is  not  in  observed  contact  with  the  Animikie  group  Avithin  the  Vermilion  chstrict,  but  there 
is  almost  certainly  an  unconformity  between  them.  The  lower-middle  Iluronian  rocks  are 
characteristically  steeply  inclined  and  schistose,  contrasting  with  the  less  folded  and  less  schis- 
tose Animikie  rocks.  Also,  rocks  similar  to  the  lower-middle  Iluronian  of  the  Vermilion  dis- 
trict are  on  satisfactory  evidence  found  in  the  Mesabi  district  to  be  unconfoi-mably  below  the 
Animikie  or  upper  Huronian. 

THICKNESS. 

On  account  of  the  complicated  folding  of  the  Knife  Lake  slate  it  is  impossible  to  determine 
its  thickness  with  any  degree  of  exactness.  But  the  extent  of  the  areas  which  the  formation 
continuously  covers  in  the  eastern  and  western  parts  of  the  district — a  district  which  has  been 
profoundly  folded — leaves  no  doubt  that  the  formation  is  one  of  great  thickness,  probably 
thousands  of  feet. 

INTRUSIVE  ROCKS. 

Later  than  the  deposition  of  the  I^ife  Lake  slate,  in  several  parts  of  the  district  many 
igneous  rocks  were  intruded.  These  vary  fi-om  comparatively  small  masses  to  those  covei-ing 
very  considerable  areas.  In  chemical  character  they  include  basic,  acidic,  and  intermediate 
rocks.  In  texture  they  include  por])hyritic,  ophitic,  and  granolitic  rocks.  In  age  the  intru- 
sives  range  from  rocks  which  are  slightly  later  than  the  Knife  Lake  slate,  and  which  therefore 
suffered  orogenic  movements  and  metamorphism  with  that  formation,  to  intrusive  rocks  of 
much  later  age,  which  have  been  but  comparatively  little  modified. 

The  more  extensive  of  these  intrusive  masses  are  the  Giants  Range  granite,  the  Snow- 
bank granite,  and  the  Cacaquabic  granite.  In  addition  to  these  there  are  many  smaller 
areas  of  acidic  and  basic  intrusive  rocks. 

The  Giants  Range  granite  extends  for  20  miles  or  more  along  the  Vermilion  range  in  contact 
with  various  formations.  It  includes  a  series  of  granites  ranging  m  color  from  light  gray  to  very 
dark  gray,  to  flesh  color,  puik,  and  red.  The  rock  varies  from  very  dense  fuie-grained  granites 
through  medium  to  coarse-grained  ones.  Though  this  rock  is  as  a  rule  granitic  in  texture,  there 
are  also  variations  to  granite  porphyries  and  exceptionally  to  some  that  can  be  spoken  of  as 
rhyolite  porphyries.     The  constituents  of  these  granitic  rocks  as  disclosed  by  the  microscope 


136  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

are  orthoclase  (microcline),  plagioclase,  quartz,  hornblende,  niica,  zircon,  apatite,  sphene,  and 
.1  little  iron  oxide. 

This  granite  is  intrusive  into  the  iVrchean  and  the  lower-middle  Iliu'onian.  The  contacts 
will  not  be  further  mentioned,  as  descriptions  of  them  and  their  resultant  metamorphism  luive 
been  given  in  connection  with  the  formations  which  have  been  intruded. 

The  Snowbank  granite  is  confined  to  Snowbank  Lake  and  vicinity.  It  varies  from  the 
fine-grained  to  the  coarse-grained  form,  the  medium-gramed  facies  being  most  abundant. 
Porpliyritic  facies  of  the  granite  also  occur.  Mineralogically  the  Snowbank  granite  varies 
from  a  normal  mica  and  hornblende  granite  to  an  augite  granite  and,  by  loss  of  (juartz,  to  a 
syenite.  The  Snowbank  granite  is  intrusive  into  both  the  Ogishke  conglomerate  and  the 
Knife  Lake  slate.  The  character  of  the  contacts  and  the  resultant  metamorphism  have  been 
described  in  connection  with  those  formations. 

The  Cacaquabic  granite  has  been  carefully  mapped  and  described  by  U.  S.  Grant,"  and 
from  his  report  the  following  summary  is  taken. 

The  granite  occupies  an  oval  area  south  of  Kekekabic  Lake;  also  many  of  the  islands 
of  tliat  lake  and  a  few  small  isolated  areas  in  the  vicinity  of  the  lake.  Petrogra])hicullv 
the  rock  is  an  augite  granite,  ricli  in  soda.  Its  main  mass  has  a  granolitic  texture;  small 
masses  are  porphyritic.  Grant  inclines  to  the  view  that  the  latter  is  somewhat  later  than  the 
former.  He  also  regards  the  granite  as  intrusive  in  the  Ogishke  conglomerate  and  the  Knife 
Lake  slate,  because  where  it  is  in  contact  with  the  conglomerate  the  granite  is  uniformly  finer 
grained  than  elsewhere,  and  because  the  slate  at  one  place  on  the  north  shore  of  Kekekabic 
Lake  is  cut  by  "a  small  irregular  dike  of  granite,  which  sends  many  stringers  into  the  argillite 
and  also  mcludes  fragments  of  it."  *  Grant  mentions  no  metamorphic  effects  of  the  granite 
on  tlie  Ogislike  conglomerate  and  Knife  Lake  slate. 

In  addition  to  these  granites,  acidic  dikes  have  been  found  cutting  tlirough  the  formations 
of  the  district.  They  are  supposed  to  have  relations  with  the  large  eruptive  masses,  but  for 
the  most  part  this  connection  has  not  been  definitely  traced,  though  it  is  very  strongly  indicated 
by  the  greater  abundance  of  the  acidic  intrusive  rocks  adjacent  to  the  large  granite  masses 
already  described  than  at  points  remote  from  them. 

At  many  places  in  the  district  are  basic  intrusive  rocks  which  have  a  more  or  less  well- 
developed  schistose  structure  and  are  otherwise  metamorphosed.  These  intrusives  evidently 
reached  their  present  position  before  the  strong  orogenic  movements  following  upper  Huronian 
time  had  ceased.  A  considerable  body  of  these  rocks  occurs  near  Epsilon  Lake  and  is  called 
porphyrite  by  Grant.*^  Metamorphosed  basic  intrusive  rocks  of  upper  Huronian  age  are  known, 
but  they  are  very  subordinate  and  unimportant  in  this  district. 

UPPER    HURONIAN    (aNIMIKIE    GROtIP)   AND    KEWEENAWAN    SERIES. 

The  upper  Huronian  (Animikie  group)  occurs  in  a  small  area  in  the  eastern  part  of  the 
district  just  west  of  Gunflint  Lake  and  in  a  few  patches  between  the  lower-middle  Huronian 
and  the  Keweenawan  as  far  west  as  Gabimichigami  Lake.  The  relations  of  the  Archean  and 
lower-middle  Huronian  to  the  upper  Huronian  in  this  region  are  interesting,  but  they  are  dis- 
cussed more  appropriately  in  Chaj)ter  XX  (pp.  599  et  seq.).  It  is  here  merely  to  be  remarked 
that  in  the  Vermilion  district  these  relations  are  not  clear,  and  that  for  a  time  it  was  supposed 
that  the  Animikie  group  represented  rocks  equivalent  to  the  lower-middle  Huronian  but  less 
metamorphosed.  Later  studies  of  the  relations  of  these  rocks,  especially  in  the  Mesabi  and 
I^oon  Lake  districts,  show  clearly  that  between  the  lower-middle  Huronian  and  the  ,\jiimikie 
groups  there  is  a  very  marked  unconformity.     (See  Chapter  VIII,  pp.  198-210.) 

a  Kept.  Geol.  Survey  Minnesota,  vol.  4, 1S99,  pp.  442-448. 
6  Idem,  p.  444. 

f  The  geology  of  Kekequabic  Lake  in  northeastern  Minnesota,  with  special  reference  to  an  atig)te>soUa  granite;  Twenty-first  Ann.  Rept.  GeoL 
anil  Nat.  Hist.  Sun'ey  Minnesota.  1S93,  p.  53. 


VERMILION  IRON  DISTRICT.  137 

As  has  been  noted,  the  Keweenawan  Duluth  gabbro  bounds  the  eastern  half  of  the  Ver- 
mihon  district  on  the  south.  In  the  lower-middle  Huronian  and  Archean  rocks  are  numerous 
comparatively  fresh  dolerite  dikes  and  bosses.  There  are  also  more  sparingly  late  acidic  dikes 
in  the  Archean  and  the  Huronian.  It  is  supposed  that  these  fresh  rocks,  showing  compara- 
tively little  orogenic  movement,  are  of  Keweenawan  age,  although  they  have  not  been  con- 
nected areall}^  with  the  greater  masses  of  Keweenawan  rock^.  The  metamorphosing  effects 
of  the  Keweenawan  gabbro  upon  the  Archean  antl  Huronian  have  already  been  considered. 
The  Keweenawan  rocks  themselves  are  discussed  in  Chapter  X^'  (])p.  866-426). 

THE  IRON   ORES   OF  THE  VERMILION   DISTRICT,  MINNESOTA. 

By  the  authors  and  \V.  J.  Mead. 
DISTRIBUTION,  STRUCTtTRE,  AND  RELATIONS. 

The  iron  ores  of  the  Vermilion  district  occur  in  the  Soudan  formation,  belonging  to  the 
Keewatin  series  of  the  Archean  system.  This  formaticm  rests  upon  the  Ely  greenstone,  is  in 
places  interbedded  with  it,  is  interbedded  with  and  intruded  by  acidic  porphyries,  and  as  a 
whole  has  been  closely  folded,  with  the  result  that  the  iron-bearmg  formation  stands  with 
contorted  and  steeply  inclined  bedding,  with  steep  walls  and  bottoms  of  green  schist  and  mashed 
porphyry.  These  constitute  deep,  narrow,  pitchmg  troughs  in  which  the  ores  are  found.  The 
jaspers  constitute  for  the  most  part  the  hanging  wall  of  the  ore. 

The  total  area  of  the  ores  is  but  a  minute  fraction  of  that  of  the  iron-bearing  formation  of 
the  district.  It  is  significant  that  notwithstanding  the  enormous  sums  of  money  spent  in  the 
exploration  of  the  district  no  ore  deposit  of  magnitude  has  been  developed  outside  of  the  two 
principal  series  of  deposits  at  Tower  and  Ely,  which  were  the  first  discoveries  in  the  district. 

One  additional  deposit  in  sec.  30,  T.  63  N.,  R.  11  W.,  about  4  miles  east  of  Elj^,  has  been 
considerably  explored,  leadmg  up  to  the  first  shipment  of  ore  in  1910. 

On  Soudan  Hill  near  Tower  the  structural  relations  of  the  iron-bearing  formation  to  the 
green  schists  and  mashed  porphyries  are  so  complex  that  it  is  extremely  difficult  to  follow  the 
ore  bodies.  The  steeply  pitching  troughs  branch,  change  their  pitch,  and  are  duplicated  by 
parallel  troughs  to  such  an  extent  that  m  spite  of  the  enormous  amount  of  underground  explora- 
tion to  which  the  hiU  has  been  subjected  it  is  not  certain  yet  that  all  the  ore  deposits  have  been 
found.  The  Soudan  ores  may  have  (a)  "paint  rock"  or  "soapstone"  as  foot  wall,  below  which 
is  jasper,  and  similar  paint  rock  or  jasper  as  the  hanging  wall;  or  (&)  they  may  have  jasper 
both  as  a  foot  and  a  hanging  wall,  and  hence  may  lie  within  it  and  grade  in  all  dhections  into 
the  Soudan  formation.  Deposits  of  this  kind  are  small.  The  Soudan  ores  are  mainly  of  the 
first  form.     They  have  now  been  found  to  a  depth  of  2,000  feet. 

At  Ely  there  is  a  single  trough  of  the  iron-bearing  formation  in  the  greenstone,  beginning 
as  a  comparatively  wide  body  at  the  west  and  narrowing  and  deepening  toward  the  east.  The 
northeast  side  of  the  trough  seems  to  be  formed  in  part  by  lower  Huronian  slates  or  graywackes. 
The  greenstones  associated  with  the  ores  are  altered  to  paint  rock  along  the  contacts.  This 
trough  is  a  comparatively  simple  one,  but  there  is  also  a  minor  parallel  anticline  separating 
the  Zenith  ore  deposits  into  two  portions  and  separating  the  trough  longitudinally  into  two 
great  s\Ticlines,  one  between  the  Zenith  and  Pioneer  mines  and  the  other  between  the  Zenith 
and  Savoy  mines.  (See  fig.  14.)  Here  also  parts  of  the  formation  are  found  separated  from 
the  main  mass  by  greenstone  masses  in  such  a  manner  as  to  make  it  difficult  to  explain  them 
on  the  basis  of  occurrence  in  troughs  alone.  It  would  seem  that  the  main  mass  of  the  ormation 
here  has  been  infolded  in  such  a  manner  as  to  give  a  steep  monoclinal  trough  dipping  northward, 
but  that  in  addition  to  this  main  mass,  which  originally  rested  upon  the  greenstone,  minor 
masses  of  the  iron-bearing  formation  may  be  mterbedded  with  the  greenstone,  so  that  after 
the  folding  they  would  be  separated  from  the  main  mass  by  la3-ei's  of  greenstone. 


138 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


The  deposits  of  Soudan  Hill  come  to  the  surface  near  the  crest  at  an  elevation  of  1,660  feet, 
about  150  feet  above  a  cross  valley  to  the  east  between  Sf)U(l!Ui  Hill  and  Jasper  Peak.  The  Ely 
ore  dcj)osits  are  below  comparatively  low-lyinj;  <j:roun(l,  the  upper  part  of  the  dejiosits  being  at 


about  the  1,400-foot  contour,  and  are  surrounded  on  the  north,  west,  and  south  by  an  amphi- 
theater of  hisjli  ground  comijoscd  of  the  Ely  greenstone,  the  higher  points  of  which  rise  to  an 
elevation  of  1,500  feet.     Farther  east  is  a  cross  valley  wliich  is  somewhat  less  than  1,400  feet 


VERMILION  IRON  DISTRICT. 


139 


high.  To  what  extent  the  cross  valley  is  filled  is  unknown,  hut  the  drift  covering  is  moder- 
ately thick.  The  pitch  ol'  the  ore  deposits  is  parallel  to  the  range,  as  it  is  in  the  Menominee, 
Martjuette,  and  Penokee-Gogebic  districts  and  toward  tliis  valley.  The  ores  in  general  are 
located  below  crests  and  slopes. 

The  newly  developed  Section  30  mine,  in  sec.  30,  T.  63  N.,  R.  11  W.,  is  located  on  a  bend  of 
the  iron-bearing  Soudan  formation,  trenchng  a  httle  east  of  south.  The  jasper  is  bounded  on 
both  sides  by  greenstone,  that  to  the  south  probably  being  basal  and  that  to  the  north  being 
overlying.  The  bend  in  the  jasper  seems  to  represent  the  result  of  shearing  between  these  two 
gi-eenstones.  Outcrops  of  rich,  highly  contorted  jasper  led  to  the  sinking  of  the  shaft.  Below 
the  surface  the  jasper  becomes  in  general  softer  and  small  leads  of  ore  in  the  jasper  widen 
out  into  shoots  of  commercial  value.  Mining  operations  have  shown  the  ore  to  come  to  the 
surface  where  covered  hj  the  drift.  The  ore  body  is  yet  too  little  developed  to  permit  an 
accurate  description  of  tlie  structure.  The  ore  thus  far  developed  seems  to  be  in  two  main 
masses — one  in  the  soutlieast  with  an  easterly  or  southeasterly  linear  trend,  pitching  west  at 
the  west  end,  apparentlj^  east  at  the  east  end,  and  with  minor  rolls  between;  and  another  ore 
body  north  of  the  west  eml  of  this  one,  having  a  similar  trend  and  seeming  to  pitch  to  the 
west.  There  is  little  doul^t  that  these  ore  bodies  are  developed  along  the  axial  lines  of  the 
pitcliing  drag  folds  in  the  jasper.     Their  greatest  dimension  is  in  the  direction  of  the  pitch. 

CHEMICAL  COMPOSITION. 

The  average  composition  of  all  ore  niinetl  in  the  Vermilion  district  in  1909,  obtained  by 
combining  average  cargo  analyses  in  proportion  to  their  respective  tonnages,  is  shown  below. 
The  range  for  each  constituent  is  from  the  cargo  analyses  and  represents  the  variation  in  com- 
position of  the  marketed  ore  and  not  of  the  ore  in  the  mine. 

Composition  of  ore  shipped  from  the  Vermilion  district  in  1909  {1,108,790  tons). 


Range. 


Moisture  (loss  on  drying  at  212°  F.) 

Analysis  of  dried  ore: 

Iron 

Phosphorus 

Silica 

Manganese 

Alumina 

Lime 

Magnesia 

Loss  on  ignition 


0.75  to  5.06 


60.91  to  65. 34 
.  037  to  .  168 
3.06  to 
to 
to 
to 
to 
to 


.09 
1.40 
.20 
.04 
.40 


;.07 
.15 

3.27 
.47 
.15 

2.18 


Partial  average  analyses  of  ores  and  jaspers. 


Average 
analyses  of 

ore  from 
Soudan  Hill 

in  1906. 

Average 
analyses  of 

ore'from 
Soudan  Hill 

inl909. 

.\verage 

analyses  of 

ore  from 

mines  at  Ely 

in  1906. 

Average 

analyses  of 

ore  from 

mines  at  Ely 

in  1909. 

Average  of 
10  partial 

analyses  of 
jasper  from 
Soudan  Hill. 

Average  of 
,^.ve^age  of     20  analyses  of 
several  par-      iron  forma- 
tial  analyses      tion  bands 
of  jasper  from,  from  sees.  13 
mines  at  Ely.   and  14,  T.  02 
N.,  R.  13  W. 

Moisture  (loss  on  drying  at  212°  F.) 

1.37 

0.79 

5.26 

5.37 

Analysis  of  dried  material: 

Iron : 

00.07 
.088 

3.43 
.10 

1.27 
.13 
.02 
.55 

64.85 
.107 

4.70 
.09 

1.57 
.36 
.08 
.47 

65.00 
.046 

3.39 
.11 

1.90 
.34 
.06 

1.24 

03.70 
.049 

4.91 
.10 

3.03 
.22 
.05 
.90 

38.27 

28.97 
.022 

16.20 

.042 

70.95 

a  Loss  on  ignition  includes  both  water  of  hydration  and  CO2,  as  well  as  minor  amounts  of  organic  matter.    The  ores  from  the  mines  near  Ely 
contain  an  appreciable  amount  of  iron  carbonate  and  the  loss  on  ignition  in  these  ores  is  probably  largely  CO3. 


140 


GEOLOGY  OF  THE  LAKE  SUPERIOR  liEGION. 


MINERAL  COMPOSITION  OF  THE  ORES  AND  CHERTS. 

The  priucipal  ii-on  miiierals  of  the  ores  urc  licmatiU'  and  minor  amounts  of  magnetite  and 
siderite.  The  siderite  is  noticeably  abundant  in  tlie  ore  from  the  Savoy  and  Sibley  mines.  In 
addition  to  the  iron  minerals  are  quartz,  chlorite,  calcite,  kaolin,  pyrite,  and  small  amounts  of 
minerals  bearing  phosphorus,  magnesium,  and  manganese,  not  suflieiently  abuiulant  to  i)e  iden- 
tified. A  variety  of  copper  minerals,  including  native  copper,  malachite,  azurite,  cuprite,  and 
several  sulphides  of  eoj)])er,  are  found  locally  in  small  amounts  in  l)oth  the  Ely  mines  and  the 
mines  at  Soudan  Hill.  These  copj^er  minerals  are  not  sufhciently  abundant,  however,  to  afiect 
the  average  composition  of  the  ores. 

Approximate  mineral  composition  of  ores  and  jaspers,  calculated  from  the  partial  analyses  given  above. 


Ore  from  mines  at 
Ely. 

Ore  from  minrs  at 
Soudan  Hill. 

Ely 
jasper. 

Soudan 

1906. 

1909. 

1900. 

1909. 

92.85 
1.15 
4.85 
1.15 

91.00 

1.34 

7.18 

.48 

94.50 

1.9:( 

3.22 

.35 

92.75 

2.92 

3.97 

.36 

41.40 
I      58.60 

54.50 

45.50 

100.00 

100.00 

100.00 

100.00 

100.00 

100.00 

All  the  ores  of  the  Ely  district  are  dark  red  and  blue  hematites,  with  a  small  amount  of 
magnetite  and  siderite.  They  are  practically  anhydrous,  the  water  of  hydration  averaging  less 
than  1  per  cent. 

The  jasper  is  a  dense,  brittle  rock  made  up  of  layers  of  nearly  pure  anliydrous  hematite 
separated  by  layers  of  comparatively  barren  chert.  The  jaspers  contain  more  or  less  magnetite; 
in  places  nearly  all  of  the  iron  is  in  that  form. 

PHYSICAL  CHARACTERISTICS  OF  VERMILION  ORES. 

TEXTURE. 

In  texture  the  VermiUon  ores  show  a  complex  gradation  from  dense  massive  hematite 
through  brecciated  or  broken  ore  to  fine  blue  granular  ore. 

The  Soudan  ore  is  massive  hematite,  all  of  the  ore  requiring  crushing. 

The  Ely  ores  exliibit  a  complete  range  from  dense  hematites  with  jiractically  no  pore  space 
to  fine  granular  hematites  with  large  porosity. 

The  textures  of  the  ores  of  the  VermiUon  tlistrict  are  shown  in  the  following  table  of 
screening  tests.  These  screening  tests  were  made  by  the  Ohver  Iron  Mmuig  Company  on  three 
of  the  typical  grades  of  Vermilion  ore  representing  a  total  of  1,034,221  tons.  For  each  of  the 
grades  samples  were  taken  biweekly  and  quartered  down  monthly  in  proportion  to  the  tonnage 
mined,  and  at  the  end  of  the  season  the  entire  sample  was  quartered  down  to  100  pounds  and 
screened.  A  comparison  of  the  textures  of  the  ores  of  the  several  Lake  Superior  districts  is 
shown  in  figure  72  (p.  481). 

Textures  of  Vermilion  ores  as  shown  bij  scrccnitirj  tests. 


Held  on  J-inch  sieve 

J-inch  sieve 

No.  20  sieve 

No.  40  sieve 

No.  60  sieve 

No.  80  sieve 

No.  100  sieve 

Passed  through  No.  100  sieve. 


Ely  ore. 

Soudan 
ore. 

Per  cent. 

Per  cent. 

16.93 

62.40 

41.76 

28.10 

16.23 

4.40 

6.96 

1.10 

3.81 

.60 

.59 

.40 

9.32 

.40 

4.  as 

2.30 

VERMILION  IRON  DISTRICT.  141 

These  screening  tests  show  phiinly  the  difl'erejice  in  texture  between  the  ores  from  the 
mines  at  Soudan  Hill  and  the  ores  from  the  vicinity  of  Ely.  The  former  are  dense  massive 
ores  with  practically  no  line  material  except  what  results  from  tlie  blasting  and  crushmg  due 
to  mining.     The  latter  ores  are  of  much  Imer  texture,  being  easily  broken  down  with  a  pick. 

The  average  ore  of  the  Ely  district  is  well  described  by  the  local  term  "broken  ore,"  as  it 
is  a  rubble  of  more  or  less  unconsolidatetl  fragments  of  hard  hematite,  which  range  in  size  from 
small  grains  to  large  masses.  This  rubble  or  brecciated  material  is  cemented  locally  by  infil- 
trated hematite  and  iron  carbonate,  in  some  places  the  infiltrated  minerals  almost  completely 
filling  the  voids.  The  bedding  of  the  jasper  is  plainh'  preserved  in  the  ore  where  slumping  has 
not  destroyed  it.  In  the  Zenith  mine  a  fresh  surface  of  the  ore  showed  perfectly  the  folded 
structure  of  the  jasper.  No  sharp  line  of  contact  exists  between  tlie  ore  and  jasper,  the  gra- 
dation being  complete.  The  slumping  of  tlie  ore  has  in  most  places  produced  a  drag  which 
destroys  the  bedding  texture  in  this  transitional  zone,  producing  a  mixtm-e  of  broken  ore  and 
jasper. 

DENSITY. 

The  mineral  density  of  the  ores  varies  with  the  iron  content  and  ranges  from  5.10  for  the 
pure  hematites  to  4.40  in  the  lower-grade  ores.  The  average  for  the  district  (1906  production) 
is  approximately  4.SS.  As  the  ores  consist  essentially  of  hematite  and  quartz,  the  approximate 
mineral  density  may  be  readily  calculated  from  chemical  analj'ses. 

POROSITY. 

Owing  to  the  texture,  porosity  determinations  on  the  Ely  ores  are  rather  difficult  to  obtain. 
Ten  determinations  made  on  typical  specimens  of  the  cemented  brecciated  ore  showed  an  aver- 
age pore  space  of  approximately  20  per  cent  of  the  volume  of  the  ore.  If  the  average  moisture' 
content  of  the  ore  as  given  above  is  assumed  to  be  the  moisture  of  saturation  of  the  ore,  cal- 
culation shows  that  it  represents  a  porosity  of  21.5  per  cent.  This  moisture  content,  however, 
is  probably  less  than  the  moisture  of  saturation  of  the  ores;  hence  the  porosity  of  the  average 
ore  may  be  assumed  to  be  greater  than  the  moisture  determination  indicates.  Engineers  esti- 
mate 9  to  10  cubic  feet  of  the  ore  in  place  to  the  ton.  If  the  average  mineral  specific  gravity 
for  the  ore  is  4..93,  as  calculated  above,  this  figure  indicates  an  average  porosity  of  from  20  to 
28  per  cent.  Though  these  estimates  of  porosity  are  all  approximations,  their  rather  close 
accordance  indicates  their  probable  correctness. 

The  Soudan  ores  are  much  more  compact  than  the  Ely  ores  and  have  an  average  porosity 
of  less  than  10  per  cent. 

CUBIC    CONTENTS. 

Calculations  based  on  the  mmeral  density,  porosity,  and  moisture  of  the  ores  give  an  aver- 
age of  8.75  cubic  feet  ])cr  lung  ton  for  Soudan  ore  and  approxinuitely  9.5  cubic  feet  per  ton  for 
Ely  ore. 

SECONDARY  CONCENTRATION  OF  VERMILION  ORES. 
.  PRECEDENT    CONDITIONS. 

In  the  Vermilion  chstrict  the  steeply  pitching  foot  walls  of  Keewatin  greenstone  and  asso- 
ciated jiorphyry  afford  impervious  basements  and  troughs  for  the  concentration  of  waters  from 
the  surface.  This  is  especially  well  shown  in  the  western  end  of  the  eastward-pitching  Ely 
trough.     (See  p.  137.) 

Originally  the  iron-bearmg  Soudan  formation  consisted  largely  of  cherty  iron  carbonate 
interlayered  with  more  or  less  of  sideritic  slates  and  perhaps  l)anded  chert  and  iron  oxide. 


142 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


MINERALOGICAL    AND    CHEMICAL    CHANGES. 

Tlie  alteration  of  the  clicrty  iron  carbonate  to  ore  has  been  accomplished  in  the  general 
manner  described  (p.  529)  as  typical  for  the  region — (1)  oxidation  and  hydration  of  the  iron 
minerals  in  place;  (2)  leaching  of  silica;  and  (.3)  introduction  of  secondary  iron  oxide  and 
minor  amounts  of  iron  carbonate  from  other  parts  of  the  formation.  These  changes  may  start 
simultaneously,  but  the  first  step  is  usually  far  advanced  or  complete  before  the  second  and 
third  are  conspicuous.  The  early  products  of  alteration,  therefore,  are  ferruginous  cherts — 
that  is,  rocks  in  wliich  the  iron  is  oxidized  and  hydrated  and  the  silica  is  not  removed.  The 
later  removal  of  silica  is  necessary  to  produce  the  ore,  except  in  layers  originally  rich  enough 
in  iron  to  make  ores  without  the  removal  of  siUca. 

It  is  shown  in  discussing  the  secondary  concentration  of  the  Mesabi  and  Gogebic  ores  that 
the  degree  of  hydration  of  the  iron-oxide  laj^ers  in  the  cherts  and  jaspers  is  not  changed  by  the 
alteration  to  ore.  This  appears  to  be  true  also  of  the  Vermilion  ores,  as  both  the  jaspers  and 
the  ores  are  practically  anhydrous. 


SEQUENCE    OF    SECONDARY    ALTERATIONS    AND    DEVELOPMENT    OF    TEXTURES. 

Before  lower  Huronian  time  the  iron-bearing  formation  at  the  surface  at  Soudan  liad  been 
altered  to  iron  ores,  cherts,  and  jaspers,  for  all  these  substances  were  yielded  to  the  conglomerate 
at  the  base  of  the  lower  Huronian.  The  concentration  of  the  ore  may  be  supposed  to  have 
stopped  while  the  formation  was  covered  by  the  lower  Huronian  sediments.  Close  folding 
following  the  lower  Huronian  deposition  rendered  the  ores  hard,  anhydrous,  and  ciystalline, 
developed  green  schists  out  of  the  basaltic  wall  rocks  and  talcose  and  sericitic  schists  from  the 
porphyry  wall  rocks,  and  Ln  general  developed  a  steep,  vertical  structure  in  both  ore  and  wall 
rock,  making  it  difficult  to  decipher  the  structural  relations.  Erosion  later  exposed  the  iron- 
bearing  formation,  but  owing  to  its  refractory  character  it  was  not  further  concentrated. 

At  Ely  also  the  concentration  began  before  the  deposition  of  the  lower  Huronian  and  was 
interrupted  when  the  iron-bearing  formation  was  covered  by  lower  Huronian  sediments.  Later, 
when  the  formation  was  again  exposed  to  weathering,  the  concentration  continued,  and  then 

accomplished  the  greater  part  of  its  work, 
the  process  difTering  m  this  respect  from 
that  undergone  by  the  ores  at  Soudan, 
which  were  comparatively  little  affected  by 
the  later  concentration.  The  fact  that  the 
iron-bearing  formation  at  Ely  was  less 
closely  folded  anil  rendered  less  scliistose 
than  the  iron-bearing  formation  at  Soudan, 
thereby  retaining  more  openings  through 
which  concentratmg  solutions  might  work, 
may  explain  why  concentration  was  so 
effective  after  the  folding.  That  at  least  a 
part  of  the  concentration  followed  the  fold- 
ing is  shown  by  the  retention  in  the  ore  of 
the  folded  bedded  structures  of  the  jasper 
and  by  the  development  of  pore  space  as  a 
result  of  the  leachmg  of  siUca  from  the 
folded  jasj)er,  discussed  below. 

VOLUME    CHANGE    IN    ELY    ORE. 


X 

X 

\ 

Pore  space 

N. 

N. 

\ 

>v 

\ 

Quartz 

N 

Quartz  and  other 

and  other 
minerals 

minerals 

/ 

/ 

/ 
/ 

Hematite 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

Hematite 

Jaspe 


Ore 


Figure  15.— Diagram  illustrating  volume  changes  involved  in  the  altera- 
tion of  jasper  to  ore  at  Ely,  Minn.  From  average  analyses  and  porosity 
detemiinations. 


Comparison  of  tlie  volume  compositions 
of  the  ore  ami  jasper  shows  the  removal  of 
a  large  amount  of  silica  from  the  jasper.  In  order  to  suliiciently  reduce  the  silica  content  it  is 
necessary  that  silica  equivalent  to  63.7  per  cent  of  the  volume  of  the  jasper  be  removed.     The 


VERMILION  IRON  DISTRICT. 


143 


ayerage  porosity  of  the  ore  is  approximately  22  per  cent  of  its  volume;  hence  the  remaining 
space  left  by  the  removal  of  silica,  or  41.7  per  cent  of  the  volume  of  the  jasper,  has  been  filled 
by  infiltration  of  iron  and  by  mechanical  slumping  of  the  ore.  The  relative  importance  of  these 
two  factors  can  not  be  definitely  determined,  but  it  is  known  that  both  have  been  efl'ective. 
The  broken  and  brecciated  condition  of  the  ore  and  the  drag  at  the  jasper  contacts  give  abun- 
dant evidence  of  slump,  and  secondary  hematite  and  siderite  cementing  the  ores  indicate  that 
a  considerable  amount  of  iron  has  been  introduced  in  solution.  Figure  15  illustrates  the  vol- 
ume changes  above  discussed. 

DISTRIBUTION  OF  PHOSPHORUS. 

Phosphorus  and  iron  contents  of  the  Vermilion  ores  and  associatetl  ri^cks  are  as  follows: 

Phosphorite  and  iron  contents  of  Vermilion  ores  and  associated  rocks. 


Phos- 
phorus. 


Relation 
of  phos- 
phorus to 
iron. 


Average  ore  at  Ely 

Average  jasper  at  Ely 

Average  ore  at  Soudan 

Average  jasper  at  Soudan 

Paint  rock  from  Pioneer  mine 


Per  cent. 
65.00 
28.97 
65.21 
38.27 
16.32 


Per  cent. 
0. 0469 
.0213 
.108 


Per  cent.. 

0. 0707 

.0759 

.1655 


.127 


.778 


As  calculated  from  the  figures  of  the  Lake  Superior  Iron  Ore  Association,  89. 3  per  cent  of 
the  total  production  of  the  Vermilion  range  in  1906  was  of  Bessemer  grade.  The  lowest  phos- 
phorus grade  was  Pilot  lump  (iron  67.22  per  cent,  phosphorus  0.0297  per  cent),  and  the  high- 
est phosphorus  grade  was  Vermilion  lump  (iron  66.07  per  cent,  phosphorus  0.0878  per  cent). 
The  phosphorus  contents  of  individual  samples  show  a  much  greater  range  than  the  grade 
analyses,  the  ore  containmg  as  high  as  0.500  per  cent  of  phosphorus  locally. 

The  paint  rock  is  an  altered  phase  of  the  greenstone  and  porphyry,  consisting  principally 
of  kaoUn  more  or  less  stained  with  hon  oxide.  It  is  similar  both  m  appearance  and  composition 
to  the  altered  dike  rocks  of  the  Gogebic  range.  These  altered  igneous  rocks  are  higher  m  phos- 
phorus than  the  ores  (the  above  analysis  being  a  typical  one),  owing  probably  to  the  high 
phosphorus  m  the  greenstone. 

"Chemical  maps"  have  been  made  by  the  chemists  and  engineers  of  the  Oliver  Iron  Mining 
Company  of  the  mines  on  the  Vermilion  range  operated  by  that  company,  the  phosphorus  and 
iron  contents  of  the  ore  being  entered  hi  the  proper  place  directly  on  the  nfine  maps.  Study 
of  these  maps  fails  to  show  any  relation  between  the  distribution  of  phosjihorus  and  the  wall 
rocks.  The  only  general  conclusion  that  may  be  drawn  is  that  in  general  the  phosphorus  content 
is  lowest  in  the  largest  ore  bodies  and  has  a  tendency  to  be  liigh  m  the  small  shoots  of  ore  away 
from  the  mam  ore  body.  The  maps  show  no  relation  between  high-phosj)horus  ore  and  i)amt 
rock;  in  fact,  in  several  places  ore  running  as  low  as  0.030  per  cent  of  phosphorus  occurs  m 
the  immediate  vicinity  of  high-phosphorus  paint  rock. 

Owmg  to  the  very  small  amomit  of  phosphorus  even  m  what  are  termech  "  high-phosphorus  " 
ores,  very  little  is  known  as  to  its  mmeral  occm-rence.  So  far  as  is  known,  no  phosphorus  minerals 
have  been  identified  in  the  ores  or  jaspers;  hence  any  conclusions  regarding  the  chemical  combi- 
nations m  which  phosphorus  exists  are  necessarily  based  entirely  on  chemical  evidence.  Phos- 
phorus is  present  in  the  ores  in  at  least  two  different  forms,  knowTi  to  the  Iron-ore  chemists  as 
"soluble"  and  "insoluble"  phosphorus,  part  of  it  being  easily  dissolved  in  hydrocliloric  acid 
and  the  remamder  requiring  ignition  before  it  can  be  dissolved.  Chemical  analysis  of  the 
insoluble  residue  shows  it  to  be  an  alummum  phosphate.  This  occurrence  of  both  soluble  and 
insoluble  phosphorus  is  common  to  ores  of  the  other  Lake  Superior  districts,  particularly  those 
of  the  Marquette  range. 


CHAPTER  VI.    THE   PRE-ANIMIKIE  IRON    DISTRICTS   OF  ONTARIO. 
LAKE   OF   THE   WOODS  A^'D   RAINY  LAKE  DISTRICT. 
INTRODUCTORY   STATEMENT. 

The  Lake  of  the  Woods  and  Rainy  Lake  distriet  includes  these  hirge  hikes  and  the  sur- 
rounding lands.  The  district  may  be  considered  as  being  bounded  on  the  south  by  j)arallel 
48°  30',  on  the  north  by  parallel  50°,  on  the  east  by  meridian  92°  30',  and  on  the  west  by  merid- 
ian 95°  30'.  The  area  which  has  been  most  closety  studied  is  an  angular  one  running  north- 
west and  southeast.  The  Canadian  Survey  has  published  detailed  reports  by  A.  C.  Lawson 
on  the  Lake  of  the  Woods  "  and  Rainy  Lake  *  district  and  one  by  W.  H.  C.  Smith  <^  on  Hunters 
Island. 

The  geology  of  tliis  region  may  be  said  to  duplicate  in  most  essential  respects,  save  the 
distribution  of  the  formations,  the  geology  of'the  Vermilion  district  of  Minnesota.  The  rocks 
therefore  include  lower-middle  Huronian,  Laurentian,  and  Keewatin. 

ARCHEAN   SYSTEM. 

KEEWATIN  SERIES. 

The  series  of  Keewatin  rocks  in  the  district  of  the  Lake  of  the  Woods  is  that  to  wliich  the 
term  was  first  applied.  Lawson's  study  of  it,  supplemented  by  later  work  of  others,  shows 
that  the  Keewatin  series  is  dominantly  igneous  but  includes  subordinate  amounts  of  sediments, 
precisely  as  in  the  Vermilion  district.  The  igneous  rocks  comprise  ancient  lava  flows,  volcanic 
elastics,  and  contemporaneous  and  subsequent  intrusives.  They  are  dominantly  of  basic  and 
intermediate  varieties,  exactly  as  in  the  Vermilion  district,  and  among  these  the  characteristic 
ellipsoidal  greenstones  are  conspicuous.  Locally  felsites  and  quartz  porphyries  occur.  In 
many  areas  subsequent  dynamic  action  has  gone  very  far,  so  that  the  rocks  uncommonly  have  a 
slaty  or  scliistose  structure.  These  belts  of  slaty  and  schistose  rocks  Lawson  has  separated 
into  two  divisions,''  one  of  which  he  describes  as  hydromicaceous  schists  and  nacreous  schists, 
with  some  associated  chloritic  schists  and  micaceous  schists  and  included  areas  of  altered  quartz 
porphyry,  and  the  other  of  which  he  calls  clay  slate,  mica  schist,  and  quartzite  with  some  fine- 
grained gneiss.  Subsequent  examinations  of  the  areas  by  other  geologists  have  led  to  the  con- 
clusion that  large  areas  of  these  rocks  are  but  altered  facies  of  the  ordinars^  varieties  of  the 
Keewatin  igneous  rocks.  Thus  the  slates  are  to  a  large  extent  mashed  varieties  of  the  ellij>- 
soidal  greenstones  and  tuffs.  At  various  places  the  transition  between  the  ellipsoidal  green- 
stones and  slaty  varieties  of  rocks  produced  from  them  by  metamorphism  is  well  shown. 
However,  there  are  present  with  the  slaty  and  scliistose  rocks  of  igneous  origin  subordinate 
amounts  of  sedimentary  graywacke  and  slate,  including  small  belts  of  ordinary  black  slate 
which  are  in  some  parts  carbonaceous.  There  has  not  yet  been  discovered  in  the  Lake  of  the 
Woods  district  any  iron-bearing  formation  corresponding  with  the  iron-bearing  vSouilan  forma- 
tion of  the  Vennilion  district,  and  tliis  is  the  chief  cUfference  between  the  two  series.  The  only 
rocks  which  could  possibly  be  regarded  as  a  correlative  of  the  iron-bearing  Soudan  formation 

o  Geology  of  the  Lake  of  the  Woods  region,  with  special  reference  to  the  Keewatin  (Huronian?)  belt  of  the  Arehean  rocks:  Ann.  Rept.  Geol.  and 
Nat.  Hist.  Survey  Canada  for  1S85.  vol.  1  (new  ser.),  1.S8H,  Rept.  CC,  pp.  5-l.*)l.  with  map. 

!>  Geology  of  the  Rainy  Lake  region:  .\nn.  Rept.  Geol.andNat.  Hist.  Survey  Canada  for  IS87-18S8.  vol.  3  (new  ser.),  pi.  1. 18S.S.  Rept.  F,  pp.  1-182, 
with  two  maps. 

c  Geology  of  Hunters  Island  and  adjacent  counlry:  .\nn.  Rept.  Geol.  Survey  Canada  for  hst«HS91,  vol.  5  (new  ser.).  pi.  1.  1S92.  Rept.  G» 
pp.  1-7G.    S(H^  also  The  .\ri-hean  rocks  west  of  Lake  Superior:    Bull.  Geol.  Soc.  .\merica.  vol.  4,  1S93,  pp.  3.'13-34S. 

d  Geology  of  the  Lake  of  the  Woods  region,  p.  5G. 

144 


PRE-ANIMIKIE  IRON  DISTRICTS  OF  ONTARIO.  145 

are  very  subordinate  beds  of  limestone  which  occur  at  various  phiccs.  The  nature  of  this  lime- 
stone is  represented  by  that  at  Scotty  Islands,  where  there  are  narrow  bands  from  a  fraction 
of  an  inch  to  2  feet  wide  in  a  schistose  and  banded  greenstone.  The  layers  are  usually  lens-shaped, 
and  along  their  strike  they  may  become  narrow  and  pinch  out.  Commonly  the  division  between 
the  limestone  and  the  greenstone  is  rather  sharp. 

For  the  Lake  of  the  Woods  district  Lawson  "  gives  various  sections  of  the  Keewatin,  ranging 
from  6,500  feet  to  23,756  feet  in  thickness.  As  tliis  is  a  volcanic  series  and  practically  all  the 
structures  are  secondaiy,  it  may  be  doubted  whether  these  figures  have  any  real  significance. 

In  conclusion  it  may  be  well  to  give  the  statement  of  the  International  Geological  Committee,'' 
consisting  of  Messrs.  Frank  D.  Adams,  Robert  Bell,  A.  C.  Lane,  C.  K.  Leith,  W.  G.  Miller,  and 
Charles  R.  Van  Hise,  concerning  the  Keewatin  of  the  Lake  of  the  Woods: 

In  the  Lake  of  the  Woods  area  one  main  section  was  made  from  Falcon  Island  to  Rat  Portage,  with  various  traverses 
to  the  east  and  west  of  the  line  of  section.  The  section  was  not  altogether  continuous,  but  a  number  of  representatives 
of  each  formation  mapped  by  Lawson  were  visited.  We  found  Lawson's  descripti<')ns  to  be  substantially  correct.  We 
were  unable  to  find  any  belts  of  undoubted  sedimentary  slate  of  considerable  magnitude.  At  one  or  two  localities 
subordinate  belts  of  slate  which  appeared  to  be  ordinary  sediment  and  one  belt  of  black  slate  which  is  certainly  sedi- 
ment are  found.  In  short,  the  materials  which  we  could  recognize  as  water-deposited  sediments  are  small  in  volume. 
Many  of  the  slaty  phases  of  rocks  seemed  to  be  no  more  than  the  metamorphosed  ellipsoidal  greenstones  and  tuffs, 
but  some  of  them  may  be  altered  felsite.  However,  we  do  not  assert  that  larger  areas  may  not  be  sedimentary  in  the 
sense  of  being  deposited  under  water.  Aside  from  the  belts  mapped  as  slate,  there  are  great  areas  of  what  Lawson 
calls  agglomerate.  These  belts,  mapped  as  agglomerates,  seem  to  us  to  be  largely  tuff  deposits,  but  also  include  exten- 
sive areas  of  ellipsoidal  greenstones.  At  a  number  of  places,  associated  and  interstratified  with  the  slaty  phases  are 
narrow  bands  of  ferruginous  and  siliceous  dolomite.  For  the  most  part  the  bands  are  less  than  a  foot  in  thickness,  and 
no  band  was  seen  as  wide  as  3  feet,  but  the  aggregate  thickness  of  a  number  of  bands  at  one  locality  would  amount  to 
several  feet. 

LAURENTIAN  SERIES. 

The  Laurentian  series  is  represented  mainly  by  granite,  sj-enite,  granite  gneiss,  and  syenite 
gneiss.  These  rocks  occur  in  masses  varying  from  small  areas  to  those  many  miles  in  diame- 
ter. They  are  intrusive  in  the  Keewatin  series  and  comprise  batholiths,  bosses,  dikes,  and 
stringers.  The  nature  of  the  contacts  between  the  Laurentian  and  Keewatin  in  the  Lake  of 
the  Woods  area  is  identical  with  that  of  the  contacts  in  the  Vermilion  district.  Along  the 
borders  of  the  batholiths  the  Keewatin  is  metamorphosed  into  hornblende  schist  or  gneiss, 
exactty  as  it  is  in  the  Vermilion  district.  Indeed,  between  the  more  metamorphosed  varieties 
of  these  rocks  and  their  less  metamorphosed  forms  there  are  all  gradations.  Included  in  the 
great  batholiths  of  granite  are  various  masses  of  Keewatin  which  have  generally  been  pro- 
foundly metamorphosed  and  in  many  places  partly  absorbed. 

The  chemical  and  mineralogical  compositions  of  the  batholiths  have  thus,  to  some  extent  at 
least,  been  affected  by  the  included  material.  Similarly  the  chemical  and  mineralogical  charac- 
ters of  the  Keewatin  have  been  affected  by  the  material  derived  from  the  granite.  Indeed, 
there  are  few  better  examples  of  endomorphic  and  exomorpliic  effects  than  those  furnished  by 
this  district.  All  these  relations  may  be  conveniently  studied  in  the  vicinity  of  Rat  Portage. 
Intrusive  into  both  the  Keewatin  and  Laurentian  are  later  masses  of  granite  and  also  various 
basic  rocks,  including  diabase,  gabbro,  and  peridotite. 

Lawson's  maps  of  the  Keewatin  and  Laurentian  in  the  Lake  of  the  Woods  and  Rainy  Lake 
district  show  certain  interesting  features  which  have  here  been  better  worked  out  than  any- 
where else  in  the  Lake  Superior  region.  The  great  batholiths  have  a  tendency  to  a  schistose 
structure,  which  is  parallel  to  their  borders  and  is  more  marked  at  their  exteriors  than  at  their 
interiors.  The  Keewatin  schists  around  the  borders  are  in  bands,  the  schistosity  of  which  is 
rouglily  parallel  to  the  batholith  boundary.  Very  commonly  a  band  of  Keewatin  widens  or 
narrows  within  a  short  distance  or  separates  into  two  or  more  bands.  This  subdivision  may  go 
on  until  a  band  is  lost  in  stringers  in  a  granite  mass.  With  many  large  areas  of  schists  there 
appear  subordinate  granite  batholiths,  bosses,  and  dikes. 

o  Geology  of  the  Lake  of  the  Woods  region,  pp.  104-112. 

l>  Report  of  the  special  committee  on  the  Lake  Superior  region,  with  introductory  note:  Jour.  Geology,  vol.  13, 1905,  pp.  96-96. 

47517°— VOL  52—11 10 


146  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION.       . 

Tlio  area  covcrod  by  the  Laurentian  granites  is  muc'li  jjreater  tlian  tliat  covered  by  tlie 
Keowatin.  It  is  certain  that  after  the  Keewatin  volcanic  rocks  were  once  spread  over  tlie 
entire  region,  as  tliey  doubtless  were,  the}^  must  have  been  raised  in  great  domes,  pushed  aside, 
and  jammed  in  between  the  batholithic  intrusions.  It  is  probable  that  the  greater  areas  of 
Keewatin  which  once  overlaid  tliese  batholiths  have  been  removed  by  erosion  anrl  that  the 
existing  masses  of  Keewatin  are  but  mere  renmants  of  a  great  volcanic  formation  which  once 
covered  the  entire  district.  It  has  been  suggested  also  tliat  parts  of  the  Keewatin  may  have 
foundered  and  suidv  in  tlie  granite  batholiths  at  thi^  time  of  intrusion." 

The  foregoing  facts  in  reference  to  tlu!  relations  of  tlie  Laurentian  and  Keewatin  have  led 
Lawson  ''  to  his  subcrustal  fusion  theory,  his  idea  being  that  the  Laurentian  represents  the 
fused  and  recrystallized  nuxsses  of  the  Keewatin.  There  is  no  doubt  tliat  along  the  border  of  the 
batholitJiic  masses  a  certain  amount  of  Keewatin  material  has  been  aljsorbed,  and  no  doubt  that 
the  Keewatin  along  tlie  borders  of  the  granites  has  derived  material  from  them;  thus  there 
appears  in  some  places  to  be  an  approach  to  chemical  gradation  between  the  two. 

The  known  facts,  then,  are  these:  The  Keewatin  volcanic  period  antedated  the  Laurentian. 
The  Keewatin  rocks  were  intruded  by  the  various  Laurentian  granites  and  syenites,  extending 
tlu-ough  an  enormous  period  of  time.  There  were  important  exoniorphic  and  endomorphic 
effects.  There  is  difference  of  opinion  as  to  the  amount  of  tlie  Keewatin  which  has  been  absorbed 
by  the  Laurentian.     Our  own  view  tends  toward  conservatism  in  this  matter. 

ALGONKIAN   SYSTEM. 

HURONIAN  SERIES. 

The  Huronian  rocks  in  this  district  belong  to  the  lower-middle  Huronian.  They  are 
chiefly  confined  to  the  southern  part  of  the  Rainy  Lake  area.  From  west  to  east  their  northern 
boundary  roughly  follows  Rainy  River,  the  central  body  of  Rain}-  Lake,  and  Seine  River  with 
its  various  enlargements.  From  this  line  the  Huronian  extends  across  the  mternational  bound- 
ary into  Minnesota  for  distances  not  yet  determined,  except  at  a  few  points.  Tliis  is  the  main 
mass  of  rocks  to  which  Lawson  gave  the  name  "Coutchiching."  In  the  area  imder  discussion 
the  rocks  consist  dominantly  of  mica  schists,  but  there  are  argillaceous  slates,  mica  slates,  gray- 
wackes,  and  conglomerates  at  the  bottom.  All  the  evidences  of  imconformable  relations 
between  these  rocks  and  the  Laurentian  and  Keewatin  series  are  found  in  many  places  along 
the  contact.  Where  the  underlj'ing  rocks  are  Keewatin  detritus  is  mainl}'  derived  from  that 
series;  where  they  are  granite,  as  at  Bad  Vermilion  Lake,  the  detritus  is  mainly  derived  from 
granite.  In  intervenmg  areas  both  granite  and  greenstone  are  found.  Also  with  these  mate- 
rials is  found  detritus  of  other  kinds,  such  as  felsites,  quartz  porphyries,  and  gneiss.  The 
materials  include  practically  all  the  varieties  of  the  Keewatin  rocks.  In  places  the  conglom- 
erate passes  up  into  a  feldspathic  quart zite  and  tliis  mto  a  micaceous  graywacke  or  slate. 
Wherever  the  bedding  can  be  recognized  the  dip  is  steep  to  the  south. 

The  main  areas  of  the  lower-middle  H\u"onian  micaceous  schists  have  been  intruded  by 
large  masses  of  granite  which  maj'  be  especially  well  seen  at  the  end  of  the  southeast  arm  of 
Ramy  Lake  and  in  Namakon  Lake  along  the  international  bouudarj-.  From  masses  of  very 
considerable  size  the  intrusive  granite  varies  to  masses  of  much  smaller  size,  and  cutting  through 
the  mica  schists  are  very  numerous  granite  dikes,  a  large  number  of  which  are  roughly  parallel 
to  the  foliation.  Large Ij"  in  consequence  of  the  mtrusions  of  the  granite  the  mam  mass  of  the 
lower-middle  Huronian  has  been  transformed  to  a  well-crystallized  mica  schist.  As  a  result 
of  these  intrusions,  from  the  end  of  the  southeastern  arm  of  Rainy  Lake  northwestward  to  the 
base  of  the  series  the  rocks  are  less  anil  less  metamorphosed.  Possibly  this  grailation  was  one 
of  the  factors  in  Lawson's  conclusion  that  the  Keewatin  series  was  higiicr  and  rested  uncon- 
formably  upon  his  "Coutcliiching,"  which  exactly  reversed  the  true  relation.     As  the  relation 

<•  Daly,  R.  .\.,  The  mechanics  of  igneous  intrusion:  .\ni.  Jour.  Sci.,  4th  ser.,  vol.  26, 190S,  p.  30. 
6  Geology  of  the  Rainy  Lake  region,  p.  131. 


PRE-ANIMTKIE  IRON  DISTRICTS  OF  ONTARIO.  147 

of  the  great  mica  schist  series  to  th(>  Keewatin  is  one  about  \vlii(  li  tliere  is  difference  of  opinion, 
the  statement  of  the  International  Geological  Committee/'  consisting  of  Messrs.  Frank  D. 
Adams,  Robert  Bell,  A.  C.  Lane,  C.  K.  Leith,  W.  G.  Miller,  and  Charles  R.  Van  Hise,  who 
visited  this  district  and  examined  the  contact,  is  here  quoted: 

In  the  Rainy  Lake  district  the  party  observed  the  relations  of  the  several  formations  along  one  line  of  section  at 
the  east  end  of  Shoal  Lake  and  at  a  number  of  other  localities.  The  party  is  satisfied  that  along  the  line  of  section 
most  closely  studied  the  relations  are  clear  and  distinct.  The  Coutchiching  schists  form  the  highest  formation.  These 
are  a  series  of  micaceous  schists  graduating  downward  into  green  homblendic  and  chloritic  schists,  here  mapped  by 
Lawson  as  Keewatin,  which  pass  into  a  conglomerate  kiio\vn  as  the  Shoal  Lake  conglomerate.  This  conglomerate  lies 
upon  an  area  of  green  schists  and  granites  known  as  the  Bad  Vermilion  granites.  It  holds  numerous  large  well-rolled 
fragments  of  the  underlying  rocks,  and  forms  the  base  of  a  sedimentary  series.  It  is  certain  that  in  this  line  of  section 
the  Coutchiching  is  stratigraphically  higher  than  the  chloritic  schists  and  conglomerates  mapped  as  Keewatin.  On 
the  south  side  of  Rat  Root  Bay  there  is  also  a  great  conglomerate  belt,  the  dominant  fragments  of  which  consist  of  green 
schist  and  greenstone,  but  which  also  contain  much  granite.  The  party  did  not  visit  the  main  belts  colored  by  Lawson 
as  Keewatin  on  the  Rainy  Lake  map,  constituting  a  large  part  of  the  northern  and  central  parts  of  Rainy  Lake.  These, 
however,  had  been  visited  by  Van  Hise  in  a  previous  year,  and  he  regards  these  areas  as  largely  similar  to  the  green- 
schist  areas  intruded  by  granite  at  Bad  Vermilion  Lake,  where  the  schists  and  granites  are  the  source  of  the  pebblee 
and  bowlders  of  the  conglomerate. 

As  to  the  existence  of  areas  of  sediments  equivalent  in  age  to  the  lower-michlle  Huronian  in 
other  parts  of  the  Rainy  Lake  and  Lake  of  the  Woods  district,  no  defmite  statements  can  yet 
be  made.  It  is  probable,  however,  that  close  structural  studies  of  tliese  areas  will  disclose  such 
sediments.  Indeed,  a  traverse  of  the  Rainy  Lake  section  by  the  senior  author  led  him  to  think 
that  in  the  belt  of  rocks  mapped  as  Keewatm,  running  from  the  southeastern  end  of  Crow  Lake 
to  Manitou  Lake,  there  are  representatives  of  this  upper  series,  but  the  area  was  not  sufficientlv 
studied  for  its  areal  distribution  to  be  given.  On  the  other  hand,  it  is  certain  that  some  areas 
which  have  been  mapped  as  "Coutchiching"  on  the  Ramy  Lake  sheet  of  the  Geological  Survey  of 
Canada,  and  especially  on  adjacent  sheets,  are  but  the  chloritic  and  hornblendic  schists  of  the 
Keewatin  metamorphosed  by  the  intrusive  granite.  It  is  plain  that  the  term  "Coutchiching," 
if  it  is  to  have  any  structural  significance,  must  be  restricted  to  the  sedimentary  series  of  Ramy 
Lake,  its  extensions  and  equivalents.  It  must  not  be  used  as  a  term  to  cover  the  more  schis- 
tose varieties  of  rocks  of  the  region  without  reference  to  their  stratigraphic  position. 

As  to  the  thickness  of  the  so-caUed  "Coutchichmg,"  Lawson ''  gives  estimates  varying 
from  23,760  feet  to  28,754  feet.  These  measurements,  however,  are  clearly  based  on  cleavage 
structures  rather  than  on  bedding,  and  close  examination  shows  that  the  two  do  not  conform; 
hence  there  is  grave  doubt  whether  the  thickness  of  the  series  is  more  than  a  fraction  of  these 
estimates. 

It  has  already  been  indicated  that  in  the  lower-middle  Hiu-onian  schists  ("Coutclaiching" 
of  Lawson)  there  are  intrusive  masses  of  granite  which  have  produced  metamorphic  effects. 
In  addition  to  these  granitic  masses  cutting  all  the  formations  of  the  district  are  later  diabases, 
dikes,  and  bosses  which  are  supposed  to  be  of  Keweenawan  age. 

STEEP  ROCK  LAKE  DISTRICT. 
OENfiRAL   GEOLOGY. 

The  Steep  Rock  Lake  district  has  been  described  and  mapped  by  II.  L.  Smyth  "^  and  W.  N. 
Merriam.'*  The  authors  have  visited  the  district  for  general  correlation  purposes  but  have 
not  studied  it  in  detail.     The  following  account  is  based  principally  on  Merriam's  work. 

ajour.  Geology,  vol.  13,  1905,  p.  95. 

6  Geology  ol  the  Rainy  Lake  region,  pp.  131-102. 

(■Structural  geology  of  Steep  Rock  Lake,  Ontario:  Am.  Jour.  Sci.,  3(i  ser.,  vol.  42,  1891,  pp.  317-331. 

d  Private  report. 


148  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

The  geology  of  the  Steep  Rock  Lake  district  is  similar  in  essential  respects  to  that  of  the 
Vermilion  and  Rainy  Lake  districts.     The  succession,  in  descending  order,  is  as  follows: 

Algonkian  system: 
Intrusive  rocks. 
Huronian  series: 

Lower  Huronian... I iilerbedded  sediments  and  eruptive  rocks:  Dark-gray  slate,  agglomerate, 
greenstones  and  green  schists,  conglomerates,  and  limestone,  .forming 
part  of  Steep  Rock  "series"  of  Smyth,  estimated  by  Smyth  to  be  5,000 
feet  thick. 
Unconformity. 
Archean  system: 

Laurentian  series Granites  and  gneisses  intrusive  into  Kecwatin. 

Keewatin  series Ellipsoidal  greenstones  and  green  schists  containing  iron  formation. 

The  lake  resembles  an  irregular  letter  M,  of  wliich  the  western  arm  runs  north  and  south 
and  the  eastern  arm  northwest  and  southeast. 

The  Keewatin  greenstones  have  a  wide  distribution  on  the  south  side  of  the  lake,  especially 
near  Straw  Hat  Lake.  They  are  in  isolated  areas  surrounded  and  overlapped  ])y  the  lower 
Huronian  sediments.  The  principal  showing  of  iron-bearing  formation  is  southwest  of  Straw 
Hat  Lake.  It  is  in  contact  with  elhpsoidal  greenstone  on  the  west  side,  but  the  relation  on 
the  cast  is  not  Ioiowti.  Lean  iron  ore  also  outcrops  on  the  west  side  of  the  lake  and  in  various 
other  parts  of  the  district.  Glacial  fragments  of  iron  ore  have  been  found  on  the  south  side 
of  the  lake  opposite  Mosher's  Point. 

The  Laurentian  granites  and  gneisses  are  exposed  principally  on  the  north  and  east  sides 
of  the  lake.  Along  the  contact  of  the  Laurentian  and  Keewatin  in  the  southeastern  part  of 
the  district  there  is  a  great  series  of  hornblende  schists  intricately  associated  with  both  Kee- 
watin and  Laurentian  rocks.  These  are  regarded  as  contact  phases  of  the  Keewatin  where  it  is 
intruded  by  the  Laurentian,  similar  in  all  respects  to  those  of  the  Verniihon  district.  Smyth 
regards  them  as  overlying  the  lower  Huronian  sediments  and  as  passing  upward  into  the  schists 
of  Atikokan  River,  which  he  designates  as  the  "Atikokan  series." 

The  lower  Huronian  fringes  the  Laurentian  on  the  southwest.  Its  principal  exposure 
is  on  the  south  and  west  shores  of  the  lake,  but  small  patches  of  it  rest  against  the  granite  on 
the  points  projecting  from  the  east  and  north  sides  of  the  lake.  It  dips  at  60°  to  80°  away 
from  the  Laurentian.  The  basal  conglomerate  carries  fragments  of  various  phases  of  Lauren- 
tian and  Keewatin  rocks.  Where  the  conglomerate  rests  against  the  granite  it  is  made  up  so 
largely  of  granitic  debris  and  has  been  so  metamorphosed  that  it  is  frequently  difficult  to  deter- 
mine the  exact  contact  of  the  granite  and  the  sediments.  According  to  Smyth,  the  succession 
above  the  conglomerate  is:  Lower  limestone,  ferruginous  horizon,  interbedded  crystalhne  traps, 
calcareous  green  scliists,  upper  conglomerate,  greenstones  and  greenstone  schists,  agglomerate, 
and  dark-gray  clay  slate.  Some  of  the  greenstones  and  green  scliists  included  by  Smyth  in 
the  lower  Huronian  are  regarded  by  Merriam  and  by  the  authors  as,  at  least  in  "part,  Keewatin 
unconformable  beneath  the  Huronian. 

According  to  Smyth,  the  Steep  Rock  group  is  folded  into  an  eastern  syncUnal,  a  middle  anti- 
clinal, and  a  western  synclinal,  the  latter  being  faulted.  The  axes  of  these  folds  have  a  liigh 
pitch  to  the  south,  varying  from  60°  to  nearly  90°.  Throughout  the  whole  area  is  a  regional 
cleavage  which  has  a  nearly  uniform  direction  transverse  to  all  the  members  of  the  Steep  Rock 
group  and  also  to  the  contact  between  this  group  and  the  basement  complex.  This  has  largeh* 
obliterated  the  original  lamination  of  the  sediments  and  is  now  the  donunant  structure.  It  is 
therefore  the  effect  of  the  last  force  which  has  left  its  marks  upon  the  rocks  of  the  lake. 
Before  this  last  force  acted  upon  tlic  rocks  the  Steep  Rock  group  had  been  folded  into  a  south- 
westward-tlipping  monoclinal,  which,  under  the  action  of  the  cleavage-prochicing  force  in  a 
northeast  and  southwest  direction,  caused  the  present  fluted  outcrop  of  the  formations  of  the 
Steep  Rock  group.  That  the  basement  complex  itself  yielded  to  tliis  latter  force  is  sliowni  by 
the  irregular  outcrops  of  the  dikes  cutting  it. 

At  least  three  varieties  of  intrusives  cut  the  Laurentian  ami  Keewatin  and  have  supplied 
pebbles  to  the  conglomerate  at  the  base  of  the  lower  Huronian.     Other  iiitrusivcs  cut  the 


PRE-ANIMIKIE  IRON  DISTRICTS  OF  ONTARIO.  149 

Keewatin,  Laurentian,  and  lower  Iluronian  rocks  but  have  been  subjected  to  folding.     Finally, 
a  single  massive  dike  appears  to  be  subsequent  to  the  latest  period  of  folding. 

IRON   ORES. 

Lean,  banded  iron-bearing  rocks  appear  in  the  Keewatin  of  the  Steep  Rock  Lake  district. 
The  principal  showing  is  southwest  of  Straw  Hat  Lake.  The  rocks  are  in  contact  mth  ellipsoidal 
greenstone  on  the  west  side,  but  the  relation  on  the  east  is  not  known.  Lean  iron  ore  also 
outcrops  on  the  west  side  of  the  lake  and  in  various  other  parts  of  the  district.  Glacial  fragments 
of  iron  ore  have  been  found  on  the  south  side  of  the  lake  opposite  Mosher's  Point.  Explorations 
southwest  of  Straw  Hat  Lake  are  reported  to  have  recently  disclosed  an  ore  deposit. 

ATIKOKAN  DISTRICT. 

The  existence  of  iron  ore  along  Atikokan  River  and  Sabawe  Lake  to  the  east  of  Steep 
Rock  Lake  requires  mention  of  the  geology  of  tliis  area.  The  area  has  not  been  geologically 
mapped  in  detail. 

As  a  result  of  visits  to  the  district  Mr.  Merriam  and  the  authors  behevo  the  geology  to  be 
similar  in  all  essential  features  to  that  of  the  Steep  Rock  Lake  and  VermiHon  districts — that 
is,  there  are  represented  in  this  district  Keewatin,  Laurentian,  and  lower  Huronian  rocks. 
The  ores  are  in  the  Keewatin  series. 

The  Atikokan  iron  ores  are  3  miles  north  of  the  Canadian  Northern  Railway,  on  the  north 
side  of  Atikokan  River,  just  east  of  its  expansion  into  Sabawe  Lake.  Here  is  a  ridge  of  magnet- 
ite, green  scliist,  massive  greenstone,  and  iron  carbonate  running  approximately  parallel  to  the 
river.  The  greenstone  is  essentially  a  diorite  with  a  large  proportion  of  hornblende.  The 
magnetite  is  coarsely  crystalline  and  dense  and  carries  abundant  ampliibole  and  iron  pyrites 
and  small  amounts  of  the  nickel  minerals.  The  relations  of  the  magnetite  and  greenstones 
are  complex,  as  in  the  VermiUon  district,  but  as  a  whole  the  greenstone  seems  to  be  intrusive 
into  the  magnetite.  To  the  west  of  the  main  magnetite  exposure  iron  carbonate  appears  in 
similar  relations  to  the  greenstone.  So  intricate  are  the  relations  of  the  ore  to  the  greenstone 
that  it  is  difficult  to  determine  the  true  shape  of  the  magnetite  deposit  from  the  surface  outcrop. 
The  bands  are  narrow,  at  most  not  more  than  44  feet,  and  extend  along  the  bluff  for  more  than 
400  yards.  They  are  now  being  opened  for  mining.  The  ores  will  be  roasted  and  used  in 
furnaces  at  Port  Arthur. 

To  the  west,  down  the  river,  the  iron-bearing  formation  is  exposed  with  similar  association 
to  greenstone  and  green  scliist  at  a  number  of  places. 

KAMINISTIKWIA  AND  MATAWIN   DISTRICT. 

The  Kaministikwia  and  Matawin  district  is  characterized  by  lean,  slightly  magnetic  cherts 
and  jaspers  in  vertical  bands  and  lenses,  very  irregular,  closely  associated  with  green  schist  and 
ellipsoidal  basalt  typical  of  the  Keewatin  series.  Granite  and  quartz  poii^hyry  intrude  the 
Keewatin  at  many  places.  The  association  of  the  jasper  and  greenstone  and  porphj'ries  pre- 
sents all  the  problems  of  the  Vermilion  district.  The  principal  exposures  are  along  Kaministi- 
kwia River  between  Kamjnistikwia  and  Mokoman.  Just  north  of  the  railway,  a  mile  north  of 
Mokoman,  is  a  jasper  and  greenstone  breccia  and  conglomerate.  The  rock  here  exposed  has 
essentially  the  features  of  a  breccia,  but  parts  of  it  contain  fragmental  quartz  and  are  truly 
conglomerate,  suggesting  that  it  is  perhaps  the  basal  conglomerate  of  the  lower  Iluronian. 

Still  farther  south,  near  Kakabeka  Falls,  the  flat-lying  iron  formation  and  slates  of  the 
Animikie  group  (upper  Huronian)  are  exposed  along  the  river  and  at  Kakabeka  Falls. 

Farther  west  in  the  same  township  (Conmee)  are  more  extensive  outcrops  of  banded  jasper 
m  chert  containing  impure  siderite.     It  is  strongly  magnetic. 

Farther  south  in  Conmee  Township,  on  the  south  half  of  lot  7  in  the  sixth  concession,  the  iron  range  is  found  again 
with  a  trend  of  about  northwest  and  southeast  and  a  nearly  vertical  dip  on  a  long  ridge  about  150  feet  wide.  The 
silica  is  mainly  jasper,  often  of  beautiful  color,  banded  with  magnetite,  the  bands  often  folded  in  complex  ways,  and 
here  also  there  is  more  or  less  of  a  peculiar  breccia  of  grained  silica  or  jasper  in  a  fine  gray  matrix.  ' 


150  GEOLOGY  OF  THE  LAKE  SLtPERIOR  REGION. 

In  the  southeast  end  of  lot  7  in  the  fifth  concession  there  is  finely  banded  jasper  and  some  impure  carbonate 
intermixed,  but  on  lot  4  in  the  third  concession  the  rock  is  unusually  black  from  the  presence  of  magnetite,  and  some 
specimens  are  heavy  enough  to  make  fairly  good  ore.  Bands  having  a  width  of  1  or  2  feet  appear  to  be  nearly  solid 
masnotite  and  seem  rich  enough  to  work,  though  a  small  amount  of  pyrite  present  would  lower  the  grade  of  the  ore. 
The  banding  varies  in  direction  from  southeast  to  south;  and  here  again  a  conglomerate  or  breccia  is  commonly  found 
mixed  with  the  ore,  the  wliole  having  a  length  of  10  chains  and  a  width  of  135  feet. 

Altogether  this  series  of  iron  dejjosits  has  been  traced  for  about  8  miles,  running  parallel,  it  is  said,  to  a  similar 
range  located  by  Tumpelly  and  Smyth  L'  miles  to  the  southwest;  and  probably  both  are  continuations  of  the  Matawin 
ranges,  though  curvdng  in  a  somewhat  different  direction."! 

The  Matawin  iron  bolt  e.xtends  from  Kaministikwia  station  westerly  beyond  Greenwater 
Lake.  Banded  magnetic  and  hematitic  cherts  and  jaspers  outcrop  at  many  places  on  Matawin 
and  Shebandowan  rivers.  West  of  this  belt  banded  iron  ores  were  seen  outcropping  at  Copper 
Lake,  south  of  Shebandowan  Lake,  and  on  the  eastern  shore  of  Greenwater  Lake.  Ores  which 
probably  form  an  extension  of  the  same  belt  occur  south  of  Moss  Township,  on  the  farther  side 
of  the  gneiss  area  of  Greenwater  Lake. 

MICHIPICOTEN   DISTRICT. 

The  following  account  of  the  Michipicoten  district  is  taken  largely  from  the  writings  of 
A.  P.  Coleman  and  A.  B.  Willniott''  and  of  J.  M.  Bell."^  The  present  writers  have  made  no 
detailed  survey  of  the  district,  but  have  visited  the  area  and  agree  with  the  essential  conclusions 
reached  by  the  men  named. 

GEOGRAPHY  AND  TOPOGRAPHY. 

The  Michipicoten  district,  on  the  northeast  shore  of  Lake  Superior,  is  about  25  miles  in 
length  from  southwest  to  northeast,  with  a  greatest  width  of  about  7  miles,  and  runs  from  the 
mouth  of  Dore  River,  a  few  miles  beyond  Parks  Lake  on  the  northeast.  It  lies  northwest  of 
Michipicoten  River  near  its  entry  into  the  bay  of  the  same  name  on  the  northeast  side  of  Lake 
Superior  and  shows  the  rugged  topography  so  characteristic  of  that  shore. 

The  country  rises  rapidly  from  the  lake  in  steep  hills,  often  ridgelike,  with  the  general  direction  of  the  strike  of 
the  schists  about  70°  east  of  north,  and  culminates  in  the  ridge  of  iron-range  rock  just  east  of  the  Helen  mine,  called 
Hematite  Hill  or  Mountain,  which  reaches  a  height  of  1,100  feet  above  the  lake  or  1,700  feet  above  the  sea.  This  is 
the  highest  point  for  many  miles  around  and  makes  a  conspicuous  landmark,  though  other  hUls  reach  a  level  of  800 

or  900  feet. 

As  Hematite  Mountain  is  only  7  miles  from  Lake  Superior,  the  rLse  is  rapid,  and  the  location  of  the  railway  to  the 
Helen  mine,  which  is  at  a  level  of  G50  feet,  just  at  the  foot  of  the  mountain,  required  some  skill  in  the  choice  of  a 
route,  old  lake  beaches  and  sand  plains  being  utilized  where  possible. 6 

SUCCESSION. 

The  succession  of  formations  here  given  is  that  of  Coleman  and  WLllmott,  but  the  names 
of  the  series  are  changed  m  accordance  with  the  recommendation  of  the  special  committee  on 
the  Lake  Superior  region  and  the  series  are  grouped  into  the  Algonkian  and  Archean  systems.  "^ 

Algonkian  system: 
Huronian  series: 

Lower-middle     Huronian  fBasic  eruptive  rocks.  ^ 

("Upper  Hiuonian"  of  |  Acidic  eruptive  rocks. 
Coleman  and  Willmott).  [Dor6  conglomerate. 
Unconformity. 
Archean  system: 

Laurentian  series Granites  and  gneisses  intrusive  into  Keewatin  series. 

Eleanor  slate. 

Helen  formation  (iron-bearing). 

W'awa  tuff. 

Gros  Cap  greenstone. 


Keewatin  series  ("Lower  Hu- 
ronian' '  of  Coleman  and  Will- 
mott). 


0  Coleman,  ,\.  P.,  Iron  ores  of  northwestern  Ontario:  Eleventh  Rept.  Ontario  Bur.  Mines,  1902,  p.  130. 

!>  Tlie  Midiiplcoten  iron  ranges:  fniv.  Toronto  Studies,  geol.  ser.,  No.  2,  1902.  47  pp.     See  also  Rept.  Ontario  Bur.  Mines,  1902.  pp.  152-llW. 

c  Iron  ranges  of  Michipicolen  West;  Rept.  Ontario  Bur.  Mines,  vol.  14,  1905,  pt.  1.  pp.  278-3.''»,  with  geologic  map. 

rf  Report  of  the  special  committee  on  the  Lalte  Superior  region:  Jour.  Geology,  vol.  13. 1905,  pp.  89-104. 


PRE-ANIMIKIE  IRON  DISTRICTS  OF  ONTARIO.  151 

The  geology  of  the  Michipicoten  district  is  remarkably  similar  to  that  of  the  Vermilion 
district  of  Minnesota  in  regard  to  hthology,  succession,  and  structure. 

ABCHEAN   SYSTEM. 

KEEWATIN  SERIES. 

GROS  CAP  GREENSTONE. 

DISTRIBUTION. 

The  Gros  Cap  greenstone  is  well  exhibited  just  west  of  Michipicoten  Harbor  and  on  the 
trail  to  the  old  fishing  station  at  Gros  Cap. 

The  most  extensive  area  of  the  Gros  Cap  greenstones  is  the  one  extending  from  Gros  Cap  eastward  to  Magpie  River 
and  thence  north  from  Michipicoten  River  to  the  eastward  bend  of  the  Magpie.  Other  large  areas  exist  northeast  of 
Eleanor  Lake,  including  most  of  the  shore  of  Loonskin  Lake,  and  along  the  Josephine  branch  railway  from  mile  LH 
to  mile  17. 

Numerous  smaller  areas  have  been  mapped.  There  are  bands  of  greenstone  and  green 
schist  in  the  Wawa  tuff  that  have  the  same  characteristics. 

PETROORAPHIC   CHARACTER. 

The  Gros  Cap  greenstone  consists  of  ellipsoidally  parted  basic  igneous  rocks  formed  partly 
of  lava  flows,  in  all  respects  similar  to  the  Ely  greenstone  of  the  Vermilion  district  of  Minnesota. 

Many  parts  of  the  greenstones  do  not  show  the  ellipsoidal  structure  and  are  apparently  greatly  weathered  dia- 
bases, while  still  other  parts  are  distinctly  schistose;  but  the  three  varieties  run  into  one  another  and  can  hardly  be 
separated  in  mapping.  The  chloritic  schists  are  probably  tuffs  of  the  volcanoes  which  poured  out  the  lavas.  The 
whole  series  is  greatly  weathered  and  saussuritic  in  thin  sections. 

WAWA  TUFF. 

DISTRIBUTION. 

The  extent  of  the  Wawa  tuffs  and  their  boundaries  can  be  given  only  approximately,  partly  because  of  the  sand 
plains  covering  them  and  partly  on  account  of  the  intermixed  later  eruptive  rocks.  Beginning  at  the  southwest  is  a 
narrow  band  of  quartz  porphyry  schist  and  felsite  schist  along  the  northern  boundary  of  the  Dore  conglomerate  area, 
between  the  latter  rock  and  the  Laurentian.  Where  the  Dore  conglomerates  narrow  toward  the  northeast,  the  northern 
fringe  of  quartz  porphyry  schist  seems  to  widen  correspondingly,  though  greatly  interrupted  by  later  acid  and  basic 
eruptives.  Still  farther  northeast  the  sand  plains  of  the  Magpie  Valley  hide  the  rocks  almost  completely,  not  to  reap- 
pear until  near  Talbott  Lake,  where  the  Wawa  schists  are  extensively  developed.  From  here  to  the  northeast  end  of 
the  region  mapped  the  Wawa  schists  are  found  on  each  side  of  the  bands  of  the  iron  range  as  the  immediately  inclosing 
rocks,  except  where  broken  by  masses  of  greenstone  or  later  diabase,  and  they  extend  northeast  to  the  end  of  the  region 
mapped. 

PETROGRAPHIC   CHARACTER. 

The  Wawa  tuff  generally  has  the  composition  of  quartz  porphyry  or  felsite,  and  in  some  places 
evidently  consists  of  mashed  and  rearranged  rocks  with  crystals  of  quartz  and  feldspar  still  to  be 
seen  in  them.  In  general,  however,  the  formation  apparently  consists  of  tuffs  or  ash  rocks,  prob- 
ably erapted  in  connection  with  the  quartz  porphyry  and  deposited  in  water,  so  that  they  have 
a  more  or  less  stratified  character.  A  few  of  them  are  brecciated,  some  crashed  breccias,  others 
perhaps  agglomerates  formed  of  volcanic  fragments  larger  than  the  ash.  Some  rare  forms 
have  much  the  appearance  of  water-formed  conglomerates  with  rounded  pebbles,  one  singular 
example  of  the  sort  occurring  on  a  steep  hill  slope  at  the  west  end  of  Lake  Wawa.  In  a  gen- 
eral way  this  resembles  a  beach  deposit  with  pebbles  cemented  by  a  finer-grained  greenish 
or  yellowish  matrix,  but  on  closer  examination  the  apparent  pebbles  are  found  to  be  really 
concretions. 

There  is  no  sharp  line  between  this  phase  of  the  rock,  which  occurs  in  sipaller  amounts  at  other  points  also,  and 
varieties  like  ordinary  quartz  porphyry  schist,  so  that  one  may  suppose  it  to  be  merely  a  phase  of  the  series  of  acid 
schists  in  which  there  has  been  concretionary  action. 


152  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION 

Since  the  materials  forming  the  schists  were  laid  down,  or  else  during  their  deposit,  important  chemical  changes 
have  taken  place  in  them,  probably  by  circulating  hot  water,  bo  that  sheared  and  crushed  quartz  porphyry  or  porphy- 
rite  has  been  greatly  silicified,  at  times  even  transformed  into  thick  bands  of  pale-gray  or  green  chert  or  chalcedony, 
with  a  small  amount  of  eericite.  This  phase  is  similar  to  parts  of  the  Palmer  gneiss  of  the  Marquette  district.  In  other 
cases  a  considerable  amount  of  siderite  or  of  a  carbonate  like  aiikerite,  dolomite,  or  calcite  has  been  deposited  with 
cryptocrystalline  or  microcrystalline  silica,  suggesting  a  change  to  the  iron-range  rocks  which  form  the  uppermost 
series  of  the  lowor  Iluronian.  It  is  probable  that  this  change  went  on  at  the  time  when  the  original  iron-range  rocks 
were  deposited  and  under  the  same  conditions. 

In  a  gcneial  way  it  may  l>o  stated  that  tlie  Wawa  tuff  is  accompanied  by  lenses  or  bands 
of  carbonates,  inckiding  impure  siderite.s,  dolomites,  and  limestones.  In  most  places  some 
granular  silica  also  is  present,  and  it  may  be  that  these  lenses  or  bands  are  chemical  sediments. 

In  a  general  way  the  Wawa  tuffs  tend  to  bo  more  siliceous  and  to  contain  more  siderite  as  they  approach  the  iron 
range,  and  to  be  somewhat  coarser  in  grain  and  gneissoid  in  look  on  the  sides  toward  the  I.aurentian,  as  though  the 
proximity  of  these  rocks  had  influenced  their  crystalline  character  and  chemical  composition.  The  boundary  between 
them  and  the  Helen  iron-range  rocks  is  sometimes  quite  sharp,  a  thin  sheet  of  black  slate  occasionally  intervening 
between  the  two,  but  in  other  cases  there  are  schistose  varieties  of  the  siderite  of  the  iron  range  which  form  a  transition 
toward  the  quartz-porphyry  schists. 

STRUCTURE   AND   THICKNESS. 

The  Wawa  tuffs  have  on  the  average  a  strike  of  70°  east  of  north,  though  with  considerable  local  variations,  and  a 
dip  toward  the  south  of  from  50°  to  verticality.  Near  the  Helen  mine  they  are  shown  to  form  a  syncline  pitching 
toward  the  east  and  inclosing  in  their  trough  the  iron-range  rocks.  As  the  dip  is  much  the  same  on  each  side  of  this 
synclinal  axis,  the  fold  must  have  been  a  closed  one;  and  since  it  was  formed  erosion  has  eaten  down  the  Archean  sur- 
face until  at  various  points,  such  as  west  of  the  Helen  mine  and  south  of  Lake  Eleanor,  the  iron  range  in  the  central 
trough  has  been  completely  removed,  leaving  the  lower  schists  across  the  whole  width. 

The  greatest  measured  thickness  of  the  schists  is  to  the  south  of  Sayers  Lake,  where  they  are  known  to  reach 
across  Lake  Wawa,  a  distance  of  about  two  miles  and  a  quarter,  which  at  a  dip  of  70°  would  give  more  than  11,000  feet. 

HELEN  FORMATION. 
DISTRIUUTION. 

Beginning  at  ttie  southwest,  several  bands  of  the  granular  sUica  variety  occur  on  the  Gros  Cap  Peninsula,  the  largest 
being  at  the  Gros  Cap  mine  on  the  south  shore  of  the  peninsula."  The  materials  here  are  chert  and  granular  silica 
interbanded  with  hematite,  and  the  width  is  in  all  about  150  feet.  To  the  east  another  narrower  band  of  rusty  siliceous 
rock  is  seen,  and  just  around  the  eastern  point,  near  the  beacon,  is  a  third  still  narrower  band,  differing  from  the  others 
in  containing  magnetite  and  much  pyrite.  All  of  these  bands  of  iron  range  run  about  northwest  and  southeast  and 
have  a  dip  of  perhaps  50°  to  the  southwest.  A  similar  band  is  seen  on  the  west  shore  somewhat  south  of  the  portage 
across  the  neck  of  the  peninsula,  probably  an  extension  of  one  of  the  bands  mentioned  before.  About  150  yards  north 
of  the  portage  are  several  narrow  bands  of  the  rock,  usually  very  pyritous,  associated  with  quartz  porphjTy  schist  and 
striking  about  east  and  west  with  a  dip  to  the  south.  This  belt  probably  extends  to  the  east,  where  an  outcrop  of 
brown  sandy-looking  grained  silica  occiu's  a  little  inland  fi'om  the  old  fishing  station.  The  band  just  mentioned  is  nearly 
parallel  to  the  great  area  of  schist  conglomerate  to  the  north  and  is  the  nearest  part  of  the  iron  range  to  it,  so  that  it 
may  ha\-e  furnished  part  of  the  pebbles  of  granular  silica  in  the  conglomerate. 

Two  or  three  small  patches  of  the  iron  range  are  found  in  the  greenstone  east  of  Michipicoten  Harbor,  after  which  no 
more  is  known  for  about  8  miles,  when  the  Helen  iron  range  begins.  All  of  the  outcrops  mentioned  thus  far  appear 
to  be  inclosed  in  the  greenstones  as  if  swept  off  eruptively. 

The  prmcipal  belt  outcrops  near  the  Helen  mine. 

Beginning  on  the  west,  the  iron  range  as  found  at  the  Helen  mine  is  in  two  long  fingers  reaching  the  shore  of  Talbott 
Lake,  but  not  crossing  it.  The  southern  finger,  long  and  narrow,  possibly  reaches  a  short  distance  into  the  water  of 
the  lake,  but  does  not  appear  on  the  opposite  side.  It  extends  eastwardly  uji  the  valley  of  a  small  creek  until  it  reaches 
the  main  body  of  the  formation  near  Sayers  Lake.  Following  the  boundary  northward  are  several  minor  folds  which 
are  seen  to  rest  on  Wawa  tuffs.  Then  crossing  the  railway  track  near  the  outlet  of  the  lake,  the  range  extends  westward 
down  to  the  shore  of  the  lake,  where  it  comes  to  an  end  within  a  few  feet  of  the  shore,  being  bottomed  by  Wawa  tuffs. 

A  comparatively  recent  development  has  been  the  finding  of  a  large  band  of  iron-bearing 
formation  witliin  2  miles  to  the  northwest  of  the  Helen  mine  in  an  area  Avhich  was  supposed 
to  have  been  carefully  explored.  It  is  associated  with  a  thick  belt  of  black  slate,  but  most 
of  the  inclosing  rock  is  of  the  Gros  Cap  and  Wawa  formations. 

o  Kept.  Geol.  Survey,  Canada  1863-1869,  p.  131;  also  Eighth  Kept.  Ontario  Bur.  Mines,  pp.  145, 254. 


PRE-ANIMIKIE  lEON  DISTRICTS  OF  ONTARIO.  153 

On  the  north  side  the  range  seems  to  extend  quite  regularly  toward  the  east,  the  formation  standing  almost  ver- 
tically. [From  the  Helen  mine  the  range]  runs  for  a  mile  and  three-quarters  a  little  north  of  east,  when  another  inter- 
ruption occurs,  thought  by  some  to  be  caused  by  a  fault.  The  evidence  for  this  does  not  seem  conclusive,  and  more 
careful  exploration  may  bring  to  light  in  the  heavily  wooded  region  to  the  east  some  links  connecting  it  with  the 
Lake  Eleanor  band,  which  commences  after  a  gap  of  a  mile  and  a  half  and  runs  northeast  to  the  Grasett  road  between 
Lakes  Wawa  and  Eleanor.  The  road  follows  a  depression  between  hills  that  probably  represents  a  line  or  zone  of  fault- 
ing, for  the  iron  range  here  jogs  three-eighths  of  a  mile  to  the  north  and  then  continues  the  usual  strike  of  about  60°. 
Between  the  two  main  outcrops  and  just  east  of  the  road  are  two  small  ridges  of  rusty  granular  silica  pointing  a  little 
east  of  north,  perhaps  remnants  left  during  the  dragging  of  the  strata  in  faulting. 

The  iron  range  south  of  Lake  Eleanor  gives  the  best  exposure  of  the  range  between  the  Helen  and  Josephine  mines. 
In  a  general  way  it  suggests  that  of  the  Helen  mine,  though  on  a  smaller  scale. 

The  strike  of  the  ii-on-range  rocks  at  the  extreme  southwest  end  is  not  far  from  north  and  south,  with  a  dip  running 
from  30°  to  90°  to  the  east,  pointing  toward  the  two  hills  of  granular  silica  to  the  east  of  the  road.  Less  than  100  paces 
eastward  along  the  top  of  the  ridge  the  strike  becomes  G0°  to  80°  and  keeps  this  direction  until  the  east  end  of  the  little 
lake  is  passed,  when  it  changes  to  45°  for  a  short  distance,  and  the  range  ends  abruptly  in  a  mass  of  greenstone.  Beyond 
this  it  has  not  been  traced,  but  the  country  is  very  mossy  and  forest  covered,  so  that  it  is  hard  to  say  positively  that  there 
may  not  be  exposures  of  the  iron  range  yet  undiscovered. 

The  next  point  at  which  the  iron-bearing  rocks  have  been  found  is  2J  miles  to  the  northwest  of  the  Lake  Eleanor 
range,  where  they  begin  just  east  of  a  long  unnamed  lake  and  run  about  60°  east  of  north  past  the  north  side  of  Brooks 
Lake  almost  to  Bauldry  Lake,  a  distance  of  about  2  miles.  Here  again  a  fault  of  great  magnitude  has  been  suggested, 
the  plane  of  faulting  running  northwest  and  southeast;  and  there  is  much  in  favor  of  this  view,  though  it  can  not  be 
said  to  have  been  proved,  since  very  little  work  has  been  done  on  the  geology  of  the  country  between  the  two  iron 
ranges.  The  only  rocks  known  to  exist  between  them  are  greenstones  and  green  schists. 

The  iron-bearing  formation  appears  again  near  the  south  side  of  Baukhy  Lake  and 
extends  eastward  past  the  south  side  of  Long  Lake.  Beginning  at  Goetz  Lake  and  rimning 
east  through  Brooks  Lake  and  Kimball  Lake  is  a  considerable  belt  of  iron  formation,  on  vvliich 
the  Josephine  mine  is  located.  Ore  has  been  foimd  here  in  small  amount  by  driUing,  but  has 
not  been  mined. 

STRUCTURE   AND   THICKNESS. 

In  a  general  way  the  rocks  of  the  Helen  iron  formation,  though  so  narrow,  rarely  exceeding  1,000  feet  in  width, 
are  the  most  distinctive  features  of  the  lower  Huronian,  since  they  are  very  easily  recognized  and  nearly  always  rise 
as  sharp  ridges  above  the  sui'rounding  region.  Except  on  Gros  Cap,  where  the  bands  strike  about  northwest  and  south- 
east, the  different  ridges  have  a  surprising  uniformity  of  strike,  about  N.  60°  to  70°  E.,  the  same  direction  as  one  finds 
prevalent  in  the  adjoining  schists.  Though  the  general  strike  is  so  uniform,  it  is  evident  that  along  with  the  other 
rocks  of  the  region  the  u'on  formation  has  been  interrupted  frequently  by  eruptive  masses,  and  apparently  also  by 
faults  of  great  magnitude,  the  effect  always  being  to  shift  the  part  east  of  the  fault  plane  toward  the  north. 

It  is  probable  that  the  bands  of  iron  range  are  not  simple  tilted  strips  of  rock  but  closely  folded  sheets,  only  the 
lower  portion  of  which  is  still  preserved,  and  it  may  be  that  the  apparent  gaps  between  the  ranges  are  really  due  to  the 
erasion  of  the  general  rock  surface  so  far  down  as  to  cut  off  the  folded  upper  part  of  the  lower  Huronian  altogether,  leaving 
only  the  schists  beneath.  If  this  is  the  case  the  depth  to  which  the  iron-bearing  rocks  descend  may  be  quite  limited, 
though  the  amount  of  mining  and  diamond  drilling  done  on  the  range  does  not  give  very  certain  evidence  in  this 
respect. 

The  iron-bearing  formation  at  the  Helen  mine  underlying  the  Boyer  Lake  basin  is  peculiar 
in  that  the  lake  bottom  is  much  below  the  outlet.  The  origin  of  tliis  is  discussed  elsewhere 
(p.  158). 

PETKOGBAPHIC   CHARACTER. 

Five  species  of  rock  may  be  distinguished  in  the  iron-bearing  Helen  formation — banded 
granular  silica  or  ferruginous  cherts  with  more  or  less  iron  ore,  black  slate,  sitlerite  with  varying 
amounts  of  sihca,  griinerite  schist,  and  pyritic  quartz  rock.  All  are  found  well  developed  at  the 
Helen  mine,  and  all  but  the  griinerite  schist  have  been  found  in  the  Lake  Eleanor  iron  range 
also ;  granular  sihca  and  siderite  occur  in  large  quantities  in  every  important  part  of  the  range, 
though  small  outcrops  sometimes  show  the  silica  alone.  The  ferruginous  cherts  are  in  many 
places  soft  and  sandy,  like  the  ferruginous  cherts  or  taconites  of  the  western  Mesabi.  Jaspery 
varieties  have  not  been  found  on  this  range,  but  they  occur  only  a  few  miles  to  the  north. 

RELATIONS  TO    OTHER   FORMATIONS. 

The  Helen  formation  is  very  closely  related  to  the  Gros  Cap  greenstone  and  Wawa  tuff. 
Its  relations  to  the  associated  rocks  of  the  Keewatin  series  are  almost  identical  with  the  rela- 
tions of  the  Soudan  formation  of  the  Vermilion  district  of  Minnesota  to  the  adjacent  Keewatin 


154  GEOLOGY  OF  THE  LAKE  SLTEKIOK  REGION. 

rocks.  In  general  the  iron-bearing  formation  from  its  structure  seems  to  be  at  upper  hori- 
zons of  the  Keewatin  and  to  rest  on  the  other  rocks,  being  fol'ded  in  with  (hem;  bvit  tlie  forma- 
tion lias  been  also  intrudeti  by  basic  rocks  wliich  have  been  mapped  as  Gros  Cap  greenstone, 
and  some  of  them  may  be  intei-bedded  with  the  surface  flows  of  the  Gros  Cap  greenstone.  For 
a  discussion  of  the  problem  the  reader  is  referred  to  the  chapter  on  the  Vermilion  district  and 
also  to  the  (Hscussion  of  the  origin  of  tlie  ores.  (Sec  Chapters  V,  pp.  118  et  seq.,  and  XVII, 
pp.  460  et  seq.) 

ELEANOR  SLATE. 

In  addition  to  the  slates  of  the  Wawa  formation,  i^lates  of  a  distinctly  sedimentary  kind  occur  as  thin  bands  in 
the  northeastern  part  of  the  region  near  Eleanor  Lake  and  elsewhere.  .Slate  or  shale  of  the  kind  described  is  traceable 
at  intervals  for  a  mile  along  the  north  shore  of  Parks  Lake,  and  is  found  underlying  the  Dot6  conglomerate  north  of 
Eleanor  Lake  on  the  Grasett  road.  They  are  buff  to  dark -gray  or  black  rocks  with  slaty  cleavage,  sometimes  forming 
an  angle  of  25°  with  the  well-marked  bedding.  Some  varieties  of  them  are  carbonaceous,  and  at  a  point  east  of  Wawa 
Lake  such  a  slate  was  taken  up  as  a  coal  mine.  \\'hether  the  black  graphitic  slate  often  cormected  with  the  iron 
ranges  belongs  with  Eleanor  slates  is  not  certain,  nor  has  it  been  determined  positively  whether  the  slates  are  older  or 
yotmger  than  the  adjoining  iron-bearing  rocks. 

LAURENTIAN  SERIES. 

The  Laurentian  series  includes  various  types  of  granite,  quartz  porphyry,  quartz  por- 
phyrite,  felsite,  and  quartzless  porphyry.  They  are  intrusive  mto  the  Keewatin  series  and 
in  part  into  the  overlymg  lower-middle  Huronian  sediments,  but  in  large  part  also  they  he 
miconformably  below  the  Hm-onian,  as  is  shown  by  the  numerous  pebbles  of  Ijaurentian 
gneiss  and  granite  included  in  the  basal  conglomerate  of  the  Huronian.  It  is  not  desirable 
that  all  these  mtrusives  should  be  classed  with  the  Laurentian,  as  that  term  is  properly  appUed 
only  to  the  pre-Huronian  rocks,  but  they  have  not  been  sufficiently  w^eil  discriminated  and 
mapped  to  warrant  a  separate  classification. 

ALGONKIAN   SYSTEM. 

HTJBONIAN  SERIES. 

LOWER-MIDDLE    HUKOXIAX. 

dor£  COITGLOMEKATE. 

distribution,  topography,  and  stritcture. 

The  conglomerate  occurs  fi-om  point  to  point  along  the  shore  as  far  as  Dog  River,  10  miles  to  the  west,  and  east- 
ward to  about  the  third  milepost  on  the  railway  from  Michipiroten  Harbor  to  the  Helen  mine,  a  distance  of  4  miles; 
while  the  greatest  width  measured  during  last  summer's  work  is  about  a  mile  and  a  half,  on  a  line  due  north  of  the 
harbor. 

In  general  the  topography  of  the  conglomerate  band  is  very  rugged  and  hilly,  with  numerous  successive  ridges 
running  parallel  to  the  strike,  which  averages  about  70°;  and  with  very  steep  slopes  on  each  side,  but  especially  toward 
the  north,  where  the  narrow  hills  often  drop  off  vertically  or  even  overhang.  The  cause  of  this  is  to  be  found  in  the 
unequal  resistance  of  the  different  layers  to  weathering  and  in  the  fact  that  the  dip  is  usually  very  steep,  from  60°  to 
90°,  averaging  about  75°  to  the  south.  Dips  to  the  north  have  only  rarely  been  noticed.  The  steep  cliffs  formed  in 
the  way  described  often  have  a  height  of  50  or  more  feet,  and  on  the  north  side  are  frequently  unscalable  for  consider- 
able distances.  Perhaps  the  most  rugged  portion  of  the  region  is  directly  north  of  Michipicoten  Harbor,  where  within 
2  miles  of  the  shore  there  are  several  of  these  ridges,  with  Valleys  between,  rising  finally  to  over  600  feet  above  Lake 
Superior. 

\\Tiile  the  general  strike  is  about  70°  there  are  great  local  \'ariations,  especially  in  the  vicinit)^  of  eruptive  masses. 
Near  the  second  mile  on  the  railway  the  strike  is  nearly  north  and  south  for  more  than  400' yards,  but  on  each  side  the 
usual  directions  of  from  70°  to  75°  are  found.  There  is  good  reason  to  believe  that  in  general  the  strike  of  the  schistosity 
corresponds  to  that  of  the  sedimentation,  for  bands  of  rock  free  from  pebbles  follow  the  same  direction,  but  in  a  few  cases 
the  schistose  structure  seems  to  cross  the  direction  of  sedimentation,  having  a  bearing  of  about  45°,  while  the  general 
course  of  the  ridges  is  70°  to  80*". 

PETROGRAPHIC   CHARACTER. 

Tilie  conglomerates  are  for  the  most  part  large  and  well  rounded.  They  consist  of  dark- 
green  schist,  granite,  ferruginous  chert,  spotted  gray-green  scliist,  porpiijTj-,  felsite,  and  con- 
glomerate or  breccia.  All  have  been  more  or  less  flattened  during  the  development  of  schistosity 
in  the  rock. 


PEE-ANIMIKIE  IRON  DISTRICTS  OF  ONTARIO.  155 

The  conglomerate  is  in  many  places  penetrated  by  dikes  of  quartz  porphyry,  or  sometimes  quartzless  porphyry, 
running  parallel  to  the  stratification  as  a  rule,  and  in  many  cases  squeezed  or  sheared  into  felsite  schist  in  which  the 
porphyritic  structure  is  almost  lost. 

In  addition  to  the  porphyry  dikes  there  are  numerous  masses  and  dikes  of  diabase  rising  through  the  conglomer- 
ate, apparently  later  in  date  than  the  porphyries,  since  they  are  seldom  squeezed  into  schists  so  far  as  ob.served.  The 
diabase  seems  to  be  the  most  resistant  rock  of  the  region  with  the  exception  of  the  iron  range  of  the  Helen  mine,  and 
accordingly  forms  in  many  cases  the  tops  of  the  highest  ridges. 

THICKNE.SS. 

The  general  attitude  of  the  large  area  of  schist  conglomerate  just  described  suggests  a  continuous  series  of  strata, 
as  supposed  by  Logan,  since  in  most  cases  the  dip  and  strike  are  fairly  uniform;  and  any  marked  variations  maybe 
accounted  for  by  the  presence  of  eruptive  rocks.  This  would  give  them  a  thickness  of  about  7, .500  feet,  for  the  greatest 
width  is  8,000  feet,  with  an  average  dip  of  about  7.5°. 

However,  it  is  not  easy  to  imagine  the  mass  as  tilted  bodily,  and  it  is  more  natural  to  think  of  the  series  as  form- 
ing a  close  fold,  most  probably  a  syncline  with  the  two  sides  closely  squeezed  together,  and  tilted  slightly  against  the 
Laurentian  mass' to  the  north.  In  this  case  we  may  suppose  that  the  schists  were  to  some  extent  pulled  asunder  at  the 
base  of  the  fold,  which  was  in  tension,  allowing  the  felsites  and  diabases  to  penetrate  parallel  to  the  cleavage.  There 
is  no  doubt,  however,  that  some  of  the  diabase  dikes  are  later  in  age  and  cut  diagonally  across  the  schistose  structure. 

One  feature  of  the  arrangement  of  the  conglomerates  supports  the  view  that  they  form  a  syncline.  Toward  the 
western  end  of  the  series  of  rocks  we  find  bands  of  well-defined  conglomerate  along  each  side  with  gray  and  green  schists 
showing  few  or  no  pebbles  between,  as  if  there  was  an  upper  layer  of  finer  sediments  nipped  in  between  the  two  sides 
of  the  conglomerate.  The  absence  of  pebbles  in  this  central  area  may,  however,  be  due  merely  to  a  greater  amount 
of  compression,  flattening  them  beyond  recognition.  Toward  the  eastern  end  there  are  very  few  gaps  where  pebbles 
have  not  been  seen. 

RELATIONS   TO    UNDERLYING    ROCKS. 

The  T)or6  conglomerate  near  Michipicoten  Harbor  is  nowhere  found  in  contact  with  undoubted  Archean  rocks, 
though  what  look  like  Wawa  tuffs  and  have  been  mapped  as  such  occur  as  a  narrow  band  to  the  north  between  the 
conglomerate  and  the  Laurentian;  and  schists  with  some  granular  silica,  certainly  lower  Huronian,  are  found  near  the 
north  end  of  the  peninsula  of  Gros  Cap,  though  a  small  sand  plain  separates  them  from  the  conglomerate.  The  Lauren- 
tian eruptives  have  not  been  seen  in  actual  contact  with  them  on  the  north,  though  some  belts  of  green  schists  in  the 
Laurentian  a  little  way  from  the  hidden  contact  may  be  gi'eatly  metamorphosed  conglomerate  swept  off  at  the  time  of 
eruption. 

The  relationship  to  the  south  is  more  distinct,  and  the  Gros  Cap  greenstones  appear  to  be  the  underlying  rock 
folded  into  a  syncline  with  them;  so  that  south  of  the  railway  half  a  mile  from  the  harbor  the  greenstone  seems  to  over- 
lie the  conglomerate,  both  having  a  dip  of  about  70°  to  the  south. 

The  pebbles,  however,  are  clearly  derived  from  the  rocks  of  the  adjacent  Keewatin  and 
Laurentian.  Their  variety  and  large  size  characterize  the  conglomerate  as  a  basal  conglomerate 
marking  a  great  unconformity. 

MICHIPICOTEN  EXTENSIONS. 

Many  areas  of  iron-bearing  rocks  near  the  Michipicoten  district  have  been  reported  and 
mapped  by  Coleman,  Bell,  and  others.  Their  lithology  and  association  are  similar  to  those  of 
the  Michipicoten  district.  No  attempt  Avill  be  made  here  to  describe  in  detail  the  individual 
belts.     None  of  them  are  productive  and  in  few  of  them  have  detailed  geologic  maps  been  made. 

J.  M.  Bell"^  has  reported  on  the  iron  ranges  of  Micliipicoten  West,  covering  the  northern 
and  western  extensions  of  the  producing  Micliipicoten  iron-range  district,  adjacent  to  Micliipi- 
coten Bay.  The  northern  range  lies  between  Magpie  River  and  the  western  branch  of  Pucaswa 
River,  practically  continuous  with  the  old  Michipicoten  range.  The  western  range,  separated 
from  the  other  by  granite,  lies  between  Otter  Head  and  Bear  River,  on  the  Lake  Superior  shore, 
and  extends  but  a  short  distance  north  of  Lake  Michi-Biju.  The  lithology  and  succession  are 
essentially  the  same  as  in  the  Micliipicoten  district.  The  Helen  formation  consists  of  sideritic 
and  pyritous  cherts,  jaspers,  amplubolitic  scliists,  siderite,  iron  ores,  cjuartzite  phyllites,  and 
biotitic  and  epidotic  sclusts.  For  the  most  part  the  iron-bearing  bands  are  lean  ore.  Explora- 
tion has  been  carried  on  somewhat  extensively  at  Iron  Lake,  Frances  mine,  and  Brotherton  Hill, 

a  Iron  ranges  of  Micliipicoten  West:  Rept.  Ontario  Bur.  Mines,  vol.  14, 1905,  pt.  1,  pp.  278-355. 


156  GEOLOGY  OF  THE  LAKE  SLTERIOR  REGION. 

at  the  Leach  Lake  bands  in  the  northern  range,  and  in  Laird's  claims,  the  Julia  River  bands, 
tlie  David  Katossin  claims,  and  the  Lost  Lake  claims  in  the  western  range,  but  no  important 
ore  deposits  have  yet  been  found. 

THE  IRON   ORES   OP  THE   MICHIPICOTEN   DISTRICT. 

By  the  authors  and  W.  J.  Mead. 
GENERAL  STATEMENT. 

The  Micliipicoten  district  has  one  producing  mine,  the  Helen.  The  Helen  ore  bodv  Hes 
in  a  great  anipliitlieater  opening  westward  on  Boyer  Lake,  the  east  wall  of  wliicli  is  formed  by 
iron  carbonate,  the  north  by  ferruginous  cherts,  and  the  south  by  Wawa  tuff.  The  tuffs  and 
ferruginous  cherts  stand  vertical.  Boyer  Jjake  has  been  drained,  and  the  basin,  a  (juarter  of  a 
mile  long  and  130  feet  tleep,  is  apparently  cut  out  of  solid  rock.  A  dike  of  diabase  crosses  the 
basin  from  north  to  south  near  its  east  end,  as  shown  by  mining  operations,  and  its  outcrop  on 
the  edge  of  the  basin  can  now  be  seen.  Most  of  the  ore  mined  is  east  of  the  dike,  but  ore^  is 
known  west  of  it.  Alining  operations  are  300  feet  below-  the  original  level  of  Boyer  Lake. 
The  ore  body  seems  to  dip  eastward  as  if  passing  under  the  siderite  hill.  A  drift  under  this 
hill  has  developed  several  hundred  thousand  tons  of  iron  pyrites.  Along  the  south  margin  of 
the  ore  body  ocher  or  paint  rock  marks  the  limit  against  the  green  schists.  To  the  north  the 
ore  runs  gradually  into  lean  material,  with  too  much  white  silica  to  be  worth  mining. 

CHEMICAL  COMPOSITION. 

Following  is  the  average  analysis  of  all  ore  sliipped  from  the  Micliipicoten  district  in  1907: 

Average  composition  of  all  ore  shipped  from  the  Michipicoten  district  in  1907. 

Moisture  (loss  on  drying  at  212°  F.) 5.  70 

Analysis  of  dried  ore  : 

Iron 58.  20 

Phosphorus 127 

Silica 4.  40 

Manganese 165 

Alumina 88 

Lime 23 

Magnesia 14 

Sulphur 127 

Loss  by  ignition 10.  40 

Chemically  the  ore  most  closely  resembles  some  of  the  more  hydrous  Mesabi  ores.  It  is 
low  in  alumina  and  high  in  combined  water,  which  makes  almost  all  of  the  loss  on  ignition. 

MINERAL  COMPOSITION. 

Mineralogically  the  ores  are  made  up  of  hydrated  iron  oxide  and  silica,  with  small  amounts 
of  clay  and  other  minor  constituents.  The  following  approximate  mineral  composition  Vvas 
calculated  from  the  above  average  analyses: 

Mineral  composition  of  Michipicoten  ores,  calculated  from  above  analyses. 

Hematite 23.  60 

Limonite 69.  60 

Quartz ' 3.  36 

Kaolin 2.  23 

Iron  sulphide 24 

Apatite 41 

Miscellaneovis .56 

100.00 


PRE-ANIMIKIE  IRON  DISTRICTS  OF  ONTARIO.  157 

The  hydrated  ferric  oxide  is  calculated  as  hematite  and  limonite  in  order  to  afTord  com- 
parisons with  other  ores  similarly  calculated.  It  is  known  that  liydrated  iron  oxides  other 
than  limonite  are  present,  and  it  is  probable  that  practically  all  of  the  ore  is  more  or  less 
hydrated;  hence  the  amount  of  hematite  present  is  probably  less  than  is  indicated  above.  No 
phosphorus  minerals  have  been  identified,  but  the  presence  of  calcium  sugg:ests  calcium  phos- 
phate (apatite).  Calculation  shows,  however,  that  sufficient  calcium  is  not  present  to  account 
for  all  the  phorphorus  as  apatite.  Iron  sulphide  is  locally  abundant  in  the  ores,  occurrmg  in 
pockets  of  "pyritic  sand." 

PHYSICAL  CHARACTERISTICS. 

Color  and  texture. — In  color  the  ore  ranges  from  liglit-yellow  ocber,  suitable  for  paint, 
tlu-ough  a  variety  of  shades  of  red  and  brown  to  dark  brown  or  nearly  black.  In  texture  the 
ore  varies  from  soft  earthy  material  to  rough,  slaglike  limonitic  ore,  and  locally  hard  blue 
hematite  is  found. 

Density. — The  average  mineral  density  of  the  ore,  calculated  from  the  average  mineral 
composition  given  above,  is  approximately  3.85. 

Porosity. — The  porosity  of  the  ore  varies  to  an  extreme  degree,  ranging  from  a  minimum  of 
less  than  5  per  cent  m  the  dense  ore  to  a  maximum  of  over  50  per  cent  (locally  60  per  cent)  in 
the  limonite.  The  average  is  rather  difficult  to  estimate,  but  is  probably  between  .30  and  40 
per  cent. 

Cubic  feet  per  ton. — Owing  to  the  extent  to  wliich  it  varies  in  density,  jiorosity,  and  moisture, 
the  cubic  content  of  the  ore  ranges  within  wide  limits.  The  average  is  approximately  1.3.5 
cubic  feet  a  ton. 

SECONDARY  CONCENTRATION  OF  THE  MICHIPICOTEN  ORES. 

The  Helen  mine  has  impervious  walls,  but  the  direction  and  nature  of  the  concentrating 
waters  are  not  yet  clear. 

Tlie  u'on-bearing  formation  was  originally  cherty  iron  carbonate.  The  hill  east  of  the  ore 
body  exhibits  one  of  tlie  largest  masses  of  unaltered  carbonate  known  in  the  Lake  Superior 
region.  The  alteration  of  the  iron  carbonate  can  be  seen  in  all  its  stages,  first  into  ferruginous 
chert  and  then  into  ore,  and  locally  directly  into  ore.  The  bottom  of  the  lake  basin  is  partly 
covered  with  large  masses  of  yellow  ocher  dissolved  from  the  carbonate  and  redeposited. 

The  iron  carbonate  is  thoroughly  impregnated  with  sul])hide  minutely  disseminated  tlirough 
the  carbonate  and  m  veins  in  it.  During  the  alteration  of  the  iron  carbonate  to  the  iron  ore  this 
sulphide  has  remained  relatively  intact,  for  it  is  included  with  the  oxides  of  iron  in  large  masses. 
In  the  deeper  levels  of  the  Helen  mine  the  iron  sulphide  is  in  such  large  masses  as  to  constitute 
a  great  obstacle  to  mining.  Nevertheless,  some  of  the  sulphide  has  been  altered  and  is  rep- 
resented in  the  limonite  forming  the  lake  bottom.  The  waters  of  the  lake  are  liighly  charged 
with  sulphuric  acid,  which  has  a  strong  deleterious  effect  on  the  pipes. 

Associated  with  the  limonite  in  the  lake  bottom  is  a  peculiar  green  mud,  the  composition 
of  which  is  as  follows: 

Analysis  of  dark-green  mud  from  lake  bottom. 

SiOj 47.  58 

Fe 11.  23 

Mn 14 

CaO 95 

CO, 3.19 


158 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


Embedded  in  tliis  imid  was  found  a  glacial  bowlder  consisting  largely  of  serpentine,  showing 
peripheral  alteration  to  a  depth  of  several  inches.  Analyses  made  ]>y  R.  D.  Hall  of  the  center 
and  outer  ]iortious  are  as  follows: 

Analyses  oj  altered  bowlder  jrom  bottom  o/  Boyer  Lake. 


SiO;... 

AljOs.. 
Fe.Oj. 
FeO... 
MgO.. 
CaO... 
NasO.. 
KjO... 
HaO-. 
H2O+. 
TiOs. . 
SO3... 
COs... 


Center  of 
bowlder. 


39.36 
3.48 

0.S4 
6.82 
31.04 
3.22 

.n 

.90 
.20 
7.44 
.13 
.18 
Trace. 


Altered 
portion. 


37.80 

3.76 

9.13 

7.76 

28.02 

3.50 

.06 

.10 

.62 

8. 58 

.38 

.40 

.10 


Altered 
portion. 

assuming 
FejOa 

constant. 


28.30 

2.82 

(1.84 

5.81 

21.00 

2.62 

.04 

.07 

.46 

6.41 

.28 

.30 

.07 


The  inner  portion  of  the  bowlder  was  a  dense  dark-green  rock  and  the  altered  portion  a 
lighter-green  earthy  material.  The  alteration  appe.u-s  to  have  been  brought  about  essentially 
by  solution.  Oxidation  has  been  practically  nil,  as  has  also  earbonation.  The  ferric  iron 
occurs  essentially  as  magnetite.  If  this  mineral  is  assumed  to  have  been  unaltered,  it  follows 
from  a  comparison  of  the  first  and  third  columns  in  the  above  table  that  there  has  been  a  loss 
of  all  constituents  except  ferric  iron,  SO3  and  CO,.  The  nature  of  the  alteration  differs  essen- 
tially from  typical  \yeathering. 

The  abundant  evidence  of  decomposition  of  the  substances  of  the  lake  bottom  and  the 
presence  of  sulphuric  acid  in  the  lake  waters  have  suggested  to  Coleman  and  Willmott"  that 
Boyer  Lake  represents  a  solution  basin.  The  bottom  of  the  lake  is  considerably  below  its 
outlet.  Though  ilecomposition  has  undoubtedly  aided  in  the  erosion  of  the  lake  bottom, 
there  is  also  evidence,  summarized  by  Martin  (see  pp.  430-431),  that  the  lake  basin  is  a  glacial 
cirque  developed  largely  by  mechanical  means. 


1  The  Michipicoten  iron  ranges:  Univ.  Toronto  Studies,  geol.  ser.,  No.  2,  1902,  p.  23. 


CHAPTER  VII.  THE  MESABI  IRON  DISTRICT  OF  MINNESOTA.^ 

GENERAL  DESCRIPTION. 

The  Mesabi  iron  district  lies  in  the  jiart  of  Minnesota  northwest  of  Lake  Superior.  In  shape 
and  trend  it  is  simiLar  to  the  other  iron  districts  of  the  Lake  Superior  re(:i;ion.  (See  Ph  VIII, 
in  pocket.)  It  extends  from  a  point  west  of  Pokegama  Lake,  in  T.  142  N.,  R.  25  W.,  east-north- 
east to  Birch  Lake,  a  distance  of  approximately  110  miles,  with  a  width  varj-ing  from  2  to  10 
miles.  Its  area  is  about  400  square  miles.  To  the  east  from  Birch  Lake  to  Gimflint  Lake 
and  beyond  are  small  patches  of  u-on-bearing  rocks,  constituting  remnants  of  an  eastward 
extension  of  the  ^lesabi  district. 

The  main  topographic  feature  of  the  district  is  a  ridge  or  "range"  parallel  to  the  longer 
direction  of  the  district,  known  as  the  Giants  or  Mesabi  Range.''  Mesabi  (spelled  also  Mesaba 
and  Missabe)  is  the  Chippewa  Indian  name  for  "giant."  In  the  west  end  of  the  district  the 
Giants  Range  merges  insensibly  into  the  level  of  the  surrounding  country,  about  1,400  feet 
above  sea  level,  or  800  feet  above  Lake  Superioi-.  Toward  the  east  the  elevation  with 
reference  both  to  Lake  Superior  and  to  the  surrounding  country  increases;  from  range  18  to 
range  12  elevations  of  1,800  and  1,900  feet  above  sea  level,  or  400  and  500  feet  above  the  level 
of  the  surrounding  country,  are  reached.  For  many  miles  both  north  and  south  of  the  range 
there  is  a  comparatively  low,  flat  area,  and  the  Giants  Range,  particularly  its  eastern  portion, 
is  a  conspicuous  feature  in  the  landscape. 

While  the  general  trend  of  the  range  is  east-northeast,  there  are  many  gentle  bends  in 
the  crest  line,  and  m  range  17  a  spur  known  locally  as  the  "Horn"  projects  in  a  southwesterly 
direction  for  6  miles.  The  crest  of  the  range  is  in  places  broad  and  flat,  in  others  comparatively 
narrow  and  sharp.  The  southern  slope  is  very  gentle;  the  northern  slope  is  somewhat  less  so. 
At  short  intervals  both  crest  and  slopes  are  notched  by  dramage  channels. 

The  Giants  Range  for  the  most  part  forms  a  drainage  divide,  although  it  is  crossed  by 
drainage  channels  at  several  places.  The  dramage  of  the  district  is  apportioned  among  three 
of  the  great  river  systems  of  the  country — tJie  Mississippi,  St.  Lawrence,  and  Nelson. 

The  succession  of  formations  in  the  Mesabi  district  appears  in  the  following  statement: 

Quaternary  system: 

Pleistocene  series Deposits  of  late  Wisconsin  age. 

Unconformity. 
Cretaceous  system. 
Unconformity. 
Algonkian  system: 

Keweenawan  series Great  basal  gabbro   (Duluth  gabbro)   and   granite   (Embarrass 

granite),  intrusive  in  all  lower  formations. 
Unconformity. 
Huronian  series: 

fAcidic  and  basic  intrusive  rocks. 
Upper   Huronian    (Animi-J  Virginia  slate. 

kie  group) 1  Biwabik  formation  (iron-bearing). 

[Pokegama  quartzite. 
Unconformity. 

(Giants  Range  granite,  intrusive  in  lower  formations. 

Lower-middle  Huronian.  .I^'''''''"'^'''^^^^^'^'^'^"™""'''™®'''^''®     formation     (equivalent    to    the 

Ogishke  conglomerate  and  Rnife  Lake  slate  of  the  Vermilion 
district). 


a  For  turther  detailed  description  of  the  geology  of  this  district  see  Men.  V.  S.  Geol.  Survey,  vol.  43,  and  references  there  given.  Mining 
men  and  others  have  cooperated  cordially  in  the  preparation  of  this  chapter,  hut  we  would  acknowledge  jjarticularly  our  indebtedness  to  Mr.  J.  U. 
Sebenius,  who,  having  been  in  charge  of  explorations  in  the  Mesabi  district  since  its  discovery  and  being  now  chief  engineer  of  the  United  States 
Steel  Corporation,  has  perhaps  closer  knowled^'e  of  the  geology  of  the  iron-tearing  rocks  here  than  any  other  person. 

i>For  the  use  of  the  terms  "Giants  Range"  and  "Mesabi  range"  in  this  report,  see  footnote  on  p.  41,  also  Mon.  U.  S.  Geol.  Survey,  vol,  43 
1903,  p.  21. 

159 


160  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Unconformity. 
Archean  system: 

Liiurontian  series Granites  and  porphyries. 

Keewatin  scries Greenstones,  green  schists,  and  porphyries. 

The  core  of  the  Giants  Ranee  is  made  up  principally  of  j^ranitc  of  lower-middle  Iluronian 
and  Keweenawan  a<;e  and  subordinately  of  Archean  igneous  rocks.  To  the  south  of  the  igneous 
core,  for  a  part  of  the  district,  arc  lower-middle  Huronian  sedimentar}'^  rocks,  with  bedding 
approximately  vertical.  Against  the  southern  boundary  -f  the  lower-middle  Iluronian,  or, 
where  the  lower-middle  Huronian  is  lackmg,  against  the  igneous  core,  he  the  upper  Iluronian 
sedimentary  rocks  (Animikie  group).  They  dip  gently  to  the  south  and  underlie  the  greater 
portion  of  the  southerly  slopes  of  the  range.  On  the  southeast  the  Huronian  rocks  are  limited 
by  the  Keweenawan  Duluth  gabbro,  the  north  edge  of  which  cuts  across  the  Huronian  forma- 
tions diagonally  from  southwest  to  northeast.  The  Archean,  lower-middle  Huronian,  and 
upper  Huronian  are  separated  from  one  another  by  unconformities.  Glacial  drift  cover.s  the 
district  so  thickly  that  rock  exposures  are  rare  on  the  lower  slopes  of  the  range  and  only  fairly 
numerous  near  the  crest. 

ARCHEAN   SYSTEM   OR   "BASEMENT   COMPLEX." 

DISTRIBUTION. 

The  Archean  rocks  of  the  Mesabi  district  are  confined  to  its  central  portion.  They  are 
found  north  and  northwest  of  Nashwauk;  northwest  of  Hibbing;  north  and  northeast  of  Motm- 
tain  Iron;  in  the  southerly  projection  of  the  Giants  Range  known  as  the  "Horn,"  bounded  by 
the  cities  of  Virginia,  Eveleth,  Sparta,  and  Mcliinley;  north  of  Biwabik;  and  eastward  nearly 
to  the  east  Hue  of  R.  16  W.  With  the  exception  of  the  portion  of  the  Archean  area  east  of 
Embarrass  Lake,  exposures  are  sufficiently  common  to  allow  a  fairly  close  determination  of 
the  boundaries.  East  of  Embarrass  Lake  the  mapping  is  based  on  the  presence  of  abundant 
Archean  fragments  in  the  drift. 

Included  in  the  areas  mapped  as  Archean  north  of  Mountain  Iron  are  several  small  patches 
of  lower-middle  Huronian  rocks.  Exposures  are  so  few,  they  are  so  mixed  in  t*  e  same  exposure 
.with  Archean  rocks,  and  they  are  metamorphosed  to  such  difficultly  recognizable  forms  that 
their  accurate  delimitation  on  the  general  map  is  not  possible. 

KINDS  OF  BOCKS. 

The  Archean  is  represented,  about  in  order  of  abundance,  by  micaceous,  chloiitic,  and 
hornblendic  scliists,  basalts,  dolerites,  porphyritic  rhyolites,  granites,  and  diorites.  The  basic 
rocks  have  commonly  a  green  color  and  are  usually  referred  to  locally  as  greenstones  or  green 
schists.  They  are  given  one  color  on  the  general  map  of  the  ilesabi  district  and  are  to  be 
-correlated  with  the  Keewatin  series  (PI.  VIII).  The  acidic  igneous  rocks,  consisting  of  the 
porph}-iitic  rhyoHtes  and  the  granites,  are  mapped  under  another  color  and  are  correlated  with 
the  Laurentian. 

All  these  rocks  have  their  counterparts  in  other  iron  districts  of  the  Lake  Superior  region. 
In  the  .Vermihon  and  Crystal  Falls  districts,  where  especially  well  developed,  Clements  has 
descril>ed  each  i)hase  in  great  detail.  For  details  of  petrography  the  reader  is  referred  to  the 
description  of  the  Archean  rocks  in  the  monographs  on  the  Crystal  Falls  and  the  Vermilion 
districts." 

Nowhere  iu  the  district  have  sediments  been  found  which  are  demonstrably  of  Archean 
age,  but  slate  fragments  in  the  basal  conglomerate  of  the  lower-midcUe  Huronian  point  to  the 
former  existence  of  Archean  sediments. 

a  Mon.  U.  S.  Geol.  Survey,  vols.  36  and  45. 


MESABI  IRON  DISTRICT.  161 

STRUCTURE. 

Most  of  the  Archean  rocks  show  some  cleavage,  and  perhaps  about  half  have  enough  cleavage 
to  warrant  calling  them  scliists.  In  general  the  plane  of  cleavage  is  nearly  vertical  and  strikes 
parallel  to  the  range,  about  N.  60°  E.  The  hornblendic  schists  north  of  Mountain  Iron  have 
a  cleavage  of  a  linear  parallel  type,  and  the  lines  of  the  cleavage  dip  steeply  to  t-he  northeast. 
In  addition  to  cleavage,  there  are  many  joints  and  faults  ^\^th  displacements  of  a  few  inches  or 
feet,  but  no  regular  systems  have  been  determined. 

ALGONKIAN  SYSTEM. 

HURONIAN  SERIES. 
LOWER-MIDDLE     HURONIAN. 

DISTBIBUTION. 

Sedimentary  rocks  of  lower-middle  Huronian  age  appear  in  two  considerable  areas  in  the 
Mesabi  district.  One  vnth  an  average  width  of  perhaps  a  mile  extends, from  Eveleth  northeast 
to  Biwabik ;  the  other,  somewhat  less  than  a  mile  in  width,  extends  from  near  the  Duluth  and 
Iron  Range  Railroad  northeast  to  near  the  center  of  sec.  11,  T.  59  N.,  R.  14  W.  In  the  former 
belt  there  are  areas  of  green  schist  forming  the  cores  of  the  hills.  One  of  them  has  been  mapped, 
but  others,  though  their  presence  is  known  by  isolated  exposures,  are  not  sufRcientl}^  exposed 
to  warrant  their  separation  on  the  map.  A  number  of  small  patches  of  lower-middle  Huronian 
sediments  are  known  also  in  other  parts  of  the  district. 

Granite  of  lower-middle  Huronian  age  forms  most  of  the  core  of  the  Giants  Range  and, 
except  north  of  Mountain  Iron,  where  it  is  interrupted  for  a  short  distance  by  Archean  horn- 
blendic schists,  is  exposed  continuously  along  the  crest  to  where  it  is  succeeded  on  the  east  by 
the  younger  Embarrass  granite  in  R.  14  W.  This  lower-middle  Huronian  granite,  known  as 
the  Giants  Range  granite,  thus  bounds  on  the  north  the  other  formations  for  most  of  the  district. 
Detailed  work  has  not  gone  farther  north  than  the  granite  boundary. 

GRATWACKES  AND  SLATES. 

• 

The  interbedded  graywackes  and  slates  form  the  greater  part  of  the  lower-middle  Huronian 
sediments.  They  are  dull  dark-gray  and  dark-green  rocks  which  usually  weather  to  a  somewhat 
lighter  green  or  gray  or  to  a  dirty  hght  yellow.  The  grain  is  usually  fine,  although  it  varies 
considerably.  The  bedding,  shown  by  both  color  and  texture,  is  conspicuous.  Parallel  to 
the  bedding  a  secondary  cleavage  has  been  developed.  As  a  result  of  variation  in  texture, 
bedding,  and  secondary  cleavage,  there  appear  all  gradations  between  metamorphosed  coarse 
graywackes,  banded  graywackes,  and  finely  fissile  slates.  Along  the  parting  plane  of  some  of 
the  graywackes  and  slates  may  be  seen  glistening  plates  of  mica  or  chlorite,  conspicuous  because 
of  the  fact  that  they  appear  in  separate  spangles  on  the  dark  background  rather  than  in  con- 
tinuous layers,  although,  indeed,  some  of  the  more  fissile  slates  show  mica  and  chlorite  in  the 
continuous  layers  characteristic  of  slates. 

The  graywackes  and  slates  above  described  have  resulted  from  the  alteration  of  fine  mud 
and  feldspathic  sand  deposits.  Some  of  the  mica,  especially  that  in  separate  clear-cut  plates, 
may  have  been  originall}'  deposited  in  its  present  position,  but  most  of  it,  and  especiallj^  that 
in  continuous  sheets  on  the  parting  surfaces,  is  undoubtedly  a  secondary  development  due  to 
dynamic  movement  in  the  rock. 

The  intrusion  of  granite  below  described  has  further  greatly  metamorphosed  the  graywackes 

and  slates.     In  approacliing  the  granite  they  become  more  chloritic,  hornblendic,  and  micaceous, 

and  a  marked  and  usually  much  contorted  schistosity  obhterates  the  bedding.     Under  the 

microscope  may  be  seen  abundant  development  of  secondary  chlorite  and  hornblende  and  a 

47517°— VOL  52—11 11 


162  GEOLOGY  OF  THE  LAKE  SLTPERIOR  REGION. 

lesser  development  of  secondary  biotite  and  muscovite.  Accessories  inclmlo  tourmaline,  stauro- 
lite,  <rarnet,  rutiio,  ilmeiiitc,  magnetite,  and  apatite.  The  alteration  of  tlie  ilmenite  and  rutile 
to  sphene  (titanoniorphic)  is  well  exhibited. 

CONGLOMERATES. 

The  conglomerates  are  abundantly  and  typically  exposed  in  a  belt  nmning  from  the  cut 
along  the  Duluth  and  Iron  Range  Railroad,  in  sec.  22,  T.  58  N.,  R.  17  W.,  southwest  through 
sees.  22  and  21  into  sees.  20  and  29,  T.  58  N.,  R.  17  W.  Similar  conglomerates  are  known  in 
small  patches  bordermg  the  greenstones  north  of  the  Genoa  mine  at  Sparta. 

The  conglomerates  are  massive  rocks  for  the  most  part,  with  various  shades  of  green  on 
fresh  surface  and  a  lighter  green  on  the  weathered  surface.  The  pebbles  vary  in  diameter  from 
6  inches  to  a  small  fraction  of  an  inch.  In  kind  they  are,  for  the  most  part,  identical,  both  macro- 
scopically  and  microscopically,  with  the  rocks  of  the  Archean  above  described,  including  diabases, 
basalts,  and  granite  porphyries.  The  more  basic  pebbles  are  in  greater  quantity  than  the  acid 
ones. 

The  conglomerates,  in  common  with  the  rest  of  the  lower-middle  Iluronian  rocks,  have 
sufl'ered  metamorphism,  but  the  extent  of  the  alteration  varies  greatly  from  place  to  place. 
East  of  Mariska,  in  the  railway  cut  referred  to,  the  rocks  show  only  recrystallization  of  the 
mineral  particles,  without  marked  development  of  schistosity.  The  alteration  of  the  minerals 
is  the  same  as  that  described  above  for  the  various  rocks  of  the  Archean.  To  the  southwest  of 
this  cut  the  conglomerates  have  been  much  srjueezed  and  are  now  very  schistose.  The  recrys- 
tallization accompanymg  the  squeezmg  has  made  the  rocks  very  chloritic  and  micaceous, 
and,  m  many  places  at  least,  has  completely  obliterated  the  clastic  texture  in  the  finer-grained 
portions.  The  pebbles  have  been  elongated  in  the  plane  of  schistosity  (vertical  and  striking 
N.  60°  E.),  and  on  the  weathered  surface  stand  out  in  lenticular  and  oval  forms  from  the  finer, 
more  schistose,  and  more  easily  eroded  matrix.  Rocks  of  this  character  may  be  traced  into 
schistose  rocks  in  which,  in  pebbles  and  matrix  alike,  nearly  every  vestige  of  sedimentary  texture 
has  been  lost. 

GIANTS   RANGE   GRANITE. 

At  Birch  Lake  the  lower-middle  Iluronian  granites  are  coarse  gray  and  pink  hornblende 
granites.  From  the  east  line  of  R.  14  W.  to  the  neighborhood  of  Mountain  Iron  the  granites  are 
similar  to  those  on  Birch  Lake.  It  is  noticeable  that  the  coarser  phases  appear  in  the  eastern 
end  of  this  area.  The  hornblende  varies  m  abundance,  but  is  usualh'  conspicuous.  Rarely, 
as  near  the  Mailman  camps,  the  dark  constituent  is  augite  instead  of  hornblende,  or,  again,  it 
may  be  partlj^  biotite.  In  places  the  rock  becomes  very  slightly  gneissic,  and  immediately  next 
to  its  contact  with  the  lower-middle  Hui'onian  sediments  it  becomes  very  fine  grained.  Xext 
to  the  contact  of  the  granite  with  the  Keweenawan  Duluth  gabbro  on  Birch  Lake  is  a  meta- 
morphic  rock  resembling  granite,  wliich  is  described  in  connection  with  the  gabbro. 

From  the  neighborhood  of  Mountain  Iron  westward  to  the  west  end  of  the  district  the  pre- 
ponderating granite  is  somewhat  finer  grained  than  the  granite  to  the  east,  possibly  somewhat 
more  gneissic,  and  usuallj^  of  a  pink  color.  Certain  phases  of  this  finer  granite  are  similar  to  the 
hornblende  granite  to  the  east,  but  by  far  the  larger  poi'tion  shows  a  considerabl}^  greater  con- 
tent of  (piartz  and  a  smaller  content  of  the  basic  minerals. 

Associated  with  these  two  prevailing  types  are  dikes  of  exceedingly  fine-grained  pink  granite 
showing  very  little  biotite.  They  may  be  well  observed  in  the  cuts  along  the  main  lino  of  the 
Duluth  and  Iron  Range  Railroad.  Other  dikes  are  pegmatitic  granite  consisting  of  a  pink  feld- 
spar with  very  abundant  quartz,  and  with  the  ferromagnesian  minerals  almost  totally  lacking. 
They  may  be  seen  to  advantage  at  the  upper  falls  of  Prairie  River. 

RELATIONS    OF    GIANTS    RANGE    GRANITE    TO    THE    LOWER-MIDDLE    HURONIAN    SEDIMENTS    AND    OF    BOTH    TO 

OTHER   ROCKS. 

The  Giants  Range  granite  is  throughout  intrusive  into  the  lower  miildle  Iluronian  sedi- 
ments. Actual  intrusive  contacts  are  to  be  observed  in  a  number  of  jiJaces.  The  lower-middle 
Iluronian  sedimentary  rocks  show  the  metamorphic  eli'ects  of  the  hitrusion,  and  near  the  con- 


MESABI  IRON  DISTRICT.  163 

tactg  no  conglomerates  are  to  be  observed.  The  contact  of  the  granite  and  the  sediments  is 
well  exposed  northwest  of  Mesaba  station. 

Though  the  evidence  is  conclusive  that  the  great  mass  of  the  granite  is  intrusive  into  the 
lower-middle  Iluronian  sedunents  and  not  into  the  upper  Iluronian  (Animikie  group),  it  is  likely 
that  in  minor  areas  the  granites  here  mapped  and  described  as  lower-middle  Huronian  may  con- 
tain granite  of  later  date,  which  is  known  to  be  present  in  the  district. 

The  conglomerate  forming  the  great  part  of  lower-middle  Huronian  sediments  affords 
conclusive  proof  that  the  lower-middle  Iluronian  sedunents  rest  unconformably  upon  the'Archean 
rocks.  Every  kmd  of  pebble  found  in  this  conglomerate,  with  the  possible  exception  of  a  few 
cherty  slate  pebbles,  can  be  matched  among  the  Archean  rocks. 

Both  the  lower-middle  Iluronian  sediments  and  the  Giants  Range  granite  are  unconformably 
underneath  the  upper  Huronian  (Animikie  group),  as  shown  both  by  structure  and  by  con- 
glomerates at  the  base  of  the  upper  Huronian  sediments.  This  unconformity  is  described  in 
connection  with  the  upper  Huronian. 

STRUCTURE   AND   THICKNESS. 

The  lower-middle  Huronian  beds  now  stand  on  edge,  the  dip  seldom  varjdng  more  than 
5°  or  10°  from  vertical.  Superposed  upon  the  original  bedding  structure  is  an  excellent  secondary 
cleavage.  The  cleavage  planes,  for  the  most  part,  are  approximately  parallel  to  the  bedding 
planes.  The  strike  of  both  bedding  and  cleavage  is  imiform,  about  N.  60°  E.,  though  locally 
var3dng  10°  to  20°  from  this  direction. 

Both  the  lower-middle  Huronian  sediments  and  the  Giants  Range  granite  are  jointed,  the 
sediments  particularly  so.  The  sediments,  moreover,  show  conspicuous  faulting  and  brecciation. 
The  breccias  at  some  places  might  be  mistaken  for  conglomerate.  A  thickness  of  3,000  to  5,000 
feet  is  probably  as  great  as  can  safely  be  assigned  to  the  lower-middle  Iluronian  sediments  of  the 
Mesabi  district. 

CONDITIONS  OF  DEPOSITION. 

It  is  suggested  in  Chapter  XX  (pp.  603  et  seq.)  that  the  lower-middle  Iluronian  deposits 
of  the  north  shore  may  be  in  part  subaerial  continental  deposits. 

UPPER    HUBONIAN    (aNIMIKIE    GROUP). 
GENERAL   CHARACTER  AND   EXTENT. 

The  sedimentary  rocks  of  upper  Huronian  age  occujiy  practically  all  the  southern  slopes 
of  the  range  from  one  end  of  the  district  to  the  other  and  extend  also  an  unknown  tlistance 
south  beneath  the  glacial  drift.  The  surface  width  of  the  Animikie  group  in  the  area  included 
in  the  district  described  varies  from  less  than  1  mUe  to  5  miles  or  more.  Tlie  beds  have  a 
flat  dip  to  the  south.  Their  upper  edges  being  truncated,  they  appear  in  belts  \vinding  along 
parallel  to  the  range,  the  northerly  belts  representing  the  lower  beds  and  the  southerly  belts 
the  higher  beds  of  the  series. 

The  exposures  of  the  upper  Iluronian,  particidarly  on  the  lower  slopes,  are  so  widely 
separated  that  the  mapping  of  the  rocks  woidd  have  been  an  impossibility  had  it  not  been 
for  numerous  drill  holes  and  pits  sunk  in  search  for  ore,  which  were  bottomed  in  the  upper 
Huronian.  These  are  particularly  numerous  along  the  central  portion  of  the  range  and  have 
enabletl  the  distribution  of  the  upper  Huronian  rocks  to  be  mdicated  within  rather  close 
limits  for  this  part  of  the  range. 

The  upper  Huronian  comprises  from  the  base  up  (1)  the  Pokegama  quartzite,  consist- 
ing mamly  of  quartzite  but  contaming  also  conglomerate  at  its  base;  (2)  the  Biwabik  forma- 
tion, consisting  of  ferruginous  cherts,  iron  ores,  slates,  greenalite  rocks,  and  carbonate  rocks, 
with  a  small  amount  of  coarse  detrital  material  at  its  base;  and  (3)  the  Virginia  slate.  Between 
the  Pokegama  quartzite  and  the  Biwabik  formation  there  is  a  slight  break  indicated  by 
conglomerate.  The  Biwabik  formation  grades  conformably  into  the  Virginia  slate  both  ver- 
tically and  laterally. 


164  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

POKEGAMA  QTJARTZITE. 

The  Pokegama  quartzitc  is  the  basal  formation  of  (lie  upper  Huronian  (Animikie  group). 
Because  of  the  southcrl_y  dip  and  truncation  of  the  rocks,  (lie  cjuartzite  appears  as  a  beU  imme- 
diately south  of  and  contiguous  to  tlie  lower-middle  lluronian  antl  Archean  formations.  The 
belt,  varying  from  a  few  steps  to  half  a  mUe  or  more  in  width,  extends  from  the  west  end 
of  the  Mesabi  district  continuously  to  a  point  north  of  Mountain  Iron.  From  here  on  to  the 
east  end  of  the  range  data  are  insullicient  for  mapping  the  quartzitc  as  a  continuous  belt, 
and  it  is  accordingly  mapped  as  a  number  of  discontinuous  areas  of  varying  width  and  length. 
It  is  likely  that  future  exploration,  as  in  the  past,  will  result  in  extending  and  connecting 
some  of  these  areas,  but  it  is  also  certain  that  some  of  them  are  really  cut  off  from  one  another 
because  of  the  overlapping  of  the  iron-bearing  formation. 

The  Pokegama  consists  of  vitreous  quartzites  of  various  colors  and  textures,  with  some 
micaceous  quartz  slates  and  conglomerates. 

The  thickness  of  the  Pokegama  quartzitc  ranges  up  to  200  feet. 

BIWABIK  FORMATION. 
DISTRIBUTION. 

The  Biwabik  formation  extends  along  the  slopes  of  the  range  for  its  entire  length,  from 
T.  142  N.,  R.  25  W.,  west  of  Grand  Rapids,  to  Birch  Lake,  a  distance  of  nearly  110  miles. 
The  width  of  exposure  averages  perhaps  1^  miles,  but  is  in  places  as  great  as  3  miles  and  in 
others  as  little  as  a  quarter  of  a  mUe.  The  total  area  is  approximately  135  square  miles. 
The  boimding  formation  on  the  north  is  for  the  most  part  the  Pokegama  quartzitc,  but  where 
this  is  lacking  the  Biwabik  formation  comes  into  contact  with  the  lower-middle  Huronian 
and  Archean  rocks.  To  the  south  the  iron-bearing  formation  is  bounded  by  the  Virginia 
slate,  except  in  range  12  and  a  part  of  range  13,  at  the  east  end  of  the  range,  where  the  Kewee- 
nawan  Duluth  gabbro  laps  up  over  the  formation.  On  the  east  the  iron-bearing  formation 
is  cut  off  by  the  overlapping  Duluth  gabbro;  on  the  west  it  gradually  tliins  out,  the  overlying 
slate  and  underlj'uig  quartzitc  coming  together. 

On  account  of  the  covermg  of  glacial  drift,  exposures  of  the  iron-bearing  formation,  except 
in  the  eastern  end  of  the  district,  are  few.  But  the  formation  has  been  reached  and  pierced 
in  thousands  of  places  by  drUls  and  mining  excavations,  and  it  is  therefore  possible,  particu- 
larly along  the  part  of  the  range  at  present  productive,  to  delimit  it  with  a  fair  degree  of 
accuracy. 

Much  attention  has  been  paid  in  recent  j-ears  to  following  up  the  westward  extension  of 
the  iron-bearing  formation,  which,  in  the  vicinity  of  Grand  Rapids  and  westward,  becomes 
deeply  buried  under  glacial  drift.  By  drilling  a  large  number  of  holes  it  has  been  possible 
to  follow  the  formation  into  T.  142  N.,  R.  25  W.,  where  it  becomes  thin  and  appariently  dis- 
appears, the  slate  and  quartzitc  coming  together.  These  results  have  not  seemed  to  warrant 
continuation  of  drilling  in  this  direction,  but  until  suflicient  drilhng  has  been  done  to  demon- 
strate clearly  what  the  structure  and  distribution  are  there  it  can  not  be  said  that  the  pos- 
sibilities at  this  end  of  the  district  have  been  exliausted.  Folding  or  faulting  or  changes  in 
sedunentation  might  easily  cause  variations  wliich  would  make  it  difllcult  to  follow  the  forma- 
tion. Twelve  miles  to  the  northwest  of  the  westernmost  Biwabik  formation  (iron-bearing) 
of  the  Mesabi  district  there  begins  a  magnetic  belt  which  extends  from  T.  144  N.,  R.  26  W., 
through  Leech  Lake  to  T.  142  N.,  R.  35  W.,  a  distance  of  about  50  miles.  This  belt  has  not 
been  proved.  The  few  holes  that  have  been  put  down  seem  to  indicate  that  the  formation 
is  of  Vermilion  type,  but  the  continuity  and  the  breadth  and  length  of  the  belt  arc  exceptional 
for  Vermilion  iron-bearing  rocks.  It  has  been  thought  possible  that  this  belt  might  rep- 
resent an  extension  of  the  Mesabi  district  thrown  to  the  north  by  a  fold  or  a  fault.  Wli!i(- 
•ever  it  is,  its  trend  indicates  that  the  same  general  lineaments  of  structure  of  the  Vermilion 
and  Mesabi  districts  are  following  out  here  to  the  west,  and  even  if  the  belt  ultimately  proves 


MESABI  IRON  DISTRICT.  165 

to  be  Vermilion  it  would  then  serve  to  limit  the  distribution  of  the  Animikie  (including  the 
Biwabik  iron-bearing  formation)  on  the  north,  and  thus  serve  as  a  guide  to  further  exploration. 
The  iron-bearmg  formation  in  general  occupies  the  middle  slopes  of  the  Giants  Range, 
and  its  north  and  south  boundaries  have  fairly  uniform  altitudes  for  considerable  distances. 
By  an  examination  of  the  map,  however,  it  may  be  seen  that  the  elevation  of  the  formation 
increases  from  the  west  end  of  the  district  to  the  east,  the  total  dill'ercnce  amounting  to  as  much 
as  500  feet.  This  corresponds  with  the  increased  elevation  of  the  range  as  a  whole  in  this  direc- 
tion, although  the  higher  elevation  of  the  southern  limit  of  the  formation  at  the  east  end  of  the 
range  is  in  part  due  to  the  fact  that  the  lower  parts  of  the  formation  are  overlapped  by  gabbro. 
It  may  be  further  seen  that  the  elevations  of  the  north  and  south  boundaries  show  local  fluctu- 
ations as  great  as  200  feet,  due  to  the  folding  of  the  formation  and  to  differences  in  depth  of 
erosion. 

KINDS    OF    ROCKS. 

The  great  bulk  of  the  Biwabik  formation  is  ferruginous  cliert  more  or  less  am])hiholitic, 
calcareous,  or  sideritic  and  gray,  red,  yellow,  brown,  or  green,  with  bands  and  shots  of  iron  ore. 
It  is  analogous  to  the  jaspers  of  the  other  iron  ranges,  but  differs  in  certain  particulars,  as  is 
shown  on  pages  461-462. 

Associated  with  the  chert,  mainly  in  the  middle  zone,  are  the  iron  ores.  Their  surface 
area  is  only  about  5  per  cent  of  the  total  area  of  the  iron-bearing  formation,  and  the  proportion 
of  their  bulk  to  that  of  the  iron-bearing  formation  is  much  less.  Near  the  bottom  of  the  Biwabik 
formation  is  a  small  amount  of  conglomerate  and  quartzite — that  is,  coarsely  clastic  sediments. 
A  minute  conglomeratic  layer  has  also  been  observed  in  the  Mahoning  mine,  in  about  a  central 
horizon  of  the  formation.^  In  thin  layers  and  zones  throughout  the  iron-bearing  formation, 
and  particularly  in  its  upper  horizons,  are  layers  of  slate  and  of  paint  rock,  the  paint  rock  usually 
resulting  from  the  alteration  of  the  slate.  Between  the  slate  and  the  paint  rock  and  the  ferru- 
ginous chert  are  numerous  gradational  varieties,  most  of  which  come  under  the  head  of  ferru- 
ginous slate.  Associated  with  the  slaty  layers  in  the  iron-bearmg  formation  or  closely  adjacent 
to  the  overlying  Virginia  slate  are  green  rocks  made  up  of  small  green  granules  of  ferrous  silicate 
which  are  li.ere  called  greenalite,  in  a  fine-grained  cherty  matrix.  It  will  be  shown  later  that 
these  are  the  original  rocks  from  which  most  of  the  other  phases  of  the  iron  formation,  mcluding 
the  ores,  have  resulted  by  alteration.  Finally,  certain  calcareous  and  sideritic  rocks  are  present 
in  small  quantity,  particularly^  near  the  upper  horizons,  associated  with  the  greenalite  rocks. 
The  rocks  of  the  iron-bearing  formation  are  described  below,  beginning  with  the  original  type — 
the  greenalite  rock. 

The  origin  of  the  ores  and  iron-bearing  rocks  is  discussed  in  Chapter  XVII  (pp.  499  et  seq.). 

GREEN.\LITE    ROCKS. 

In  moderate  quantity,  just  below  the  Virginia  slate  or  associated  with  some  slate  layer 
in  the  iron-bearing  formation,  are  didl  dark-green  rocks  of  rather  uniform  fine  grain  and  con- 
choidal  fracture.  Layers  of  slate,  iron  ore,  and  other  phases  of  the  iron-bearing  foi'mation 
usually  mark  their  bedding.  On  close  examination,  particularly  when  the  surface  is  wet, 
there  may  be  observed  numerous  ellipsoidal  granules  of  a  green  substance  of  a  very  slightly 
lighter  green  than  the  matrix  in  which  they  lie.  They  are  so  small  and  of  a  color  so  nearly  like 
that  of  the  matrix  that  they  are  likely  to  be  overlooked  unless  especially  selrched  for.  An 
occasional  one  is  of  much  greater  size  than  the  average  and  looks  like  a  conglomerate  pebble 
in  the  rock. 

Under  the  microscope  the  gramdes  are  conspicuous.  Their  cross  sections  are  roinid, 
oval  (in  some  cases  with  much  elongation),  crescent-shaped,  lens-shaped,  gourd-shaped,  or 
even  sharply  angular.  Here  and  there  a  curved  "tail"  seems  to  connect  one  granule  with 
its  neighbor.  Wliere  in  contact  with  a  layer  of  iron  carbonate  or  calcium  carbonate,  as  many 
of  them  are,  the  granules  are  more  irregular  in  shape  and  project  into  or  are  included  in  the 
carbonate  layers  as  irregular  filaments  and  fragments.     The  carbonate  is  largely  secondary 


166  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

and  clearly  replaces  the  fjraniiles,  but  some  of  it  is  |)erhaps  orii^inal,  and  in  this  case  the  variation 
in  shape  of  the  granules  where  associated  with  the  carbonate  layers  has  a  bearing  on  the  origin 
of  the  ores,  which  is  discussed  elsewhere  (p.  187).  One  hundred  and  twenty  measurements  of 
the  granules  show  an  average  greater  diameter  of  0.4.5  millimeter  and  an  average  least  diameter 
of  0.21  millimeter,  with  average  ratio  of  greatest  to  least  of  100  to  47.  The  diameters  rarely 
reach  1  miUimeter  and  few  are  below  0.1  millimeter.  Occasionally  certain  of  the  granules  may 
be  seen  to  be  aggregated  into  larger  granules  with  well-rounded  outlines,  making  the  conglom- 
erate-like fragments  above  mentioncnl.  The  greater  diameters  of  the  granules,  for  the  most 
part,  are  parallel  to  the  bedding,  and  in  fact  this  arrangement  largely  determines  the  bedding. 
In  ordinary  light  the  granules  are  green,  greenish  yellow,  brown,  or  black.  The  green  and 
yellow  ones  are  transparent  and  the  brown  and  black  are  nearly  or  quite  opaque.  Under 
crossed  nicols  the  granules  are  either  entirely  dark  or  show  a  very  faint  lightening,  hardly 
sufficient  to  disclose  a  color.  Here  and  there  incipient  alterations  to  chert,  griinerite,  ciunmiiig- 
tonite,  or  actinolite,  scarcely  discernible  in  ordinary  light,  give  low  polarization  colors  in  minute 
spots  and  make  the  term  "aggregate  polarization"  applicable.  In  reflected  light  the  transpar- 
ent green  and  yellow  granules  appear  black,  dark  green,  or  dark  yellow,  while  the  opaque  brown 
and  black  granules  exliibit  a  rough  light-green  surface.  Were  it  not  for  the  light-green  surface 
in  reflected  light,  certain  of  the  opaque  dark-brown  granules  would  be  mistaken  for  iron  oxide 
in  ordinary  and  polarized  light. 

The  matrix  of  the  rocks  containing  the  unaltered  green  granules  varies  widely  in  amount, 
from  a  mere  interstitial  filling  to  an  abundant  mass  in  which  granules  are  widely  separated. 
The  matrix  may  be  almost  pure  chert;  it  may  be  nonaluminous,  monoclinic  amphibole,  actinolite, 
griinerite,  or  cummingtonite ;  it  may  be  largel}"  iron  or  calcium  carbonate,  although  where  the 
carbonate  is  abundant  the  granules  are  usually  sparse  and  irregular;  it  maj'  consist  of  any 
combination  of  chert,  amphibole,  and  carbonate  with  a  small  amount  of  accessor}-  iron  oxide. 

Origmally  the  matrix  may  have  had  a  somewhat  different  character.  In  the  rocks  con- 
taining the  least  altered  granules  the  matrix  is  predominantly  chert  and  subordinately  light- 
colored  amphiboles  and  carbonate.  As  the  rocks  become  altered  they  contain  more  iron  oxide 
and  dark  amphiboles,  wliich  will  be  shown  on  a  subsequent  page  to  develop  from  the  alteration 
of  the  granules.  The  lighter  amphiboles  are  themselves  known  to  be  a  secondary  development 
from  chert  and  carbonate  rocks.  It  seems  likely,  therefore,  that  the  original  matrix  of  the 
green  granules  was  largely  chert  and  in  small  part  carbonate.  In  the  freshest  rocks  now  found 
the  chert  is  much  recrystallized  and  the  original  carbonate  is  largely  leached  out  or  replaced 
by  actinolite. 

The  specific  gravity  of  the  unaltered  granules  can  not  be  satisfactorily  determined  because 
of  the  practical  impossibility  of  separating  the  granules  from  the  matrLx.  Determinations  of 
the  specific  gravity  of  the  rock  as  a  whole  give  results  ranging  from  2.7  to  3.  As  the  matrix 
is  largely  quartz  in  the  form  of  chert,  which  is  known  to  have  a  specific  gravity  in  the  neighbor- 
hood of  2.65,  the  figures  above  given  for  the  unaltered  rock  are  too  low  for  the  granules  them- 
selves, although  their  incipient  alterations  to  iron  oxide  and  amphiboles  tend  to  raise  the  specific 
gravity.  So  far  as  the  matri.x  is  colorless  amphibole  it  is  apparent  that  the  specific  gravity 
of  the  green  granules  is  lower  than  the  figures  obtained  for  the  rock,  for  the  specific  gravity  of 
the  colorless  amphiboles  is  above  3.  One  exceptionally  fresh  specimen  in  wliich  the  granules 
lie  in  a  matri.x  (^  chert  gave  a  result  of  2.7.  The  matrLx  in  this  case  makes  up  something  more 
than  half  of  the  rock  mass,  and  it  therefore  seems  probable  that  the  true  specific  gravity  of  the 
granules  is  a  little  above  2.75. 

Four  analyses  of  rocks  containing  the  least  altered  granules  observed  have  been  made 
by  George  Steiger,  of  the  United  States  Geological  Survey.  He  found  that  by  treatment 
with  hot  concentrated  hydrochloric  acid  most  of  the  granules  and  their  associated  alteration 
products  dissolved  out,  lea\-ing  a  residue  of  almost  clear  silica,  which  probably  mainly  repre- 
sents the  matrix. 


MESABI  IRON  DISTRICT. 

Analyses  of  greenalite  rocks. 


167 


1. 

2. 

3. 

4. 

Soluble. 

Insoluble. 

Soluble. 

Insoluble. 

Soluble. 

Insoluble. 

SiO"                                                

13.45 

.37 

15.00 

10.28 

2.33 

.28 

None. 

None. 

2.60 

4.17 

None. 

2.04 

None. 

48.45 
.04 

O19.30 

.01 

1.3.  ,83 

17.57 

3.22 

None. 

None. 

None. 

2.38 

5.74 

None. 

None. 

None. 

36.50 
.70 

33.11 
..56 

6.44 
30.93 

5.35 
None. 
None. 
None. 

1.34 

6.13 
None. 
None. 
None. 

13.01 
2.60 

150.96 

Al-Oa         

1.09 

5.01 

FeO                                                                              

30.37 

jjgO                                                                      

5.26 

CaO                                                         

.04 

Na"0                  

K^O                                                         

HoO                                                                                                

.75 

H'.0+                                                    

6.41 

TiOo                                                                                    



None. 

COt                                             

Pjds 

None. 

s                            

Trace. 

MuO                                                                             

None. 

BaO                          

.21 

.52 

.15 

.38 

50.42 
49.01 

49.61 

62. 05 
.37. 41 

37.41 

83.86 
15.99 

15.19 

100.10 

100.03 

100.06 

99.85 

"  Ot  which  3.3  was  found  in  the  rocli  upon  treatment  with  HCl  (probably  opal). 


i>  Of  which  23.96  is  soluble. 


1.  Specimen  45758.  From  250  paces  west.  83  paces  north,  of  the  west  quarter  post.  sec.  35,  T.  59  N.,  R.  15  W.  The  finely  ground  rock  was 
evaporated  on  the  water  bath  to  dryness  with  50  cc.  of  1-1  UCl.  taken  up  with  water  slightly  acidified  with  HCl,  and  filtered.  Soluble  sdica  was 
then  determined  in  this  residue  by  ijoiling  with  5  per  cent  solution  of  NasCOa.  A  determination  of  soluble  SiOs  was  then  made  in  the  rock  before 
treatment  with  HCl  and  subtracted  from  the  first  soluble  SiOs  found,  which  gave  the  figure  for  SiO^  in  the  soluble  portion. 

2.  Specimen  45705.  From  test  pit  in  Cincinnati  mine.  The  soluble  portion  was  found  by  evaporating  to  dryness  on  the  water  bath  with  50  cc. 
of  1-1  HCl,  and  taking  up  with  water  slightly  acidified  with  HCl.  The  residue  was  then  boiled  fifteen  minutes  with  a  5  per  cent  solution  of  NaaCOa 
to  dissolve  any  soluble  silica,  this  silica  determined  and  placet!  with  the  soluble  portion.  The  residue  was  ignited  and  finally  heated  for  fifteen 
minutes  over  the  blast  lamp,  weighed,  and  then  a  rough  analysis  made,  which  is  found  in  the  second  column.  The  small  amount  of  iron  shown 
in  the  insoluble  portion  could  easily  have  been  carried  down  mechanically.  A  determination  of  soluble  silica  was  then  made  in  the  rock  before 
treatment  with  HCl  and  found  to  be  3.3  per  cent.  Subtracting  this  from  the  total  soluble  silica,  10  per  cent  of  soluble  silica  remains  for  the  part 
dissolved  in  HCl. 

3.  Specimen  45766.  From  test  pit  in  Cincinnati  mine.  The  finely  ground  rock  was  evaporated  on  the  water  bath  to  dryness  with  50  cc.  of 
1-1  HCl.  taken  up  with  water  slightly  acidified  with  HCl,  and  filtered.  Soluble  sihca  was  then  detennined  in  this  residue  by  boiling  with  5  per 
cent  solution  of  Na^COa.  A  determination  of  soluble  Si02  was  then  made  in  the  rock  before  treatment  with  HCl  and  subtracted  from  the  first  soluble 
SiOo  found,  which  gave  the  figure  for  SiOa  in  the  soluble  portion. 

4.  Specimen  45180.  From  500  paces  west,  100  paces  north  of  the  southeast  comer  of  sec.  22,  T.  59  N.,  R.  15  W.  Owing  to  presence  of  organic 
matter,  tlie  determination  of  ferrous  iron  is  probably  high. 

From  the  detailed  consideration  of  these  results,  which  is  not  I'epeated  here,  it  appears 
that  the  ferric  iron  occurs  in  the  rock  mainly  as  sesquioxide,  for  the  soluble  silica  is  accounted 
for  by  the  ferrous  iron  and  magnesia  present,  leaving  none  for  the  ferric  iron;  that  in  tliree 
slides  of  the  four  of  the  rocks  analyzed  the  ferric  oxide  may  be  observed  to  be  present  and  to 
be  probably  secondary,  and  hence  that  the  iron  oxide  shown  by  the  analyses  is  mamly  sec- 
ondary and  not  to  be  considered  as  belonging  with  the  substance  of  the  unaltered  granules. 
It  appears  further  that  the  alumina  and  lime  are  in  such  small  quantity  as  to  be  practically 
negligible.  It  appears  still  further  that  there  is  far  more  than  enough  combined  water  to  com- 
bine with  the  ferric  iron  to  form  ferric  hydrate,  and  thus  that  a  considerable  portion  of  combined 
water  shown  by  the  analyses  may  be  taken  to  belong  to  the  green  granules.  Finally,  it  appears 
that  the  substances  which  can  not  be  accounted  for  in  any  other  way  and  which  clearly  belong 
with  the  green  granules  are  silica,  ferrous  iron,  magnesium  oxide  in  small  proportions,  and  water. 
It  is  therefore  concluded  that  the  substance  of  the  green  granules  is  essentially  a  hydrous 
ferrous  silicate  with  a  subordinate  amount  of  magnesium,  and  that  if  ferric  iron  is  present  at 
all  as  an  original  constituent  of  the  green  granules  it  is  in  small  cjuantity. 

This  conclusion  is  essentially  in  accord  with  that  reached  by  J.  E.  Spurr  in  his  report  on 
the  Mesabi  district  published  in  1894." 

Having  concluded  the  substance  of  the  green  granules  to  be  mainly  silica,  ferrous  iron, 
magnesium  oxide,  and  water,  we  may  ascertain  whether  or  not  there  is  any  uniformity  in  the 
proportions  of  these  elements.  The  ratios  of  the  silica,  ferrous  iron,  and  magnesium  in  the  four 
analyses,  calculated  on  the  basis  of  100,  appear  in  the  table  on  page  168.  The  percentage 
of  water  is  not  included  for  the  obvious  reason  that,  while  it  is  certain  that  much  of  it  belongs 
with  the  granules,  no  quantitative  estimate  can  be  made  of  its  amount  because  of  the  uncer- 
tainty as  to  the  portion  which  belongs  with  the  ferric  hydrate. 

a  Bull.  Geol.  Nat.  Hist.  Survey  Minnesota  No.  10. 


168 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


1. 

2, 

3. 

i. 

Average. 

SiOi : 

55.1 
42.1 
2.8 

43.7 
47.5 
8.8 

47.7 
44.6 
7.8 

40.2 

50.9 

8.9 

46.8 

FeO 

46.3 

MgO .       . 

7.1 

The  relative  proportion  of  the  ferrous  iron  and  silica  above  shown  suggests  a  combina- 
tion of  the  two  on  the  basis  of  one  molecule  of  each.  Theoretically  the  percentages  of  the  two 
in  such  a  combination  would  be — 

Silica '. 45.  62 

Ferrous  iron 54.  38 

The  average  of  the  ferrous  iron,  46.3,  is  about  8  per  cent  less  than  tne  theoretical  percent- 
age. The  magnesium  oxide,  which  has  a  higher  combining  power  than  the  iron,  more  than 
makes  up  for  this  deficiency. 

On  a  subsequent  page  is  given  an  analysis  of  a  rock  in  which  the  green  granules  have  been 
altered  to  a  dark-green  and  brown  amphibole,  probably  griinerite,  apparently  through  simple 
recrystallization  and  dehydration.  The  alteration  has  occurred  under  deep-seated  conditions, 
and  it  is  probable  that  little  if  any  addition  or  subtraction  of  material  has  taken  place  other 
than  that  involved  in  dehydration.  The  composition  of  the  amphibole  ought  to  give  a  clue 
to  the  composition  of  the  original  green  substance.  It  is  there  found  that  the  principal  constit- 
uents of  the  amphibole  are  silica  and  ferrous  iron  in  the  following  proportions: 


SiOo. 
Fed- 


47.5 
52.5 


The  correspondence  of  these  percentages  with  those  above  given  is  evident. 

The  above  results  are  not  sufficiently  accordant  to  show  that  the  substance  under  dis- 
cussion has  a  definite  and  uniform  composition.  On  the  other  hand,  the  impurities  and  altera- 
tions cause  such  variations  that  it  can  not  be  said  that  the  green  granules  do  not  have  definite 
chemical  composition.  If  the  granules  do  have  a  definite  composition,  the  above  results  indi- 
cate the  most  probable  formula  to  be  Fe(Mg)O.Si02.nH20. 

Dr.  Spurr,  after  his  study  of  the  green  granules,  concluded  to  call  them  "glauconite."  In 
view  of  the  fiict  that  potash  is  by  most  mineralogists  insisted  upon  as  one  of  the  essential  con- 
stituents of  glauconite,  the  entire  absence  of  potash  in  the  substance  under  discussion  is  taken 
to  preclude  the  application  of  the  term  "glauconite."  The  substance  apparently  corresponds  to 
no  known  mineral  species.  As  a  convenient  term  by  which  to  refer  to  it  the  name  "greenalite" 
was  coined  for  use  in  the  monograph  on  the  Mesabi  district  and  is  used  in  this  report  also. 

The  origin  of  greenalite  and  the  details  of  the  similarities  and  differences  between  greenalite 
granules  and  granules  of  glauconite,  concretions  of  iron  oxide  and  chert,  and  other  granule 
and  concretionary  structures  are  discussed  in  Chapter  XVII,  on  the  origin  of  the  iron  ores. 

FERRUGINOUS,  AMPHIBOLITIC,  SIDERITIC,  AND   CALCAREOUS   CHERTS. 

The  following  description  applies  to  the  normal  types  of  chert  occurring  through  the  central 
and  western  portions  of  the  range.  The  highly  metamorphosed  chert  characteristic  of  the  east 
end  of  the  range  is  given  a  separate  description  on  a  subsequent  page. 

The  cherts  are  gray,  yellow,  red,  brown,  or  green  rocks,  mth  irregular  bands  and  shots 
and  granules  of  iron  oxide  varying  in  quantity  from  predominance  almost  to  disappearance. 
A  slight  brecciation  thoroughly  recemented  may  be  occasionally  observed,  and  a  pitted  surface, 
due  to  the  solution  of  certain  of  the  constituents,  is  not  uncommon.  The  iron  oxide  is  mainly 
intermediate  between  hematite  and  limonite,  and  to  a  subordinate  extent  is  magnetite,  and  its 
color  accordingly  ranges  from  red  to  yellow  or  to  black.  The  variety  of  colors  of  the  chert  and 
the  iron  oxide,  their  irregular  association,  and  their  variation  in  relative  abundance  give  the 
cherts  most  highly  varied  aspects;  yet  no  phase  of  the  cherts  is  likely  to  be  mistaken  for  any 


MESABI  IRON  DISTRICT.  169 

other  rock  by  anyone  reasonably  familiar  with  the  iron-bearing  rocks  of  the  Lake  Superior 
region.  To  the  casual  observer  the  massive  lighter-colored  cherts,  containing  little  iron  oxide, 
resemble  quartzite,  and  indeed  have  been  frequently  so  called.  However,  the  splintery  frac- 
ture of  the  chert  and  the  absolute  lack  of  rounded  clastic  grains,  aside  from  the  usual  content  of 
iron  oxide  in  layers  or  spots  or  minute  grains,  are  unfailing  criteria  for  the  discrimination  of  the 
two.  The  ferruginous  cherts  difi'er  from  the  jaspers  or  jaspilites  of  the  old  ranges  of  Lake 
Superior  ia  lacking  their  even  banding  and  brilliant  red  color  as  well  as  the  microscopic  features 
described  below. 

When  studied  under  the  microscope  it  appears  that  all  the  rocks  hero  described  as  chert 
are  genetically  connected.  In  lookmg  over  250  slides  but  few  have  been  observed  which  do  not 
show  some  evidence  of  the  derivuuon  of  the  rock  from  the  greenalite  rocks  above  described. 
The  granule  shapes  are  stUI  largely  preserved,"  but  the  alterations  have  tended  in  some  places 
to  make  the  shapes  more  irregular  and  partly  or  wholly  to  obliterate  them.  The  alteration  of 
the  granules  has  been  almost  entirely  metasomatic,  for  thero  is  little  evidence  of  dynamic  move- 
ment resulting  in  the  breaking  up  of  the  constituents  of  the  rock. 

The  greenalite  has  been  replaced  by  cherty  quartz,  magnetite,  hematite,  limonite,  siderite, 
calcite,  grunerite,  cummingtonite,  actinolite,  epidote-zoisite,  or  any  combination  of  them. 
The  extent  and  nature  of  the  alteration  replacement  vary  withm  wide  limits.  The  granule 
may  be  mainly  greenalite,  showing  incipient  crystallization  of  quartz,  griinerite,  or  actinolite, 
visible  only  under  crossed  nicols.  The  granules  may  be  represented  almost  wholly  by  hematite, 
limonite,  magnetite,  intermediate  varieties,  or  any  combination  of  them.  The  oxides  may  be 
arranged  irregularly  or  concentrically.  In  the  iron  ores  the  granules  are  entirely  represented 
by  iron  oxide,  although  their  shapes  are  in  part  obliterated.  The  granules  may  be  represented 
almost  wholly  by  chert,  which  may  be  distinguished  from  that  of  the  matrix  by  its  coarser  or 
finer  texture,  or,  if  not  by  texture,  by  distribution  of  pigment.  In  ordinary  light  chert  granules 
may  be  marked  by  the  pigments  which  in  parallel  polarized  light  are  completely  obscured  by  the 
crystallization  of  the  chert,  or  the  granules  may  not  be  seen  in  ordinary  light  and  be  conspic- 
uous under  crossed  nicols  because  of  the  crystallization.  Or  the  crystallization  of  the  chert 
may  have  entirely  obliterated  the  granules  for  much  of  the  slide,  both  m  ordinary  and  polarized 
light.  The  granules  may  be  represented  entirely  by  green,  yellow,  and  brown  grunerite,  cum- 
mingtonite, or  perhaps  actinolite,  or  aU,  which  in  ordinary  light  may  be  scarcely  distinguishable 
from  the  unaltered  greenalite  granules  but  which  become  apparent  under  crossed  nicols  by 
their  double  refraction.  The  granules  may  be  represented  by  calcite  or  siderite  in  rhombs  or 
irregular  grains,  sometimes  showing  zonal  growth,  which  for  the  most  part  are  clearly  replace- 
ments of  the  granules.  Most  commonly  the  granules  are  represented  by  a  combination  of  any 
or  all  of  the  minerals  above  named.  Of  these  combinations,  that  of  chert  and  iron  oxide  stands 
first.  The  two  substances  occur  in  all  proportions  with  a  great  variety  of  arrangement.  The 
two  may  be  irregularly  intermingled,  or  the  iron  oxide  may  form  a  rim  about  a  cherty  interior, 
or,  though  not  commonly,  the  chert  and  iron  oxide  may  be  in  concentric  layers  in  the  manner 
of  normal  concretions,  or  polygonal  areas  of  fine  chert  may  contain  spots  of  iron  oxide  in  the 
center  of  each  as  well  as  a  rim  of  iron  about  the  periphery,  suggesting  an  organic  structure. 
The  alteration  and  replacement  of  the  greenalite  and  the  conditions  favoring  the  development 
of  the  different  minerals  are  discussed  under  the  origin  of  the  ores  (pp.  187  et  seq.). 

In  addition  to  the  derivatives  of  the  greenalite  granules,  there  are  present  a  few  concentric 
concretions  of  iron  oxide  and  chert  about  quartz,  which  may  have  been  secondarily  developed 
from  some  substance  other  than  the  greenalite.  These  are  similar  to  concretions  in  the  iron- 
bearing  formation  of  the  Penokee-Gogebic  district,  where  they  have  developed  from  the  alteration 
of  an  iron  carbonate.  The  secondary  concretions  in  the  Mesabi  district  may  also  be  develop- 
ments from  iron  carbonates,  which  are  now  associated  with  unaltered  portions  of  the  formation 
and  probably  existed  formerly  in  the  portions  which  are  at  present  altered.  The  secondary 
concretions  are  different  from  the  greenalite  granules  in  their  beautifully  developed  concentric 

1  Spuir  (Bull.  Geol.  and  Nat.  Hist.  Survey  Minnesota  No.  10)  has  applied  to  this  texture  the  term  "spotted  granular." 


t> 


170  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

structure.  Though  a  few  of  the  granules  themselves  have  a  concentric  structure  resulting  from 
zonal  alteration,  this  is  usually  poorly  developed  and  there  is  ordinarily  little  difficulty  in  distin- 
guishing it  from  that  of  the  secondary  concretion,  though  in  some  places  it  is  possible  that  some  of 
the  supposed  secondary  concretions  formed  from  carbonate  may  be  really  secondary  alterations 
of  original  granules. 

Spherulites  of  epidote,  rarely  to  be  observed,  though  in  part  replacements  of  the  granules, 
are  also  clearly  secondary  developments  in  the  matrix. 

The  matrix  of  the  chert  may  be  a  sparse  interstitial  filling  between  the  granules  or  it  may 
form  most  of  the  rock  mass  and  contain  but  few  isolated  granules.  The  matri.x  is  similar  to 
that  of  the  unaltered  greenahte  rocks  in  that  it  is  mainly  chert,  but  it  differs  in  containing  far 
more  actinolite,  gri'inerite,  cummingtonite,  iron  oxide,  calcite,  and  siderite,  and  rarely  epidote- 
zoisite  in  spherulitic  form.  Sometimes  also  green  chloritic  substances  are  abundant,  either 
irregularly  distributed  tlirough  the  matrix  or  forming  a  definite  rim  about  the  granule.  In  the 
latter  case  the  chlorite  is  in  part  in  the  fibrous  form  known  as  delessite  and  much  resembles 
uralite.  The  recrystallization  of  the  rock  has  in  some  places  made  the  chert  in  the  matrix 
coarser  than  that  of  the  granules  and  in  other  places  the  reverse.  The  leaching  out  of  the  car- 
bonates and  greenahte  from  the  matrix  has  occasionally  left  cavities  which  give  the  pitted  char- 
acter to  the  weathered  surface  of  the  cherts. 

Accompanying  the  recrystaUization  of  the  chert  has  been  its  frequent  adoption  of  radial  or 
sheaf-hke  forms,  giving  black  crosses  under  crossed  nicols.  These  sheaves,  as  well  as  the 
sheaves  of  actinolite,  griinerite,  and  cummingtonite,  and  rarely  epidote,  frequently  lie  with  their 
butts  against  the  outhnes  of  the  granules  and  send  their  points  outward  until  they  interlock 
with  similar  projections  from  adjacent  granules.  Commonly  also  one  or  more  of  the  constitu- 
ents of  the  matrix  may  be  observed  to  lie  partly  in  the  matrix  and  partly  in  the  granule,  thus 
helping  to  obhterate  the  granule.  Indeed,  under  crossed  nicols  the  granules  may  not  be  observed, 
while  in  ordinary  light  their  position  may  be  indicated  by  the  distribution  of  the  fine  pigment. 

All  of  the  constituents  in  the  matrix  are  secondary  except,  perhaps,  a  part  of  the  chert, 
and  even  this  has  been  thoroughly  recrystaUized.  The  amphiboles  and  iron  oxide  may  be 
observed  to  have  developed  by  the  alteration  of  the  granules  and  some  of  the  lighter  amphiboles 
by  the  alteration  of  carbonate  and  chert  in  the  matrix.  The  carbonate  is  largely  though  not 
entirely  replacement  from  without,  for  it  may  be  observed  replacing  nearly  all  the  other  con- 
stituents of  the  rock  and  occurring  in  minute  veins  crossing  the  rock. 

The  composition  and  origin  of  the  ferruginous  cherts  are  discussed  on  pages  186-187. 

SILICEOUS,  FERRUGINOUS,  AND    AMPHIBOLITIC    SLATES. 

Under  this  head  are  grouped  a  variety  of  slaty  rocks  which  are  interstratified  with  the 
other  phases  of  the  iron-bearing  formation.  They  include  dense  black,  dark-gray,  green,  or 
reddish  rocks  with  a  tendency  toward  conchoidal  fracture  and  the  slaty  parting  poorly  devel- 
oped, if  at  all;  rocks  showing  banding  of  dark-green,  black,  gi-ay,  red,  or  brown  layers  parallel 
to  the  bedding  and  a  well-developed  cleavage  parallel  to  the  same  structure;  gradational 
varieties  between  these  two,  between  them  and  the  ferruginous  cherts,  and  between  them  and 
the  iron  ores.  Any  of  them  may  be  hard  or  soft,  carbonaceous  or  noncarbonaceous,  fine  grained 
or  medium  grained. 

Under  the  microscope  the  slates  are  seen  to  contain  principally  cherty  quartz,  iron  oxide, 
either  hematite  or  magnetite,  usually  in  octahedra,  or  some  hydrated  oxide,  monoclinic  amphi- 
bole  which  may  be  griinerite,  cummingtonite,  or  actinolite,  ami  possibly  even  common  horn- 
blende, a  small  amount  of  carbonate  of  calcium  or  iron,  a  little  zoisite,  and  possibly  also  a 
httle  chlorite.  From  the  optical  properties  and  from  the  analysis  of  the  rock  it  is  thought  that 
the  ampliibole  is  mainly  griinerite  and  cummingtonite.  There  is  much  variation  in  the  relative 
proportion  of  the  principal  constituents.  Some  of  the  slates  consist  almost  entirely  of  fine 
cherty  quartz,  with  subordinate  quantities  of  dark  amphibolo  in  radial  aggregates  or  in  irregular 
masses,  and  of  the  iron  oxides.  Others  are  composed  mainly  of  in)n  oxide,  showing  but  small 
quantities  of  the  quartz  and  dark  amphibole.     Others  are  composed  of  a  tangled  mass  of  yel- 


MESABI  IRON  DISTRICT.  171 

lowish,  brownish,  and  greenish  amphibole  fibers  containing  minute  particles  of  iron  oxide,  siUca, 
and  other  subordinate  constituents.  The  griinerite  is  far  more  abundant  than  the  actinoUte. 
The  banding  shown  in  many  specimens  is  due  to  the  segregation  of  the  above-named  elements 
into  layers.  ^AHiile  it  may  be  convenient  in  description  to  refer  to  tliis  or  that  slaty  rock  as  a 
ferruginous  slate,  a  siliceous  slate,  an  amphibolitic  slate,  or  an  actinolite  slate,  depending  upon 
the  relative  abundance  of  the  constituents,  usually  all  tliree  constituents  are  present  in  one  rock, 
and  the  rocks  are  really  amphibolitic,  siliceous,  and  ferruginous  slates.  Perhaps  the  most  char- 
acteristic feature  of  the  slates  as  a  group  is  the  abundance  of  the  dark  amphibole. 

PAINT    ROCK. 

Tlu-oughout  the  iron-bearing  formation,  and  particularly  adjacent  to  the  ore  deposits,  are 
thin  seams  of  paint  rock,  wliich  have  resulted  from  the  alteration  of  the  slates  above  described. 
The  paint  rocks  are  essentially  soft  red  or  yellow  or  white  clay.  They  retain  the  original  bedding 
of  the  rocks  from  wliich  they  were  derived,  the  structure  being  marked  by  alternation  of  bands 
of  dift'erent  color.  In  place  the  paint  rocks  are  moist  and  soft.  When  taken  out  and  dried  they 
become  harder  but  retain  a  soft,-  greasy  feel. 

The  alteration  of  the  paint  rocks  from  slates  is  proved  by  the  numerous  intermediate  phases 
to  be  observed.     For  analyses  of  paint  rock  see  page  191. 

SIDEEITIC    AND    CALCAREOUS    ROCKS. 

Associated  with  the  slaty  layers  in  the  iron-bearing  formation,  and  particularly  with  the 
greenalite  rocks,  are  carbonates  of  iron  and  calcium  in  small  quantity.  Most  of  the  carbonate 
reacts  readily  with  cold  dilute  hydrochloric  acid  and  is  certainly  limestone,  which,  from  the 
analysis  of  rocks  containing  it,  is  doubtless  magnesian.  Some  of  the  carbonate,  however,  is 
certainly  siderite,  as  shown  by  analysis.  The  carbonates  occur  in  minute  clear-cut  layers 
interbedded  M-ith  the  other  rocks  of  the  iron-bearing  formation,  in  veins  cutting  across  the 
bedding,  and  in  irregular  aggregates  and  well-defined  rhombohedral  crystals  in  the  layers  of  the 
iron  formation.  In  the  carbonate  bands  are  small  quantities  of  iron  oxide,  ferrous  silicate,  and 
chert,  and  in  the  bands  of  these  minerals  are  small  quantities  of  the  carbonate.  In  some  places 
the  carbonates  are  coarsely  crystalhne  and  fresh  and  clearly  have  resulted  from  the  replacement 
of  the  other  constituents  in  the  rock,  particularly  the  ferrous  silicate,  or  fi-om  infiltration  along 
cracks  and  crevices.  In  other  places,  especially  where  in  distinct  layers  interbedded  with 
unaltered  ferrous  sihcate  phases  of  the  formation,  the  carbonate  layers  seem  certainly  to  be 
original.  At  the  top  of  the  iron-bearing  formation  and  closely  associated  with  the  basal  horizons 
of  the  Virginia  slate  are  several  feet  of  clear  calcium  carbonate,  which  is  described  in  connection 
with  the  Virginia  slate. 

CONGLOMERATES   AND   QUARTZITES. 

At  the  base  of  the  iron-bearing  formation  is  a  thin  layer  of  fairly  coarse  fragmental  material 
consisting  in  places  of  conglomerate  alone  and  in  other  places  of  conglomerate  and  quartzite. 

THICKNESS. 

The  average  thickness  of  the  iron-bearing  Biwabik  formation  is  about  800  feet.  This 
figure  is  based  on  average  dips  of  the  formation,  width  of  outcrop,  and  drUl  records.  Local 
averages  are  likely  to  be  either  larger  or  smaller.  In  both  the  east  and  west  ends  of  the  district 
the  thickness  diminishes  somewhat,  the  iron-bearing  formation  apparently  giving  way  along  the 
strike  to  slate. 

ALTERATION   BY   THE   INTRUSION   OF   KEWEENAWAN    (iRANITE    AND   GABBRO. 

Through  ranges  12  and  13,  near  Birch  Lake,  the  Biwabik  formation  is  intruded  on  the  north 
by  granite  and  on  the  south  by  the  Duluth  gabl^ro  and  has  undergone  considerable  meta- 
morphism  in  consequence.  This  metamorphism  has  extended  even  farther  west,  for,  though 
the  gabbro  does  not  come  into  actual  contact  with  the  iron-bearing  formation  through  range 


172  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

14,  it  abuts  against  the  overlying  Virginia  slate  and  has  metamoq^hosed  both  the  slate  and  the 
iron-bearing  formation  in  this  area." 

In  general  through  the  western  and  central  portions  of  the  Mesabi  district  the  iron  oxide 
of  the  iron-bearing  formation  is  mainly  hydrated  hematite,  and  magnetite  is  in  subordinate 
quantity.  Eastward  from  Mesaba  station  the  iron  oxide  is  mainly  magnetite,  and  hematite 
is  in  subordinate  quantity.  Westward  from  Mountain  Iron  the  amijliiboles  are  almost  entirely 
lacking;  from  Mountain  Iron  eastward  to  Mesaba  station  the  amphiboles  are  present  in  the 
iron-bearing  formation  but  are  not  a])un(lant  until  Mesaba  station  is  approached ;  eastward  from 
Mesaba  station  they  become  abundant  and  make  up  an  important  constituent  of  the  formation. 
In  the  eastern  portion  of  the  range  the  chert  is  correspondingly  less  abundant  than  in  the  west- 
ern and  central  portions  of  the  district,  and  in  some  places  is  almost  entirely  absent.  The  chert 
becomes  also  distinctly  coarser  in  this  area.  In  range  12  the  grains  commonly  reach  a  diameter 
of  3  or  4  miUmieters,  and  there  are  a  few  smaller  particles,  and  in  the  central  and  western  por- 
tions of  the  district  they  are  seldom  greater  than  0.1  millimeter  and  almost  invariably  are  asso- 
ciated with  smaller  particles.  Toward  the  east  there  is  a  tendency  for  the  texture  to  become 
more  even,  although  there  are  many  wide  variations  from  uniformity.  The  chert  grains,  instead 
of  being  in  irregular,  roundish,  and  scalloped  cherty  forms,  as  in  the  central  and  western  por- 
tions of  the  district,  are  in  rouglily  polygonal  shapes  and  united  in  a  fairly  uniform  mosaic. 
Accompanying  these  changes  is  a  more  pronounced  segregation  of  the  magnetite  and  the  ampliib- 
olitic  chert  into  irregular  laj-ers  and  lenses,  with  the  result  that  the  iron-oxide  layers,  instead 
of  contairdng  various  other  minerals,  are  comparatively  free  from  them.  The  characteristic 
granules  of  the  ferruginous  cherts  are  still  conspicuous  in  the  east  end  of  the  district,  but  in  the 
most  highly  metamorphosed  phases  of  the  rocks,  as  in  range  12,  they  have  entirely  disappeared, 
being  obscured  by  magnetite,  amphibole,  and  chert.  In  the  phases  not  showing  the  maximum 
alteration  they  are  marked  by  magnetite,  either  as  a  rim  about  the  granule,  as  a  solid  mass 
filling  it,  or  in  evenly  disseminated  particles  through  it.  Not  unconunionly  the  granules  may 
be  observed  only  in  ordmary  light  and  then  by  distribution  of  the  magnetitic  particles;  in  parallel 
polarized  light  they  are  obscured  by  the  polarization  of  the  amphibolitic  and  cherty  constituents. 
Finally,  in  the  eastern  portion  of  the  district  certain  minerals  have  developed  which  have  not 
been  found  in  the  less  altered  rocks  of  the  central  and  western  portions  of  the  Mesabi  district. 
In  the  latter  areas  the  amphiboles  are  entirely  grunerite  and  actinohte,  with  little  or  no  horn- 
blende. In  the  eastern  portion  of  the  district  the  amphiboles  include  grunerite  and  actinolite, 
and  in  addition  green  and  brown  hornblende  in  considerable  quantity.  Associated  with  these 
minerals  are  small  quantities  of  biotite,  glaucophane,  andalusite,  zoisite,  and  garnet.  Though 
hypersthene,  augite,  and  olivine  are  abundant  and  characteristic  in  the  true  gabbro  of  range  12 
and  westward,  these  minerals  are  nearly  if  not  quite  lacking  in  the  Biwabik  formation. 

Although  to  the  east  toward  Gunflint  Lake  the  gabbro  alone  has  been  able  to  produce  even 
greater  metamorphic  effects  on  the  iron-bearing  rocks,  it  is  probable  that  the  metamorphism  of 
the  iron-bearing  rocks  in  the  region  untler  description  has  been  produced  jointly  bj'  Kewee- 
nawan  gabbro  and  granite. 


VIRGINIA  SLATE. 


DISTRIBUTION. 


The  Virginia  slate  bounds  the  iron-bearing  Biwabik  formation  on  the  south  from  the  west  end 
of  the  district  nearly  to  the  east  side  of  sees.  5  and  8,  T.  59  N.,  R.  13  W.,  where  the  slate  is  overlapped 
by  the  gabbro.  Still  farther  east,  in  the  SW.  i  sec.  25,  T.  60  N.,  R.  13  W.,  drilling  has  shown 
altered  slate  to  lie  between  Keweenawan  Duluth  gabbro  on  the  south  and  Kewecnawan  diabase 
on  the  north,  but  whether  it  is  an  isolated  mass  at  this  point  in  the  Keweenawan  area  or  is 
continuous  with  the  slate  to  the  west  explorations  or  exposures  do  not  yet  tell.  The  slate 
underlies  the  lower  slopes  of  the  Giants  Range  and  continues  south  under  the  low-lying  swampy 


a  The  metamorphism  of  the  Biwabik  formation  by  the  Duluth  gabbro  in  the  area  adjacent  to  Birch  Lalte  and  to  the  east  in  the  ^•ieinit5•  of 
.\lceley  and  Ciinllint  lakes  has  been  described  in  detail  by  U.  S.  Grant,  W.  S.  Bayley,  and  Carl  Zaplle  and  has  been  briefly  considered  or  mentioned 
by  N.  U.  Winchell.  11.  V.  Winchell.  A.  U.  Elftmann.  J.  E.  Spurr.  J.  Morgan  Clements.  C.  R.  Van  Hise.and  others.  The  reader  is  referred  to  Chap- 
ter Vin,  on  the  Cunllint  district  (pp.  198-204),  for  a  fuller  account  of  the  alterations  near  the  gabbro. 


MESABI  IRON  DISTRICT. 


173 


area  south  of  the  Giants  Range  for  an  unknown  distance.  The  area  overlain  by  slate  is  so 
thickly  covered  with  drift  that  exposures  of  the  slate  are  almost  entirely  lacking;  its  presence 
and  distribution  have  been  determined  by  drilling  and  test  pitting  in  the  search  for  iron. 
Tlirough  the  central  portion  of  the  district  enough  of  such  work  has  been  done  to  show  the  posi- 
tion of  the  slate  boundary  with  a  fair  degree  of  accuracy,  although  even  here  there  are  con- 
siderable stretches  where  records  of  the  occurrence  of  slate  are  wanting.  In  the  western  and 
eastern  portions  of  the  district  the  distribution  of  the  slate  is"  less  well  known,  particularly 
in  the  western  end  of  the  district.  In  drawing  the  slate  line  on  the  map  of  this  portion  of 
the  area  all  that  can  be  done  is  to  connect  the  separated  explorations  which  reveal  slate. 
Wherever  exploration  has  been  detailed  it  is  found  that  the  slate  boundary  is  not  straight  but 
in  gentle  curves,  and  it  is  reasonable  to  expect,  therefore,  that  future  work  will  show  numerous 
additional  undulations  in  the  slate  boundary  for  the  area  at  present  not  completely  explored. 


The  normal  Virginia  slate  is  usually  a  gray  rock,  though  in  part  black,  reddish,  or  brown, 
with  bedding  shown  by  alternating  bands  of  varying  color  and  texture.  Some  of  the  beds  are 
almost  coarse  enough  to  be  called  graywackes.  Indeed,  in  the  field  the  rock  has  been  called  a 
banded  slate  and  graywacke.  Some  of  the  slate  is  hard  and  siliceous;  other  phases,  especially 
the  nonsiliceous  and  carbonaceous  ones,  are  soft,  and  can  be  wliittled  with  a  knife.  Near  the 
contact  of  tlie  slate  with  the  iron  deposit  in  the  underlying  iron-bearing  formation,  as  at  Biwabik 
and  in  sec.  3,  T.  58  N.,  R.  15  W.,  the  slate  becomes  iron  stained  and  soft  and  grades  into  paint 
rock.  The  slate  in  general  has  a  very  poor  parting  parallel  to  beddmg  planes,  and  there  is  little 
or  no  development  of  secondary  cleavage.  Wliat  there  is  of  secondary  cleavage  has  been 
developed  parallel  to  the  bedding  planes  and  is  marked  by  minute  particles  of  mica  there  found. 
The  rock  in  general  aspect  and  mineralogical  and  chemical  composition  looks  like  slate,  but  it 
differs  from  true  slate  in  lacking  a  true  cleavage,  and  as  this  is  one  of  the  essential  characteristics 
of  slate  it  ma}'  be  doubted  whether  the  term  "slate"  ought  to  be  applied  to  the  rock.  Yet  the 
rock  is  not  a  shale,  for  it  is  too  much  metamorphosed  and  lias  too  poor  a  partuag  parallel  to  the 
bedding.  In  the  Cuyuna  district  the  same  formation  shows  the  charactci-istics  of  a  true  slate, 
and  the  formation  both  there  and  in  the  Mesabi  district  proper  has  been  known  locally  and 
in  geologic  literature  as  slate.     Hence  the  term  is  here  retained. 

Analyses  of  Virginia  slate. 


1. 

2. 

SiOs 

62.26 

10.  S9 

1.76 

4.55 

2.95 

.42 

2.29 

3.02 

.70 

3.8S 

.60 

None. 

.20 

56  Gl 

AlsOs.              

17.76 
3.29 
5.15 

Fe-Oa 

FeO    

MgO 

CaO .     •  

1.00 

NajO 

K2O 

4  04 

HjO- 

H2O  + 

4  18 

TiOj 

COj 

PjOs 

Organic  undetemiincd 

C  and  c 

99.52 

99.56 

1.  Analysis  by  Oeorge  Steiger.  of  the  United  States  Geological  Survey,  of  a  composite  sample  of  the  Virginia  slate  made  up  bv  assembling  several 
specimens  from  two  localities  (specimen  45767  from  excavation  for  water  tank  of  Eastern  Railway  of  Minnesota,  at  Virginia;  specimen  45463  from 
a  point  south  of  the  Biwabik  mine). 

2.  Analysis  of  Virginia  slate  by  R.  D.  Hall,  University  of  Wisconsin,  of  a  sample  representing  900  feet  of  drill  core  from  a  drill  hole  at  the  south- 
east comer  of  sec.  S,  T.  58,  E.  15. 


CORDIERITE   HORNSTONE    RESULTING    FROM    THE   ALTERATION   OF  THE    VIRGINIA    SLATE    BY   THE    DULUTH   GABBRO. 

In  approaching  the  Duluth  gabbro,  which  overlaps  the  Virgmia  slate  in  ranges  14  and  13, 
the  slate  becomes  more  crystalline,  harder,  and  characteristically  breaks  with  a  conchoidal 
fracture,  and  the  color  becomes  darker  and  in  many  places  is  a  bluish  black.     The  rock,  indeed, 


174  GEOLOGY  OF  THE  LAKE  SLTPEKIOR  REGION. 

becomes  a  hornstonc'.  Moreover,  there  appear  minute  light-colored  specks  which  on  tlie 
weatlicred  surface  arc  likely  to  have  disappeared  and  to  be  represented  by  pits.  Under  the 
microscope  the  wlute  specks  are  found  to  be  cordierite  in  typical  development,  standing  as 
numerous  phenocrysts  in  a  fine-grained  matrix  of  biotite,  feldspar,  magnetite,  and  certain 
doubtful  microlites  wliich  may  be  actinolite  or  sillimanite,  or  botli.° 

RELATIONS   TO   THE    BIWABIK    FORMATION. 

Reference  has  already  been  made  to  the  fact  that  tlic  relations  of  tlie  Virginia  slate  to  the 
underlying  Biwabik  formation  are  those  of  gradation,  both  lateral  and  vertical.  It  remains 
to  discuss  tliis  gradation  somewhat  fully.  The  iron-bearing  formation  contains  slate  layers 
tlu-oughout.  At  upper  and  middle  horizons  they  are  perhaps  more  numerous  than  at  lower 
horizons.  Just  below  the  solid  black  Virginia  slate  there  is  a  zone  in  which  there  are  many 
interlaminations  of  iron-bearing  formation  and  slate,  the  layers  varying  in  thickness  from  several 
feet  to  a  fraction  of  an  inch.  Tliis  zone  is  of  varying  and  uncertain  thickness.  In  many  places 
at  least  the  zone  of  minute  interbanding  is  thin,  not  more  than  15  or  20  feet,  but,  as  already 
noted,  layers  of  slate  are  found  well  down  in  the  iron-bearing  formation  and  layers  of  the  iron- 
bearing  formation  are  found  well  up  in  the  slate,  so  that  in  a  broad  way  the  gradation  zone  m.'iy 
be  several  hundred  feet. 

Drilling  shows  much  irregularity  in  the  alternation  of  layers.  Slate  layers  are  more  abun- 
dant in  the  eastern  end  of  the  district,  and  westward  from  Grand  Rapids  the  iron-bearing 
formation  rapidly  thins,  its  place  being  taken  by  slate  in  T.  142  N.,  R.  25  W.  Wliether  the 
iron-bearing  formation  extends  indefinitely  southward  under  the  slate  or  gives  place  to  slate 
in  that  direction  is  not  known.  All  di-ill  holes  put  down  near  the  northern  margin  of  the  Vir- 
ginia slate  in  the  Mesabi  district  have  shown  the  Biwabik  formation  below.  For  reasons  cited 
on  pages  517-518,  however,  it  is  regarded  as  not  impossible  that  farther  south  the  iron-bearing 
formation  thins  and  becomes  discontinuous,  its  place  being  taken  by  the  black  slate. 

An  examination  of  the  map  will  show  the  Vii'ginia  slate  to  encroach  on  the  south  margin 
of  the  iron-bearing  formation  to  greatly  varying  distances,  with  the  result  that  the  surface  outcrop 
of  the  iron  formation  ranges  in  width  from  2  miles  or  more  to  less  than  a  cjuarter  of  a  mile. 
This  is  due  in  part  to  steeper  dips  at  the  narrow  places  than  at  the  wide  places  in  the  iron- 
bearing  formation,  erosion  having  thus  uncovered  less  of  the  iron  formation  where  the  dips  were 
steep;  it  is  due  in  part  to  faulting,  as  at  Biwabik  and  eastward;  it  is  due  in  part  to  the  greater 
dip  of  the  present  plane  of  surface  erosion,  either  atmospheric  or  glacial,  in  places  where  the 
formation  is  wide  than  where  narrow,  the  greater  dip  of  the  surface  bringing  it  more  nearly 
parallel  with  the  dip  of  the  iron-bearing  formation,  and  thus  uncovering  more  of  it;  but  so  far 
as  present  evidence  goes  these  factors  are  not  adequate  to  account  for  the  observed  variations 
in  width  of  the  iron  formation.  The  known  irregular  alternation  of  iron-bearing  formation  and 
slate  both  across  and  along  the  beds  is  therefore  regarded  as  a  cause  of  the  varying  widths 
of  the  iron-bearing  formation. 

STRICTURE. 

Opportunities  for  studying  the  structure  of  the  Virginia  slate  in  place  are  so  few  that  if 
the  obsei-ver  were  dependent  upon  such  obsei-vations  alone  he  would  be  unable  to  make  any 
statements  concerning  the  structure  of  the  formation  beyond  the  fact  that  it  dips  at  low  angles 
away  from  the  high  land  adjacent. 

THICKNESS. 

The  thickness  of  the  Virginia  slate  can  not  be  determined  in  the  Mesabi  district.  The 
drift  covering  is  thick,  mining  exploration  stops  to  the  south  where  the  slates  are  encountered, 
and  the  southerly  extent  of  the  slate  belt  is  thus  unknown. 

o  Cordierite  in  this  fonnation  was  first  noted  and  described  by  N.  II.  Winchell,  Final  Kept.  Geol.  and  Nat.  Hist.  Surrey  Minnesota,  vol.  S,  1900. 


MESABI  IRON  DISTRICT.  175 

STRUCTURE   OF   THE   UPPER   HURONIAN  (ANIMIKIE  GROUP). 

As  a  whole  the  upper  Huronian  (Animikie  group)  is  a  well-bedded  series  of  sediments. 
The  bfedding  is  most  pronounced  in  the  mitldle  and  upper  horizons.  The  beds  have  gentle  dips, 
averaging  between  5°  and  20°,  though  locally  greater  or  less,  in  southerly  and  southeasterly 
tlirections  away  from  the  older  rocks  forming  the  core  of  the  Giants  Range,  but  locally  the  dips 
show  much  variation  both  in  degree  and  direction.  About  the  southerly  projecting  tongue  of 
the  Giants  Range,  in  the  vicinity  of  Virginia,  Eveleth,  antl  McKinley,  the  dips  are  westerly  on 
the  west  side  of  the  tongue,  southerly  at  the  end  of  the  tongue,  and  southeasterly  on  the  south- 
east side — thatis,  throughout  approximately  normal  to  its  periphery.  Even  more  conspicuous 
than  the  change  of  dip  at  such  a  place  are  the  minor  variations  between  exposures.  Seldom  is 
it  possible  to  get  two  identical  readings  in  dip  at  exposures  of  rock  separated  by  even  short 
intervals,  although  the  direction  and  amount  of  the  dip  come  within  the  above  limits.  These 
facts  indicate  that  the  upper  Huronian  beds  are  tilted  away  from  the  core  of  the  Giants  Range 
in  directions  normal  to  its  trend  and  that  the  gently  tilted  beds  are  not  plane  surfaces  but  are 
gently  flexed.  By  tabulation  and  comparison  of  the  dips  it  becomes  further  apparent  that  the 
greater  flexures  are  not  random  ones  but  generally  have  their  axes  normal  to  the  trend  of  the 
range.  On  examination  of  the  attitudes  of  the  beds  still  more  in  detail  it  appears  that  the 
great  flexures  themselves  are  not  simple  but  have  many  subordinate  flexures,  some  of  them 
transverse  to  the  major  ones.  The  complexity  of  the  structure  may  be  likened  to  that  of  water 
waves.  On  the  great  swells  and  troughs  there  are  smaller  waves,  on  the  smaller  waves  there 
are  stUl  smaller  ones,  and  so  on  down  to  the  tiniest  disturbance  of  the  surface.  Though  perhaps 
the  majority  of  the  minor  flexures  in  tlie  upper  Hui'onian  rocks  have  attitudes  similar  to  the 
larger  ones,  many  of  them  vary  greatly  in  direction.  They  may  be  observed  at  almost  any 
single  exposure  of  the  upper  Huronian. 

The  great  flexures  are  ver\'-  gentle,  involving  very  small  changes  in  degree  and  direction  of 
dip.  Many  of  the  minor  flexures  superimposed  upon  the  greater  ones  are  sharp  and  conspicuous. 
The  local  dips  may  vary  as  much  as  50°  witliin  a  few  hundred  feet  and  change  their  direction 
considerably.  Dips  as  liigh  as  45°  or  even  60°  may  be  seen  in  the  layers  of  the  iron-bearing 
formation  in  some  of  the  open  pits  of  the  mines,  as  at  the  Stevenson,  the  Sauntry-Alpena,  the 
Kanawha,  and  the  Sparta.  At  the  Hawkins  and  Agnew  mines  the  iron-bearing  formation 
exliibits  steep,  sharp  fokls.  The  iron-bearing  formation  shows  more  minor  contoi'tions  than 
the  rest  of  the  upper  Huronian  rocks,  because  of  the  great  chemical  changes  which  it  has  under- 
gone, but  it  is  not  probable  that  there  is  any  great  dift'erence  in  the  major  folding. 

The  prevailing  gentle  southern  tflt  of  the  upper  Huronian  and  the  manner  in  which  it 

laps  around  the  salients  in  the  older  rocks  suggest  that  the  major  features  of  upper  Huronian 

"Structure  may  be  due  partly  to  initial  dip  as  well  as  to  subsequent  folding — in  other  words, 

that  the  upper  Huronian  sediments  are  essentially  in  the  position  in  which  they  were  deposited 

against  an  old  shore  and  have  undergone  minor  deformation  since. 

Accompanying  the  tilting  and  minor  folding  of  the  upper  Huronian  there  has  been  a  very 
considerable  amount  of  fracturing,  especially  in  the  comparatively  brittle  Pokegama  and 
Biwabik  formations.  Indeed,  it  seems  likely  that  the  folds  of  the  two  lower  formations  of  the 
upper  Huronian  are  mainly  the  result  of  lelatively  small  displacement  along  fractures,  and  only 
to  a  small  degree  the  result  of  the  actual  bending  of  the  strata  without  breaking.  The  pondmg 
of  water  beneath  the  Virginia  slate  would  seem  to  indicate  that  this  formation  has  been  less 
fractured  than  the  iron-bearing  formation  because  of  its  less  brittle  character,  and  has  thus 
yielded  to  deformation  by  actual  bending  rather  than  by  bi'eaking.  On  almost  every  exposure 
of  Pokegama  and  Biwal^ik  formations  joints  and  minute  faults  are  to  be  obsei'ved  cutting  almost 
perpendicularly  across  the  bedding.  In  each  case  the  joints  seem  to  make  up  two  or  more 
systems  crossing  each  other  at  various  angles,  but  such  sets  have  little  constancy  of  direction  in 
widely  separated  exposures,  unless  we  except  a  set  of  joints  which  at  a  number  of  places  have 
an  average  direction  of  somewhere  between  N.  60°  and  70°  E. — that  is,  approximately  parallel 
to  the  trend  of  the  range.     In  the  massive  rocks  the  joints  are  clear  cut  and  continuous  for 


176  GEOLOCn'  OF  THE  LAKE  SUPERIOR  REGION. 

considerable  distances.  In  the  well-bedded  rocks — as,  for  instance,  in  the  thin-bedded  portions 
of  the  iron-bearing  formation — the  joints  are  usually  more  irregular,  less  continuous,  and  less 
conspicuous.  In  such  jjIuccs  each  individual  bed  may  be  more  or  less  jointed  witliout  reference 
to  the  la^'ers  above  or  below. 

The  displacement  or  faulting  along  joints  has  been,  in  general,  small.  The  displacement 
is  rarel}'  3  or  4  feet,  and  commonly  it  is  measured  by  a  few  uiclies. 

There  is  a  displacement  of  about  200  feet  along  a  nearly  vertical  fault  strike  running  east- 
ward along  the  north  side  of  the  Biwabik  mine  parallel  to  the  northern  margin  of  the  upper 
Iluronian  past  Embarrass  Lake.  The  south  side  of  the  fault  has  droppetl,  with  the  result  that 
the  layers  of  the  u'on-bearing  formation  are  somewhat  tilted  along  the  contact  and  the  width 
of  the  outcrop  lessened.  The  eastward  extension  of  tliis  fault  carries  it  tlirough  the  peculiar 
point  of  Pokegama  quartzite  projecting  eastward  into  the  iron-bearing  formation  cast  of  Embar- 
rass Lake.  Though  the  structure  has  not  been  worked  out  in  detail  east  of  Embarrass  Lake, 
it  seems  not  unlikely  that  the  peculiar  features  of  the  distribution  of  the  quartzite  and  iron- 
bearing  formation  there  may  be  partly  explained  by  faulting,  though  original  configuration  of 
the  shore  line  in  upper  Huronian  time  may  have  something  to  do  with  it.  Other  great  faults 
are  almost  certainly  present  in  the  district,  but  evidence  for  them  has  not  been  correlated. 

Certain  of  the  joints  and  faults  have  been  filled  with  vein  quartz  and  others  have  not.  It 
is  rather  siu'prising  that  so  little  vein  quartz  is  to  be  observed.  In  the  harder  rocks,  where  the 
joints  are  clear  cut  and  continuous,  the  quartz  veins  also  appear  so.  In  the  well-bedded  por- 
tions of  the  iron-bearing  formation,  where  the  joints  are  irregular  and  discontinuous,  the  distri- 
bution of  the  vein  quartz  is  also  irregular  and  discontinuous,  being  rather  in  a  confused  zone 
than  in  a  well-defined  plane. 

After  the  upper  Huronian  was  tilted  and  folded  the  upper  edges  of  the  beds  were  eroded 
awaj',  with  the  result  that  the  rock  surface  is  now  in-egular,  ^\^th  dips  corresponding  roughly 
in  direction  but  not  in  degree  with  those  of  the  underlying  rock  strata,  being  in  general  less 
steep. 

RELATIONS   OF  THE   tTPPER  HURONIAN  (ANIMIKLE   GROUP)  TO   OTHER   SERIES. 

The  upper  Huronian  lies  unconformably  upon  the  Archean  and  lower -middle  Huronian 
rocks.     The  proof  of  unconformity  is  as  follows: 

1.  The  conglomerates  at  the  base  of  the  upper  Huronian"  contain'  fragments  derived 
from  the  underlying  rocks. 

2.  There  is  discordance  in  dip.  The  underlying  formations,  where  they  have  any  parallel 
structure  at  all,  are  almost  vertical.  The  upper  Huronian  is  well  bedded,  with  a  low  dip. 
Moreover,  in  approaching  the  contact  no  change  of  dip  is  to  be  observed  either  in  the  upper 
Huronian  or  in  the  underlying  rocks. 

3.  There  is  a  difference  in  the  amount  of  minor  folding,  fracturing,  secondary  cleavage, 
and  further  consequent  metamorphism  of  the  two  boches,  the  upper  Huronian  being  much 
less  affected  than  the  older  rocks. 

4.  The  upper  Huronian  belt  overlies  Archean  and  lower-middle  Huronian  rocks  indiscrim- 
inately. Near  Biwabik,  for  instance,  the  northern  edge  of  the  upper  Huronian  lies  diagonally 
across  the  contact  of  the  Archean  and  lower-middle  Huronian  rocks. 

5.  The  lower-middle  Huronian  sediments  are  intruded  by  the  Giants  Range  granite,  which 
composes  most  of  the  core  of  the  Giants  Range.  The  u])per  Huronian  is  not  intruiletl  by  the 
Giants  Range  granite,  and,  moreover,  in  the  conglomerate  at  its  base  it  beare  fragments  of 
this  granite.  The  ui)per  Iluronian  in  ranges  12  and  13  is  in  eruptive  contact  with  the  Kewee- 
nawan  granite  and  gabbro. 

CONDITIONS   OF   DEPOSITION   OF   THE   UPPER   HURONIAN  (ANIMIKIE   GROUP). 

The  conditions  under  which  the  upper  Huronian  0\jiunikic  group)  was  de|)osited  are  dis- 
cussed for  the  Lake  Superior  region  in  Chapter  XX.  It  may  be  noted  here  that  the  rocks  of  this 
group  are  believed  to  be  subaqueous  deposits  grading  upward  into  delta  deposits.     The  Mesabi 

"Listed  in  Mon.  U.  S.  Geol.  Survey,  vol.  43,  pp.  94-9S. 


MESABI  IRON  DISTRICT.  177 

district  may  represent  shore  conditions'  of  deposition  as  contrasted  with  the  Cuyuna  district 
farther  soutli,  wliich  may  represent  offshore  conditions.  The  well-assorted  sands  at  the  base  of 
the  group  in  the  Mesabi  district  seem  to  show  variation  in  tliickness  and  area  corresponding 
to  the  coiiliguration  of  the  older  rock  surface.  For  instance,  the  point  of  Pokegama  quartzite 
extending  eastward  from  Embarrass  Lake  suggests  a  sand  spit,,  though  distribution  may  be 
complicated  by  faulting.  The  peculiar  conditions  determining  the  deposition  of  the  iron- 
bearing  formation  are  discussed  on  pages  499  et  seq. 

KEWEENAWAN  SERIES." 
DULUTH  CABBRO 

A  portion  of  the  great  mass  of  Keweenawan  gabbro  of  northern  Minnesota  comes  within 
the  limits  of  the  Mesabi  tlistrict.  The  northern  edge  of  the  mass  lies  diagonally  across  the  east- 
ern end  of  the  district,  extending  from  near  the  Duluth  and  Iron  Range  track,  in  range  14, 
northeastward  through  ranges  13  and  12  to  Birch  Lake.  Through  range  14  the  gabbro  is  in 
contact  with  Virginia  slate;  in  ranges  13  and  12  it  is  in  contact  with  the  Biwabik  formation, 
and  north  of  Birch  Lake  it  is  in  contact  with  lower-middle  Huronian  granite.  The  northern 
edge  of  the  gabbro  forms  a  conspicuous  northward-facing  escarpment  overlooking  the  low- 
lying  area  of  the  Virginia  slate  and  of  iron-bearing  formation  immediately  to  the  north.  To 
this  the  name  "Mesabi  Range"  was  first  applied.  In  the  neighborhood  of  Birch  Lake  the 
gabbro  comes  well  up  on  the  crest  of  the  Giants  Range,  and  here  it  does  not  stand  above  the 
adjacent  rocks. 

DIABASE. 

There  are  in  the  Mesabi  district  certain  rocks  associated  with  the  Duluth  gabbro  which 
are  not  covered  in  the  above  general  account.  In  range  13  exposures  of  fine-grained  diabase 
appear  in  the  SW.  i  sec.  25,  T.  60  N.,  R.  13  W.,  and  in  the  central  and  northern  portions  of 
sec.  35,  T.  60  N.,  R.  13  W.  Bowlders  of  the  same  material  indicate  its  extension  for  several 
miles  east  and  west,  and,  taken  together  with  the  exposures,  indicate  a  belt  with  a  possible 
width  of  somewhat  less  than  a  mile,  a  length  of  at  least  3  miles  and  probably  much  more,  and 
a  trend  northeast  and  southwest — that  is,  parallel  to  the  general  strike  of  the  formation  bound- 
aries in  this  part  of  the  district.  The  diabase  is  a  fine-grained  dark-gray  rock  which  under  the 
microscope  shows  a  weU-developed  ophitic  arrangement  of  plagioclase  feldspar  crystals  and  the 
presence  of  abundant  hornblende  and  less  abundant  ilmenite  and  magnetite.  The  diabase  corre- 
sponds Uthologically  to  the  diabase  sills  intruded  in  the  iron-bearing  formation  in  the  neighbor- 
hood of  Gunflint  Lake,  and  there  supposed  to  be  either  offshoots  of  the  gabbro  or  intrusives  both 
in  the  gabbro  and  adjacent  rocks.  The  trend  of  recent  opinion  is  toward  the  former  conclu- 
sion. In  the  SW.  {  sec.  25,  T.  60  N.,  R.  13  W.,  south  of  the  diabase,  drill  holes  have  recently 
penetrated  altered  slate  (cordierite  hornstone).  The  relations  of  the  slate  to  the  surrounding 
rocks  are  unknown  because  of  lack  of  exposures  and  exploration.  If  the  slate  is  continuous 
with  that  to  the  west,  which  had  not  heretofore  been  known  to  extend  farther  east  than  sees. 
5  and  8  of  the  same  range,  the  diabase  must  be  a  sill  intruded  in  the  upper  Huronian  (Animilde 
group).  If  the  slate  is  not  continuous  with  the  main  belt  of  slate  to  the  west,  it  must  be  an 
isolated  mass  in  the  Keweenawan  rocks,  and  the  diabase  would  belong  with  the  main  mass  of 
the  Keweenawan.  From  the  analogy  of  its  hthologic  character  with  that  of  the  diabase  sills 
to  the  east,  from  its  distribution,  and  from  the  occurrence  of  slate  to  the  south  it  is  thought 
that  the  diabase  is  probably  a  siU,  but  lack  of  exposures  and  of  sufficient  exploration  makes 
it  quite  impossible  at  present  to  show  its  boundaries  on  the  map.  The  area  south  of  the  diabase, 
including  that  in  wliich  the  slate  has  been  found,  is  therefore  mapped  as  Keweenawan. 

A  httle  southeast  of  the  northwest  corner  of  sec.  34,  T.  59  N.,  R.  14  W.,  E.  J.  Longyear 
found  diabase  at  the  depth  of  984  feet,  in  a  drill  hole  which  had  passed  through  16  feet  of  drift, 
392  feet  of  black  slate,  and  576  feet  of  iron-bearing  formation.     Diabase  was  penetrated  for  309 

o  For  a  general  account  of  the  Keweenawan  series  of  Minnesota  see  Chapter  XV  (pp.  366  et  seq.) 
47517°— VOL  52—11 12 


178  GEOT.OGY  OF  THE  LAKE  SUPERIOR  REGION. 

feet  before  the  work  was  stopped.  The  iron-bearing  formation  is  IioiiihIciI  on  the  north  by 
lower-middle  Huronian  graj^wackes  antl  slates,  upon  the  eroded  edges  of  which  lies  the  iron- 
bearing  formation,  with  perhaps  a  tliin  layer  of  Pokegama  (juartzite  between.  The  fact  that 
the  diabase  rather  than  the  Pokegama  quartzite  or  lower-middle  Iluronian  graywacke  and 
slate  was  reached  by  the  drill  below  the  iron-bearing  formation  would  be  in  accord  with  the 
supposition  that  the  diabase  formed  a  sill  intruded  into  the  iron  formation  at  this  place. 

In  the  NE.  i  SE.  i  sec.  13,  T.  57  N.,  R.  22  W.,  drilUng  has  penetrated  20  feet  of  diabase 
with  iron-bearing  formation  both  above  and  below. 

EMBARRASS   GRANITE. 

Through  ranges  12  and  13  and  as  far  west  as  sec.  2,  T.  59  N.,  R.  14  "W.,  a  distance  of  15 
miles,  the  granite  forming  the  core  of  the  Giants  Range  is  intrusive  into  the  upper  Huronian. 
Wliether  it  was  intruded  at  the  close  of  the  upper  Huronian  epoch  or  during  the  succeeiling 
Keweenawan  is  a  matter  of  doubt  and  indeed  is  a  matter  of  small  consequence.  The  fact  that 
granite  dikes  cut  the  Keweenawan  series  in  other  parts  of  northern  Minnesota  makes  it  a  plausible 
assumption  that  the  granite  was  intruded  in  Keweenawan  time,  but  no  relations  of  the  granite 
to  the  Keweenawan  have  been  observed  in  the  Mesabi  district.  The  granite  is  named  the 
Embarrass  granite  from  its  lithologic  similarity  to  granite  exposed  at  Embarrass  station  on  the 
Duluth  and  Iron  Range  Railroad,  just  north  of  the  Giants  Range. 

The  Embarrass  granite  is  a  pink  hornblende  granite.  It  is  usually  of  coarse  grain  but 
shows  much  variation.  In  general  the  grain  becomes  finer  toward  the  west.  The  character- 
istic feature  of  the  granite  is  its  large  content  of  quartz  in  small  and  large  grains,  which  are  verj^ 
conspicuous,  especially  on  the  weathered  surface.  The  quartzes  range  in  diameter  from  a  few 
miUimeters  to  more  than  a  centimeter.  The  large  one>s  have  a  characteristic  purplish-blue 
color.  In  its  content  of  quartz  the  Embarrass  granite  is  readily  distinguished  from  the  lower- 
middle  Huronian  granite  (Giants  Range  granite)  in  the  central  and  western  parts  of  the  range, 
in  which  the  quartz  is  exceedingly  rare  or  entirely  lacking.  Other  constituents  are  pink  ortho- 
clase  feldspar,  which  sometimes  occurs  as  porphyritic  crystals  almost  an  inch  long,  and  a  rather 
small  amount  of  hornblende.  The  relative  abundance  and  coarseness  of  aU  the  constituents 
of  the  granite  of  course  show  the  usual  variations  of  a  large  granitic  mass. 

Cutting  the  granite  are  a  few  dikes  of  finer-grained,  lighter-colored  quartzose  granite, 
wluch  under  the  microscope  is  found  to  differ  from  the  one  just  described  only  in  lacking 
hornblende  and  the  rare  elements  mentioned. 

In  the  Mohawk  mine  and  elsewhere  near  Aurora  granite  forms  the  foot  wall  of  the  ore 
bodies,  in  one  place  coming  within  16  feet  of  the  rock  surface.  From  tliis  vertical  dikes  cut 
across  the  formation.  The  relations  seem  to  be  those  of  intrusion  of  granite  principally  parallel 
to  the  bedding  but  partly  across  it.  These  relations  may  be  correlated  with  those  of  the 
Embarrass  granite  at  the  east  end  of  the  range. 

CRETACEOUS   ROCKS. 

'>'■■  ^-' ' 
Distribution  and  character. — Recent  explorations  have  showTj  Cretaceous  conglomerates, 

shales,  or  iron  ores  as  a  tliin  mantle  over  most  of  the  western  part  of  the  district  and  in  isolated 
patches  as  far  east  as  Embarrass  Lake.  It  is  therefore  thought  inadvisable  to  attempt  to 
show  Cretaceous  deposits  on  the  map.  Especially  noteworthy  is  the  discovery  of  small  con- 
glomeratic Cretaceous  ore  bodies  overlying  the  contact  of  the  iron-bearing  Biwabik  formation 
and  the  Virginia  slate.  From  the  distribution  of  the  remnants  now  known  it  is  certain  that 
Cretaceous  rocks  once  overlay  all  of  the  district  west  of  range  16,  that  they  may  have  extended 
farther  east,  and  that  erosion  has  largely  removed  thein»from  the  area  they  did  occupy.  It  is 
not  unlikely  that  some  of  these  remnants  have  been  protected  because  faulted  do^Tn  in  post- 
Cretaceous  time. 

The  rocks  consist  of  conglomerate  and  shale.  The  conglomerate  in  the  occurrences  known 
overlies  iron-bearing  rocks  and  in  some  places  iron  ore.     As  woukl  be  expected,  therefore,  the 


MESABI  IRON  DISTRICT.  179 

fragments  of  the  conglomerate  are  (.lerivcd  from  the  iron-bearing  formation;  in  the  western 
part  of  the  range  the  conglomerate  is  locally  rich  enough  to  mine.  The  conglomerate  fragments 
consist  mamly  of  heavy  ferruginous  chert  and  iron  ore,  both  hematite  and  limonite.  Except 
locally,  and  especially  where  the  pebbles  are  of  hard  material,  they  are  not  well  rounded.  There 
are  present  in  the  conglomerate  also  fossils  which  are  described  below.  The  fragments  are  but 
loosely  cemented.  When  broken  out  of  the  ledge  the  rock  is  fairly  compact,  but  on  being  exposed 
to  weathering  it  soon  disintegrates.  The  cement  is  largely  ferruginous,  but  there  is  present  also 
a  considerable  amount  of  white  or  yellow  substance  which  A.  T.  Gordon,  chemist  of  the  Mountain 
Iron  mine,  found  to  consist  of  silica  and  alumina  and  hence  to  be  essentially  a  clay.  Occasion- 
ally there  may  be  observed  also  minute  greenish-yellow  particles  in  the  cement  which  may  be 
glauconite  grains,  so  common  in  the  Cretaceous.  Analyses  disclose  abundant  phosphorus. 
The  general  appearance  of  tliis  Cretaceous  iron-ore  conglomerate  is  very  like  that  of  "canga" 
or  rubble  ores  formed  subaerially  on  the  surface  of  iron  formations  in  the  Minas  Geraes  district 
of  Brazil. 

The  shales  are  soft,  thm-bedded  rocks  of  a  bluish-gray  color  when  fresh  but  in  many  places 
are  of  a  light  color  due  to  bleaching.     These,  too,  contain  fossils. 

Fossils. — Selected  specimens  of  the  shale  and  conglomerate  containing  fossils  were  sub- 
mitted to  T.  W.  Stanton,  paleontologist,  of  the  United  States  Geological  Survey,  for  examina- 
tion. He  pronounced  them  to  be  "unquestionably  Upper  Cretaceous  forms,  not  older  than  the 
Benton  and  probably  not  younger  than  the  Pierre." 

In  addition  to  the  fossils  above  noted,  the  Cretaceous  of  the  Mesabi  district  has  been  found 
to  contain  small  shreds  of  lignitic  material.  The  presence  of  this  material  well  up  on  the  Mesabi 
range  suggests  the  possibility  of  finding  lignite  deposits  in  the  low  area  to  the  west,  north,  or 
south  of  the  Mesabi  range. 

PLEISTOCENE  GLACIAL  DEPOSITS. 

The  Mesabi  district  is  covered  by  a  mantle  of  glacial  drift,  of  the  late  Wisconsin  epoch,  which 
effectually  conceals  the  greater  part  of  the  imderlymg  rocks.  On  lower  slopes  the  drift  is  thick, 
sometimes  reaching  150  to  200  feet,  and  here  of  course  rock  exposures  are  rare:  on  middle  slopes 
the  thickness  commonly  does  not  exceed  50  or  60  feet,  and  20  to  50  would  measure  much  of  it;  on 
the  upper  slopes  of  the  range  the  drift  is  thin  or  altogether  lacking  and  rock  exposures  are  corre- 
spondingly abundant.  In  the  eastern  portion  of  the  district  also,  where  the  Giants  Range 
granite  has  a  higher  elevation  than  to  the  west,  the  drift  is  thm  and  allows  numerous  rock  masses 
to  project  through;  toward  the  west,  as  the  elevation  of  the  Giants  Range  decreases,  the  drift 
becomes  thicker,  until  westward  from  Grand  Rapids  it  buries  even  the  crest  of  the  Giants  Range 
to  a  depth  of  more  than  100  feet. 

The  Pleistocene  deposits  are  fully  discussed  in  Chapter  XVI  (pp.  427-459). 

THE  IRON   ORES   OF  THE   MESABI  DISTRICT. 

1 
By  the  authors  and  W.  J.  Mead.  . 

DISTRIBUTION,  STBTJCTTJRE,  AND  RELATIONS. 

The  iron-bearing  Biwabik  formation  rests  on  the  middle  south  slope  of  the  Giants  Range, 
with  a  low  dip  to  the  south,  4°  to  20°,  affording  an  exposure  of  considerable  width  at  the  surface. 
The  elevation  of  this  exposure  varies  between  1,400  and  1,600  feet.  The  distribution  of  the 
Biwabik  formation  and  the  possibilities  of  westward  extension  are  discussed  on  pages  164-165. 
Possibilities  of  extension  southward  are  mentioned  on  page  174.  The  ore  bodies  are  in  patches 
along  the  erosion  surface  of  the  iron-bearing  formation,  generally  less  than  200  feet  thick,  but 
reaching  500  feet  at  greatest. 

The  aggregate  area  of  all  the  iron-ore  deposits  of  present  commercial  grade  known  at  this 
writing  at  the  surface  is  about  15  square  miles,  constituting  a  little  less  than  8  per  cent  of  the 
exposed  surface  of  the  iron-bearing  formation  in  its  productive  portion  between  the  east  line  of 


180 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


> 


4 


i\i 


H 


X 


1^ 


range  14  on  the  east  and  west  side  of  nintje  26  on  tlie  west.     If  low-grade  ores  were  counted  the 

area  would  be  approximatelj'  douMcd, 

East  of  ran^c  14  the  nature  of  the  formation  is  influenced  by  tlic  great  Keweenawan  Duluth 

gabbro  mass  overlying  the  east  end  of  the  district.  The 
ore  bodies  are  few  and  snuill  and  are  more  largely  niagnet- 
itic  and  amphibolitic  than  hematitic.  Toward  the  west  end 
of  the  district  also  the  ores  become  lower  in  grade,  owinf 
to  increasing  content  of  loosel}'  disseminated  chert,  locally 
called  sand,  so  abundant  in  certain  of  the  ores  that  they 
require  washing  to  attain  the  present  commercial  grade. 

The  rocks  immediiitely  associated  with  the  ores  are 
mainl}^  ferruginous  cherts,  locally  called  "taconite, "  form- 
ing both  the  walls  and  basements  of  the  deposits.  The 
ores  usually  do  not  rest  directly  upon  the  quartzite  under- 
lying the  iron-bearing  formation.  Their  lower  hmits  are 
locally  marked  by  thin  layers  of  paint  rock  a  few  inches 
thick.  A  horizontal  plan  of  the  Mesabi  ore  dej>osits  is 
exceedingly  irregular  both  in  major  outline  and  in  minor 
features.  The  deposits  are  in  many  places  bounded  hj 
intersecting  plane  surfaces  of  joint  or  fault  planes.  In 
A^ertical  section  the  ore  deposits  in  general  are  widest  at  the 
top  and  narrow  below,  in  the  form  of  a  shallow  basin.  The 
slopes  of  the  basin  are  rarely  symmetrical  and  few  slopes  are 
uniform;  a  slope  is  generally  a  series  of  steps,  some  of  them 
overhanging  the  ore  or  projecting  into  it.  The  bedding 
of  the  iron  ores  is  continuous  with  that  of  the  adjacent 
ferruginous  cherts  of  the  iron-bearing  formation  except 
where  there  has  been  local  slump  or  faulting  at  the  contact. 
The  shunp  is  sometimes  accompanied  by  close  crumpling 
of  the  layers  of  the  iron-bearing  formation  (PI.  IX).  It 
will  be  shown  later  that  the  slump  results  from  the  leaching 
of  silica.  Obviously  the  layers  have  been  originally  too 
long  for  their  present  positions  and  have  crumpled  to  ac- 
commodate themselves  to  the  new  conditions.  The  bed- 
ding of  the  ores  is  thus  essentially  parallel  to  that  of  the 
upper  Huronian  of  this  district — that  is,  sloping  gently  south- 
ward at  angles  from  4°  to  20°,  with  minor  gentle  folds  whose 
axes  pitch  in  that  direction.  A  good  general  conception  of 
the  structural  relations  of  the  Mesabi  ores  may  be  obtained 
by  thinking  of  the  ores  as  irregular  rotted  upper  portions  of 
the  slightly  tilted  and  beveled  iron-bearing  formation,  the 
rotting  having  been  favored  in  certain  spots,  as  will  be  shown 
later,  by  the  fracturing  of  the  formation  or  by  the  minor 
folds  in  which  the  formation  rests.  (See  fig.  16;  PI.  X, 
which  is  a  north-south  cross  section;  and  PI.  XI.) 


a  a 

a  a 


I 


CHEMICAL  COMPOSITION  OF  FERRUGINOUS  CHERTS 
AND  ORES. 

AX.VLTSES. 

of  theores  and  related  rocks  is 


«  The  chemical  composit  ion 

g        here  exhibited  by  partial  and  comi)lcte analjses from  vari- 

'^'"'"""°'~'"  ous  sources.      A  large  number  of  the  analyses  employed 

were  kindly  furnished  ])V  the  several  mining  companies.     .\.ll  the  other  analyses  except  those 

previously  published    were  made   hj  Lerch  Brothers  in  their  laboratories   at   Ilibbing   and 


U.  S.  GEOLOGICAL  SURVEY 


MONOGRAPH   Lll     PL.  IX 


*^. 


s. 


>^, 


'm^^^ 


^■ijf«y',^^.-*;:;f-:.v.s_ 


A.     HAWKINS     MINE. 


B.     MONROE    MINE. 


SHARP   FOLDING   OF   BEDS  OF   IRON-BEARING    BIWABIK    FORMATION    IN 

MESABI    DISTRICT,    MINN. 

See  page  180. 


U.  8.  GEOLOGICAL  SURVEY 


MONOGRAPH   LM      PL.  X 


5BN..n.20E.    ,     Sec  2e,T,  53N.,R.E0E. 


Datum  9(J0ft  aboy  LakeSiipenof 


■.^-  .   _     -        -^^  -.Sfe^i 


^ 


Decoinpoaedlaconile  Tacoiiile 

(uaJDt  rock)  (tl^composedat 

(with  rurruffinous  slate  and  points  maihed  Ct) 
3om^green  alats  ut  h) 


NORTH-SOUTH    CROSS    SECTION    THROUGH     IRON-BEARING    BIWABIK     FORMATION,     MESABI     DISTRICT,     MINNESOTA. 

Compiled  by  0.  B.  Warren  from  drill  records. 


U.  S.  GEOLOGICAL  SURVEY 


MONOGRAPH   LI1     PL.  Xr 


PANORAMIC    VIEW    OF    THE    MOUNTAIN     IRON     OPEN-PIT     MINE,     MESABI     DISTRICT,     MINN. 
Looking  east.      From  photograph  presented  by  J.  F.  Lindberg,  Hibbing,   Minn-     See  pages  180,  497. 


B.     PANORAMIC    VIEW    OF    THE    SHENANGO    IRON     MINE,     MESABI     DISTRICT,     MINN. 

See  pages  180,  497. 


MESABI  IRON  DISTRICT. 


181 


Virginia,  Minn.     The  average  cargo  analyses  for  the  various  grades  of  ore  were  obtained  from 
the  hst  pubHshed  by  the  Lake  Superior  Iron  Ore  Association. 

Nine  tj'jjical  analj'ses  of  taconite  are  given  in  the  followmg  table.     These  analyses  include 
carefully  selected  samples  from  several  drill  holes  giving  complete  sections  through. the  formation. 

Partial  anali/sts  of  ferruginous  chert  {laconilf)  from  the  Memhi  range. 
(Samples  dried  at  212°  F.I 


La  Rue  mine,  see.  29,  T.  57  N.,  R.  22  W 

Stevenson  mine,  sees.  7  and  8,  T.  67  N. ,  R.  21  W 

Crosby  mine,  sec.  32,  T.  57  N.,  R.  21  W 

Do 

Drill  core  from  three  holes  in  T.  57  N.,  R.  22  W.,  in  all  800  feet 

Drill  core,  30) feet 

La  Rue  mine,  sec.  29,  T.  67  N.,  R.  22  W 

Burt,  mine,  see.  31 ,  T.  6S  N . ,  R.  20  W 

Do 

Average 


32.24 
24.99 
11.79 
19. 5(i 
30.24 
23.80 
32.26 
23. 98 
32.62 


25.71 


SiOj. 


68.70 


P. 


0.021 
.024 
.010 
.013 
.038 
.030 
.018 
.013 
.020 


.021 


-MiO.. 


Loss  on 
ignition. 


0.37  i 

.21 

.29  I 

.%i 

.84 
1.20 

.30 

.91 

.42 


..54 


0.62 

.60 

.67 

.25 

5.16 

7.52 

.45 

1.33 

1.07 


1.96 


The  large  loss  on  ignition  in  tlie  drill-core  samples  is  in  part  due  to  the  presence  of  CO2  in 
carbonates.  The  samples  represent  the  hard  phases  of  the  formation,  showing  little  concentration 
to  ore.  When  all  of  the  iron-bearing  formation  outside  of  the  available  non-ore  deposits  is  aver- 
aged, including  both  the  hard  lean  parts  shown  in  the  above  table  and  the  partly  concentrated 
portions  of  the  formation,  the  average  iron  content  runs  higher.  An  average  of  1,094  analyses, 
representing  5,400  feet  of  drilling  in  the  district  away  from  the  available  ores,  gives  38  per 
cent.  This  does  not  include  the  ores.  Because  of  the  great  mass  of  such  rocks  as  compared 
with  the  ores,  this  figure  of  38  per  cent  represents  approximately  the  general  average  u'on 
content  of  the  entire  formation. 

The  average  composition  of  the  Mesabi  ore  for  the  years  1906  and  1909  was  obtained  by 
combining  average  cargo  anah'ses  of  all  grades  mined  for  each  of  those  j^ears  in  proportion  to 
the  tonnage  represented  by  each  grade.  In  this  manner  an  average  analysis  was  obtained 
which  represents  as  exactly  as  possil)le  the  composition  of  all  of  the  ore  mined  in  the  Mesabi 
district  during  the  years  1906  and  1909. 

Average  composition  of  all  ore  mined  in  the  Mesabi  district  during  the  years  1906  and  1909. 


Moisture  (loss  on  drying  at  212°  F.). 
Analysis  of  dried  ore: 
Iron. 


Phosphorus. 

Silica 

Manganese . . 
Lime. 


Alumina.. 
Magnesia. 


Sulphur. 

Loss  on  ignition . 


60.70 

.0559 
5.58 


1.58 
'4.' 57" 


1909. 


12.27 


58.83 
.062 

6.80 
.816 
.32 

2.23 
..32 
.  069 

4.72 


Range  in  composition  of  ores  mined  in  the  Mesabi  district,  as  shown  by  average  cargo  analyses  for  1909. 
Moisture  (loss  on  drying  at  212°  F.) 7.  15     to  15.  79 


Analysis  of  dried  ore: 

Iron 52.  40    li .  64.  05 

Phosphorus 019  to     .105 

Silica 2.50    to  19. 90 

Manganese 20    to    2.  84 

Alumina 16    to    5.67 

Lime 0         to    1.82 

Magnesia 0         to    2. 06 

Sulphur 004  to      .440 

Loss  on  ignition 1.  71    to    9. 45 


182 


GEOLOGY  OF  THE  LAKE  SUPEKTOI!  I!EGTOX. 


KEPRESENTATION    BY    MEANS    OF    TRIANGITLAK    DIAGRAM. 

In  figure  17  the  triangular  method  of  phitting  is  employed  to  show  the  chemical  cf)mposi- 
tion  of  the  various  phases  of  taconite  and  ore  studied.  Here  actual  percentage  weights  of  the 
constituents  are  indicated,  and  no  account  is  taken  of  volume  or  porosity.  Each  point,  by  its 
position  in  the  triangle,  indicates  an  individual  analysis.  The  diagram  consists  of  an  equilateral 
triangle  crossed  by  equally  spaced  lines,  100  parallel  to  eacii  side.  Distances  measured  i)er- 
pendicularly  from  the  three  sides  to  any  point  within  the  triangle  (by  means  of  the  divisions  in 
the  triangle)  represent  severally  percentages  of  ferric  oxide,  silica,  and  the  remaining  constit- 

FERRIC  OXIDE 


MINOR  SILICA 

CONSTITUENTS 
Principally  alumina  and 
water  of  hydration 

Figure  17.  — Trian;ular  diagram  showing  composition  of  various  phases  of  Mesabi  ores  and  ferruginous  cherts  in  terms  of  ferric  oxide,  silica,  and 
minor  constituents  (essentially  alumina  and  combined  water).  The  ores  and  cherts  here  represented  are  shown  in  flguro  21  in  terms  of 
percentage  volumes  of  iron  minerals,  silica,  and  pore  space. 

uents.  Thus  any  point  in  the  triangle  indicates  a  certain  definite  combination  of  these  three 
factors.  The  grouping  of  the  points  in  the  triangle  shows  that  the  principal  variation  in  com- 
position lies  between  the  iron  and  the  silica.  In  the  process  of  concentration  of  ore  from  the 
ferruginous  chert  the  percentage  of  iron  increases  in  proportion  to  the  decretisc  in  sUica,  while 
the  percentage  of  minor  constituents  remains  practically  constant;  hence  this  concentration 
would  be  represented  by  a  series  of  j)()ints  in  a  line  parallel  to  the  right-hand  side  of  the 
triangle.  A  taconite  with  a  higii  content  of  alumina  produces  an  ore  high  in  kanlin.  tmd 
conversely. 


MESABI  IRON  DISTRICT. 


183 


MINEBALOGICAL  COMPOSITION  OF  FERRUGINOUS  CHERTS  AND  ORES. 

Mineralogicall}'  both  the  cherts  and  the  ores  consist  essentially  of  hydrated  oxides  of  iron, 
chert,  or  quartz,  aluminum-bearing  minerals,  usually  kaolin,  and  a  small  amount  of  minor 
constituents.  In  the  calculation  of  the  approximate  mineral  composition  of  the  various  rocks 
and  ores  these  minor  constituents — alkalies,  sulphur,  phosphorus,  etc. — were  disregarded,  the 
error  thus  introduced  being  small.  The  iron  is  present  in  the  ores  and  cherts  as  a  partly 
ly'drated  ferric  oxide.  To  ascertain  in  each  case  the  particular  hydrated  iron-oxide  mineral 
present  would  be  impracticable,  but  by  calculating  the  iron  as  hematite  and  limonite  the 
degree  of  hydration  is  expressed  by  relative  amounts  of  the  two  minerals.  The  amount  of 
limonite  is  found  by  assigning  to  the  volatile  matter  or  water  of  hydration  available  the  proper 
amount  of  iron,  the  remainder  of  the  iron  being  calculated  as  hematite.  The  practice  of  assign- 
ing to  the  iron  mineral  all  the  water  of  hydration  not  in  aluminum  silicates  may  introduce  minor 
inaccuracies  because  of  the  possible  slight  hydration  of  the  chert. 

The  mineralogical  compositions  of  the  ores  and  ferruginous  cherts  of  the  Mesabi  range 
calculated  from  the  average  analysis  by  the  methods  describetl  above  are  as  follows: 

Approximate  mineral  compositions  of  average  ores  and  ferruginous  cherts. 


Ferrugi- 
nous 
cherts. 

Ores. 

1906. 

1909. 

Hematite  .     . 

26.30 
12.22 
58.07 
1.37 
2.04 

as.oo 

27.00 
4.10 
4.08 
1.82 

61.81 

Limonite 

25.95 

Quartz                                               .                                                                                                                    ..  .- 

'4.10 

5.30 

Miscellaneous                                                                                                                    .                  .                

2.84 

100.00 

100.00 

100.00 

PHYSICAL  CHARACTERISTICS  OF  THE  ORES. 


TEXTURE. 


The  Mesabi  iron  ores  are  for  the  most  part  soft,  somewhat  hydrated  hematite,  though 
approximately  pure  limonite  ores  are  present  in  subordinate  quantity.  The  ores  as  a  whole 
are  of  finer  texture  than  those  of  any  other  Lake  Superior  district.  Their  texture  varies  from 
exceedingly  fine-grained  "flue  dust"  to  a  fairly  coarse,  hard,  and  granular  ore  breaking  into 
parallelepiped  blocks.  Usually  the  ore  needs  but  little  blasting  to  allow  the  steam  shovel  to  take 
it  from  the  bed.  The  average  texture  of  the  Mesabi  ores  is  shown  by  the  following  table,  repre- 
senting an  average  of  screening  tests  on  eight  grades  of  typical  Mesabi  ore  totaling  18,313,570 
tons  in  1909.  These  screening  tests  were  made  by  the  Carnegie  Steel  Company  and  represent 
the  total  3'ear's  output  of  each  of  the  grades  tested.  The  textures  of  the  ores  of  the  several 
Lake  Superior  districts  are  compared  in  figure  72  (p.  4S1). 

Textures  of  Mesabi  ores  as  shown  by  screening  tests. 

Per  cent. 

Held  on  J-inch  sieve 25.  98 

^-inch  sieve 26.  24 

No.  20  sieve 11.  54 

No.  40  sieve 9.  90 

No.  60  sieve 8.  54 

No.  80  sieve 2. 16 

No.  100  sieve 2.  28 

Passed  through  No.  100  sieve 13.  68 

The  fineness  of  many  of  the  ores  has  required  mixture  with  coarser  grades  for  blast-furnace 
charges.  The  average  mixture  is  approximately  indicated  by  the  proportions  of  Mesabi  to 
other  Lake  Superior  ores,  which  has  increased  to  69  per  cent  in  1910. 


184 


GEOLOGY  OF  THE  LAKE  SUPEKlOll  KEGIO.X. 


DENSITY. 


Several  methods  were  employed  in  the  determination  of  density — (1)  determination  of 
density  of  finely  powdered  specimen  by  means  of  specific-gravity  bottle;  (2)  determinations  of 
density  from  hand  specimens  by  the  common  metliod  of  weighing  in  air  and  in  water,  the 
pores  of  the  rock  being  filled  with  water  by  prolonged  boiling  before  weighing  under  water; 
(3)  calculation  of  specific  gravity  of  tlie  rock  or  ore  from  mineral  composition  by  proper  combina- 
tion of  tlie  thaisities  of  the  several  minerals  present.  The  density  of  the  ores  or  cherts  calcu- 
lated l)y  using  the  density  of  the  iron  minerals  given  by  Dana  was  uniformly  liigher  than  the 
density  iound  by  gravity  methods.  The  iron  minerals  in  an  earthy  form  have  a  lower  density' 
than  those  in  the  hard  ores,  and  it  was  found  that  the  two  methods  could  be  made  to  agree  by 
assigning  to  hematite  a  density  of  4.5  and  to  limonite  one  of  3.6. 

By  combining  the  specific  gravities  in  proportion  to  the  percentages  of  the  minerals  the 
average  density  of  the  ferruginous  cherts  is  found  to  be  3.27. 

Actual  density  determinations  on  eleven  specimens  of  ferruginous  cherts  gave  an  average 
of  3.02.  (See  table  below.)  This  figure  is  lower  than  the  average  figure  computed  above, 
for  two  reasons:  The  eleven  specimens  on  which  the  detenninations  were  made  contained  a 
smaller  percentage  of  iron  than  the  average  analysis  above.  The  close  texture  of  the  specimens 
prevented  complete  saturation  by  immersion  in  water  and  also  prevented  complete  drying; 
hence  both  density  and  porosity  determinations  are  somewhat  lower  than  they  should  be. 
For  these  reasons  it  is  believed  that  the  specific  gravity  as  calculated  from  the  average  anah^sis 
above  (3.27)  represents  most  closely  the  average  specific  gravity  of  the  taconite. 

The  average  specific  gravity  of  the  ore,  as  calculated  from  the  mineralogical  composition 
of  the  average  ore,  is  fountl  to  be  4.10. 

POROSITY. 

In  all  rocks  and  ores  of  which  hand  specimens  could  lie  collected  the  porosity  was  deter- 
mined by  comparing  the  weight  of  the  specimen  when  saturated  with  water  with  its  weight 
when  dried.     This  manner  of  determination  is  formulated  as  follows: 


Weight  of  water  absorbed 


Weight  of  rock  when  saturated 
Porosity  = 


=  moisture  of  saturation  =  M. 


M 

1  -M 


G 


+  M 


where  G  equals  specific  gravity.     From  this  formula  it  is  obvious  that  a  determmsition  of 
density  is  necessary  in  connection  with  each  porosity  determination. 

The  porosity  determinations  on  eleven  specimens  of  ferruginous  chert  by  tlie  method 
described  follow. 

Porosity  detenninations  of  chert. 


Specimen  No. 


44051. 
45S88 
45309 
4S305 
40651 
4.5021 
45596 


Specific 
gravity. 


3.25 
3.04 
2.86 
2.88 
2.90 
3.22 
2.92 


Porosity 
(per  cent 
of  total 
volume). 


6.5 

2.3 

9.45 

5.1 

6.25 

6.00 

3.75 


Specimen  No. 


45603 

45692 

4,5672.\ 

45.590 

Average 


Specific 
gravity. 


2.80 
2.87 
2.96 
3.07 


Poro*:ity 
(percent 
of  total 
volume). 


3.02 


3.50 
.3.80 
6.45 
3.55 


4.72 


To  unconsolidated  material,  such  as  a  large  part  of  the  Mesabi  ores,  the  above  method  could 
not  be  applied.  The  porosity  of  such  material  was  found  by  comparhig  its  actual  density 
when  in  place,  including  jiore  space,  with  the  calculated  mineral  diMisity,  which  does  not  include 
pore  space.     The  actual  density  of  the  material  in  ])lace  was  detcrmmed   by  weighing  the 


MESABI  IRON  DISTRICT.  185 

amount  removed  from  an  excavation  made  on  a  leveled  surface  of  the  ore,  the  volume  of  the 
excavated  material  being  determined  by  measuring  the  amount  of  grain  necessary  to  fill  the 
excavation.  Another  method  for  the  determination  of  cubic  content  of  the  Mesabi  ores  is  one 
employed  by  O.  B.  WaiTcn,  of  Hibbing,  Minn.  Mr.  Warren  used  a  bottomless  box  4  feet 
long,  3  feet  wide,  and  1  foot  deep.  These  dimensions  were  chosen  as  representing  the  average 
volume  of  a  ton  of  ore.  This  box  is  set  up  on  a  leveled  surface  and  the  ore  removed  from  the 
inside  of  the  box  until  the  sides  are  sunk  to  the  level  of  the  surface.  In  this  way  exactly  12 
cubic  feet  of  ore  are  removed  and  weighed,  a  sample  for  analysis  being  taken  at  the  same  time. 

The  porosity  of  the  ore  may  also  be  determined  by  saturating  a  portion  in  place  by  an 
abundant  application  of  water.  Placing  a  sample  of  the  saturated  material  immediatelv  in  a 
closed  vessel  permits  the  determination  of  the  moisture  of  saturation,  from  which  the  porosity 
may  be  calculated  as  shown  above.  Where  the  ore  to  be  tested  is  in  a  vertical  wall  a  small 
niche  should  be  cut  to  afford  a  horizontal  surface  for  the  application  of  the  water.  It  will  be 
seen  that  this  method  does  not  differ  essentially  from  the  determination  of  porosity  of  hand 
specimens,  except  that  the  material  is  saturated  in  place  and  not  after  removal  from  the  ground. 

More  than  100  determinations  by  the  various  methods  show  the  average  porosity  of  the  ore 
to  be  approximately  40  per  cent  of  the  volume.     (See  fig.  21,  p.  190.) 

CUBIC    CONTENTS. 

Owing  to  the  wide  variation  in  the  three  essential  factors,  densit}',  porosity,  and  moisture, 
there  is  a  wide  variation  in  the  number  of  cubic  feet  per  ton  of  the  ores.  This  number  ranges 
from  9  cubic  feet  per  long  ton  in  some  of  the  highest-grade  blue  granular  ores  to  17  or  18  in 
the  low-grade  limonites.  The  average  for  the  district  is  approximately  12  cubic  feet  per  long 
ton.     The  method  of  calculation  is  discussed  on  pages  480-484. 

MAGNETIC  PHASES  OF  THE  IBON-BEABING  FORMATION. 

OCCURRENCE.  • 

Eastward  from  the  town  of  Mesaba,  on  the  Duluth  antl  Iron  Range  Railroad,  the  iron- 
bearing  Biwabik  formation  becomes  progressively  more  magnetic,  more  coarsely  crystalline, 
and  the  red  or  bro\viiish  tones  of  the  ferruginous  cherts  give  way  to  black  and  gray  colors. 
Ore  deposits  are  rare.  Such  as  there  are  consist  of  mixtures  of  liematite  and  magnetite.  In  the 
most  magnetic  and  crystalline  parts  of  the  formation  ore  deposits  seem  to  be  entirely  lacking. 
In  addition  to  the  magnetite  and  tjuartz,  there  are  present  various  anhydrous  silicates,  such  as 
griinerite,  actinolite,  augite,  and  others.  The  parts  of  the  formation  rich  in  magnetite  are 
concentrated  into  definite  layers  a  few  inches  to  a  few  feet  in  thickness  and  interlayered  with 
layers  less  rich  in  magnetite.  Mining  would  require  not  only  hand  sorting  but  presumably  also 
crushing  and  magnetic  concentration. 

CHEMICAL    COMPOSITION. 

The  chemical  composition  of  the  amphibole-magnetite  rock  is  about  the  same  as  the  average 
of  the  iron-bearing  formation  elsewhere  in  the  Mesabi  district  outside  of  the  iron-ore  deposits, 
as  is  shown  by  the  following  average : 

Average  chemical  composition  of  amphibole-magnetite  rock  in  the  Mesabi  district- 

SiO, 60.  51 

AUO3 ] .  20 

Fe 25.  22 

MgO 52 

CaO 67 

HoO Small. 

P2O5 05 

S 59 

MnOo 92 

TiOj None. 


186 


GEOT.OGY  OF  THE  LAKE  SUPERIOR  REGION. 


The  reasons  for  the  hick  of  concentration  of  ore  in  this  part  of  tlie  formation  are  discussed 
on  page  553. 

SECONDARY  CONCENTRATION  OF  MESABI  ORES. 
STRUCTURAL    CONDITIONS. 

In  the  Mesabi  district  waters  faihng  on  the  south  slope  of  the  Giants  Range  have  flowed 
southward,  entered  the  eroded  edges  of  the  slightly  tilted  Huronian  series,  and  flowed  through 
the  iron-I)caring  formation,  following  both  bedding  and  joint  planes.  There  are  genth'  pitching 
rolls  in  the  formation,  but  they  are  so  light  that  their  control  of  the  circulation  is  small  as  com.- 
pared  with  that  of  the  bedding  and  joints.  The  result  is  the  extreme  irregularity  in-  the  shape 
and  distribution  of  the  Mesabi  ore  deposits. 

On  the  south  the  iron-bearing  formation  is  overlain  by  slate.  The  percolating  waters  un- 
doubtedly permeate  the  iron-bearing  formation  beneath  the  slate,  but  it  is  altogether  likely 
that  there  they  are  ponded  and  have  a  relatively  slow  movement.  Drill  holes  put  down  through 
the  slate  into  the  iron-bearing  formation  occasionally  meet  water  under  artesian  pressure. 
The  principal  zone  of  escape  doubtless  is  the  north  edge  of  the  slate — that  is,  the  water  over- 
flows to  the  surface  before  passing  far  under  the  slate  (fig.  18).  Tliis  doubtless  explains  the 
comparative  lack  of  alteration  of  the  iron-bearing  formation  or  the  existence  of  ore  deposits  far 
under  the  Virginia  slate. 

The  ponding  effect  of  the  slate  also  probably  aids  in  diminishing  any  possible  effect  which 
the  southward-pitching  synclines  in  the  iron-bearing  formation  might  have  on  the  localization 
of  the  ores,  for  the  reason  that  near  the  slate  flowage  of  water  is  controlled  by  the  point  of  escape 
at  the  edge  of  the  slate  rather  than  by  the  configuration  of  the  basin  in  which  it  might  othei-wise 


Iron-bearing  forma,tion 


FlQUBE  18.— Section  through  iron-bearing  Biwabik  formation  transverse  to  the  range,  showing  nature  of  circulation  of  water  and  its  relations  to 

confining  strata. 

flow,  and  this  point  of  escape  may  be  higher  than  the  anticlines  in  the  basement,  thus  ahowing 
the  waters  to  flow  equally  well  over  anticlines  and  synclines  in  the  basement. 

The  impervious  basement  in  the  Mesabi  district  is  usually  some  laj^er  in  the  iron-bearing 
formation  itself,  commonly  a  shaly  layer  which  has  subsequently  been  altered  to  paint  rock. 
In  no  place  does  the  ore  rest  directly  upon  the  underlying  quartzite. 

The  greatest  depth  of  the  Mesabi  ore  deposits  must  be  less  than  the  depth  of  the  iron-bear- 
ing formation,  and  as  the  greatest  thickness  of  the  formation  is  only  near  the  slate  margin,  where 
the  waters  are  escaping  and  are  not  doing  their  best  work,  it  follows  that  the  ore  deposits  are  not 
likely  to  reach  this  maximum  depth.  The  greatest  depth  thus  far  known  in  the  Mesabi  range 
is  500  feet.     The  common  depths  ai-e  less  than  300  feet. 

The  Giants  Range  furnishes  the  head  for  the  percolating  waters.  Toward  the  west  end 
of  the  district  the  range  becomes  lower  and  the  grade  of  the  ore  becomes  correspondingly  lower, 
suggesting  that  the  circulation  of  the  ore-concentrating  solutions  was  less  vigorous  at  the  west- 
ern end  because  of  the  lower  elevation.  The  ores  have  no  close  relation  to  the  minor  hills  on  the 
Giants  Range  slope,  though  they  tend  to  occur  in  the  depressions,  principally  because  in  such 
places  denudation  is  relatively  deep  owing  to  softness.  Were  it  not  for  the  irregular  covering 
of  glacial  drift,  their  relations  to  minor  valleys  would  be  more  apparent. 

ORIGINAL    CHARACTER    OF    THE    IRON-BEARING    FORMATION. 

The  iron-bearing  Biwabik  formation  originally  consisted  dominantly  of  greenalite  rocks  and 
subordinately  of  cherty  iron  carbonate,  the  characters  of  wliich  are  described  on  pages  165-170. 


MESABI  IRON  DISTRICT.  187 

The  alteration  of  these  rocks  to  the  ore  has  been  accompHshed  in  two  stages,  mainly  successive 
but  partly  overlapping — first,  by  alteration  to  ferruginous  chert;  second,  by  leaching  of  silica 
from  the  ferruginous  chert. 

ALTERATION  OF  SIDERITIC  OR  GREENALITIC  CHERT  TO   FERRUGINOUS   CHERT    (TACONITE)  . 

Chemical  change. — Tiie  chemical  change  consists  of  oxidation  of  the  iron  according  to  the 
following  reactions : 

For  greenalite — 

2FeSi03.nH,0  +  O  =  Fe  AnH,0  +  2SiO,  ±  H^O. 
For  siderite — 

2FeC03  +  nH,0  +  O  =  Fe^Oj.nH.O  +  2C0,. 

Mineral  change. — The  greenalitic  cherts  or  greenalite  rocks  are  composed  essentially  of 
rounded  granules  of  greenalite  in  a  matrix  of  chert.  The  tendency  to  banding  is  not  as  distinc- 
tive as  in  the  cherty  iron  carbonates.  The  greenalite  alters  to  hydrated  iron  oxide.  The  silica 
remains  or  goes  out.  Mineralogically  the  sideritic  cherts  consists  essentially  of  siderite  and  chert 
more  or  less  segregated  into  alternate  layers.  The  siderite  is  changed  to  hydrated  iron  oxide. 
Either  removal  or  retention  of  silica  may  accompany  this  change. 

Secondary  siderite,  usually  differing  from  original  siderite  in  having  coarser  grain,  is  a 
minor  product  of  alteration  of  both  greenalitic  and  sideritic  cherts. 

VoluTne  cJuinge. — Though  the  alteration  is  distinctly  of  a  katamorphic  nature,  the  change 
is  from  a  light  to  a  denser  mineral,  and  hence  involves  a  reduction  in  the  volume  of  the  iron 
mineral.  Like  the  oxidation  of  the  siderite,  the  oxidation  of  the  greenalite  involves  a  change 
from  a  lighter  to  a  denser  iron  mineral  and  a  decrease  in  the  volume.  The  volume  changes 
involved  in  the  above  alterations  are  as  follows: 

Alteration  of  siderite  to  hematite,  49.25  per  cent  loss. 

Alteration  of  siderite  to  limonite,  18.30  per  cent  loss. 

Alteration  of  greenalite  to  hematite  and  quartz,  24.50  per  cent  loss. 

Alteration  of  greenalite  to  limonite  and  quartz,  9  per  cent  loss. 

As  the  chert  is  at  first  unchanged  in  the  alteration  of  the  greenalite  and  carbonate  to  iron 
oxide,  the  volume  change  accompanying  these  alterations  is  effective  on  only  a  portion  of  the 
rock.  Chemical  analyses  of  both  the  sideritic  cherts  and  the  greenalitic  rocks  show  that 
approximately  60  per  cent  of  their  volume  is  chert.  Hence  the  change  in  volume  is  effective 
on  only  40  per  cent  of  the  total  volume  of  the  rock.  The  loss  in  volume,  then,  for  the  entire 
rock,  taking  into  account  both  the  iron  and  the  sUica,  ranges  from  3.6  per  cent  to  19.7  per 
cent,  according  as  the  original  rock  bore  siderite  or  greenalite  and  according  to  the  degree  of 
hydration  of  the  resulting  product. 

Development  of  porosity. — This  volume  change,  due  to  oxidation  of  greenalite  or  siderite, 
develops  pore  space.  Determinations  of  porosity  on  eight  typical  .specimens  of  greenalitic 
rock  and  sideritic  chert  showed  the  average  porosity  to  be  0.96  per  cent  of  the  volume  of  the 
rock.  An  average  of  twelve  determinations  on  type  specimens  of  ferruginous  chert  (taconite), 
from  which  apparently  no  silica  had  been  leached,  gave  a  porosity  of  4.72  per  cent.  The 
porosity  resulting  from  the  reduction  in  volume,  due  to  the  oxidation  of  greenalite,  in  a  rock 
containing  40  per  cent  by  volume  of  that  mineral  should  be  9.8  per  cent  of  the  volume  of  the 
rock  when  the  product  is  hematite  and  3.6  per  cent  when  the  product  is  limonite.  The  ratio 
of  hematite  to  limonite  in  the  average  taconite  is  about  three  parts  of  hematite  by  volume 
to  two  of  limonite;  hence  the  porosity  resulting  from  the  alteration  of  average  greenalite  rock 
to  average  ferruginous  chert  should  be  approximately  7.3  per  cent  of  the  volume  of  the  chert. 
This  figure  does  not  differ  greatly  from  the  observed  porosity  of  the  ferruginous  chert — 4.72 
per  cent.  It  is  to  be  expected  that  the  observed  porosity  would  be  less  than  the  porosity  as 
calculated  above,  for  several  factors,  such  as  cementation  and  mechanical  agencies,  would  tend 
to  close  openings  formed. 


188 


GEOLOGY  OF  THE  LAKE  SUPEKIOll  JJEGIOX. 


ALTERATION    OF    FEllKUGINOUS    CHEKTS    (tACONITE)    TO    ORE. 

The  alteration  of  ferruginous  chert.s  (taconite)  to  ore  consists  essentially  in  removal  of  silica. 
It  has  ah-eady  been  shown  that  the  alteration  of  ferruginous  cherts  to  ores  is  essentially  later 
than  that  of  the  original  greenalite  and  carbonate  rocks  to  ferruginous  cherts. 

During  the  change  from  the  ferruginous  cherts  to  ore  the  iron  oxide  remains  essentially  the 
same  in  absolute  quantity  (not  in  i)ercentage)  and  in  degree  of  hydration,  as  will  apjjear  from 
some  of  the  following  analyses  and  calculations. 

VOLUME    CHANGES. 

At  many  places  in  the  district  the  actual  gradation  from  ferruginous  chert  to  ore  ma}-  be 
observed.  In  the  following  table  are  several  series  of  analyses  showing  this  gradation.  Each 
series  represents  a  series  of  specimens  taken  from  the  same  layer  of  taconite.  In  no  case  were 
the  members  of  one  series  taken  from  an  area  greater  than  2  feet  in  extent,  so  that  approximately 
uniform  original  composition  was  insured  throughout  each  series. 

The  first  member  of  each  series  represents  the  least  altered  phase,  each  successive  member 
of  the  same  series  showing  a  greater  degree  of  alteration. 

Alteration  of/erniginoiis  chert.. 


Chemical  composition. 

.\pproximate  volume  composition. 

Fe. 

SiO,. 

P. 

AljOj. 

Loss  on 
igniiioa. 

Pore 
space. 

Hematite 

+ 
limonite. 

Quartz. 

Kaolin. 

f      29.47 

J      33.01 

1      35.26 

I      48.88 

f      44.33 

1       45. 30 

1       48.51 

49.18 

32.20 

38. 84 

44.49 

52.89 
50.08 
43.44 
23.03 
34.24 
31.05 
20.  42 
23.60 
50.78 
42.69 
34.33 

0.016 
.016 
.013 
.015 
.013 
.014 
.013 
.010 
.018 
.012 
.010 

0.62 
.33 
.40 
.21 
.37 
.23 
.33 
.32 
.30 
.24 
.22 

2.92 

1.65 

4.48 

3.83 

1.81 

2.73 

2.07 

2.64 

.43 

.74 

.69 

S.OO 
16.30 
20.30 
52.70 
38.00 
39.70 
42.40 
43.40 

4.00 
23.20 
24.20 

32.33 
31.23 
33.51 
30.81 
33.12 
34.65 
36.05 
35.70 
33.55 
33.73 
39.89 

57.90 
51.40 
39.30 
•16. 18 
28.10 
25.20 
20.88 
18.25 
61.10 
42.30 
35.35 

1.74 
.93 
.92 
.34 
.77 
.48 
.70 
.05 
.90 
.61 
.57 

Series  3  La  Rue  mine                             

Series  1  was  taken  near  the  top  and  to  one  side  of  the  ore  bod}-;   there  was  apparently  no 
slump,  as  is  shown  by  the  constant  volume  of  the  iron  mineral.     Figure  19  is  a  graphic  repre- 


Pore  space 

"""""^ 

Pore  space 

^— ^ 

Pore  space 

S 

PoKspzce 

saica 

Kaolin 

Silica 
KaoJin 

Silica 
Kaolin 

SUica 
Kaolin 

Iron 
minerals 

Iron 
minerals 

Iron 
minerals 

Iron 
minerals 

Iron  33.01  % 


Iron  35.26  K 


Intermediate  phases 


Iron  4S.S^  ft, 

Ix>w-grade 
ore 


Iron  29.J7  yb 

Ferruginous 

chert 

[taconite] 

Figure  19.— Diagram  showing  volume  changes  observed  in  the  alteration  of  Icmiginons  chert   to  ore.    The  four  specimens  represented  were 
collectcil  from  a  single  band  of  ferruginous  chert  in  the  .'Jlcvenson  mine,  Mesabi  district,  Minnesota.    (See  analyses,  above.) 

sentation  of  the  series.  Both  the  other  series  showed  slight  evidence  of  slumping,  the  chert 
bands  being  thinner  at  the  most  altered  end:  consequently  the  increase  in  volume  of  the  iron 
mineral  was  expected. 


MESABI  IRON  DISTRICT. 


189 


Figure  19  shows  very  well  tluit  the  essential  process  in  the  alteration  of  the  taconite  is  the 
leaching  of  silica.  This  removal  of  material  causes  an  increase  in  pore  space.  The  development 
of  porosity  beyond  certain  limits  weakens  the  rock  and  results  in  slumping  or  crushing;  lience 
the  volume  of  silica  removed  may  be  greater  than  the  porosity  observed.  In  order  properly  to 
compare  the  various  phases  of  taconite  and  ore  studied,  it  is  necessary  to  consider  them 
in  terms  of  volume  composition  rather  than  of  weight.  By  so  doing  the  factor  of  porosity  is 
included  in  each  phase  studied,  the  volume  composition  being  given  in  terms  of  hydrated  iron 
oxide,  silica,  pore  space,  and  minor  constituents  (principally  kaolin).  The  alteration  as  showTi 
by  tlie  average  analyses  of  greenalite,  taconite,  and  ore  is  expressed  diagrammatically  in  figure  20. 


.  Average  greenalite  rock 


Average  ore  - 

Figure  20.— Graphic  representation  of  the  changes  involved  in  the  alteration  of  greenalite  rock  to  ferruginous  chert  (taconite)  and  ore,  Mesabi 
district,  Minnesota.  The  mineral  composition  of  the  various  phases  is  represented  in  terms  of  volume  by  vertical  distances.  The  mineral 
composition  of  the  greenalite  rock,  ferruginous  chert,  and  ore  as  represented  was  obtained  by  averaging  a  large  number  of  analyses. 


METHOD   OF  EXPRESSING   VOLUME   CHANGES  BY   TRIANGULAR   DIAGRAM. 

While  the  method  of  representation  shown  above  (figs.  19  and  20)  expresses  well  the  average 
results,  it  is  not  a  convenient  way  of  handling  a  large  number  of  detailed  figures.  In  order  that 
many  individual  comparisons  may  be  made  on  a  single  diagram,  the  volumes  of  the  principal 
constituents — silica,  iron  minerals,  and  pore  space — are  platted  on  a  triangle  (fig.  21)  in  which  all 
these  factors  are  indicated  by  position  in  the  diagram.  Tlie  triangular  method  of  representing 
percentages  of  tliree  constituents  has  been  described  on  page  182.  In  figure  21  the  same  method 
is  employed  to  represent  the  volume  composition  of  the  various  phases  of  the  iron-bearing 
formation  studied.  As  is  indicated  on  the  triangle,  distances  measured  from  the  tliree  sides 
represent  severally  percentage  volumes  of  iron  minerals,  silica,  and  pore  space.  Tlius  any  point 
in  the  triangle  represents  amounts  of  pore  space,  quartz,  and  iron  minerals  totaling  100  per  cent. 
In  actual  analyses,  however,  it  is  found  tliat  these  three  factors  seldom  total  100  per  cent,  a 
small  percentage  of  minor  constituents  being  present,  principally  kaolin,  which  makes  it  im- 
possible to  represent  the  volume  composition  by  a  single  point  in  the  diagram.  This  difficulty 
is  obviated,  however,  by  representing  the  percentage  volume  of  each  of  the  three  principal 
constituents  by  a  short  line  drawn  parallel  to  each  side  at  the  proper  distance,  thus  constructing 
a  small  equilateral  triangle  within  the  larger  one.  The  altitude  of  this  small  triangle  repre- 
sents, by  the  divisions  in  the  large  triangle,  the  percentage  volume  of  the  minor  constituents. 
We  may  then  represent  by  the  position  and  size  of  a  small  parallel  triangle  within  the  large 
equilateral  triangle  the  volume  composition  of  any  chert  or  ore  in  terms  of  pore  space,  silica, 
iron  minerals,  and  mmor  constituents. 


DATA   USED    IN    TRIANGLE. 


Chemical  analyses,  together  with  density  and  porosity  determinations,  were  procured  for 
120  taconite  and  ore  specimens,  including  gradation  phases  between  the  taconite  and  ore,  slaty 
phases  of  the  taconite,  and  paint  rock.     These  data,  when  platted  on  the  triangular  diagram, 


190 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


show  the  relations  of  the  various  ])hases  of  the  iron-bearing  formation  and  cnah'e  one  to  deal 
with  a  large  number  of  individual  cases  as  easily  as  with  averages.  Each  of  the  small  triangles 
within  the  large  one  represents  an  actual  specimen  or  sample  from  the  iron  formation. 

CONSIDEHATION  OF  THE  TRIANGULAR  DIAGRAM. 

The  imaltered  taconite  is  repre.'icnted  by  the  small  triangles  in  the  lower  left-liand  side  of 
the  triangle,  where  porosit}'  is  low  and  silica  liigh.  If  the  taconite  represented  by  an}'  one  of 
these  triangles  is  to  be  altered  to  ore,  it  is  necessary  that  part  of  the  silica  be  removed  to  permit 
an  increase  in  the  iron  content.     If  silica  is  removed,  there  must  be  an  increase  in  the  percentage 

IRON    MINERALS 


High-grade  soft  ore 


Average  ore 

(  Cargo  analysis  for  1906 ) 


Average  unaltered 
ferruginous  chert 


Lowz-grade  ore 
ocherous  and  clayey 


SILICA  PORE   SPACE 

FiGDRE  21.— Triangular  diagram  representing  in  terms  of  pore  space,  iron  minerals,  silica,  and  minor  constituents  (clay,  etc.)  tlie  volume  compo- 
sition of  the  various  phases  of  ferruginous  cherts  and  iron  ores  of  the  Mesabi  district.    For  detailed  explanation  see  page  189. 

of  volume  either  of  pore  space  or  of  iron.  If  we  suppose  the  change  to  lie  between  silica  and 
pore  space,  iron  being  unchanged,  the  alteration  of  the  chert  ^\ill  be  represented  bj^  a  succession 
of  triangles  reaching  across  the  diagram  to  the  right  at  a  constant  distance  from  the  base,  silica 
decreasing  and  porosity  increasing.  If  the  sample  selected  is  high  enough  in  iron  and  low 
in  silica,  sufficient  siUca  may  be  removed  to  produce  ore  without  developing  an  impossible 
porosity.  On  the  other  hand,  if  the  small  triangle  selected  is  near  the  base  of  the  diagram, 
representing  a  taconite  that  is  composed  largely  of  silica  and  contains  only  a  small  amount  of 
iron,  it  is  evident  that  removal  of  sufficient  silica  to  brmg  the  percentage  of  u'on  up  to  the  ore 
grade  without  slump  would  develop  a  very  large  porosity.  It  is  probable  that  the  porositj'  would 
increase  until  the  material  became  too  porous  to  support  itself  and  the  weight  above,  when 


MESABI  IRON  DISTRICT. 


191 


slump  would -occur,  decreasing  the  pore  space  and  increasing  the  percentage  volume  of  iron. 
This  change  would  be  represented  on  the  diagram  by  an  upward  movement  of  the  triangle 
selected.  Actual  infiltration  of  iron  in  solution  would  also  cause  decrease  in  porosity  and 
increase  in  iron,  but  field  observation  shows  that  infiltration  of  iron  is  very  sHght  in  tliis  dis- 
trict, and  hence  any  shortage  of  pore  space  must  be  explained  by  slump.  Calculations  sliow 
that  on  an  average  this  slump  amounts  to  approximately  45  per  cent  of  the  volume  of  the 
original  taconite,  wliich  would  give  a  vertical  slump  of  82  feet  for  every  100  feet  depth  of  ore. 
This  figure,  though  apparently  large,  is  well  in  accord  with  the  observed  facts.  The  degree  of 
slump  in  an  ore  body  may  best  be  measured  by  observing  the  amount  of  sag  in  the  paint- 
rock  layers  which  have  been  bent  downward  by  the  slump  of  the  underlying  ore.  Figure  16 
shows  a  typical  cross  section  of  an  ore  deposit  in  the  Ilibbing  district;  the  amount  of  slump  in 
the  ore  beneath  the  paint  rock  is  seen  to  be  of  the  same  magnitude  as  the  above  figures.  The 
diagram  (fig.  21)  shows  that  where  the  original  content  of  iron  in  ferruginous  chert  is  high  the 
amount  of  siHca  to  be  leached  is  small  and  the  resulting  pore  space  is  small,  but  that  where 
tlie  iron  is  low  the  pore  space  is  proportionally  greater.  It  follows,  then,  that  ferruginous  cherts 
originally  low  in  siUca  are  much  more  easily  and  quickly  altered  to  ore  than  those  high  in  silica. 
It  is  also  seen  from  the  diagram  that  the  ores  high  in  alumina  or  clay  (represented  by  the 
larger  triangles)  have  a  greater  porosity  in  rough  proportion  to  the  alumina  content.  The 
alumina  is  very  largely  in  the  form  of  kaolin,  a  substance  characteristically  very  porous  and 
not  so  easily  affected  by  slump  as  the  coarser  and  more  granular  ores;  hence  the  larger  porosity. 


ALTERATIONS  OF    ASSOCIATED    ROCKS  CONTEMPORANEOUS   WITH  SECONDARY  ALTERATION  OF    THE 

IRON-BEARING    FORMATION. 

The  shaly  layers  in  the  original  iron-bearing  formation  become  transformed  to  paint  rock 
or  ferruginous  slates  during  the  ore  concentration.  Abinidant  phases  of  the  formation  inter- 
mediate between  the  shales  and  carbonates  or  greenalites  become,  after  alteration,  either  ores 
or  cherts  with  a  pronounced  shaly  or  slaty  structure.  These  are  variously  called  ferruginous 
slates,  slaty  ores,  or  paint  rock,  according  to  their  kon  and  clay  contents.  The  nature  of  the 
alteration  is  a  leacliing  of  silica  and  the  more  soluble  bases,  leaving  a  mixture  of  clay  and  iron 
oxide.     Following  are  typical  analyses  of  the  phases  mentioned  above: 

Typical  analyses  of  unaltered  slaty  phase  of  iron-bearing  formation  and  paint  rock. 


Unaltered  slaty  ptiase  of  iron- 
bearing  formations. 

Paint  rock. 

1. 

2. 

3. 

4. 

5. 

6. 

Si02.                

37.11 
2,41 
17.51 

53.86 
9.14 

23.80 
7.95 
5.97 

9.54 

7.00 
77.30 

20.94 
19.01 

AI2O3                                            

3  28 

FesOa 

Fe  .                                

15.90 

30.88 

25  60 

FeO 

26.13 

3.70 

.75 

.09 

.62 

.95 

2.57 

.22 

6.16 

1.21 

.73 

32.21 

5.89 

4.67 

.29 

.18 

}        4.28 

.45 

MgO       

CaO 

NajO 

K-O 

H-O  - 

]■      13.44 

H2O  + 

{ 

{:;;:;:;;: 

TiOz 

.61 

C02 

.14 

11.  84 

MnO 



c 

Volatile 

3.35 

P2O5 .' 

.09 

.04 

.20 

1.  Specimen  45461  from  Moss  mine;  analysis  by  George  Steiger. 

2.  Specimen  4.W00  from  point  near  the  southeast  corner  of  the  NE.  J  SW.  J  sec.  21,  T.  58  N.,  R.  20  W.;  analysis  by  H.  N.  Stolies 

3.  Specimen  112  (Chem.  series  No.  240).  NE.  J  SE.  J  sec.  17,  T.  58  N.,  R.  19  W.;  analysis  by  A.  D.  Meeds  for  1.  E.  Spurr.    (See  Geo!,  and  Nat. 
Hist.  Survey  Minnesota,  Bull.  No.  10,  p.  10). 

4.  Specimen  40661  from  Mahoning  mine;  analysis  by  George  Steiger. 

5.  Darlc  portion  of  banded  red  and  white  paint  rock  (specimen  4564{;)  from  Mountain  Iron  mine;  analysis  by  A.  T,  Gordon. 

6.  Paint  rock  (specimen  4,5594)  from  Penobscot  mine,  Ijeneath  ore;  analysis  by  H.  N.  Stokes. 

Where  igneous  rocks  have  been  intruded  into  the  formation  before  its  alteration  these  have 
suffered  similar  alterations  to  the  slate.  Theh-  bases  have  been  leached  and  they  remain  essen- 
tially as  clay,  retaining  the  igneous  textures. 


192  GEOLOGY  01'^  THE  LAKE  SUPElilOR  REGION, 

PHOSPHORUS  IN  MESABI  ORES. 
DISTRIBUTION    IX    THE    IKON-BEARING    FORMATION. 

Tho  distribution  of  phosphonis  in  tlie  various  phases  of  the  iron-bearing  formation  is  as 

follows  -. 

J'husphorus  in  iTon-bmrimj  formation. 


Groenulite  rock,  average  of  six  typical  specimens 

Ferruginous  chert: 

Average ■  ■  - 

Iron  layers  In  (erniginous  chert  (Specimen  440oi ) . . . 

Chert  liivers  in  ferruginous  chert  (Specimen  44031).. 

Iron  lavers  in  ferruginous  chert  (Specimen  44nriO). . 

Chert  lavers  in  ferruginous  chert  (Specunen  440.50).. 

Iron  lavers  in  ferruginous  chert  (Specimen  44071 ) . . 

Chert  lavers  in  ferruginous  chert  (Specunen  44071 ).. 

Slate  In  iron  foraiation.  typical  analysis 

Paint  rock,  typical  analysis 

Amphibole-magnetite  rock 

Iron  ore,  average  of  1906  output 


Ratio  of 

Iron. 

Phos- 

phos- 

phori:s. 

phorus 

to  iron. 

25.05 

0.012 

0.000479 

25.71 

.021 

.000820 

fil.69 

.074 

.001200 

24.50 

.019 

.000770 

51.27 

.035 

.000680 

11.55 

.010 

.000870 

58.39 

.052 

.00089 

38.33 

.018 

.00047 

29.90 

.098 

.00328 

<0.80 

.189 

.00462 

23.56 

.0394 

.00167 

60.70 

.0559 

.000920 

There  is  a  wide  variation  in  the  phosphorus  content  of  the  several  grades  of  ore.  In  gen- 
eral it  may  be  said  that  the  more  hydrous  ores  tend  to  run  high  in  phosphorus  but  are  not 
uniformly  so.  In  figure  22  the  increase  of  phosphorus  %vith  the  degree  of  hydration  of  the  ore 
is  shown,  tlie  data  being  average  cargo  analyses  of  all  grades  of  ore  shipped  from  the  Mesabi 


FlGUKE  22.— Diagram  showing  relation  of  phosphorus  lo  Uegiee  ol  hyilralion  in  .M.saM  ores. 

range  in  1906.  Percentages  of  phosphorus  and  of  water  of  hydration  are  plattcil  respectively  as 
ordinatcs  and  abscissas.  Tiie  arrangement  of  the  points  on  the  diagram  seems  to  indicate  that 
high  phosphorus  is  in  general  associated  witli  liigh  content  of  combined  water. 

In  the  Mahoning  open-pit  mine  large  round  concretions  of  rather  hard  yellow  ore  are  fouml 
embedded  in  darker  ore.     The  concretions  contain  in  their  centers  crystalline  and  botrj^oidal 


MESABI  IRON  DISTRICT. 


193 


quartz  and  yellow  hydrated  iron  oxide.  Analyses  of  the  outer  shell  and  of  the  core  of  the  con- 
cretions were  made  from  samples  representing  a  number  of  individuals.  The  results  of  the 
partial  analyses  given  in  the  following  table  show  a  marked  concentration  of  phosphoras  at 
the  center  of  the  concretion.  As  the  concretions  are  of  a  distinctly  geodal  structure,  the  phos- 
phorus in  the  interior  was  evidently  one  of  the  last  constituents  introduced. 

Analyses  of  concretions  from  Mahoning  mine. 


Outer  shell  of  concretions . 
Center  of  concretions 


Iron. 


65.77 
53.12 


Phos- 
phorus. 


0.058 
.143 


Ratio  of 
phos- 
phorus 
to  iron. 


0.00088 
.00269 


In  the  Oliver  open-pit  mine,  in  1899,  a  vein  of  limonite  could  be  seen  cutting  down  from  the 
surface,  clearly  as  a  result  of  an  alteration  by  percolating  waters  along  a  fissure,  and  the  per- 
centage of  phosphorus  within  the  vein  was  much  higher  than  in  the  ore  immediately  adjacent. 
This  occurrence  of  high  phosphorus  is  similar  to  the  high  phosphorus  in  the  Mahoning  concre- 
tions, in  that  it  occurs  with  a  more  hydrated  iron  oxide  than  the  surrounding  ore,  and  is  evi- 
dently later  than  the  concretion  of  the  ore. 

Another  instance  of  the  occurrence  of  high  phosphorus  with  hydrated  iron  oxide  was 
furnished  by  Mr.  A.  T.  Gordon,  who  analyzed  hard  black  hematite  and  soft  yellow  limonitic 
ore  in  the  same  hand  specimen  from  the  Mountain  Iron  mine  with  the  following  results:  Hard 
ore,  iron  61.00,  phosphorus  0.077;  soft  ore,  iron  57.98,  phosphorus  0.118. 

To  obtain  further  data  on  the  association  of  phosphorus  with  the  more  hydrated  phases  of 
the  ore  wasliing  tests  were  made  on  samples  of  ore  from  the  Sellers  and  Burt  mines  at 
Hibbing,  Minn.  Each  sample  was  stirred  with  water  in  a  pail  and  after  the  mLxture  had  been 
allowed  to  settle  for  ten  minutes  the  water  was  poured  off  and  filtered  and  a  very  finely  divided 
reddish-yeUow  sediment  was  obtained.  A  portion  of  the  remaining  ore  was  then  thoroughly 
washed  with  water  until  free  of  coloring  matter.  Analyses  made  in  Lerch  Brothers'  laboratory 
at  Virginia,  Minn.,  of  the  samples  thus  obtained  gave  the  following  results: 

Partial  analyses  from  washing  tests  on  Mesabi  ores. 


Iron. 

Phos- 
phorus. 

Alumina. 

Loss  by 
ignition. 

Ore  from  Sellers  mine: 

46.64 
67.92 
69.92 

49.57 
60.65 

0.  073 
.052 
.049 

.073 
.051 

9.06 

8.32 

3.  Dark-colored  residue  after  washing  with  water             .                  ... 

1.34 

8.31 
2.00 

3  54 

Ore  from  Burt  mine: 

1.    Finely  rlh-irlpd  'jed'TTipnt  hpld  in  sn<!ppnpinn  Inngpr  than  in  minntps 

7  34 

2.  Dark-colored  heavr  residue      

3.67 

A  calculation  of  the  mineralogical  composition  of  Nos.  1  and  3  from  the  Sellers  mine  and 
Nos.  1  and  2  from  the  Burt  mine  from  these  analyses  shows  that  the  material  is  of  the  same 
general  composition  as  paint  rock,  being  liigh  in  kaolin  and  hydrated  iron  oxide.  The  mineral 
compositions  follow: 

Mineral  composition  calculated  from  analyses  in  the  tabic  above. 


Sellers  mine. 

Burt  mine. 

No.l 

(toe 

material). 

No.  3 
(heavy- 
material). 

No.l. 

No.  2. 

Hematite. . . 

36.^0 

35.25 

5.45 

22.90 

67.60 
21. 10 
7.90 
3.40 

45.00 
30  25 
3.75 
21.00 

69.00 

Limonite .     ..           .                

29.60 

Quartz .    . 

5.45 

Kaolin 

5.06 

47517 


-VOL  52—11- 


194 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


In  the  Meadow  mine,  at  Aurora,  Minn.,  ore  immediately  above  an  altered  granite  dike  was 
found  to  run  higher  in  phosphorus  than  the  ore  farther  from  the  contact.  This  fact  suggests 
that  either  the  alteration  of  the  granite  contributed  phospliorus  to  the  ore  or  the  dike  acted  as 
an  imper\'ious  layer  above  which  the  phosphorus  was  concentrated.  The  tests  show  that  the 
phosphorus  is  in  some  manner  associated  with  the  kaolin  and  hydrated  iron  oxide  and  bear  out 
the  statement  that  high  phosphorus  is  related  to  the  degree  of  liydration  of  the  ore. 

Phosphorus  content  of  rocks  associated  with  iron-bearing  formations. 


Virginia  slate  (Men.  43,  p.  170) ■'■'■' 

Giants  Range  granite,  average  twelve  specimens 

Basic  intnisives  in  iron  fonnatiun  of  Gunflint  district 

Granite  ilike  in  iron-bearing  Biwabik  formation:  | 

1.  Kaolinized  and  much  iron  stained 

2.  Near  No.  1  but  farther  from  ore,  less  iron  stained 

3.  Completely  kaolinized  but  preserving  granitic  texture;  color  light  pink 


Phos- 
phorus. 

Ratio  of 
phos- 
phorus 
to  iron. 

0.0885 
.087 

0.01868 

.120 
.059 

.036 

.020 

SECONDARY    CONCENTRATION    OF    PHOSPilOBUS. 

Present  differences  in  phosphorus  content  between  various  phases  of  the  iron-bearing 
formation  may  be  due  (1)  to  original  differences  or  (2)  to  secondary  changes,  producing  differ- 
ences in  phosphorus  content  not  due  to  original  differences  in  composition.  These  secondary 
changes  may  be  actual  increase  or  decrease  in  phosphorus  due  to  infiltration  or  leaching,  or 
relative  increase  or  decrease  due  to  the  introduction  or  removal  of  other  constituents. 

A  comparison  of  the  partial  analyses  of  the  three  principal  phases  of  the  iron-bearing  forma- 
tion— greenahte  rock,  taconite,  and  ore — successively  developed  during  the  secondary  concen- 
tration, shows  a  continuous  increase  in  phosphorus  and  in  the  phosphorus-iron  ratio  during  sec- 
ondary concentration  of  the  ore.  The  percentage  of  phosphorus  increases  from  0.012  in  the 
greenalite  to  0.021  in  the  taconite  and  probably  to  more  than  0.0559  in  the  ore  (the  average  ore 
shipped  being  lower  in  phosphorus  than  the  average  ore  of  the  range).  The  corresponding 
increase  in  the  phosphorus-iron  ratio  is  from  0.00048  to  0.00082  to  0.00092.  In  spite  of  possible 
variance  of  these  figures  from  true  averages,  the  differences  are  so  marked  as  to  point  very 
strongly  to  an  actual  increase  in  the  percentage  of  phosphorus  during  the  alteration  of  the 
greenalite  rock  to  taconite  and  of  the  taconite  to  ore. 

In  the  discussion  of  the  secondary  concentration  of  the  ore  it  was  shown  that  the  concentra- 
tion was  accomplished  by  the  removal  of  sihca  and  that  the  amount  of  iron  carried  in  solution 
was  very  small.  If  the  phosphorus  were  as  insoluble  as  the  iron  and  if  no  phosphorus  had  been 
introduced,  the  ratio  of  phosphorus  to  iron  would  necessarily  have  remamed  constant  during 
the  concentration — in  other  words,  both  elements  would  have  been  concentrated  to  the  same 
degree.  As  there  is  an  actual  increase  in  the  ratio  of  phosphorus  to  iron  during  the  alteration, 
it  appears  that  phosphorus  has  been  concentrated  to  a  greater  degree  than  the  iron.  As  iron 
has  not  been  largely  removed,  this  increase  in  phosphorus  may  be  explained  only  by  actual 
introduction  of  that  element  in  solution  from"  sources  outside  of  the  iron-bearing  formation  or 
from  other  parts  of  the  formation  itself.  AU  available  evidence  seems  to  mdicate  that  at  least 
part  of  the  phosphorus  in  the  ores  is  more  soluble  than  the  iron  oxide;  hence  without  the  intro- 
duction of  phosphorus  we  should  expect  an  actual  decrease  in  the  ratio  of  phosphorus  to  iron 
during  the  concentration  of  the  ores.  This  seems  to  show  that  the  introduction  of  phosphorus 
from  without  was  even  greater  than  the  increase  in  the  phosphorus-iron  ratio  indicates. 

Most  of  the  ores  were  at  one  time  overlain  by  Cretaceous  sediments,  patches  of  which  still 
remain  as  far  east  as  Virginia,  Minn.  Analyses  from  drill  holes  and  test  pits  ilisclose  a  high 
phosphorus  content  in  the  Cretaceous  beds  overlying  the  ores.  Furthermore,  they  show  that 
there  is  a  gradation  in  the  phosphorus  content  from  the  Cretaceous  down  into  tiie  undoilying 
ore.     A  typical  series  of  analyses  from  a  drill  hole  in  the  western  part  of   the  Mesabi  district 


MESABI  IRON  DISTRICT.  195 

shows  the  phosphorus  content  of  the  Cretaceous  shale  to  be  1.353  per  cent,  that  of  the  ore 
hnmediately  underlying  to  be  O.ISO  per  cent,  and  that  of  lower  levels  to  grade  down  to  0.045 
per  cent  at  a  depth  of  50  feet  below  the  shale.  It  seems  highly  probable,  then,  that  the  most 
abundant  source  for  the  phosphorus  introduced  into  the  ores  of  the  Biwabik  formation  was  the 
Cretaceous  rocks.  As  indicated  in  the  table  of  analyses  (p.  194),  there  are  other  sources  for 
phosphorus  in  the  granites  and  slates  outside  of  the  iron-bearing  formation,  and  it  is  possible 
also  that  the  slates  of  the  iron-bearing  formation  itself  have  contributed  phosphorus  to  the  ore. 

EXPLANATION    OF    PHOSPHORUS    IN    THE    PAINT    ROCK. 

The  paint  rock  of  the  Biwabik  formation  is  a  kaolinized  alteration  product  formed  by  the 
alteration  of  interbedded  slate  layers  or  of  the  lower  layers  of  the  overlying  Virginia  slate.  The 
change  from  slate  to  paint  rock  is  of  exactly  the  same  nature  as  the  alteration  of  taconite  to  ore, 
the  soluble  bases  together  with  quartz  being  leached  and  leaving  the  insoluble  resi(hie  of  hydrated 
iron  oxide  and  kaolin.  As  there  are  all  gradations  between  slate  and  taconite,  we  find  the  same 
continuous  gradation  between  paint  rock  and  ore.  The  paint  rock  is  characteristically  high 
in  phosphorus,  the  analysis  in  the  table  on  page  194  being  typical,  though  occasionally  paint 
rock  is  found  with  comparatively  low  phosphorus  content.  Both  the  slate  of  the  iron-bearing 
formation  and  the  Virginia  slate  are  high  in  phosphorus,  so  it  is  believed  that  the  high  phos- 
phorus of  the  pamt  rock  is  to  a  large  extent  original,  phosphorus  remaining  with  the  iron  oxide 
and  kaolin  during  the  leaching  of  the  silica  and  other  constituents. 

But  it  appears  necessary  also  to  account  for  at  least  part  of  the  phosphorus  in  the  paint 
rock  as  coming  from  outside  sources,  and  the  most  obvious  source  is  the  Cretaceous,  as 
already  indicated  for  the  iron  ores. 

PHOSPHORUS    IN    THE    AMPHIBOLE-MAGNETITE    PHASES    OP    THE    IRON-BEARING    FORMATION. 

In  the  Gunflint  district  the  Gunflint  formation  appears  to  be  an  eastern  extension  of  the 
iron-bearing  Biwabik  formation,  consisting  almost  entirely  of  silicated  magnetite  rock.  An 
average  analysis  representing  834  feet  of  drill  core  in  the  formation  showed  the  average  iron 
and  phosphorus  contents  to  be  23.56  per  cent  and  0.0394  per  cent  respectively.  This  gives  an 
average  phosphorus-iron  ratio  of  0.00167.  Comparison  of  these  figures  with  the  analysis  of  the 
other  phases  of  the  iron-bearing  formation  (p.  192)  indicates  that  the  average  iron  content  is 
very  close  to  that  of  both  the  greenalite  and  taconite  phases.  This  phosphorus  content  of  the 
silicated  magnetite  rocks  is,  however,  much  higher  than  that  of  either  greenalite  or  taconite. 
The  reason  for  this  high  phosphorus  in  the  silicated  magnetite  phase  of  the  iron-bearing  forma- 
tion may  he  either  in  the  original  clifTerence  in  phosphorus  content  between  the  cHfferent  parts 
of  the  range  or  in  the  introduction  of  phosphorus  from  the  closely  associated  intrusives.  Pres- 
ent knowledge  does  not  permit  a  more  definite  conclusion.  The  average  phosphorus  content  of 
the  intrusive  gabbros  is  0.12  per  cent,  which  is  much  higher  than  the  phosphorus  content  of  the 
iron-bearing  formation,  so  that  it  furnishes  an  abundant  source  for  the  introduction  of  secondary 
phosphorus. 

MINERALS    CONTAINING    PHOSPHORUS. 

So  far  as  is  knowTi,  no  phosphorus  minerals  have  been  identified  in  any  of  the  iron-bearing 
rocks  of  the  Mesabi  range.  Obviously,  then,  discussion  of  the  mmeral  occurrence  of  phos- 
phorus is  entirely  a  matter  of  conjecture  based  on  chemical  evidence  and  on  the  nature  of  phos- 
phorus-bearing minerals  wliich  have  been  identified  m  the  other  Lake  Superior  iron-bearing 
formations.  It  is  not  unlikely  that  some  of  the  phosphorus  occurs  in  the  form  of  apatite  (cal- 
cium phosphate).  It  seems  reasonable  to  suppose  that  this  mineral  may  be  found  in  the  iron- 
bearing  Biwabik  formation,  although  careful  search  has  not  yet  revealed  it. 


196 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


In  figure  23  percentages  of  phosphorus  in  tlie  various  commercial  grades  of  ore  are  platted 
as  ordinates  and  percentages  of  lime  as  abcissas.  The  diagonal  line  crossing  the  diagram  indi- 
cates tiie  ratio  of  phosphorus  to  lime  in  apatite;  hence  all  points  ahove  the  line  denote  an  excess 
of  lime  over  the  amount  necessary  to  form  apatite  and  all  points  below  the  line  indicate  a 
deficiency  of  lime.  In  other  words,  phosphorus  in  ore  represented  by  points  aljove  the  line 
may  be  combined  with  lime  to  form  apatite,  and  ore  represented  by  points  below  tlie  line  neces- 
sarily contains  some  phosphorus  in  forms  other  than  apatite.  This  seems  to  show  conclusively 
that,  though  there  is  sufficient  calcium  in  a  large  part  of  the  ore  to  form  apatite,  in  some  grades 
of  ore  a  deficiency  of  calcium  proves  the  existence  of  other  piiosphorus  minerals,  possibly  of 
iron  or  aluminum.  Another  fact  brought  out  by  the  diagram  is  that  calcium  is  deficient  only 
in  the  ores  highest  in  phosphorus.     This  suggests  the  possibility  that  the  original  phosphorus 


FiGUBE  23:— Diagram  sho^ving  relative  amounts  of  phosphorus  and  lime  in  Mesabi  ores. 

of  the  ores  may  be  in  the  form  of  apatite  but  that  secondarj-  phosphorus  takes  some  other  form. 
The  association  of  high  phosphorus  with  the  hj^drated  forms  of  iron  and  aluminum  suggests  that 
this  excess  of  phosphorus  may  be  in  phosphates  of  iron  and  aluminum.  It  is  verj-  probable  that 
at  least  part  of  the  phosphorus  is  combined  with  the  iron  ami  aluminum  in  no  definite  mineral 
form. 

DETRITAL  ORES  IN  THE  CRETACEOUS  ROCKS. 

In  the  western  part  of  the  Mesabi  district,  in  T.  56  N.,  Rs.  23  and  24  W.,  a  considerable 
amount  of  detrital  ore  has  been  found  in  the  Cretaceous  rocks  overlying  the  Biwabik  formation 
and  the  northern  margin  of  the  Virginia  slate.  Drilling  has  sho^\^^  up  several  million  tons  of 
this  ore  of  the  following  average  composition: 


MESABI  IRON  DISTRICT.  197 

Average  composition  of  Cretaceous  ore  from  the  west  part  of  the  Mesabi  range. 

[Samples  dried  at  212°  F.] 

Iron 54. 41 

Phosphorus .  us 

Silica 6. 18 

Alumina 8.  25 

Manganese 49 

As  the  ore  has  not  been  opened  up,  the  sources  of  information  as  to  texture,  moisture 
content,  and  other  physical  characteristics  are  Hmited  to  the  results  of  th-iliing.  The  drilhng 
shows  that  tlie  ore  is  conglomeratic  in  nature,  as  is  usual  in  detrital  ores.  There  appears  to  be 
considerable  opportunity  for  further  discoveries  of  ore  of  this  character. 

SEQUENCE  OF  ORE  CONCENTRATION  IN  THE  MESABI  DISTRICT. 

The  sequence  of  ore  concentration  in  the  Mesabi  district  is  similar  to  that  in  the  Gogebic 
district  in  that  the  upper  Huronian  (Animikie  group)  was  but  slightly  tilted  and  eroded  before 
the  Keweenawan  gabbro  was  intruded  into  it.  The  gabbro  thus  came  into  contact  with  the 
iron-bearing  formation  only  at  the  east  end  of  the  district.  Here  it  found  in  very  small 
quantity  soft  ores  and  ferruginous  cherts  developed  by  weathering  and  changed  them  to  hard 
ores  and  jaspers.  The  original  greenalite  rocks  making  up  most  of  the  iron-bearing  forma- 
tion were  altered  to  amphibole-magnetite  rock.  The  principal  and  present  productive  part  of 
the  district  was  protected  from  the  gabbro  by  a  great  mass  of  slates.  The  erosion  following  the 
post-Keweenawan  folding  for  the  first  time  exposed  the  main  mass  of  the  iron-bearing  Biwabik 
formation  from  beneath  the  slates. 

By  Cretaceous  time  the  concentration  of  the  ore  was  far  advanced,  for  we  find  the  basal 
detrital  zone  of  the  Cretaceous  carrying  abundant  iron  ore  in  the  form  of  polished  pebbles. 
Since  Cretaceous  time  all  the  Cretaceous  has  been  stripped  off  except  parts  of  the  western  part 
of  the  Mesabi  district,  so  that  surface  agencies  have  had  opportunity  to  cpntinue  tlie  concen- 
tration of  the  ore. 

The  amphibole-magnetite  rocks  of  the  east  end  of  the  district  have  resisted  surface  altera- 
tion, except  local  discoloration  by  oxidation  in  a  thin  film  at  the  surface. 


CHAPTER  VIII.     THE  GUNFLINT  LAKE,   PIGEON  POINT,  AND  ANI- 
MIKIE  IRON  DISTRICTS  OF  MINNESOTA  AND  ONTARIO. 

Under  the  three  names  Gunflint  Lake,  Pigeon  Point,  and  Animikie  is  discussed  the  strip 
of  territory  extending  from  the  east  end  of  the  Mesabi  and  Vermilion  districts  in  the  vicinit}- 
of  Gunflint  Lake  to  Port  Arthur  on  Animikie  or  Thunder  Bay  and  thence  eastward  to  the  Loon 
Lake  district.  The  districts  are  geographically  continuous  and  the  principal  geologic  features 
in  each,  given  by  the  Animikie  and  Keweenawan  rocks,  are  much  the  same,  but  because  of 
slight  variations  and  because  the  districts  have  been  studied  from  different  standpoints  by 
different  men  they  are  described  under  the  above  three  headings. 

GUNFLINT   LAKE  DISTRICT." 

GEOGRAPHY. 

The  Gunflint  Lake  district  includes  the  lake  of  that  name  on  the  international  boimdary 
at  the  extreme  eastern  end  of  the  Vermilion  district  of  Minnesota,  and  extends  in  a  narrow  strip 
about  10  miles  east  and  10  miles  west  of  the  lake.  The  rock  succession  and  structiu-e  are  essen- 
tially the  same  as  in  the  Mesabi  district  to  the  west  and  the  Animikie  district  to  the  east.  It  is 
cut  off  from  the  Mesabi  district  by  the  great  overlapping  mass  of  Dulutli  gabbro.  It  is  con- 
nected with  the  Animikie  district  by  continuous  exposure  except  for  the  drift. 

SUCCESSION   OF   ROCKS. 

The  succession  of  rocks  is  as  follows: 

Quaternary  system: 

Pleistocene  series Glacial  drift. 

Unconformity. 
Algonkian  system: 

Keweenawan  series Conglomerate,   sandstone   marl,   diabase  sills   (Logan 

sills),  and  gabbro  (Duluth  gabbro). 

Unconformity. 

Huron ian  series: 

Upper  Huronian  (Animikie  group).  .<  „      a'-\.  c         »•      /•        i  i 

*^*^  ^  °       ''     [Gunflmt  formation  (u-on  bearing). 

Unconformity. 

Lower-middle  Huronian Graywacke,  with  greenstone  and" granite  intrusives. 

Unconformity. 

Archean  system: 

Laurentian  series Granites  and  gneisses  intrusive  into  Keewatin. 

Keewatin  series Green  schists,  greenstone,  mashed  porphyry. 

ALGONKIAN  SYSTEM. 

HTJBONIAN  SERIES. 

UPPER    HURONIAN   (ANIMIKIE    GROUP). 
GENERAL  DESCRIPTION. 

The  district  is  occupied  principally  by  the  upper  Huronian  (iVnimilde  group),  dipping  to 
the  south  at  angles  of  10°  to  65°  (fig.  24).  The  group  laps  from  the  south  across  the  eastern 
end  of  the  Vermilion  district  and  thus  rests  on  the  north  against  the  varit)us  older  rocks  of 
that  district,  includmg  the  granite  of  Saganaga  Lake  in  sees.  23  and  24,  T.  65  N.,  R.  4  W.,  the 

a  See  Clements,  J.  M.,  The  Vermilion  Iron-bearing  district  of  Minnesota:  Mon.  U.  S.  Geol.  Survey,  vol.  45, 1903. 
198 


GUNFLINT  LAKE  DISTKICT. 


199 


Ely  greenstone  west  of  the  granite,  and  the  Ogishke  conglomerate  and  the  Eaiife  Lake  slate 
still  farther  west.  These  rocks  have  already  been  described  in  connection  with  the  Vermilion 
district  and  will  not  again  be  referred  to. 

The  base  of  the  upper  Huronian  (Animikie  group)  is  marked  by  a  thin  conglomerate  which 
in  some  places  is  almost  lacking.  The  unconformity  with  the  underlying  rocks  is  determined 
principally  by  the  general  structure  and  distribution.  The  Animikie  group  has  uniform  strike 
and  dip,  differing  widely  from  those  of  sedimentary  beds  to  the  north  and  contrasting  with  the 
igneous  rocks  in  wliich  no  strike  and  dip  are  found.  It  also  laps  successively  across  several 
members  of  the  older  rocks  without  losing  its  continuity.  Contacts  are  so  poor  in  the  Gunflint 
district  that  these  alone  fail  to  give  sufficient  evidence  of  unconformity.  In  view  of  the  broader 
features  indicated  and  also  indubitable  facts  in  the  Mesabi  district  to  the  west  and  the  Animikie 
district  to  the  east,  the  unconformity  may  be  regarded  as  certain. 

The  lowest  formation  is  the  iron-bearing  Gunfhnt  formation.  Above  tills  and  outcropping 
in  a  belt  south  of  it  is  the  Rove  slate,  named  from  its  abundant  exposures  on  Rove  Lake  to 
the  east.  Intrusive  sills  of  diabase  (Logan  sills)  are  found  parallel  to  the  bedding  of  the  slate 
and  the  iron-bearing  formation.  Above  and  south  of  the  Animikie  group  is  the  Keweenawan 
Duluth  gabbro,  closely  related  in  age  to  the  Logan  sills.  The  gabbro  at  the  western  end  of 
the  district  laps  directly  across  the  Animikie  group  upon  the  underlying  lower-middle  Huronian 
and.  Archean.  Eastward  it  laps  successively  against  the  Gunflint  formation  and  the  Rove 
slate.  Thus  the  outcrop  of  the  Animikie  group  widens,  V-shape,  eastward.  In  the  vicinity  of 
Gunflint  Lake  itself  only  the  iron-bearing  formation  is  exposed.  Eastward  more  and  more  of 
the  slate  appears. 


Keewatin  series 
(greenstones) 


Gunflint  formation    (iron-bearing) 


Duluth     5 
gabbro 


^0/Vsv:x'-;v)Sif^\\\\\\\\\\\\^^^^^^ 


500 


1000  feet 


FiGxmE  24.— Cross  section  of  iron-bearing  Gunflint  formation  east  of  Paulson  mine,  Gunflint  district,  Minn. 


GUNFLINT    FORMATION. 

Distribution. — The  iron-bearing  Gunflint  formation  is  exposed  in  a  nearly  east-west  belt 
600  feet  to  half  a  mile  wide.  Northeast  of  the  Paulson  mine,  sees.  21  and  22,  T.  65  N.,  R.  4  W., 
there  is  an  east-west  tongue  of  the  Gunflint  formation  projecting  westward  into  Ely  greenstone. 
About  three-fourths  of  a  mile  east  of  the  Paulson  mine,  in  sec.  27,  T.  65  N.,  R.  4  W.,  where  a 
great  north-south  valley  cuts  directly  across  the  Gunflint  formation,  the  narrow  belt  of  iron- 
bearing  formation  joins  a  wider  area  of  the  same  rock  wluch  extends  over  the  greater  portion 
of  sees.  23  and  26,  T.  65  N.,  R.  4  W.  The  Gunflint  formation  is  widest  in  these  sections,  its 
great  width  being  due  cliiefly  to  the  fact  that  a  fairly  wide  synclinal  fold  has  here  been  stripped 
of  higher  formations,  leaving  exposed  an  unusually  large  area  of  the  iron-bearing  formation. 
East  of  these  sections,  toward  the  international  boundary,  the  width  exposed  is  less. 

Structure. — The  structure  of  the  Gunflmt  formation  is  not  very  complicated.  A  small 
northeast-southwest  trending  area  of  Gunflint  formation  is  exposed  on  the  southeast  shore  of 
Disappointment  Lake.  Here  the  sediments  have  a  strike  corresponding  very  closely  to  the 
trend  of  the  area  itself — that  is,  northeast-southwest — and  they  dip  to  the  south.  In  rocks  of 
similar  age  on  Gabimichigami  Lake  the  structure  is  somewhat  more  complicated.  Here  the 
sediments  have  been  folded,  and  as  a  result  they  form  in  the  main  a  syncline  plunging  toward 
the  northwest,  but  with  a  subordinate  anticline  near  the  center  which  has  an  axis  plunging  to 
the  southeast.  In  the  narrow  belt  extending  from  sec.  34,  T.  65  N.,  R.  5  W.,  eastward  to  the 
great  cross  valley  in  sec.  27,  T.  65  N.,  R.  4  W.,  the  members  of  the  formation  rest  upon  the 
older  rocks  and  uniformly  dip  to  tlie  south.  The  regularity  of  this  dip  is,  however,  internipted 
by  a  number  of  mmor  flexures  whose  axes  plunge  southeast.     As  a  result  the  amount  of  the 


200  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

dip  varies  considerably,  ranging  from  about  10°  to  65°  to  the  south,  all  the  greater  dips  occur- 
ring at  the  west  end  of  the  belt,  the  dips  becoming  flatter  within  short  distances  eastward. 
The  gradual  diminution  in  the  angle  of  dip  as  the  sediments  are  followed  to  the  east  corresponds 
to  their  less-folded  condition  in  the  eastern  part  of  the  area.  Attention  has  already  been 
called  to  the  areal  distribution  of  the  sediments  and  the  westward-trending  tongue  of  sediments 
occurring  in  sees.  21  and  22,  T.  65  X.,  R.  4  W.,  which  is  good  evidence  of  an  infolded  syncline 
of  the  sediments  at  this  place.  The  dip  of  the  sediments  as  observed  on  tiie  outcrop  in  tliis 
area  gives  further  evidence  of  the  existence  of  tliis  syncUne. 

Some  very  considerable  Irregularities  have  been  noted  in  a  few  places  along  the  margins 
of  certain  enormous  masses  of  dolerite  wliich  occur  in  the  midst  of  tlie  seilimentary  area.  These 
dolerites,  it  may  be  stated  here,  are  intrusive  in  the  sediments,  and  this  fact  sufficiently  explains 
the  contorted  character  of  the  adjacent  sediments,  for  this  contorted  character  is  confined  to 
their  immediate  vicinity,  the  uniform  low  southerly  dip  appearing  at  a  short  distance  from 
such  centacts. 

PctrograpJiic  character. — Near  Gunflint  Lake  the  iron-bearing  formation  consists  of  sideritic 
cherts  grading  into  ferrodolomites  associated  with  minor  amounts  of  ferruginous  cherts  and 
ferruginous  slates.  Westward  toward  the  Paulson  mine  the  rocks  become  black  or  dark-gi-een, 
coarsely  crystalline,  banded  rocks  consisting  essentially  of  magnetite,  fayalite,  cordierite,  cjuartz, 
and  iron  carbonate,  in  varying  proportions  in  different  bands  and  in  different  parts.  Where 
the  iron  carbonate  is  present  the  other  minerals,  aside  from  quartz,  are  absent.  The  iron  car- 
bonate is  regarded  as  the  original  phase  and  the  other  minerals  as  their  alteration  products. 
(See  p.  529.)  In  small  and  highly  varying  ciuantities  are  hedenbergite,  bronzite,  gri'merite, 
pyrrliotite,  anthophyllite,  hypersthene,  actinolite,  biotite,  apatite,  diopside,  hornblende,  augite, 
perthite,  pleonaste,  and  crocidoUte.  Fayalite  is  conspicuous  in  association  wdth  magnetite 
layers.  Cordierite,  so  rare  the  world  over,  is  perhaps  the  most  conspicuous  mineral  of  the 
whole  series,  in  many  places  forming  a  third  of  the  volume  of  the  rock.  It  has  the  pseudo- 
hexagonal  twinning  and  staurolite  inclusions  oriented  in  a  definite  manner  with  regard  to  the 
optic  axes  of  the  cordierite,  which  are  characteristic  of  this  mineral.  The  cordierite  for  some 
time  has  been  recognized  in  the  Huronian  slates  as  an  intrusive  contact  effect  but  has  been 
discovered  in  the  Gunflint  formation  only  recently  by  Zapffe,"  who  has  also  distinguished  a 
number  of  the  minor  minerals  noted.  The  texture  is  xenomorphic  and  the  minerals  include 
one  another  in  poikilitic  fashion. 

Contact  mctamorphism. — Perhaps  nowhere  else  in  the  Lake  Superior  country  is  there  so 
good  an  opportunity  to  study  the  metamorphic  effects  of  the  great  gabbro  intrusion  and  its 
associated  sills.  The  iron-bearing  formation  has  been  coarsely  recrystalhzed  and  siUcated  to 
such  an  extent  that  it  can  be  distinguished  from  the  intrusives  only  with  great  difficulty.  De- 
tailed study  of  tliis  metamorpliism  has  been  made  by  Bayley,''  Clements, <^  Grant,''  and  Zapffe." 
Their  conclusion  in  general  has  been  that,  though  there  has  been  minute  intrusion  of  igneous 
masses  parallel  to  the  bedding,  there  has  been  no  considerable  transfer  of  solutions  from  the 
gabbro  to  the  iron-bearing  formation  during  the  alteration.  This  subject  is  fiu'ther  discussed 
in  connection  with  the  origin  of  the  ores.     (See  p.  548.) 

TkicTcness. — The  thicloiess,  so  far  as  it  can  be  determined  in  this  district,  is  approximately 
the  same  as  that  of  the  Biwabik  formation  in  the  Mesabi  district — that  is,  somewhat  less  than 
1,000  feet. 

ROVE  SLATE. 

Distribution. — The  westernmost  exposures  of  the  Rove  slate  in  the  VermiUon  district  are 
found  in  sec.  21,  T.  65  N.,  R.  4  W.,  where  the  formation  underlies  a  very  narrow  area  in  the 
south-central  part  of  the  section.     Eastward  it  rapidly  wddens.     The  northern  boundarj'  of  the 

«  Unpublished  thesis,  University  of  Wisconsin,  1908. 

6  Bayley,  W.  S.,  The  basic  massive  rocks  of  the  Lalce  Superior  region:  Jour.  Geolof^-.  vol.  1,  1S93,  pp.  433-436.  3S7-596,  6S8-716;  vol.  2,  1S94, 
pp.  814-825;  vol.  3,  1895,  pp.  1-20. 

o<"lement^  J.  M.,  The  Vermilion  iron-bearing  district,of  Minnesota:  Mon.  U.  S.  Geol.  .Survey,  vol.  45.  1903.  pp.  389-390.  419. 
d  Grant,  U.  S.,  Contact  metamorpliism  of  a  basic  igneous  rock;  BuU.  Geol.  Soo.  America,  vol.  11,  1900,  pp.  50.3-510. 


GUNFLINT  LAKE  DISTRICT.  201 

slate  extends  northeastward  and  is  limited  by  the  Gunflint  formation  and  a  great  dolerite  sill. 
The  southern  boundary,  marked  by  the  Duluth  gabbro,  trends  east-southeast.  At  the  eastern 
limit  of  the  area  mapped  (PI.  VI,  p.  1  IS)  the  extreme  width  of  the  Rove  slate  area  in  the  United 
States  is  only  about  2  miles,  and  a  great  deal  of  this  width  is  taken  up  by  intrusive  sills  of 
dolerite.  Beyond  the  limits  of  the  district  the  slates  have  an  enormous  development  in 
Minnesota  and  in  the  adjacent  portion  of  Canada. 

Structure. — The  slates  have  a  very  uniform  dip  of  from  5°  to  25°  SSE.  As  indicated  by 
the  variation  in  dip,  the  monocline  is  occasionally  varied  by  minor  southward-pitching  rolls, 
which  may  be  noted  by  close  examination  of  almost  any  of  the  great  cliffs  that  give  good  ex- 
posures. 

PetrograpTiic  character. — Slates  form  the  bulk  of  the  Rove  formation,  but  with  them  are 
associated  graywackes,  some  slaty,  others  very  massive,  and  also  some  fairly  pure  quartzite. 
These  sediments  have  been  divided  by  Grant,"*  of  the  Minnesota  Survey,  into  a  "black  slate 
member"  and  an  overlying  "graywacke  slate  member."  In  our  work  no  attempt  has  been 
made  to  discriminate  between  these  two  petrograpliic  facies  of  the  Rove  slate.  They  are  not 
separable  by  any  time  interval  but  represent  merely  slight  changes  in  the  conditions  of  depo- 
sition. Macroscopically  they  are  very  fine-grained  black  carbonaceous  slates  grading  up  into 
dark-gray  gra3^wacke  of  medium  grain,  with  occasional  bands  of  material  almost  sufficiently 
pure  to  be  called  quartzite.  Nowhere  were  any  conglomerates,  even  fine-grained  ones,  found 
associated  with  these.  The  slates  are  unquestionably  the  predominant  kind  of  rock.  These 
carbonaceous  rocks  are  commonly  very  fissile,  but  in  places  they  are  fairly  massive. 

Contact  metamorphism. — The  sediments  of  this  formation  have  been  found  within  3  feet 
of  the  gabbro — at  the  southeast  end  of  Loon  Lake — but  not  nearer.  Here  the  rocks  are  inter- 
banded  slates  and  graywackes,  quite  crystalline  and  hard.  Microscopic  examination  of  them 
shows  that  the  gabbro  has  effected  a  partial  recrystallization  of  the  sediments  and  disclosed  in 
the  sediments  a  large  amount  of  secondary  biotite  and  muscovite.  Both  of  these  occur  in 
relatively  large  porphyritic  plates  inclosing  grains  of  the  other  materials  constituting  the  slate, 
recognizable  quartz,  and  ferruginous  material.  Down  the  slope  the  rocks  are  less  indurated, 
and  near  tJie  bottom  of  the  section,  at  the  water's  edge,  about  50  feet  below  the  gabbro,  the 
sediments  do  not  appear  essentially  different  from  the  ordinary  rocks  of  the  same  character 
and  age. 

Along  the  southern  and  southeastern  shores  of  Loon  Lake  the  slate  shows  a  spotted  char- 
acter and  is  a  spilosite,  such  as  is  fairly  common  in  sediments  near  the  contact  with  the  great 
mass  of  gabbro  ajid  such  as  occurs  also  in  other  districts  near  great  dolerite  dikes.  This  spilosite 
contains  a  large  amomit  of  chlorite  in  spots  in  a  matrix  of  quartz  and  presumably  some  feldspar. 
In  the  Mesabi  range  some  of  the  slates  near  the  gabbro  contact  show  clearly  recognizable  cor- 
dierite,  which  forms  the  white  spots;  these  slates  have  been  metamorphosed  to  a  cordierite 
hornstone.''  In  general  the  slate  adjacent  to  these  sills  in  the  Gunfhnt  district  shows  its  normal 
characters,  with  at  most  a  little  metamorphism  due  to  cementation. 

TMclcness. — Within  the  district  only  about  2,600  feet  of  the  Rove  slate  is  exposed  beneath 
the  gabbro,  but  eastward  the  thickness  rapidly  increases. 

KEWEENAW  AN  SERIES. 

DULTTTH   GABBKO. 

The  Duluth  gabbro  forms  the  southern  boundary  of  the  pre-Keweenawan  rocks  throughout 
the  greater  portion  of  the  Vermilion  district.  The  westernmost  points  at  which  the  Duluth 
gabbro  touches  the  district  are  in  sees.  26  and  35,  T.  63  N.,  R.  10  W.,  and  sec.  3,  T.  62  N.,  R.  10 
W.  From  these  sections  on  along  Kawishiwi  River  the  gabbro  swings  off  to  the  northeast  with 
a  broad  sweep,  extending  just  within  the  area  mapped  on  Plate  VI  (p.  118)  as  far  east  as  the 

o  Twenty-second  Ann.  Kept.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  1894,  p.  74;  Final  Eeport,  vol.  4, 1S99,  p.  470. 
6  Leith,  C.  K.,  The  Mesabi  iron-bearing  district  of  Minnesota:  Mon.  U.  S.  Geol.  Survey,  vol.  43, 1903,  pp.  171-172. 


202  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

vicinity  of  the  Paulson  mine,  in  sec.  28,  T.  65  N.,  R.  4  W.  From  this  place  its  edge  trends  to  the 
southeast,  ])assin<j  beyond  the  limits  of  the  area  mapped  toward  I^ake  Superior.  Two  small 
isolated  outliers  have  been  found  north  of  Gabimichigami  Lake.  Tlie  southernmost  one  is  only 
a  quarter  of  a  mile  from  the  northern  edge  of  the  main  mass  of  the  gabbro,  northwest  of  Paul 
Lake,  and  the  other  is  about  tliree-fourths  of  a  mile  from  tlie  nearest  point  on  the  edge  of  the 
gabbro  and  lies  in  the  NW.  \  sec.  29  and  the  NE.  J  sec.  30,  T.  65  N.,  R.  5  W. 

The  petrographic  character  of  the  Duluth  gabbro  is  described  on  page  372,  in  the  chapter 
on  the  Keweenawan  series. 

LOGAN  SILLS. 

The  Logan  sills  lie  well  within  the  district,  at  varjnng  distances  north  of  the  edge  of  the 
gabbro  mass.  The  first  exposure  of  such  a  sill  was  noticed  on  the  southwest  side  of  Gabi- 
michigami Lake,  but  this  can  not  be  traced  far.  The  next  one  was  seen  near  Bingoshick 
Lake.  This  sill  has  been  followed  to  the  east  for  several  miles  to  a  point  east  of  tlie  Paulson 
mine,  having  throughout  this  distance  an  almost  continuous  outcrop.  Parallel  to  this  sill 
several  small  and  relatively  unimportant  sills  have  been  observed.  Beyond  the  Paulson  mine 
the  upper  Huronian  sediments  (Animikie  group)  begin  to  widen,  rapidly  increasing  in  width 
eastward,  as  already  described.  Corresponding  with  this  widening  there  is  an  increasing 
number  of  sills  which  in  general  trend  east  and  west  and  lie  approximately  parallel  to  one 
another.  During  several  trii)s  to  Gunflint  Lake  and  to  the  country  to  the  south  a  number  of 
these  sills  were  followed  along  their  strike  for  short  distances  and  were  also  crossed  at  right 
angles  to  the  strike.  Their  relations  to  the  sediments  were  thus  clearly  seen.  No  attempt  was 
made  to  trace  out  the  individual  sills.  This  work  has  been  done  in  previous  years  bj-  Chau- 
venet "  and  Merriam,*  of  the  United  States  Geological  Survey,  and  in  more  recent  years  by 
U.  S.  Grant,''  of  the  Minnesota  Surve}'. 

RELATIONS   OF   THE   KEWEENAWAN  ROOKS  TO  ONE  ANOTHER  AND  TO  ADJACENT  FORMATIONS. 

Geologic  relations. — The  general  features  of  the  relations  of  the  Keweenawan  rocks  are 
described  in  Chapter  XV,  on  the  Keweenawan  series.  Here  are  described  certain  features  of 
these  relations  especially  well  exhibited  in  this  district.  These  are  particularly  the  superposi- 
tion of  the  Duluth  gabbro  upon  all  underljnng  rocks  and  the  relations  of  the  gabbro  to  the 
Logan  sUls  intrusive  in  the  Animikie  group. 

The  gabbro  and  the  sills  are  petrographically  the  same,  and  textural  gradations  have  been 
observed  which  indicate  their  close  relationship.  The  gabbro,  though  predominanth"  coarse- 
grained and  granular,  is  locally  fine-grained  and  poikilitic;  in  one  place  it  was  foxmd  as  a  dike 
in  the  Animikie  and  there  graded  into  a  porphyritic  facies  and  even  into  a  fine-grained  ophitic 
dolerite.  Locally  in  the  midst  of  the  thick  sills  the  rock  is  a  good  granular  gabbro  in  texture, 
and  it  ranges  fi-om  this  through  ophitic  poikilitic-textured  dolerites  into  fine-grained  aphanitic 
intersertal-textured  basalts  upon  the  selvage.  Mineralogicallj-  they  are  the  same,  except  that 
in  the  relatively  few  specimens  from  the  sills  which  have  been  studied  no  olivine  nor  Jiyper- 
sthene  has  been  observed,  nor  do  the  sills  show  such  great  mineralogical  variation  from  titan- 
iferous  magnetite  rocks  to  enormous  anorthosite  masses,  though  there  are  small  anorthosite 
masses  in  the  sills.  Such  differences  in  variation  are,  however,  easily  expHcablc  as  due  to  the 
enormous  difference  existing  between  the  masses  of  magma  forming  the  gabbro  and  that  forming 
the  individual  sills.  The  gabbro  and  niHs  are  therefore  regarded  as  essentially  contem])oraneous 
and  geneticallj'  related. 

The  gabbro  is  believed  to  be  a  great  laccohthic  mass  which  in  general  follows  approximately 
the  contact  plane  between  the  Animikie  group  and  the  Keweenawan.  In  the  Ycrniilion  district 
there  are  local  departures  from  this  relation.     Over  a  great  part  of  the  southern  edge  of  the 

o  Chauvenet,  W.  M.,  manuscript  notes. 

6  Mon.  U.  S.  Cieol.  Survey,  vol.  19,  1892,  PI.  XXXVH. 

c  Final  Rcpt.  Geol.  and  Nat.  Hist.  Survey  Minnesota,,  vol.  4,  1899,  pp.  487-t88, 


GUNFLINT  LAKE  DISTRICT.  203 

Vermilion  district  the  gabbro  followed  essentially  along  the  surface  of  unconformity  between 
the  upper  Huronian  (Animikie  group)  and  the  lower-lying  sediments,  uplifting  thereby  the 
upper  Huronian  sediments,  for  at  several  places  on  the  edge  of  the  Vermilion  district  and  just 
south  of  it  isolated  patches  of  the  lowest  part  of  the  Gunflint  formation  are  found  included  in 
the  Keweenawan  gabbro. 

In  the  eastern  part  of  the  Vermilion  district  the  gabbro  began  to  rise  and  cut  across  the 
upper  Huronian  (Animikie  group),  reacWng  higher  and  liigher  beds  to  the  east,  and  then  spread 
out  essentiall}^  along  the  plane  between  the  Animikie  and  the  base  of  the  Keweenawan,  sending 
sills  and  dikes  into  the  Rove  slate  (upper  Huronian)  and  also  into  the  Keweenawan  rocks,  as 
can  be  seen  on  Brule  Lake. 

Topography  as  related  to  geology. — The  line  of  contact  between  the  gabbro  and  the  older 
rocks  adjacent  to  it  is  fairly  well  marked  by  a  slight  topograpliic  break.  The  gabbro  normally 
has  a  steep  north  face,  in  some  places  showing  an  escarpment  of  varying  height.  It  is  nowhere 
very  liigh  but  is  considerably  higher  than  any  topograpliic  features  in  the  area  extending  a 
considerable  distance  north  of  it.  The  contact  at  many  places  is  marked  by  a  lake  or  a  stream. 
Tliis  (Ufference  between  the  topography  of  the  gabbro  area  and  that  to  the  north  exists  at  the 
immediate  contact,  but  in  general  the  gabbro  area  is  lower  than  that  underlain  by  the  older 
formation  to  the  north.  Locally  the  gabbro  area  has  been  reduced  almost  to  base-level.  In 
fact,  this  area  may  be  described  as  very  nearly  a  plain,  with  minor  but  pronounced  irregularities. 
The  uniformity  of  the  surface  is  due  in  great  part  to  the  homogeneous  character  of  the  gabbro 
mass,  owing  to  winch  it  has  been  about  equally  affected  by  the  various  agents  which  have 
attacked  it.  Most  of  the  minor  pronounced  irregularities  are  due  to  erosion,  which  has  been 
controlled  very  commonly  by  the  joints  of  the  gabbro,  and  to  differences  in  composition  where 
they  exist.  For  example,  the  anorthosite  masses  usually  stand  out  conspicuously  from  the 
surrounding  more  basic  and  less  resistant  portions  of  the  gabbro. 

The  lakes  of  the  gabbro  area  are  as  a  rule  shallow,  and  they  are  also  very  irregular  and 
can  not  be  said  to  have  uniform  length  in  any  one  direction,  as  is  so  markedly  true  of  the  lakes 
of  the  other  portions  of  the  Vermilion  district.  On  the  contrary,  they  spread  out  in  all  direc- 
tions, sending  off  numerous  bays,  of  which  some  are  very  long  and  narrow  and  all  are  very 
irregular  in  shape. 

The  Logan  sills  exercise  a  very  material  influence  upon  the  topography  of  that  portion  of 
the  district  north  of  the  gabbro  in  wliich  they  occur.  It  will  be  recalled  that  the  upper  Huronian 
(Animikie)  sediments  in  tliis  vicinity  have  a  monoclinal  dip  to  the  south.  The  sills  have  been 
injected  essentially  parallel  to  the  bedding  of  the  sediments,  though  occasionally  they  are  found 
cutting  across  the  beds  at  low  angles.  Erosion  has  been  most  active  in  tliis  portion  of  the 
district  in  a  direction  parallel  to  the  strike  of  the  beds,  and  consequently  most  of  the  large 
valleys  and  lakes  trend  in  agreement  with  these,  approximately  east  and  west.  The  resistant 
sills  now  form  the  caps  of  the  ridges,  the  slates  having  been  removed  down  to  the  sills.  The 
massive  rock  forming  the  sills  breaks  off  along  the  joint  planes,  and  as  a  result  perpendicular 
cliffs  are  formed  below  the  foot  of  which  talus  from  the  sills  and  from  the  easily  weathering 
Rove  slate  gives  a  gentle  slope.  These  sills  are  sometimes  very  nearly  concealed  by  the  accu- 
mulated talus  derived  from  them. 

The  effects  of  erosion  have  produced  a  series  of  liills  with  very  nearly  vertical  north  escarp- 
ments and  a  gentle  slope  from  the  crests  to  the  south.  Tliis  slope  corresponds  very  closely  to 
the  dips  of  the  Rove  slate  and  the  upper  surface  of  the  dolerite  sills. 

THE  IRON   ORES   OF  THE   GUNFLINT   LAKE  DISTRICT. 

In  the  vicinity  of  Gunflint  Lake  the  iron-bearing  formation  (Gunflint  formation)  is  mainly 
cherty  iron  carbonate  more  or  less  recrystallized  and  silicated  ami  more  or  less  oxidized  and 
hydrated  at  the  surface  and  next  to  fissures  and  certain  bedding  planes.  No  attempt  at  mining 
has  been  made  here. 


204  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

The  principal  hope  for  ore  in  the  Gunlhnt  formation  has  been  centered  in  the  vicinity  of 
the  Paulson  mine,  5  miles  west  of  Gunflint  Lake.  Here  the  formation  consists  of  dark-green 
to  black,  coarsely  crystalline  rocks,  consisting  of  magnetite,  quartz,  ampliiboies,  cordierite, 
fayalite,  augite,  pjTrhotite,  etc.,  tliinly  interlayered  in  varying  proportions.  FayaUte  is  especially 
abundant  in  rocks  rich  in  magnetite. 

Titaniferous  magnetites  in  the  Duluth  gabbro  are  described  on  page  56L 

CHEMICAL  COMPOSITION. 

An  analysis  representing  an  average  of  the  rock  from  a  drill  hole  penetrating  245  feet  into 
the  iron-bearing  formation  antl  an  analysis  of  a  surface  sample  taken  across  the  entire  width  of 
the  formation  give  the  followang  average: 

Chemical  composition  of  iron-bearing  Gunflinl  /ormalion. 

SiOj 60.  51 

AI2O3 ; 1.  20 

Fe..... 25.22 

MgO 52 

CaO 67 

NajO 00 

KjO 00 

HjO Small. 

P2O, 05 

S 59 

MnO, 92 

Tliis  is  almost  exactly  the  composition  of  the  ferruginous  cherts  or  taconites  of  the  Mesabi 
district.  The  significance  of  tlus  resemblance  is  discussed  in  connection  with  the  origin  of  the 
iron  ores,  in  Chapter  XVII  (pp.  499  et  seq.).  Bands  of  the  formation  a  few  feet  tliick  run  as  high 
as  50  or  55  per  cent  in  iron.  At  the  bottom,  where  it  rests  against  the  greenstone,  a  3-foot  layer  is 
encountered  running  above  55  per  cent  in  iron.  The  tliinness  of  the  ore  bands,  the  higJily  crys- 
talline, silicated,  magnetic  character  of  the  ore,  and  the  locally  high  sulphur  preclude  the  use  of 
the  ore  under  present  conditions.  On  the  otlier  hand,  the  total  amount  is  large,  the  phosphorus 
content  is  low,  and  it  lacks  titanium,  in  tliis  respect  contrasting  with  the  titaniferous  magnetites 
within  the  gabbro  mass  immediately  adjacent.  Magnetic  concentration  may  make  these  ores 
available  for  the  future,  though  the  tonnage  of  low-grade  ores  requiring  no  concentration,  with 
which  these  would  have  to  compete,  is  so  large  that  the  time  may  be  distant,  if  it  ever  comes, 
when  these  ores  can  be  concentrated  and  used  with  a  profit. 

PHYSICAL  CHARACTERISTICS. 

Tlie  ores  are  in  some  places  very  coarse  grained.  The  iron-bearing  formation  is  medium 
to  coarse  grained,  dense,  and  tough.  The  pore  space  is  less 'than  1  per  cent  and  usually  almost 
zero.  Specific  gravity,  determined  by  the  pycnometer  method,  on  a  pulverized  di'ill  sample  of 
245  feet  of  the  formation,  is  3.62,  and  that  for  the  ore  layers  is  4.08. 

PIGEON  POINT  DISTRICT." 

The  oldest  rocks  of  the  Pigeon  Point  district  (PI.  XII)  are  interbedded  slates  and  quartz- 
ites  of  the  Animikie  group  (upper  Iluronian).  Cutting  the  Animikie  rocks  is  an  olivine  gabbro, 
which  occupies  all  the  higlier  portions  of  the  point.  It  is  in  all  probability  the  lower  portion  of  a 
large  dike  whose  u])pcr  part  has  been  removed  by  denudation.  Between  the  gabbro  and  the  bedded 
rocks  in  many  places  are  successively  a  coarse-grained  red  rock,  a  fine-grained  red  rock  (quartz 
keratojihjTo),  and  a  series  of  contact  rocks.  The  main  masses  of  the  keratopluTe  occupy  a 
position  between  the  Animikie  sediments  and  the  gabbro.     Tliis  rock  has  all  the  characteristics 

o  See  Bayley,\V.S.,  The  eruptive  and  sedimentary  rockson  Pigeon  Point,  Minnnesota,  and  tlieir  contact  phenomena:  Bull.  U.S.  Geol.  Survey 
No.  109, 1893. 


U.  S.  GEOLOGICAL  SURVEY 
GEORGE   OTIS   SMITH,  DIHECTQB 


MONOGRAPH     Lll         PLATE     XII 


7j!  /'    .<= 


0*# 


CROSS  SECTION 


G      E      o      jsr 


D'abasf   CROSS  SECTION 


GEOLOGIC  MAP 

OF 

PIGEON  POINT,  MINNESOTA 

By"W.  S.Bayley  1890 

Scale  1:22600 


QUATERNARY 


t'ontour  interval  20  feet 
101O 

ALGONKIAN 


KEWEENAWAN  SERIES 


Pl<*iHl.OL-en*'  GriiJiular         Iiitennedial'> 

riejiosits  rcdinck  ividt 


01i\-ii\e 
gabbro 


Slaies  and       SjK>tLeds!atP6 
(luarl  z,it  es       eui.I  quartzites 


Topography  from  U.  S.  Lake  Survey 


ANIMIKIE  OR  LOON  LAKE  DISTRICT.  205 

of  an  eruptive  younger  than  the  gabbro.  The  coarse-grained  rocks  between  the  gabbro  and 
the  keratophyxe  are  intermediate  in  character  between  the  two  and  grade  into  them.  They  are 
therefore  regarded  as  a  contact  product  formed  by  tlie  intermingling  of  tJie  gabbro  and  kera- 
tophyre  magmas.  Between  the  keratophyre  and  tlie  slates  and  (juartzites  of  the  Animikie 
group  there  are  tliree  zones  sliowing  different  grades  of  alteration  of  the  sedimentary  rocks  due 
to  the  contact  with  the  igneous  rock. 

ANIMIKIE  OR  LOON  LAKE  DISTRICT  OF  ONTARIO. 
LOCATION  AND  GENERAL  SUCCESSION. 

The  Animikie  district  proper  includes  the  area  about  Animikie  or  Thunder  Bay,  on  the 
northwest  coast  of  Lake  Superior,  but  detailed  study  has  been  made  ])rincipally  of  the  part  of 
the  district  near  Loon  Lake,  at  the  east  end  of  the  bay,  about  25  miles  east  of  Port  Arthur  (see 
PI.  XIII),  and  to  this  part  of  the  district  the  following  description  applies.  It  is  taken  largely 
from  descriptions  by  W.  N.  Smith  °  and  R.  C.  Allen.'' 
The  succession  of  rocks  is  as  follows: 
Quaternary  system: 

Pleistocene  series Glacial  drift. 

Algonkian  system: 

Keweenawan  series Conglomerate,  sandstone,  marl,  diabase  sills  (Logan  sills). 

Unconformity. 
Huronian  series: 

Upper  Huronian  (Animikie  aroup).  .w       , '^     .  ' 

llron-bearmg  formation. 

Unconformity. 

Lower-middle  Huronian Graywacke,  slate,  and  conglomerate,  with  greenstone  and 

granite  intrusive  rocks. 

Unconformity. 

Archean  system: 

Laurentian  series Granites  and  gneisses,  intrusive  into  Keewatin  series. 

Keewatin  series Jreen  schists,  greenstone,  mashed  porphyries. 

ARCHEAN   SYSTEM. 

The  Keewatin  series  outcrops  along  Current  River  5  or  6  miles  northeast  of  Port  Arthur, 
along  the  Canadian  Pacific  Railway,  near  milepost  119  and  west  of  it  about  a  mile.  It  com- 
prises a  variety  of  green  schists  and  mashed  porphyries.  Evidence  of  the  extreme  deformation 
to  which  these  rocks  have  been  subjected  is  found  in  their  folded  and  schistose  structures.  The 
schistosity  is  nearly  vertical  with  strike  N.  70°  E. 

Laurentian  rocks  are  not  present  in  the  district  itself,  but  form  part  of  the  granitic  hills 
to  the  north. 

ALGONKIAN   SYSTEM. 

HURONIAN  SERIES. 

LOWER-MIDDLE    HURONIAN. 

KINDS  OF  SOCKS. 

The  lower-middle  Huronian  occupies  the  central  part  of  the  area  between  the  upper 
Huronian  (Animikie  group)  and  the  Keweenawan  on  the  east  and  the  Animikie  and  the 
Keewatin  on  the  west.  The  detrital  member  of  the  lower-middle  Huronian  is  represented 
mainly  by  a  great  thiclcness  of  graywacke,  which  is  believed  to  be  correlated  with  the  Knife 
Lake  slate  of  the  Vermilion  district  of  Minnesota.  At  the  base  of  the  graywacke  is  a  considerable 
thickness  of  schistose  conglomerate  carrying  fragments  of  black  jasper  and  of  a  great  variety 
of  green  schists.     It  marks  the  unconformity  between  the  Keewatin  and  lower-middle  Huronian. 

a  Loon  Lake  iron-bearing  district  of  Ontario:  Rept.  Ontario  Bur.  Mines,  vol.  14, 1905,  pt.  1,  pp.  254-2'>n. 

6  Unpublished  thesis,  University  of  Wisconsin.    See  also  Silver,  L.  P.,  The  Animikie  iron  range:  Rept.  Ontario  Bur.  Mines,  vol.  15, 1906,  pt.  1, 
pp.  156-172. 


206  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

The  conglomerate  grades  up  into  the  graj^wacke,  which  in  its  lower  horizons  is  quartzose.  The 
actual  contract  l)ctwccn  the  lower-middle  Iluronian  and  Keewatin  was  not  observed,  the  two 
usually  being  separated  by  a  slight  topographic  depression,  which  near  milepost  119  is  but 
14  paces  broad. 

The  metamorphism  of  the  graywacke  has  almost  obliterated  the  bedding,  but  where 
bedding  was  ol)scrved  it  was  found  to  be  more  or  less  discordant  with  the  cleavage,  which 
varies  in  dip  from  65°  S.,  a  mile  or  more  south  of  the  Canadian  Pacific  Railway,  through  the 
vertical  to  6.5°  \.  on  and  north  of  the  graywacke  ridge  which  runs  |)arallel  to  and  a  short  distance 
south  of  tiic  Canadian  Pacific  Railway. 

Near  the  base  of  the  graywacke  is  found  locally  a  considerable  thickness  of  volcanic  tuff 
and  amygdaloid.  This  formation  is  best  exposed  about  1^  miles  south  of  milepost  110  on  the 
Canadian  Pacific  Railway,  in  a  strip  several  hundred  yards  wide  and  a  mile  or  more  long.  In 
places  it  appears  to  be  conglomeratic,  showing  a  decided  banding  which  looks  very  much 
like  bedding,  and  in  other  places  it  is  vesicular,  the  vesicules  being  filled  with  secondary'  minerals. 

The  gradation  was  but  imperfectly  observed  in  a  single  outcrop,  but  these  tuffs  and 
amj'gdaloids  seem  to  grade  both  parallel  to  the  strike  and  across  it  into  the  normal  phase  of  the 
graywacke. 

INTRUSIVES.  1 

The  graywacke  is  intruded  by  a  variety  of  granites  and  greenstones.  All  the  granites 
and  some  of  the  greenstones  are  massive  and  cut  across  the  strike  of  the  cleavage  in  the  graj-- 
wacke.  Near  some  of  these  intrusive  masses  the  graywacke  is  decidedly  more  schistose, 
especially  in  the  area  north  of  the  Canadian  Pacific  Railway,  where  the  intrusion  of  the  granites 
is  more  intimate  than  elsewhere.     Here  the  graywacke  locally  becomes  a  hornblende  schist. 

The  granite  forms  the  hUls  north  of  the  Anunikie  district  and  is  correlative  in  age  and 
topography  with  the  Giants  Range  granite  of  the  Mesabi  district. 

UPPER    HtmONIAN    (ANIMIKIE    GROUP). 
GENERAL  DESCRIPTION. 

The  iron-bearing  Animikie  group  dips  gently  to  the  southeast  across  the  steeply  inclined 
structures  of  the  underlymg  series  at  angles  locally  varying  widely,  but  averaging  from  2°  to  7°. 
It  outcrops  in  two  mam  areas,  the  first  between  Loon  Lake  and  the  head  of  Thunder  Bay,  the 
second  along  the  shores  of  Thunder  Bay  in  the  vicinity  of  Port  Arthur. 

The  Animikie  sediments  comprise  two  distinct  zones,  as  follows: 

Thickness. 

A  black  slate  formation  (total  thickness  not  present) 50  to    60  feet. 

An  iron-bearing  formation,  including: 

An  upper  iron-bearing  member 250  to  300  feet. 

An  interbedded  black  slate 25  to    30  feet. 

A  lower  iron-bearing  member 50  to    60  feet. 

A  thin  basal  conglomerate 5  to    18  inches. 

The  sediments  are  intruded  by  diabase  sills  varying  up  to  35  or  40  feet  in  thickness. 

IRON-BEARING  FORMATION. 

The  iron-bearing  formation  of  this  district  is  believed  to  be  the  same  as  the  Gunflint  for- 
mation of  the  Vermilion  district,  for  it  has  been  seen  in  almost  continuous  exposure  between 
this  district  and  the  Vernulion  district. 

Conglomerate. — The  base  of  the  Animikie  group  is  marked  bj-  a  thin  but  persistent  layer 
of  conglomerate,  wiiich,  as  shown  in  open  pits  and  by  drill  cores  from  bormgs  m  the  vicinitj- 
of  Loon  Tjake,  varies  from  5  to  18  inches  in  thickness.  The  pebbles  m  the  conglomerate  are 
small  anil  predominantly  of  vein  c(uartz.  Small  patches  of  it  found  on  the  graywacke  ridge 
east  of  McKenzie  antl  on  the  Keewatin  schists  near  Current  River,  about  5  miles  northeast  of 
Port  Arthur,  attest  the  original  extension  of  the  Animikie  group  over  the  entire  area. 


U.  S.  GEOLOGICAL  SURVEY 


MONOGRAPH  LM     PL.  XIII 


0  ;••  ••  J 

"^  ■•'•■"'  -la 

V '. ■■ '.  •t'.h-ci '.' '■<■  '■- p. '.9 :'■■'?■ 


Conglomerate, 
sandstone,and  marl 


Diatase  sills 
(Log'aa  sills) 

HURONIAN   SERIES 


'  UPPER  huronian(animikiegroup) 


LOWER-MIDDLE   HURONIAN 


3  Miles 


+  +  +  +  + 

h  +  +  +  -r  H 
+  +  +  +  + 
f  +     +     +     -t-    ^ 


vCLl-'--;' 


IroTi-l:>cririii'^'  I'Muiation 
ajad  slate 


Gra>Tvacke  and 
gre  ens  tone 


Granite 


GEOLOGIC    MAP    OF    THE    ANIMIKIE   IRON-BEARING    DISTRICT,    NORTH    OF    THUNDER    BAY,    ONTARIO. 

By  W.  N.  Smith  and  R,  C.  Allen-     See  page  205.  - 


ANIMIKIE  OR  LOON  LAKE  DISTRICT.  207 

Lower  iron-bearing  member. — In  appearance  the  lower  iron-bearing  member  resembles  the 
ferrugmous  chert  or  "taconite"  of  the  Mesabi  district  of  Minnesota,  but  it  is  peculiar  in  that 
it  carries  a  large  amount  of  calcium-magnesium-iron  carbonate.  The  carbonate  may  be 
wholly  secondary.  It  occurs  in  large  part  as  coarsely  crystalline  siderite.  A  smgle  hand 
specimen  may  be  found  to  contain  crystalline  siderite,  iron  ore,  and  typical  "taconite,"  which 
contains  small  granules  embedded  in  a  cherty  matrix,  thus  closely  resembling  the  altered 
greenalite  rock  of  the  Mesabi  district.  However,  it  may  be  that  both  the  iron  silicate  and 
most  of  the  iron  carbonate  were  deposited  simultaneously.  In  the  Mesabi  district  the  original 
iron-bearing  rock  was  predominantly  a  ferrous  silicate,  in  the  Penokee-Gogebic  district  a 
ferrous  carbonate  with  very  subordinate  ferrous  silicate.  The  lower  iron-bearing  member  in 
the  Animikie  district  may  have  been  originally  made  up  of  approximately  equal  amounts  of 
the  sUicate  and  carbonate. 

Certain  of  the  layers  of  this  member  are  sufficiently  rich  in  iron  oxide  or  low  in  siliceous 
bands  to  give  thin  zones  of  iron  ore.  Bands  6  to  8  feet  thick  contain  30  to  46  per  cent  of  iron. 
The  grade  may  be  easily  raised  by  sorting  out  the  siliceous  bands.  The  possible  commercial 
value  of  these  deposits  is  in  their  wide  horizontal  extent.  Ores  also  appear  in  small  irregular 
bodies,  following  the  fault  plane  north  of  Deception  Lake  and  extending  eastward  to  Silver 
Lake  and  south  and  east  of  Bittern  Lake. 

Interbedded  slate. — Near  the  top  of  the  "taconite"  zone  is  found  a  black  slate  interbedded 
at  more  or  less  irregular  intervals  with  the  "taconite"  below  and  the  Iron  carbonate  above. 
The  relations  are  those  of  gradation  through  continuous  deposition. 

Upper  iron-bearing  member. — The  rock  making  up  almost  the  whole  of  the  upper  iron- 
bearing  member  is  a  cherty  iron  carbonate  similar  in  every  way  to  the  iron  carbonate  of  the 
Penokee  district.  It  exhibits  all  phases  of  alteration  from  iron  carbonate  to  iron  ore.  Some 
of  it  is  coarsely  crystallized,  as  though  from  secondary  metamorphism. 

The  iron  ores  occur  principally  along  the  fault  zones  already  mentioned  in  connection 
with  the  lower  iron-bearing  member.     These  also  cut  the  upper  iron-bearing  member. 

UPPER  BLACK  SLATE. 

In  its  normal  phase  the  upper  slate  is  made  of  thinly  bedded  layers,  black  but  weathering 
to  a  rusty  bro'svn.  Locally  it  bears  an  abundance  of  mica.  Most  of  the  mica  plates  lie  with 
their  greatest  and  mean  diameters  in  the  plane  of  bedding,  but  many  of  them  cut  across  the 
bedding  at  various  angles.  This  phase  of  the  rock  has  not  been  studied  microscopicaUy,  but 
the  mica  plates  look  more  like  detrital  fragments  than  secondary  minerals  developed  in  place, 
for  they  occur  in  separated  spangles  and  not  in  continuous  layers,  as  commonly  sho%vn  in  rocks 
having  a  development  of  secondary  mica.  Furthermore,  where  outcrops  of  the  micaceous 
slate  occur  there  is  no  evidence  of  metamorphic  conditions  such  as  commonly  develop  mica; 
and  where  it  occurs  in  contact  with  intrusive  diabase  sills  the  metamorphic  eifects  of  the  intru- 
sion are  seen  not  to  extend  more  than  a  fraction  of  an  inch  from  the  plane  of  contact,  so  the 
mica  is  probably  not  a  product  of  metamorphism  attendant  upon  the  intrusion  of  the  diabase. 
Therefore  it  is  believed  to  be  clastic  in  origin. 

KEWEENAWAN  SERIES." 
GENERAL  DESCRIPTION. 

Unconformably  above  the  upper  Iluronian  (Animikie  group)  is  a  succession  of  conglom- 
erates, sandstones,  and  impure  marls,  to  which  the  term  "Nipigon"  series  has  been  applied 
by  the  Canadian  Survey.  These  rocks,  however,  are  now  known  to  belong  to  the  Keweenawan 
series,  and  the  name  "Nipigon"  has  been  abandoned  by  the  United  States  Geological  Survey. 
This  series  is  most  fully  developed  east  of  Loon  Lake.  The  unconformity  between  it  and  the 
underlying  rocks  is  marked  in  various  ways.  At  the  base  of  the  Keweenawan  is  a  coarse  con- 
glomerate containing  waterworn   pebbles    and  bowlders  of  all  the  underlying  rocks,  among 

a  See  Chapter  XV  (pp.  366-426)  for  general  discussion  of  Keweenawan  series. 


208  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

which,  however,  granite  and  the  iron-bearing  formation  are  predomiiumt.  The  Keweenawan 
series  shows  comparatively  little  metamorphism,  ev(!n  less  than  the  Aniniikie  grotij).  The 
strikes  and  dips  of  the  Keweenawan  are  always  more  or  less  discordant  witii  the  strikes  and 
dips  of  the  underlying  formations.  The  strongest  evidence  of  the  great  time  interval  repre- 
sented by  tlie  unconformity  is,  however,  the  fact  that  the  Keweenawan  is  found  successively 
overlying  both  tiie  Animikie  group  and  the  lower-middle  Humnian  rocks,  thus  showing  that  the 
entire  Animikie  group  and  part  of  the  lower-middle  Huronian  had  been  truncated  by  erosion 
before  the  Keweenawan  series  was  deposited. 

LOGAN  SILLS. 

The  Animikie  group  is  intruded,  mainly  parallel  to  the  bedding,  by  a  series  of  diabase 
sills  of  Keweenawan  age,  which  seem  to  follow  j)referably  the  slate  horizons.  By  jointing, 
these  sills  have  been  broken  up  into  great  columnar  blocks,  the  breaking  off  of  which  where 
the  sills  are  exposed  maintains  vertical  cliffs,  a  characteristic  feature  of  the  topography  in  this 
district.  These  sills  are  laccolithic  in  character."  At  one  locality  about  half  a  mile  south  of 
Deception  Lake  the  diabase  outcrojis  in  the  shape  of  a  great  flat  dome,  the  overlying  slates 
dipping  away  from  it  in  all  directions. 

The  metamorphic  effect  of  the  intrusion  on  the  slates  and  iron-bearing  formation  is  hardly 
perceptible  more  than  a  fraction  of  an  inch  away  from  the  plane  of  contact.  In  certain  localities 
the  iron-bearing  formation  in  the  vicuiity  of  the  diabase  is  very  slightly  magnetic,  indicating 
some  development  of  magnetite.  The  slight  metamorphic  effect  of  the  diabase  intrusions 
may  be  ascribed  to  rapid  cooling  of  the  magma.  The  fmeness  of  grain  of  the  diabase  suggests 
that  the  sills  were  not  deep-seated  intrusives.  Thus,  being  thin  and  also  near  the  surface,  they 
cooled  rapidly,  the  heat  being  conducted  away  from  them  by  the  cooler  rocks  adjacent. 

The  diabase  which  forms  the  laccolithic  Logan  sills  of  the  Animikie  group  is  also  found 
both  overlying  and  cutting  the  Keweenawan  sediments. 

STRUCTURAL,  FEATURES. 

The  main  structural  characteristic  of  the  area  is  the  general  dip  to  the  southeast ;  in  this 
it  conforms  to  its  geographic  position  as  a  portion  of  the  north  side  of  the  Lake  Superior  syn- 
clinal basin.  The  upper  surface  of  the  Keewatin  series  and  lower-middle  Huronian  rocks 
shares  in  the  general  slope  to  the  south,  although,  as  previously  noted,  this  does  not  apply  to 
the  bedding  and  schistosity  of  the  rocks.  The  normal  strike  of  the  Animikie  group  is  to  the 
northeast,  with  an  average  dip  of  about  7°  SE.  Locally,  however,  the  rocks  have  been  closely 
folded  and  the  resulting  strikes  and  dips  are  widely  divergent  from  the  normal.  The  general 
strike  of  the  Keweenawan  is  east  of  north,  with  flat  dip  to  the  southeast,  although  it  also  locally 
shows  the  same  severe  folding  and  fracturing  as  the  Animikie. 

Faulting  has  been  an  important  factor  in  producing  the  present  structural  and  topogi-aphic 
features  of  the  district.  The  faulting  is  believed  to  have  been  caused  by  the  same  general 
forces  that  produced  the  Lake  Superior  basin.  (See  pp.  622-623.)  The  major  fracturmg 
occurred  along  certain  approximately  parallel  zones,  and  in  the  vertical  displacements  that 
followed  the  several  fracture  blocks  acted  as  independent  units,  in  which  the  northern  units 
became  depressed  relative  to  the  southern  units,  thus  producing  a  system  of  "block"  faults. 

The  greatest  vertical  displacement  defmitely  determined  is  about  300  feet,  as  shown  from 
diamond-drill  records  and  surface  exposures  along  the  east-west  fault  a  short  distance  south 
of  Loon  Lake. 

GENERAL    TOPOGRAPHIC    FEATURES    IN    THEIR    RELATIONS    TO    GEOLOGY. 

As  seen  from  a  point  north  of  Loon  Lake  on  the  high  range  of  hills  extending  from  Pearl 
River  station  beyond  McKenzie,  the  region  as  a  whole  presents  a  general  slope  toward  Lake 
Superior.     To  the  north  the  country  rises,  the  granite  hills  towermg  one  above  another,  and 

a  Lawson,  A.  C,  The  laccolitic  sills  of  the  northwest  coast  of  Lake  Superior:  Buli.  Geol.  and  Nat.  Hist.  Sun-ey  Minnesota  No.  S,  1893, 
pp.  2-1-48. 


ANIMIKIE  OR  LOON  LAKE  DISTRICT.  209 

to  the  south  the  hikeward  slope  is  interrupted  by  the  long,  narrow  McKenzie  Valley,  beyond 
the  southern  rira  of  which  the  general  slope  is  continued  down  to  the  shores  of  Thunder  Bay. 
East  of  Loon  Lak§  the  range  of  Keweenawan  sandstone  hills  forming  the  southern  side  of  the 
valley  swings  at  a  right  angle  to  the  southeast,  and  the  valley  emerges  on  a  broad  flat  timbered 
with  spruce  and  tamarack  and  sloping  gently  down  to  Black  Bay.  To  the  southeast  the  ele- 
vated and  much  dissected  area  of  Keweenawan  sandstone  projects  into  the  lake  a  distance  of 
20  or  25  miles,  forming  a  peninsula  separating  the  waters  of  Black  and  Thunder  bays.  This 
peninsula,  crowned  at  its  lakeward  end  by  a  great  protective  cap  of  diabase,  terminates  in  a 
bold  headland  over  1,300  feet  high,  laiown  as  Thunder  Cape.  The  great  escarpment  of  sand- 
stone 600  to  800  feet  high  forming  the  northwestern  side  of  this  peninsula  and  extending  2  or  3 
miles  inland  is  one  of  the  most  striking  scenic  features  of  the  north  shore.  West  of  Thunder 
Cape,  Pie  Island,  with  its  great  flat  protecting  top  of  diabase  rising  700  or  800  feet  above  the 
water,  stands  like  a  sentinel  at  the  entrance  to  Thunder  Bay.  North  of  the  island,  on  the  main- 
land south  of  Fort  William,  McKays  Mountain,  another  great  flat  sheet  of  diabase,  supported 
on  Animikie  sediments,  rises  abruptly  from  the  plain  of  Kaministikwia  River  to  a  height  of 
over  1,000  feet.  Thunder  Cape,  Pie  Islantl,  and  McKays  Mountain  are  magnificent  examples 
of  the  mesa  type  of  topography,  which  is  a  distinct  characteristic  of  the  Thunder  Bay  region. 

The  origin  of  this  mesa-like  topography  is  found  in  the  prevalence  of  diabase  sills  underlain 
at  varying  altitudes  by  strata  of  weaker  rocks,  the  sapping  of  which  maintains  a  progressive 
undermining  of  the  great  columnar  blocks  above  them,  thus  producing  vertical  cUffs  with 
talus  slopes  beneath. 

WESTWARD   EXTENSION   OF   THE  ANIMIKIE  DISTRICT. 

The  Animikie  group,  containing  the  iron-bearing  formation,  extends  westward  from 
Animikie  Bay  to  the  Gunflint  Lake  district,  with  structural  and  lithologic  features  like  those 
at  its  east  end,  although  in  the  vicinity  of  Port  Ai-thur  and  thence  westward  the  amount  of 
slate  exposed  to  the  south  and  above  the  iron-bearing  formation  is  much  larger.  The  slates 
with  their  intrusive  sills  are  beautifully  exposed  in  Pie  Island  and  McKays  Mountain  and  many 
of  the  hills  to  be  observed  along  the  line  of  the  Port  Arthur  and  Western  Railway.  The  saw- 
toothed  topography  characteristic  of  both  the  Gunflint  and  the  Loon  Lake  districts  is  every- 
where to  be  seen,  with  its  gently  dipping  slopes  to  the  south,  usually  capped  l)y  diabase  sills, 
and  abrupt  slopes  to  the  north.     The  drainage  for  the  most  part  follows  parallel  to  the  strike. 

The  older  rocks  on  which  the  Animikie  group  rests  include  the  same  kinds  as  were 
observed  in  both  the  Animikie  and  Loon  Lake  districts,  but  they  have  not  been  mapped  in 
detail  for  all  of  this  intervening  area. 

THE  IRON   ORES   OF  THE  ANIMIKIE  DISTRICT   OF   ONTARIO. 

OCCURRENCE. 

Iron  ores  approaching  commercial  grade  are  known  only  in  a  small  area  near  Loon  Lake, 
25  miles  east  of  Port  Arthur.  The  ore  deposits  are  thin  but  extensive  layers  of  hematite  in 
the  ferruginous  cherts  of  the  lower  part  of  the  formation.  In  one  zone,  and  perhaps  in  others, 
ores  have  developed  along  fault  and  joint  planes.  The  thickness  of  the  ore  layers  which  can  be 
mined  will  depend  on  the  grade  wliich  can  be  utilized  and  on  the  success  with  which  chert  layers 
may  be  eliminated  by  hand  sorting.  Eight  feet  is  about  the  greatest  thickness  of  a  bed  which 
would  run  as  high  as  45  per  cent,  but  with  a  small  amount  of  hand  sorting  two  or  three  times 
this  thickness  could  be  used.  The  commercial  importance  of  the  ores  obviously  depends  on 
their  horizontal  dimensions.  The  ores  rest  upon  ferruginous  cherts  and  grade  into  them  lateraUy. 
One  of  the  beds  is  capped  by  a  diabase  sill  intruded  parallel  to  the  bedding. 

47517°— vol,  52— 11 14 


210 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


CHARACTER  OF  THE  ORE. 

The  ore  is  a  lean,  banded  siliceous  lieinatitc,  more  or  less  liydrated.  Analyses  of  samples 
taJien  every  3  inches  from  four  exposures  representing  vertical  distances  of  6  to  8  feet  each  are 
given  below.  These  are  from  the  natural  exposures  which  showed  the  greatest  observed  con- 
centration and  include  both  the  hematite  and  associated  siliceous  material. 

Analyses  of  Animikie  ore. 


Iron 

Phosphorus 

Sulphur 

Silica 


45.81 

45.22 

30.76 

.020 

.017 

.160 

.024 

.028 

.058 

31.91 

33.13 

35.  OC 

30.21 
.256 
.036 

37.1) 


SECONDARY  CONCENTRATION  OF  THE  ANIMIKIE  ORES. 

Structural  conditions. — The  movement  of  waters  here  has  obviously  been  controlled  by  the 
bedding,  for  the  ores  constitute  merely  enriched  layers  with  irregular  lateral  extent.  To  some 
extent  also  the.  waters  have  been  concentrated  in  the  intersecting  faults.  The  formation  i? 
very  thin  and  is  subdivided  by  impervious  igneous  sills,  making  such  movement  of  water  as  i? 
possible  in  the  formation  essential!}"  a  horizontal  one. 

Original  character  of  the  iron-bearing  formation. — As  described-  on  page  207,  the  lower  part 
of  the  iron-bearing  formation  of  tlie  Animikie  group  was  originally  a  greenalite  rock  with  some 
carbonate  and  the  upper  part  was  originally  an  iron  carbonate  with  soine  greenalite. 

Nature  of  alterations. — The  original  greenalite  and  carbonate  rocks  have  altered  prin- 
cipally to  ferruginous  cherts  in  the  manner  described  for  other  ranges.  Local  and  for  the 
most  part  subsecjuent  alteration  of  the  ferruginous  cherts  by  leaching  of  sihca  has  devehjped 
the  ore.     Coarsely  crystalline  secondary  iron  carbonate  is  abundant. 


SEQUENCE  OF  ORE  CONCENTRATION. 

The  alteration  of  the  iron-bearing  formation  has  occurred  both  before  and  since  Kewee- 
nawan  time.  Evidence  of  the  pre-Keweenawan  alteration  lies  in  the  abundant  fragments 
of  ferruginous  chert  and  iron  ore  wliich  occur  in  the  Keweenawan  conglomerates.  Evidence 
of  later  alteration  is  the  fact  that  the  deformation  which  produced  fracturing  and  breccia- 
tion  of  the  iron-bearing  formation,  and  which  in  part  determined  the  localization  of  tlie  ore 
concentration,  was  later  than  Keweenawan  time,  as  is  shown  by  the  similar  phenomena  of 
deformation  in  superjacent  Keweenawan  beds. 


CHAPTER  IX.  THE  CUYUNA  IRON  DISTRICT  OF  MINNESOTA  AND  ITS 
EXTENSIONS  TO  CARLTON  AND  CLOQUET,  AND  THE  MINNESOTA 
RIVER  VALLEY  OF  SOUTHWESTERN  MINNESOTA. 

CUYUNA  IRON  DISTRICT  AND  EXTENSIONS  TO  CARLTON  AND  CLOQUET. 

GEOGRAPHY  AND  TOPOGRAPHY. 

The  Cuyuna  iron  district  is  the  most  recently  discovered  range  in  the  Lake  Superior  region, 
and  as  such  is  receivmg  a  large  share  of  attention.  It  trends  N.  50°  E.  along  the  line  of  the 
Northern  Pacific  Railway,  near  Mississippi  River,  in  the  vicinity  of  the  towns  of  Brainerd 
and  Deerwood,  Crow  Wing  County;  Aitkin,  Aitkin  County;  and  Randall,  Morrison  County, 
in  north-central  Minnesota.  (See  Pis.  XIV  and  XV.)  Its  boundaries  are  still  being  extended 
and  limits  can  not  yet  be  drawn  with  certainty  in  any  direction.  The  area  of  present 
greatest  activity  lies  south  and  east  of  Mississippi  River  in  Tps.  4.3  to  48  N.,  Rs.  28  to  32  W. 
The  length  is  more  than  60  miles  and  the  area  for  exploration  amounts  approximately  to  32,000 
acres. 

The  general  geologic  and  geographic  relations  of  the  Cuyinia  district  to  the  adjacent  terri- 
tory appear  on  Plate  XIV.  A  larger-scale  map  of  the  Cuyuna  district  itself,  showing  magnetic 
belts,  is  Plate  XV.  This  map  is  not  colored  geologically  for  the  reason  that  the  district  is  heavily 
drift  covered  and  the  distribution  of  the  underlying  rocks  is  known  only  incompletely  from 
drill  holes.  A:iy  map  attemjjting  to  show  geologic  l)oundaries  would  be  sadly  out  of  date  by 
the  time  of  publication.  However,  the  magnetic  lines  follow  approximately  the  distribution  of 
the  iron-bearing  rocks. 

The  countiy  is  flat,  being  not  less  than  1,150  feet  nor  more  than  1,300  feet  above  sea  level. 
It  is  covered  with  a  heavy  mantle  of  glacial  drift  and  dotted  with  many  glacial  hills,  lakes,  and 
swamps. 

The  rock  surface  beneath  the  drift  shows  slight  local  variations  in  elevation,  and  between 
widely  separated  points,  because  of  the  general  slope  of  the  surface,  may  show  a  difl'erence  of 
elevation  of  as  much  as  250  feet.  Frequently  the  soft  slates  are  found  to  be  at  lower  elevations, 
because  of  erosion,  than  the  hai-der  iron-bearing  formation  adjacent — as,  for  instance,  near  Pick- 
ands,  Mather  &  Co.'s  shaft  in  sec.  8,  T.  45  N.,  R.  29  W.  Notwithstanding  these  local  irregu- 
larities of  the  rock  surface,  it  is  generally  flat.  At  many  jilaces  in  the  district  and  m  adjacent 
parts  of  Minnesota  Cretaceous  deposits  are  found  just  above  the  rock  surface  and  beneath  the 
drift,  suggesting  that  this  flat  surface  may  be  part  of  a  pre-Cretaceous  base-level  or  peneplain. 

The  Cuyuna  district  has  almost  none  t>{  the  external  aspects  commonly  associated  with  a 
Lake  Superior  iron  range.  The  conspicuous  topographic  ranges  are  lacking,  as  well  as  the 
numerous  rock  exposures. 

SUCCESSION   OF   ROCKS. 

From  the  information  so  far  available,  consisting  largely  of  drill  samples,  the  succession  of 
rocks  for  the  Cuyuna  district  is  as  follows: 

Quaternary  system: 

Pleistocene  series Glacial  drift  of  late  Wisconsin  age,  35  to  400  feet  thick. 

Cretaceous  system Sediments,  thin  and  in  small  areas. 

211 


212  GEOLOGY  OF  THE  LAKE  SLTERIOR  REGION. 

Algonkian  eystem: 

Keweenawan  (?)  Herios. .  .Igneous  rocks,  extrusive  and  intrusivo,  basic  and  acidic. 


•  Upper     Iluroniiiii 
(Animikie   grou]j  t 


Huronian  series: 

Virginia  ("St.  Louis")  slate:  Chloritic  and  carbonaceous  slates,  with 
small  amounts  of  interbedded  graywacke,  quartzite  and  limestone. 
Thickness  unknown  but  great.  Where  intruded  by  Keweenawan  (?) 
igneous  rocks,  this  formation  consists  of  gametiferous  and  slaurolit- 
ifcrous  biotite  schists  and  hornblende  schists. 

Deerwood  iron-bearing  member  of  ^'irginia  slate,  consisting  princiiially 
of  iron  carbonate  where  unaltered,  but  largely  altered  to  amphibole- 
magiietite  rocks,  ferruginous  slate  and  chert,  and  iron  ore.  Found 
in  lenses  in  the  Virginia  slate,  presumably  near  the  base. 

ALGONKIAN   SYSTEM. 

HTJBONIAN  SERIES. 

UPPER    HURONIAN    (aNIMIKIE    GROUP). 

GENERAL  STATEMENT. 

The  upper  Huronian  rocks  of  this  district,  comprising  the  Virginia  ("St.  Louis")  slate 
and  its  Deerwood  iron-bearing  member,  are  not  separated  for  much  of  the  district,  but  are 
interbedded  and  have  sunilar  structure.  They  are  accordingly  described  together.  The  slate, 
hitherto  knowai  as  the  "St.  Louis"  slate,  has  been  correlated  with  the  Virginia  slate  of  the 
Mesabi  district.  The  name  "St.  Louis  "  as  apphed  to  this  slate  has  priority  over  Virginia  slate, 
but  it  is  preoccupied  by  the  well-known  Carboniferous  formation  of  the  Mississippi  Valley. 
The  formation  will  therefore  be  called  Virginia  slate  in  this  monograph.  The  iron-bearing  rocks 
in  this  district  have  not  been  satisfactorily  correlated  with  the  Biwabik  formation  of  the  Mesabi 
district,  and  for  them  the  new  name  Deerwood  iron-bearing  member  is  here  introduced,  from 
their  typical  development  at  and  near  Deerw^ood,  in  this  district.  The  iron-bearing  beds,  being 
interbedded  in  the  Virginia  ("St.  Louis")  slate,  properly  constitute  a  member  of  the  slate  and 
are  so  treated  in  this  report. 

DISTRIBTTTION   AND    STRTJCTTTRE. 

Sediments  of  upper  Huronian  age  occupy  practically  all  of  the  rock  surface  beneath  the 
drift.  They  have  been  bent  into  repeated  folds,  as  shown  by  drilling  and  magnetic  work.  In 
the  southern  part  of  the  district  the  folding  has  been  so  close  that  the  beds  generally  stand  at 
angles  of  about  80°  with  the  horizon,  though  locally  varying  at  the  ends  of  pitchmg  folds. 
Toward  the  north  the  folding  is  less  close  and  flatter  dips  are  common.  The  folding  has  been 
accompanied  by  the  development  of  cleavage  in  the  softer  layers,  especially  in  the  softer  slates. 
Wliere  the  cleavage  can  be  definitely  distinguished  from  the  bedding,  there  is  usually  a  slight 
angle  between  them  and  the  cleavage  has  the  steeper  dip.  The  iron-bearing  member  itself  is 
less  aflfected  by  the  cleavage  than  the  slate.  The  axial  lines  of  folds  and  cleavage  strike  east- 
northeast — that  is,  about  parallel  with  the  axis  of  the  Lake  Superior  synclihe. 

The  iron-bearing  member  thus  far  found  seems  to  be  in  the  fonn  of  lenses  whose  longer 
dimensions  are  parallel  to  the  higlily  tilted  bedding  of  the  series.  The  wall  rocks  are  various 
phases  of  the  Virginia  ("St.  Loiiis")  slate.  Intrusive  rocks  locally  comphcate  these  relations. 
Along  the  strike  these  lenses  pinch  out  or  widen  and  are  locally  buckled  by  the  drag  type  of 
fold  (fig.  12,  p.  123).  It  is  dillicult  to  tell  from  the  present  state  of  exploration  just  how  far  the 
parallel  lenses  are  independent  lenses  at  different  horizons  in  the  Virginia  slate  and  Ikuv  far 
the)^  may  be  the  result  of  duplication  by  folding.  The  broader  features  of  ilistribiition  are 
undoubtedly  to  be  explained  by  folding.  There  is  a  narrow  zone  of  iron-bearing  rocks  known 
locally  as  the  "south  range,"  extending  from  a  point  east  of  Aitkin  southwest  past  Deerwood 
and  Brainerd  and  west  of  Mississi])pi  River,  as  showni  by  magnetic  attractions  and  by  drilling. 
This  is  made  up  of  a  large  nuuiber  of  short  parallel  and  overlapping  belts.  \Mietiier  these 
minor  belts  are  repeated  by  folding  or  whether  they  are  parallel  independent  lenses  at  difTerent 


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MONOGRAPH  Lll      PLATE  XV 


CUYUNA  IRON  DISTRICT  AND  EXTENSIONS.  213 

horizons  in  the  slate  is  not  known.  Six  miles  to  the  north,  however,  in  the  vicinity  of  Rabbit 
Lake,  there  is  another  belt  of  iron-bearing  rocks,  known  locally  as  the  "north  range,"  which 
is  undoubtedly  brought  up  here  by  folding,  for  if  it  were  an  independent  belt  in  a  monoclinal 
succession  it  would  imply  too  great  a  thickness  of  intervening  strata  between  the  north  and 
south  ranges.  Still  farther  to  the  northwest,  between  Rabbit  Lake  and  Mississippi  River, 
are  at  least  two  more  belts  of  iron-bearing  rocks  repeated  by  folding.  Whether  the  folds 
reappear  elsewhere  prospectors  are  now  trjang  to  determine.  Inspection  of  the  map  (PI.  XV) 
tliscloses  a  westward  divergence  of  the  south  range  and  the  north  range  belts  of  iron-bearing 
rocks.  The  best  ores  of  the  district  are  found  in  the  angle  between  them.  Divergence  of 
strike  to  the  west  is  also  to  be  noted  between  certain  pairs  of  the  minor  belts,  though  not  in 
all.  These  facts  may  indicate  either  a  general  anticlinorium  with  eastward-pitcliing  axis  or 
a  syiichnorium  with  westward-pitching  axis.     The  former  is  regarded  as  the  more  probable. 

LITHOLOGY  AND  METAMORPHISM. 

So  far  as  the  sedimentary  rocks  go,  the  emphasis  in  description  should  be  placed  on  the 
altered  phases,  for  they  have  all  been  much  metamorphosed.  Failure  to  recognize  the  scliists 
as  parts  of  the  sedimentary  series  has  caused  confusion  in  the  local  interpretation  of  drill  records. 
The  changes  in  the  quartzite  and  slate  to  scliists  are  the  typical  anamorpliic  changes  of  the 
zone  of  rock  fiowage  and  igneous  contacts. 

Hall  has  shown  how  these  slates,  toward  the  south  and  west,  where  intrusive  rocks  are 
abundant,  become  garnetiferous  and  staurolitiferous  biotite  schists  and  hornblende  schists." 

When  subsequently  exposed  at  the  surface,  there  has  been  a  leaclung  out  of  all  the  basic 
constituents,  leaving  light-colored,  soft  kaohnic  and  quartzose  schists.  This  action  is  most 
conspicuous  in  their  upper  15  or  20  feet.  It  is  especially  confined  to  the  areas  near  the  iron- 
bearing  lenses.  Farther  south,  where  anamorpliism  was  more  intense,  the  rocks  were  made 
so  hard  and  resistant  that  they  have  been  affected  but  slightly  by  weathering  where  exposed 
at  the  surface. 

The  iron-bearing  member,  originally  mainly  iron  carbonate,  has  also  undergone  anamor- 
pliism, resulting  in  the  development  of  ampliibole-magnetite  rocks  essentially  similar  to 
amphibole-magnetite  rocks  wherever  they  are  found  in  other  parts  of  the  Lake  Superior 
region.  Tliis  action,  however,  was  not  sufficiently  effective  to  destroy  a  large  part  of  the  iron 
carbonate  constituting  the  original  mass  of  the  member.  Where  exposed  to  weathering  the 
amphibole-magnetite  rocks  have  been  more  resistant  than  the  iron  carbonates,  but  even  they 
have  become  softer,  owing  to  leacliing  of  silica,  which  has  resulted  practically  in  the  concen- 
tration of  the  iron,  which  remains  substantially  as  magnetite.  The  iron  carbonate  has  been 
altered  to  limonite  at  the  surface.  The  result  is  a  mixture  of  hematite,  hmonite,  and  magnetite 
in  the  iron-bearing  member,  soft  and  granular  above  and  becoming  harder  and  mofe  siliceous 
below  and  showing  more  of  the  unaltered  carbonate  phases  with  depth.  The  gradation  phases 
between  the  iron-bearing  member  and  the  slate  have  become  ferruginous  slates. 

The  anamorpliism  of  the  rocks  of  the  Cuyuna  district  is  probably  to  be  explained  in  large 
part  by  the  existence  of  intrusives  in  the  area  itself  and  west  and  south  of  it. 

CORRELATION. 

The  sedimentary  rocks  of  the  Cuyuna  district  probably  belong  in  the  same  series  with  the 
slates  and  schists  of  the  Carlton,  Cloc^uet,  and  Little  Falls  areas.  They  show  many  similarities 
in  lithology,  structure,  and  metamorphism  and  are  geographically  contiguous.  Drilling  in 
numerous  places  in  Crow  Wing  and  Aitkin  counties  shows  the  same  pyritic  and  carbonaceous 
phases  of  slate  as  have  been  explored  for  coal  in  the  vicinity  of  Mahtowa. 

Succession  and  Hthology  are  in  accord  mth  distribution  and  general  structural  relations 
in  pointing  to  the  identity  of  the  rocks  of  the  Cuj^una-Carlton-Little  Falls  area  with  the  upper 
Huronian  (Animikie  group)  of  the  Lake  Superior  region.     The  Animikie  group  as  a  whole, 

o  Hall,  C.  W.,  Keewatin  area  of  eastern  and  central  Minnesota:  Bull.  Geol.  Soc.  America,  vol.  12, 1901,  pp.  343-376. 


214  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

where  best  known  in  tlie  Mesabi  aiul  Animikie  and  Gogebic  districts,  consists  of  a  great  slate 
formation  2  miles  or  more  thick,  underlain  by  and  intorbedded  in  its  lower  portions  with  an 
iron-bearing  formation  of  var^'ing  thickness,  but  averaging  perhaps  1,000  feet,  and  this  in 
turn  underlain  by  quartzite  varying  from  1  to  200  feet  in  tliickness.  Exploration  has  not  yet 
gone  far  enough  to  warrant  a  satisfactory  estimate  of  the  tliickness  of  the  formations  in  the 
Cuyuna  district,  but  the  information  so  far  developed  is  in  accord  with  the  figures  given  for 
the  Animikie  group  as  a  whole,  except  for  the  iron-bearing  member,  which  thus  far  has  not 
been  found  to  be  as  thick  as  the  average  for  the  Lake  Superior  region.  The  Cuyuna  range  is 
separated  from  the  Mesabi  range  on  the  northeast  by  a  flat  swamp  and  lake  area  about  50 
miles  wide,  which  completely  lacks  rock  exposures.  The  Animikie  group  in  the  Mesabi  district 
dips  to  the  south  under  tliis  low,  flat  area  at  an  angle  var\nng  from  4°  to  20°.  It  has  long  been 
obvious  that  the  group  here  disappearing  under  the  surface  might  somewhere  be  brought  up 
to  the  south  by  folding. 

In  tlie  Gogebic  range,  on  the  south  side  of  Lake  Superior,  a  similar  group  dips  at  an  average 
of  60°  toward  the  northwest  beneath  the  Lake  Superior  basin,  and  it  has  long  been  thought 
that  tills  group  represents  the  Animikie  group  as  it  comes  up  again  on  the  south  side  of  the 
lake.  An  examination  of  the  general  structure  of  the  west  end  of  the  Lake  Superior  liasin, 
however,  shows  that  the  structure  of  the  area  between  these  two  districts  is  not  that  of  a  simple 
syncUne  but  of  a  syncline  in  wliich  there  are  subordinate  anticUnes — that  is,  a  synchnorium. 
One  of  these  subordinate  anticlines  runs  west  and  southwest  from  Duluth  towanl  Little  Falls 
and  vicinity  on  Jklississippi  River.  If  the  Animikie  group  conies  to  the  surface  anj-where  between 
the  Mesabi  range  on  the  north  and  the  Gogebic  range  on  the  south,  it  should  therefore  appear 
in  tliis  subordinate  anticlinal  fold  in  the  western  part  of  the  general  synchnorium  connecting 
these  two  regions,  and  it  was  on  this  hypothesis  that  the  extension  of  the  iron-bearing  formation 
of  the  Mesabi  and  Gogebic  districts  was  drawn  by  geologists,  prior  to  its  discovery,  through 
the  present  Cuyuna  district,  wliich  Ues  near  the  north  side  of  this  subordinate  anticlinal  fold. 
The  existence  of  a  cpiartzite  exposure  at  Dam  Lake,  near  Kimberly,  and  near  Rabbit  Lake, 
as  shown  by  drill  records,  points  to  the  fact  that  here  erosion  has  cut  down  to  the  lower  part 
of  the  Animikie  group  as  it  would  in  truncating  an  antichne.  The  course  of  ilississippi  River 
itself  suggests  the  existence  of  the  antichne  in  the  vicinity  of  the  Cuyima  range,  for  after 
crossing  the  Mesabi  range  it  flows  south  until  it  reaches  the  Cuyuna  district  and  then  turns  sud- 
denly westward  as  though  deflected  along  the  anticline  toward  a  lower  point  of  escape.  Where 
it  does  break  across,  as  at  Little  FaUs,  rocks  are  exposed. 

The  slates  of  the  Carlton  and  Cloquet  districts  were  early  assigned  by  Irving  and  other 
geologists  to  the  upper  Huronian,  but  they  were  later  referred  by  Spurr  to  the  lower  Iluronian 
because  of  their  greater  metamorphism  and  folding  than  that  of  the  upper  Huronian  slates  in 
the  Mesabi  district  to  the  north  and  because  they  are  intruded  by  granites  supposed  to  be  of 
lower  Huronian  age.  It  is  now  kno\ra  that  the  upper  Huronian  (Animikie  group)  of  the  Mesabi 
district  is  also  intruded  by  granite.  The  facts  developed  in  the  Cuyima  chstrict  seem  to  con- 
&m  Irving's  view  of  the  correlation. 

In  \new  of  the  probable  equivalence  of  the  rocks  of  the  Cu^nma  and  Carlton  areas  and  the 
occurrence  of  small  iron  carbonate  bands  and  nodules  in  the  slates  about  Carlton  and  Cloquet 
and  to  the  southwest  similar  to  the  broader  bands  in  the  Cuyuna  area,  the  question  naturally 
arises  why  erosion  should  not  somewhere  in  tliis  great  area  of  exposed  slate  between  Carlton, 
Cloquet,  and  Little  Falls  uncover  the  lower  part  of  the  Animilde  group — in  other  words,  the 
iron-bearing  member.  It  may  be  that  the  crest  of  the  antichne  runs  parallel  with  the  Cujiina 
district  itself,  allowing  erosion  to  cut  down  here  only  into  the  main  iron-bearing  member,  wliile 
to  the  south  and  southeast  the  tluck  capping  of  slates  has  not  been  removed,  or  it  may  be  that 
the  existence  of  great  masses  of  intrusive  granite  and  diabase  and  the  intense  metamorpliism 
wliich  they  have  accomplished  have  prevented  erosion  of  the  surface  or  have  made  the  condi- 
tions unfavorable  for  the  direct  oxidation  of  the  iron-bearing  rocks  under  surface  katamorphic 
conditions.  Certainly  enough  facts  are  not  yet  available  to  warrant  the  assertion  that  the  iron- 
bearing  member  may  not  yet  be  found  in  tliis  area. 


CUYUNA  IRON  DISTRICT  AND  EXTENSIONS.  215 

KEWEENAW  AN  SEBIES  (?). 

Igneous  rocks  are  abundant  in  the  area  of  the  upper  Iluronian  (Animikie  jjroup).  These 
inchule  granites  and  basic  rocks,  many  of  the  latter  characterized  bj'  ophitic  structure.  Part 
are  sclustose;  others  are  not.  The  granites  outcrop  conspicuously  (thereby  contrasting  with 
the  adjacent  upper  Huronian  sediments)  in  the  southern  part  of  the  district  in  a  general  belt 
extending  from  Carlton  and  Cloquet  southwest  beyond  ^lississippi  River.  Other  exposures  are 
known  northwest  of  the  district,  in  the  vicinity  of  Randall  and  Motley.  Basic  igneous  rocks 
of  diabase  and  gabbro  types  also  outcrop,  though  less  abundantly,  over  the  same  area.  Dikes 
of  the  basic  rocks,  up  to  50  feet  in  width,  are  conspicuous  in  the  Carlton  area.  The  intrusive 
character  of  these  igneous  rocks  as  a  whole  admits  of  no  doubt.  Their  metamorphic  effect  on 
adjacent  sediments  has  already  been  described.  Within  and  adjacent  to  the  Deerwood  iron- 
bearing  member  driUing  has  disclosed  much  igneous  rock,  both  basic  and  acidic,  of  yet  unknown 
extent  and  with  unknown  relations.  The  contacts  are  sharp,  the  adjacent  members  of  the 
upper  Huronian  have  been  locally  metamorphosed,  and  no  basal  conglomerates  have  been  found 
in  the  sediments  adjacent  to  the  igneous  rocks.  From  these  facts  it  is  concluded  that  the  igneous 
rocks  cut  in  drill  holes  are  probably  intrusive  into  the  upper  Huronian  sediments,  just  as  are 
the  granites  to  the  south.  The  textures  and  structural  relations  of  some  of  the  basic  igneous 
rocks  suggest  the  possibility  that  they  may  be  extrusives  contemporaneous  with  the  upper 
Huronian  rather  than  with  later  intrusives,  but  until  mining  operations  disclose  more  under- 
ground sections  tlus  can  not  be  determined.  In  only  three  localities  are  extrusives  known. 
An  acidic  extrusive  rock  with  amygdaloidal  texture,  in  beds  15  to  25  feet  tliick,  has  been 
found  by  drilling  to  rest  across  the  edges  of  the  Virginia  slate  and  Deerwood  iron-bearing 
member,  in  sec.  2,  T.  44  N.,  R.31  W.;  sec.  6,  T.  44  N.,  R.  .30  W.;  and  sec.  7,  T.  45  N.,  R.  29  W. 

The  igneous  rocks  intrusive  into  the  upper  Huronian  and  the  extrusives  resting  on  the  upper 
Huronian  are  provisionally  classed  as  Keweenawan,  because  the  Keweenawan  is  the  next 
period  of  igneous  activity,  liecause  abundant  igneous  rocks  of  Keweenawan  age  are  known 
elsewhere  in  the  region  to  cut  the  upper  Iluronian  sediments,  and  because  they  are  especially 
abundant  in  that  part  of  the  Ci:yuna  district  which  Kes  approximately  along  the  central  axis 
of  the  Lake  Superior  syncline,  largely  developed  during  Keweenawan  time.  (See  pp.  421-422, 
622-623.) 

CRETACEOUS   ROCKS. 

Immediately  below  the  surface,  in  widely  scattered  parts  of  the  district  in  Crow  Wing 
County,  remnants  of  a  conglomerate  have  been  found.  Some  consist  of  small  pebbles  of  the 
iron-bearing  member  in  a  slaty  matrix;  others  of  small  pebbles  of  an  extrusive  rock.  Gen- 
erally the  pebbles  are  about  an  eighth  of  an  inch  or  less  in  diameter,  but  on  two  widely  separated 
properties  the  oval  pebbles  measure  as  much  as  an  inch  in  their  longest  dimension.  This 
conglomerate  is  found  resting  unconformably,  apparently  in  small  depressions,  on  a  rather 
level  erosion  surface  of  the  upper  Huronian.  It  does  not  contain  fossil  remains  to  identify  it, 
but  it  is  similar  to  the  Cretaceous  of  the  Mesabi  range.  An  excellent  opportunity  to  examine 
it  was  offered  when  an  exploration  shaft  was  sunk  in  the  SW.  i  SE.  i  sec.  8,  T.  45  N.,  R.  29  W. 

More  Cretaceous  sediments  have  not  been  identified,  probably  because,  being  poorly 
cemented,  they  are  chopped  and  brought  to  the  surface  in  drilling  as  churnings.  Drillers 
frequently  report  imbroken  shells  in  the  lower  portion  of  that  which  is  reported  as  "surface, "  and 
clay  immediately  above  bed  rock  and  below  the  surface,  and  frequently  the  top  drill  samples 
are  light-colored,  unconsolidated,  and  calcareous  material,  all  of  which  might  well  be  of  Cre- 
taceous origin.  None  of  this  has  been  very  carefully  examined.  The  common  occm-rence  of 
large  amounts  of  lignitic  material  in  the  glacial  drift  indicates  a  once  wide  distribvition  of 
Cretaceous  deposits,  possibly  with  remnants  here  and  there  such  as  are  found  in  the  Mesabi 
range  to  the  north.  Cretaceous  beds  continuously  cover  the  pre-Cambrian  rocks  of  western 
Minnesota.  Those  of  the  Cuyuna  district  may  be  regarded  as  outliers  of  the  main  Cretaceous 
area. 


216  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

QUATERNARY   SYSTEM. 

PLEISTOCENE  GLACIAL  DEPOSITS. 

The  glacial  deposits  in  the  eastern  part  of  the  district  belong,  according  to  Upham,"  to 
the  eighth  moraine  and  those  in  the  western  part  of  the  district  belong  to  the  ninth  moraine, 
counted  back  from  the  outermost  moraine  of  the  late  Wisconsin  glaciation.  The}'  vary  from 
35  to  400  feet  in  tliidcness.  The  heavy  mantle  of  weathered  material  upon  the  rock  surface  is  a 
remnant  of  the  product  of  preglacial  weathering,  which  in  the  other  districts  has  been  removed 
by  glacial  erosion.  Obviously  in  the  Cuyuna  district  glacial  deposition  has  predominated  over 
glacial  erosion. 

THE  IRON   ORES   OF  THE  CUYUNA  DISTRICT. 

By  the  authors  and  Carl  Zapffe. 
DISTKIBUTION,  STBtJCTTJRE ,  AND  RELATIONS. 

The  Cuyuna  ores  are  scattered  through  a  considerable  area  beginning  a  little  east  of  Aitkin, 
Aitkin  County,  Minn.,  and  extending  southwestward  past  Brainerd  into  Morrison  County.  (See 
PI.  XIV.)  The  Umits  of  the  ore-bearing  district  are  not  yet  known.  The  district  lacks  the 
distinct  range  or  ridge  characteristic  of  the  other  iron-producing  districts,  though  in  general 
it  follows  a  drainage  divide.     The  area  is  flat,  heavily  drift  covered,  and  without  exposures. 

The  development  of  the  Cuyuna  district  is  still  in  its  exploratory  stage.  At  tliis  writing 
no  shipments  have  been  made.  In  the  absence  of  exposures,  information  is  available  from 
about  2,000  drill  holes  and  two  shafts  and  from  magnetic  readings.  The  information  is  still 
inadequate  to  warrant  any  extended  discussion.  In  the  following  general  outline  emphasis  is 
placed  on  the  facts  thus  far  developed.  No  attempt  at  proportional  treatment  is  made.  This 
may  be  possible  later. 

The  Deerwood  iron-bearing  member  is  magnetic  as  a  whole,  and  hence  its  distribution  is 
roughly  shown  by  the  magnetic  belts  outlined  on  Plate  XV  and  by  minor  belts  which  do  not 
appear  on  this  plat.  Parts  of  the  member,  however,  are  very  weakly  magnetic;  they  are  found 
beneath  very  weak  belts  of  attraction  and  extend  laterally  some  distance  away  from  the  maxi- 
mum magnetic  line.  The  ore  deposits  may  be  more  or  less  magnetic,  usually  less  magnetic, 
than  the  associated  iron-bearing  member,  and  hence  are  not  ordinarily  situated  under  the  belts 
of  maximum  variation,  though  they  are  not  far  from  them. 

Ore  deposits  of  suflicient  size  and  grade  to  be  commercial!}-  available  Iiave  been  found  in 
both  the  north  and  south  ranges,  so  called.  The  south-range  ores  occur  at  intervals  along  the 
magnetic  belts  from  a  place  a  mile  east  of  Deerwood  more  or  less  intermittently  to  the  north- 
eastern part  of  T.  43  N.,  R.  32  W.,  near  jMississippi  River  southwest  of  Brainerd,  a  distance  of 
about  30  miles.  The  north-range  ores  are  in  intermittent  deposits,  in  a  shorter  but  wider  belt, 
extending  from  Rabbit  Lake  southwestward  nearly  to  Mississippi  River.  The  tonnage  of  the 
deposits  thus  far  found  is  about  equal  in  the  two  ranges,  but  on  the  north  range  the  ores  are 
more  largely  confined  to  a  few  large  deposits  of  good  grade,  while  on  the  south  range  the 
number  of  deposits  is  larger  and  their  individual  size  smaller. 

The  ores  are  in  nearly  vertical  lenses  and  layers  from  a  few  inches  to  125  feet  or  more  wide- 
on  the  south  range  and  up  to  400  or  500  feet  on  the  north  range.  The  depths  on  the  two  ranges 
are  variable  as  the  widths.  On  the  north  range  the  greatest  depth  known  is  850  feet  and  it 
is  quite  likely  that  this  figure  may  be  exceeded,  but  up  to  the  present  time  the  average  depth 
is  about  300  feet.  On  the  south  range  the  greatest  depth  Icnown  is  about  250  feet,  and  it  does 
not  seem  likely  that  tins  will  be  greatly. exceeded.  The  average  depth  on  the  south  range  is 
about  150  feet,  but  the  higher-grade  ores  invariably  occupy  only  tlie  up])or  100  feet.  The 
strike  is  east-northeast  for  distances  varying  from  a  few  feet  to  half  a  mile  and  to  an  unlcaown 
greater  distance. 

a  Minnesota  Geol.  Survey,  vols.  2  and  4. 


CUYUNA  IRON  DISTRICT  AND  EXTENSIONS.  217 

Whether  these  lenses  pitch  in  the  chrection  of  strike,  following  the  axes  of  drag  folds,  is  not 
yet  disclosed  by  the  drilling.  (See  fig.  49,  p.  350.)  From  analogy  with  other  districts  the  ore  bodies 
are  likely  to  have  a  pitch,  and  this  ]ntch  is  likely  to  be  more  or  less  uniform  in  direction  and 
degree,  affording  a  guide  for  exploration.  The  drilling  has  not  shown  the  pitch,  because  where 
they  are  vertical  the  holes  are  stopped  as  soon  as  they  run  out  of  ore,  and  if  they  go  into  lean 
rocks  rather  than  ore  they  are  ordinarily  not  carried  far  enough  to  locate  any  possible  extensions 
of  the  pitches.  Wliere  the  holes  are  put  to  one  side  of  the  ore  body  and  inclined  they  are 
stopped  as  soon  as  they  have  penetrated  the  ore  lens.  These  pitches  are,  as  a  matter  of  fact, 
extremely  difficult  to  locate  by  drilling.  Closely  associated  with  the  ore  on  one  or  both  walls, 
or  m  layers  within  the  ore,  is  amphibole-magnetite  rock.  At  varying  depths,  but  usually 
within  125  feet  on  the  south  range,  the  ores  tend  to  grade  vertically  into  cherty  iron  carbonate 
rocks,  and  at  these  depths  also  the  amphibole-magnetite  rocivs  contam  much  more  iron 
■carbonate  than  at  the  surface.  It  may  be  found  that  down  the  pitch  the  depth  of  gradation 
to  iron  carbonate  is  much  deeper.  The  ores,  with  the  associated  amphibole-magnetite  rocks 
and  cherty  iron  carbonates,  constitute  the  iron-bearing  member  of  this  district. 

The  Deerwood  iron-bearing  member  as  a  whole  constitutes  lenses  or  layers  in  the  great 
Virgmia  ("St.  Louis")  slate  formation,  lying  parallel,  overlapping,  or  end  to  end.     Each  major  . 
lens  may  be  divided  into  minor  lenses  by  intercalated  slate  layers. 

The  wall  rocks  of  the  ore  may  therefore  be  any  of  the  phases  of  the  Deerwood  iron-bearing 
member  or  any  of  the  phases  of  the  Virginia  ("St.  Louis")  slate.  Characteristically  one  wall 
may  be  chloritic  or  black  graphitic  slate  of  the  Virginia  formation  and  the  other  wall  amphibole- 
magnetite  rock  of  the  Deerwood  iron-bearing  member.  The  association  of  ore  with  carbona- 
ceous slates  finds  its  counterpart  in  the  Iron  River,  Crystal  Falls,  and  other  districts  of  ^Michigan. 

Dikes  and  irregular  masses  of  basic  intrusive  rocks  appear  in  all  parts  of  this  series  and 
are  associated  with  almost  every  ore  deposit  yet  known.  These  may  constitute  one  wall  of 
the  ore  body  or  may  be  separated  from  the  ore  body  on  one  wall  by  amphibole-magnetite 
rock. 

A  characteristic  occurrence  of  the  ores  is  shown  i.n  plan  and  cross  section  m  figure  25.  It 
is  apparent  from  this  figure  that  the  information  furnished  from  drill  holes  would  depend  largely 
on  the  angle  at  wliich  the  drill  penetrates  the  iron-bearing  member.  In  a  vertical  lens  a 
vertical  hole  will  tell  notlung  of  the  character  of  the  material  a  few  feet  away  across  the  strike. 
An  inclmed  hole  will  mdicate  the  proportions  of  iron  ore,  amphibole-magnetite  rock,  and  slate 
layers,  but  may  not  show  the  greatest  depth  of  the  iron-ore  lenses,  or,  on  the  other  hand,  it 
may  pass  through  the  carbonate  phases  of  the  beds  beneath  the  ore. 

The  ore,  where  associated  with  magnetite  rocks,  is  in  many  places  also  magnetic.  The 
amphibole-magnetite  rocks  are  somewhat  more  magnetic  than  the  ores  themselves,  so  that  drill- 
ing on  the  maximum  magnetic  attraction  is  likely  to  show  amphibole-magnetite  rocks  with 
the  ores  a  few  feet  to  one  side  or  the  other.  A  not  uncommon  relation  is  amphibole-magnetite 
rock  on  the  maximum  attraction,  intrusive  material  on  one  side  of  the  maximum,  and  ore  on 
the  other.  The  greatest  distance  from  the  maximum  attraction  at  which  ore  has  3'et  been 
found  is  one-half  mile.  It  will  be  shown  elsewhere  (pp.  552-553)  that  the  magnetic  character 
of  the  member  is  not  favorable  to  its  richest  concentration;  this  suggests  that  the  best  parts 
of  the  Cuyuna  ore  may  yet  be  found  farther  away  from  the  magnetic  belt. 

The  fact  that  the  foot  and  hanguig  walls  of  the  ore  deposits  of  most  of  the  Lake  Superior 
ranges  are  uniformly  different  in  their  lithology  has  led  to  the  assumption  that  the  foot  and 
hanging  walls  of  the  Cuyuna  ore  deposits  are  uniformly  different.  Beginning  in  slate  a  few 
hundred  feet  either  side  of  the  magnetic  belt,  an  inclined  drill  hole  penetrates  the  iron-bearing 
member  as  the  magnetic  maximum  is  approached.  The  slate  is  ordinarily  spoken  of  as  "hang- 
ing wall."  The  drill  is  then  likely  to  penetrate  ore  more  or  less  mterbedded  with  slate  and 
amphibole  rock.  As  the  magnetic  maximum  is  approached  the  amphibole-magnetite  rock  is 
likely  to  be  more  abundant.  The  drill  may  go  beyond  the  maxinmm  attraction  into  intrusive, 
which  would  be  spoken  of  as  "intrusive  foot  wall."  (See  fig.  25.)  The  terms  "hanging-waU 
slate"  and  "magnetic  foot  wall"  or  "intrusive  foot  wall"  therefore  signify  a  certain  tendency 


218 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


toward  uiiil'ormity  of  reliitions  wliich  it  is  well  to  identil'y  by  such  terms.  But  the  assumption 
of  uniformity  imphcd  by  the  use  of  these  terms  may  lead  to  misapprehension  of  the  facts. 
Slate  similar  to  thai   of  tlio  lianjijing  wall  may  bo  on  citlicr  side  of  the  iron-bearing  member. 

If  the  drills  go  far  enough, 
they  are  likely  to  find  slate 
in  both  walls.  Slate  layers 
witidn  the  iron-bearuig  mem- 
ber itself,  if  first  penetrated 
by  tlie  drill,  would  be  likely  to 
be  called  "hanging  wall."  In 
short,  the  nature  of  the  foot 
and  hanging  w  alls  will  depend 
on  the  particular  layers  in 
wliich  the  drill  happens  to 
start  ami  where  it  stops  in  tlie 
interlaminations  of  slate  and 
iron-bearing  member.  The  re- 
lations of  the  intrusive  rocks 
to  the  ore  deposits  are  still 
obscure,  but  it  seems  not  un- 
likely that  these  ma}'  be 
found  to  constitute  a  definite 
foot  wall  for  some  of  the  oi^e 
bodies. 

The  facts  just  given  are 
disclosed  by  drilling,  but  the 
drilling  yet  done  gives  a  ver}' 
incomplete  view  of  the  struc- 
ture, and  for  the  larger  struc- 
tural featiu'es  we  must  rely 
principally  on  interpretations 
of  the  magnetic  field.  Tlie 
existence  of  five  magnetic  belts 
in  a  zone  7  miles  wide  north 
and  south  suggests  that  the 
iron-bearmg  member  is  re- 
peated by  folding.  If  the  dips 
were  monoclinal  and  the  sev- 
eral magnetic  belts  represented 
separate  irou-bearuig  zones  in 
the  slate,  the  thickness  of  the 
series  to  ho  inferred  would  be 
greater  than  is  reasonable.  On 
the  other  hand,  the  drill  cores 
show  variations  in  the  <lip  of 
the  bedding  indicative  of  fold- 
ing. The  cleavage  of  the 
siat(>s  is  inclinccl  to  the  bed- 
ding, and  tiiis  relation  is  itself 
evidence  of  fohhng.  These 
folds  have  a  strike  east-nortli- 
east  parallel  to  the  Lake  Superior  axis,  to  judge  from  the  magnetic  belts.  Moreover,  the 
discontmuity  of  these  belts,  their  distribution   en  echelon,  and  the  varying  intensity  of  the 


Figure  ; 


-Plan  and  cross  section  of  the  ironKjre  deposit  in  sei.-.  1_',  '1'.  4i  N. 
^^'ing  County,  Minn.      By  Carl  Zapffe. 


K.  32  \V.,  Crow 


CUYUNA  IKON  DISTRICT  AND  EXTENSIONS.  219 

magnetic  field  along  a  single  belt  all  accortl  with  the  distribution  recjuired  by  pitching  folds, 
which  repeat  tlie  iron-bearmg  beds,  the  number  of  times  diil'ering  witii  the  locality.  If  tlie 
■crests  and  troughs  of  the  folds  were  horizontal,  the  beds  would  appear  as  parallel  lines  upon 
the  horizontal  erosion  plane,  but  the  actual  crest  and  trough  lines  of  the  folds  usually  have  a 
pitch;  in  other  words,  they  are  cross  folded,  so  that  on  tlie  erosion  plane  the  beds  appear  to 
converge  in  the  direction  of  the  pitch.  With  folding  of  this  type  it  is  apparent  that  the  beds 
may  strike  with  a  considerable  variety  on  the  erosion  plan(^,  according  to  the  section  this 
plane  happens  to  make  through  the  folds. 

The  magnetic  belts  fail  to  give  all  the  information  desired  as  to  structure,  for  two  reasons: 
(1)  It  is  not  certain  that  the  iron-bearing  lenses  in  all  parts  of  the  district  are  at  the  same 
horizons  in  the  slate;  indeed,  it  is  known  that  within  a  few  hundred  yards  tliere  may  be  several 
iron-bearing  bands,  so  that  the  question  is  raised  whether  iron-bearing  layei"s  in  other  ])arts  of 
the  district  belong  below,  with,  or  above  them  stratigraphically.  (2)  It  is  difficult  to  tell 
whether  two  nearly  parallel  belts  close  together  represent  truncateil  iron-bearing  layers  on  the 
two  limbs  of  a  single  fold  or  the  axes  of  two  indej)endent  folds.  The  main  belts  of  attraction 
several  miles  apart  doubtless  represent  separate  folds,  but  the  closely  associated  minor  belts 
making  up  each  of  the  main  belts  may  represent  either  the  two  limbs  of  a  single  fold  or  two 
horizons  on  one  limb  of  a  fold. 

It  is  concluded,  in  general,  that  the  iron-bearing  member  constitutes  closely  associated 
lenses  and  layers  along  a  single  general  horizon  in  the  slate.  The  finding  of  quartzite  in  a  few 
places  near  the  iron-bearing  member  suggests  that  this  horizon  is  near  the  bottom  of  the  slate 
formation,  but  this  is  not  proved.  The  foldmg  of  the  slates  carrying  the  ii"on-bearing  zones, 
followed  by  erosion,  has  developed  the  present  distribution  at  the  surface. 

CHARACTER  OF  THE  ORES.a 

j' 

>  GENERAL    APPEARANCE. 

The  Cuyuna  ores  fall  into  two  main  groups,  hard  and  soft  ores. 

The  soft  ores  are  black,  brown,  and  reddish  hydrated  hematites,  soft  and  earthy  and  much 
like  the  soft  ores  of  the  Penokee-Gogebic  district.  They  have  large  pore  space.  These  soft 
ores  are  of  two  types — a  high-grade  ore  containing  55  to  63  per  cent  iron,  soft  and  powdery  and 
of  a  brown  to  very  dark  color,  and  a  lean  reddish-purple  ore  containing  45  to  50  per  cent  iron. 
The  latter  ore  is  not  so  soft  as  the  former.  It  is  easily  broken  do%vn  with  a  juck  but  retams  its 
■stratified  form  and  hangs  together  ia  fairly  large  chunks.  In  this  type  cherty  layers  are  scat- 
tered through  the  mass  at  short  intervals,  the  cherty  impurity  probably  accounting  for  its  low 
grade.     This  ore  also  has  a  large  pore  sjiace. 

The  hard  ores  are  also  of  two  types.  The  bulk  of  the  hard  ore  is  a  black  to  very  dark  brown 
hydrated  hematite.  It  is  closely  stratified  and  has  suffered  close  brecciation  as  a  result  of 
slumpmg  caused  by  the  leaching  out  of  silica.  This  ore  varies  in  iron  content,  but  is  mainly 
high  grade,  ru;ming  fi"om  50  to  60  per  cent  iron.  Although  this  ore  is  brecciated  it  holds 
together  in  large  masses,  owing  to  the  partial  cementing  of  the  brecciated  pieces  by  the  second- 
ary introduction  of  iron.  Much  of  the  ore  of  this  type  has  been  classed  as  soft  ore  by  the 
drillers  because  it  is  fairly  easily  penetrated  by  a  churn  drill  and  comes  to  the  top  broken  up 
in  very  fine  angular  pieces.  It  can  be  distinguished,  however,  from  the  true  soft  ore,  which 
is  washed  to  the  surface  of  the  hole  as  a  fine,  even-grained,  powdery  mass.  The  Cuyuna  hard 
ore  described  above  must  not  be  compared  to  Vermilion  dense  blue  hematite  of  that  range. 
It  is  much  softer  and  more  limonitic. 

The  other  type  of  Cuyuna  hard  ore,  small  in  amount  as  compared  ^\^th  that  described  above, 
is  a  hard  blue  hematite  running  about  58  to  63  per  cent  iron.  It  is  massive  and  unbrecciated. 
This  is  a  true  hard  ore  and  can  only  be  drilled  with  diamonds.  This  ore  occurs  in  layers  in 
the  softer  ores  and  is  found  more  frequently  close  to  the  intrusives.. 

ain  the  description  of  the  ores  the  writers  have  drawn  on  quantitative  data  assembled  by  F.  S.  Adams  (Econ.  Geology,  vol.  5, 1910,  pp.  729-740; 
-vol.  6,  1911,  pp.  60-70,  156-180). 


220 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


It  is  impossible  to  state  at  lliis  incomplete  stage  of  exploration  the  proportion  of  hard  to 
soft  ore  on  the  Cuyuna  nmge.  The  soft  ores  prol)al)l3-  form  flic  lar<rer  i)r()p()rti()n,  but  the  hard 
ore  must  be  counted  as  a  large  factor  ami  may  occur  in  a  mucii  larger  percentage  than  has  pre- 
viously been  supposed. 

Locally  on  the  north  or  Rabbit  Lake  range  black,  highly  manganiferous  ores  have  been 
developed  near  the  surface.  These  are  unimportant  in  amount  as  comi)ared  with  the  other 
ores. 

CHEMICAL    COMPOSITION. 

Because  of  the  minute  interbanding  of  the  ore  with  lean,  magnetic,  and  slaty  jjiiases,  the 
chemical  composition  shows  rapid  alternation  across  the  strike.  The  percentage  of  iron  of  the 
iron-bearing  member  ranges  from  less  than  30  per  cent  in  the  lean  siderite  and  amphibolc 
phases  to  60  per  cent  and  more  in  certain  of  the  iron  ores.  In  certain  estimates  of  tonnage 
which  have  been  made  it  has  been  calculated  that  of  the  ores  rumiing  above  40  per  cent  metallic 
iron  44.5  run  above  50  per  cent  in  iron  and  21.3  per  cent  above  55  per  cent  in  iron.  These 
fio-ures  are  based  on  a  sufficient  number  of  drill  holes  to  warrant  the  belief  that  this  proportion 
may  have  some  general  significance  for  the  range.  The  average  iron  content  of  all  ores  above 
50  per  cent  in  iron  on  the  north  range,  found  by  drilling  to  the  time  of  wTiting,  is  about  1  per 
cent  higher  than  that  for  such  ores  on  the  south  range.  The  chemical  character  of  the  iron 
ore  and  interlayered  masses  as  they  stand  in  the  ground  may  best  be  shown  by  the  following 
analyses  from  a  drill  hole  cutting  the  formation  at  an  angle  of  60° : 

Analyses  of  iron  ore  and  interlayered  masses  from  the  Cuyuna  district,  Minnesota. 


Deplh. 

Fe. 

P. 

Mn. 

SiOz. 

.MjOs. 

CaO. 

MgO. 

Los.*!  by 
ignition. 

Pert. 
175-180 
180-185 
185-190 
190-195 
195-200 
2PO-205 
205-210 
210-215 
215-220 
220-225 
225-230 
230-235 
23."v-240 
240-215 
245-250 
250  255 
255-200 
200-2(15 
205-270 
270-275 
275-280 
280-285 
285-290 
290-295 
295-300 

58.70 
59.73 
■     59.02 
59.11 
00.32 
59.  CO 
59.44 
(.0.93 
01.10 
57.  82 
57. 93 
55.  52 
40.  00 
49.07 
45.  07 
40.  82 
47.95 
48.74 
48.28 
41.47 
41.80 
40.  30 
30.80 
37.19 
37.  So 

0.519 
.547 
.425 
.004 
.414 
.385 
.353 
.264 
.229 
.287 
.284 
.337 

0.24 
.22 

'.io 

.20 
.40 
.51 
.30 
.40 
.47 
.44 
.-10 
.43 
.94 

4.  SO 

3.23 

3.  78 

4.10 

li.35 

7.22 

7.34 

.      fi.  OS 

7.10 

13.00 

12. 90 

15. 19 

9.79 

0.08 

1.13 

1.35 

1.25 

.03 

.00 

.02 

.49 

.44 

.47 

.48 

.49 

2.00 

O.U 
.15 
.10 
.17 
.10 
.08 
.U 
.18 
.20 
.10 
.17 
.10 
.14 

0.08 
.14 
.11 
.10 
.09 
.08 
.10 
.07 
.05 
.04 
.08 
.07 
.00 

7.34 

7.83 

5.35 

3.36 

2.75 

.48 

29.30 

.35 

.13 

.09 

2.18 

.40 

20. 04 

.59 

.10 

.10 

.50 

20.00 

.52 

.10 

■     .06 

2.62 

.39 

30.74 

.54 

.17 

.08 

1.12 

32. 50 

.73 

.42 

.30 

l.O.n 

3i.7i     1          .74 

.44 

.37 

7.11 

1 

It  will  be  noted  that  the  principal  variants  here,  as  in  other  districts,  are  iron  anil  silica. 
Phosphorus  is  usually  high,  averaging  about  0.34  per  cent,  which  brings  the  ore  into  the  class 
of  the  Iron  River  and  Crystal  Falls  ores.  The  north  range  shows  less  phosphorus  than  the 
south  range.     Locally  on  the  north  range  there  are  streaks  of  Bessemer  ore. 

Loss  by  ignition  is  high.  This  consists  princiindly  of  water  combined  in  the  hydrated 
iron  minerals  but  includes  some  carbon. 

Manganese  is  usually  in  small  amounts,  but  locally  ami  near  the  surface  may  run  up  to 
10  or  12  per  cent  or  even  up  to  28  per  cent.  One  drill  hole  on  the  north  range  averaged  13  per 
cent  for  the  upper  35  feet.     Another  had  an  average  of  1 1 .33  per  cent  for  the  ujiper  30  feet. 

The  percentage  of  free  water  in  the  ore  as  mined  can  not  be  determined  through  drilling, 
and  the  ore  has  thus  far  been  opened  up  by  shafts  to  such  a  slight  extent  that  the  average  free 
moisture  for  the  ores  can  not  yet  be  given.  Three  determinations  from  the  Rogers,  Brown  Ore 
Co.  shaft  give  moisture  of  O.SO  per  cent,  10.40  per  cent,  and  14.20  i)er  cent,  with  an  average  of 


CUYUNA  IRON  DISTRICT  AND  EXTENSIONS. 


221 


10.46  per  cent,  not  far  from  the  'average  of  the  Lake  Superior  region.  Another  determination 
by  Pickands,  Mother  &  Co.  for  the  ore  from  their  shaft  in  sec.  8,  T.  45  N.,  R.  29  W.,  gives  12 
per  cent  of  free  moisture.  In  analyses  of  ore  from  drill  holes  the  iron  content  is  usually  cal- 
culated for  the  dried  ore.  If  the  moisture  is  included  the  iron  content  is  lower.  An  average 
moistureof  10  per  cent  indicates  that  an  ore  appearing  as  a  55  per  cent  ore  in  the  drill  hole  will 
mine  as  about  50  per  cent.  As  prices  are  based  on  standard  ores  with  moisture,  this  correction 
is  an  important  consideration. 

The  slate  layers  interlayered  witli  the  iron-bearing  member  and  intermediate  phases  between 
the  iron-bearing  meml)er  and  the  slate  would  run  higher  in  alumina.  The  above  analyses  are 
confined  to  the  iron  member  itself. 

IRON     OXIDES 


SILICA 


ACCESSORIES 


FiQUEE  26.— Triangular  diagram  showing  mineralogioal  composil  ion  of  various  phases  of  iron  ores  and  ferruginous  cherts  of  the  Cuyuna  district, 
Minnesota.  After  F.  S.  Adams.  For  description  of  method  of  platting  and  interpretation  of  diagram,  see  p.  182.  1,  Hard  blue  ore  from  the 
Kabbit  Lake  section:  2,  breeciated,  hydrated  hard  ore  (Rabbit  Lake);  3,  bard  blue  ore  (sec.  21,  T.  40  N.,  R.  28  W.);  4,  soft  ore  (see.  21,  T.  4G 
N.,  R.  28  W.);  5,  lean  soft  ore  (Rabbit  Lake);  C,  dense,  black,  highly  ferruginous  chert;  7,  8,  average  ferruginous  chert;  9,  weathered  highly 
siliceous  chert. 


MINERALOGICAL    COMPOSITION. 

The  Cuyuna  ores  are  more  or  less  magnetic  hydrated  hematite  with  some  limonite.  The 
principal  impurity  is  chert  in  layers.  A  less  common  impurity  is  clay  in  layers.  In  still  smaller 
amount  are  iron  carbonate  and  amphibole,  which  also  show  a  tendency  toward  concentration 
in  layers.  The  color  varies  from  a  hght  yellow  through  various  brown  and  reddisji  tones  to 
black,  according  to  the  hydration  of  fclie  iron  and  the  amount  of  magnetite  in  it.     The  liighly 


222 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


nian^aniforous  ores  contain  both  the  carbonates  and  oxitles  of  man<ianese.     They  are  most 
al)un(hint  near  the  surface.     The  mineral  canyin<j  the  pliosphorus  is  not  known. 

The  mineraloo;ical  composition,  fifjured  from  the  foregoing  analyses  in  which  loss  by  ignition, 
was  determined,  is  as  follows: 

Mineral  composiiion  of  Deerwood  iron-bearing  member. 


Depth. 

Hematite. 

Limonitc. 

Quart  z. 

Kaolin. 

Feet. 

17r>-180 

42.10 

48.80 

4.  no 

1.72 

lSIO-195 

40.80 

50. 00 

2.  i;3 

3.  IB 

20J-210 

.54.  i;o 

35.  .So 

1..  1.1 

1.57 

21.5-220 

(i8.  20 

22. 10 

7.63 

1.11 

230-235 

Ii4.10 

17.80 

14.  (;2 

1.24 

2J5-250 

6.1. 10 

14.20 

28.89 

.89 

2(;5'270 

54.  SO 

Ui.  1:5 

25.  .35 

1.49 

295-300 

13.85 

47.23 

30.84 

1.87 

IRON    MINERALS 
ico% 


SILICA 


PORE   SPACE 


Figure  27. — Triangular  diagram  representing  volume  ooinposilion  o(  various  phases  of  iron  ores  and  ferruginous  elierts  of  Ihc  Cuyuna  district^ 
Minnesota,  .\fler  F.  S.  Adams.  For  description  of  metliod  of  platting  and  interpretation  of  diagram,  see  p.  189.  1 ,  M:is.<ive  liard  blue  hema- 
tite; 2,  brecciated  limonitic  hard  ore;  3,  hard  ore  from  jiee.  21,  T.  4i'i  N..  It.  28  W.;  4,  soft  ore  from  sec.  21,  T.  40  N.,  R.  28  W.:  .5.  lean  soft  ore, 
(Haliljit  Lake);  0,  dense,  Idacli,  highly  ferruginous  chert:  7,  .s,  banded  ferniginous  chert;  9,  weathered  chert;  10,  typical  paint  rock;  11,  average 
ferruginous  chert;  12,  13,  average  soft  ore;  11,  average  ch<*rly  iron  carbonate. 

The  ore  really  consists  of  hematite,  limonite,  iiydrates  iiilermediate  helweeu  licmatile  and 
limonite,  and  magnetite.  As  it  is  almost  impossible  to  determine  what  degree  of  hydration 
some  of  the  minerals  may  have,  tli(>  analyses  are  expressed  in  terms  of  h(>inatite  and  limonite. 


CUYUNA  IRON  DISTRICT  AND  EXTENSIONS.  223 

Tliis  is  merely  a  conventional  means  of  showing  the  degree  of  hydration  for  these  ores.     The 
amount  of  magnetite  is  so  small  that  its  calculation  as  hematite  does  not  materially  affect  the 

result. 

TEXTURE. 

The  density  of  the  hard  ores  of  standard  grade  averages  4.09.  This  includes  both  types 
of  liard  ore.  The  low  figure  is  due  to  the  hydrated  character  of  the  Cuyuna  hard  ore.  Tlic 
density  of  the  soft  ores  averages  4.19.  The  lean  soft  ore  shows  an  average  density  of  3.73.  Th(> 
hard  blue  unbroken  type  of  hematite  has  an  average  density  of  4.26.  The  limonitic  brccciatcd 
hard  ore  shows  a  density  of  3.95. 

The  pore  s\rAce.  of  the  hard  ores  averages  13.13  per  cent  l)y  volume.  This  includes  both 
types.  The  soft  ore  has  an  average  pore  space  of  36  per  cent.  The  lean  soft  ore  shows  33.3 
jier  cent  pore  space.  The  hartl  ores  show  a  range  in  porosity  varying  from  9  to  20  per  cent  by 
volume. 

The  hard  ores  of  both  types  average  10  culiic  feet  per  ton.  The  hard  blue  hematite  varies 
between  9  and  10.5  cubic  feet  per  ton.  The  hydrated  brecciated  hard  ore  ranges  from  10  to 
10.8  cubic  feet  per  ton.  The  soft  ores  average  11.5  cubic  feet  per  ton.  The  lean  soft  ore  runs 
12.6  cubic  feet  per  ton. 

An  average  figure  to  use  in  computing  tonnage  for  a  large  deposit  where  various  ores  are 
represented  and  a  tonnage  estimate  of  each  type  is  out  of  the  question  Mould  be  about  11  cubic 
feet  per  ton. 

Notwithstanding  the  fineness  of  much  of  the  ores,  the  texture  is  not  disadvantageous,  for 
there  is  probabh'  less  of  it  that  will  act  as  flue  dust  in  the  furnace  than  there  is  of  the 
Mesabi  ore,  for  the  reason  that  it  is  as  a  whole  less  crystalline  and  more  earthy  and  takes  on  a 
more  coherent  texture  when  comjiressed. 

SECOND AKY  CONCENTRATION  OP  CUYUNA  ORES. 

Structural  covAitions. — The  structural  relations  of  the  Cuyuna  ores  are  still  so  imperfectly 
known  ^that  any  statement  concerning  them  must  be  made  with  much  qualification.  It  is 
nevertheless  obvious  here  that  the  concentration  has  been  greatest  at  the  surface  and  less  with 
depth,  and  that  at  least  in  many  places  it  has  been  very  active  next  to  the  intrusives  rocks 
which  cut  the  member  or  along  foot-wall  slates  or  am])hibole  schist.  Also  it  seems  to  have 
followed  axes  of  mmor  drag  folds.  All  the  rocks  have  been  weathered  to  a  considerable  extent. 
At  present  glacial  drift  covers  them  at  depths  of  35  to  400  feet,  so  that  water  stands  much  above 
the  rock  surface.  The  present  condition  is  oliviously  quite  different  fi-om  that  under  which  the- 
ores  were  concentrated.  It  may  be  supposed  that  when  the  rock  surface  was  exposed  waters 
penetrated  into  the  iron-bearing  member  as  it  was  exposed  on  the  anticlinal  areas  betM'een 
the  impervious  hanging  wall  and  the  impervious  foot  wall  and  that  where  the  member  M'as  cut 
by  impervious  igneous  rocks  they  served  further  to  control  the  circulation.  The  depth  of  cir- 
culation is  not  yet  known,  nor  is  it  clear  what  topographic  features  may  have  been  present  in 
the  past  to  control  the  depth  of  circulation. 

Original  character  of  the  Deeru-ood  iron-hearing  memier. — The  member  was  originally  cherty 
iron  carbonate  interbedded  with  slate. 

Mineralogical  and  chemical  changes. — The  alteration  of  the  original  carbonate  rocks  Avas 
in  different  sequence  from  that  in  most  of  the  Lake  Superior  ranges,  because  before  it  was 
exposed  to  weathermg  it  underwent  folding  and  intrusion,  which  partly  altered  the  cherty  iron 
carbonate  to  amphibole-magnetite  rock.  Subsequently,  when  erosion  had  exposed  the  mem- 
ber, the  surface  agents  of  alteration  therefore  had  two  phases  of  the  mendjer  to  work  upon — 
unaltered  iron  carbonate  and  amphibole-magnetite  rocks.  The  former  went  through  the 
ordinary  cycle  of  changes  to  ferruginous  cherts  and  ore.  The  latter  lost  some  of  its  silica 
and  amphibole  but  as  a  whole  was  much  more  resistant  than  the  carbonate.  The  net  result 
of  the  alteration  is  a  soft,  hydrated  ore  containing  much  magnetite  along  certain  bands,  both 
contaming  silica  as  imj)urity  and  in  increased  amount  with  dej)th. 


224  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

PHOSPHORUS  IN  CUYUNA  ORES. 

Phosphorus  has  been  concent  rated  with  the  iron  during  the  secondary  concentration  of 
the  ores.  It  is  probable,  for  reasons  simihxr  to  those  discussed  on  pages  192-196  for  the 
Mesabi  district,  that  phosphorus,  leached  from  the  overlying  Cretaceous  rocks,  has  been  added 
to  the  ore  during  its  secondary  concentration.  In  general  there  is  not  sufficient  lime  in  the 
'ore  to  combine  with  all  the  phosphorus  as  apatite,  hence  some  phosphorus  is  ])robably  com- 
bined with  the  hydrous  aluminum  and  iron  minerals. 

MINNESOTA  RIVER   VALLEY   OF   SOUTHWESTERN   MINNESOTA." 

Pre-Cambrian  crystalline  rocks  of  the  Minnesota  River  valley  of  southwestern  Mimiesota 
appear  in  numerous  exposures  along  the  river,  protruding  from  the  drift,  from  a  point  south- 
east of  New  Ulm  to  Ortonville  on  the  northwest.  The  great  bulk  of  the  crystalline  rocks  are 
granitos  and  gneisses.  These  ai)i)ear  for  the  most  part  in  the  river  bottoms  but  stand  also 
in  a  few  isolated  knobs  on  the  higher  ground  south  and  west  of  the  river.  There  are  man}'' 
varieties  of  granites  and  gneisses  and  all  gradations  between  them.  They  are  taken  as  a 
whole  to  represent  the  Archean  or  basement  complex. 

Associated  with  the  granites  and  gneisses  are  a  much  smaller  number  of  exposures  of 
gabbros  and  gabbro  schists.  These  present  many  varieties,  all  of  which  are  believed  to  have 
resulted  from  the  alteration  of  two  original  forms  and  their  intergradations — a  hypersthene- 
bearing  gabbro  and  a  hypersthene-free  gabbro. 

Peridotite  is  found  in  one  exposure  only  in  this  valley,  3  miles  southeast  of  Morton.  The 
relations  to  the  other  rocks  of  the  area  could  not  be  determined.  Cutting  the  gneisses  and 
gabbro  schists  throughout  the  area  are  numerous  dikes  of  diabase.  They  vary  in  width  from 
a  fraction  of  an  inch  to  175  feet.     Their  age  is  probably  Kew-eenawan. 

Southeast  of  Redstone  and  near  New  Ulm  are  exposures  of  quartzite  associated  with 
coarse  c[uartzite  conglomerate.  Near  Redstone  the  strike  of  the  quartzites  is  N.  60-70°  W. 
and  their  dip  varies  from  5°  to  27°  N.  In  New  Ulm  the  strike  is  N.  15°  E.  and  the  dip  varies 
from  10°  to  15°  SE.  The  quartzite  is  beheved  to  be  the  same  as  the  quartzite  found  in  a 
tleep  well  at  Minneopa  Falls,  near  Mankato,  Minn.,  which  is  covered  by  a  quartzite  conglom- 
erate of  Middle  Cambrian  age.  The  quartzite  of  Retlstone  and  New  Ulm  is  above  the  Archean 
granite  and  gneiss.  It  is  believed  to  be  of  Huronian  age,  but  whether  upper  or  lower  is 
unknowTi.  The  crystalline  rocks  of  the  Minnesota  River  valley  are  separated  from  the  \iv- 
ginia  slate  series  of  the  Cuyuna  and  St.  Louis  River  areas  by  a  drift-covered  area  at  least 
partly  underlain  by  granite  but  partly  unknown. 

Overlying  the  crystalline  rocks  are  Cretaceous  shales  and  sandstones,  whicli  appear  in 
rare  exposures  in  the  valley,  and  glacial  drift. 

a  For  further  detailed  description  see  Hall.  C.  W.,  The  gneisses,  gabbro  schists,  and  associated  rocks  of  southwestern  Minnesota:  Bull.  U.  S. 
Geol.  Survey  No.  157. 1899,  160  pp.,  with  geologic  maps. 


CHAPTER  X.  THE  PENOKEE-GOGEBIC  IRON  DISTRICT  OF  MICHIGAN 

AND  WISCONSIN/ 

LOCATION,  SUCCESSION  OF  ROCKS,  AND  TOPOGRAPHY. 

The  Pcnokee-Gogebic  district  lies  soutli  of  the  west  lialf  of  Lake  Superior,  in  the  States  of 
Michigan  and  Wisconsin.  It  extends  from  Lake  Numakagon  in  Wisconsin  about  N.  30°  E. 
to  Lake  Gogebic  in  Michigan,  a  distance  of  about  80  miles. 

In  the  accompanying  geologic  map  of  the  Gogebic  range  (PI.  XVI)  the  only  essential 
change  noted  from  earlier  maps  is  in  the  vicinity  of  Sunday  Lake,  where  faulting  and  perhaps 
folds  have  caused  a  marked  effect  in  the  iron-bearing  formation. 

The  succession  of  formations  in  the  district  is  as  follows: 

Cambrian  system Lake  Superior  sandstone. 

Unconformity. 
Algonkian  system: 

Keweenawan  series Gabbros,  diabases,  conglomerates,  etc. 

Unconformity. 

Huronian  series: 

Greenstone  intrusives  and  extrusives. 


Upper  Huronian  (Animikie  group). 


Tyler  slate. 

Ironwood  formation  (iron-bearing). 
Palms  formation. 
Unconformity. 

Lower  Huronian fBad  River  limestone. 

ISunday  quartzite. 
Unconformity. 
Archean  system: 

Laurentian  series Granite  and  granitoid  gneiss. 

Eruptive  unconformity. 

Keewatin  series Greenstones  and  green  srhista. 

This  chapter  mainly  deals  with  the  Huronian  series  and  especially  with  the  upper  Huronian 
(Animikie  group).  The  Huronian  series  for  most  of  the  district  has  a  breadth  varying  from 
less  than  half  a  mile  to  2  or  3  miles. 

The  Huronian  series  has  a  simple  structure.  It  consists  of  water-deposited  sediments, 
the  origin  of  which  has  been  for  the  most  part  determined.  The  rocks  have  simply  been  tilted 
to  the  north  at  an  angle  which  is  convenient  for  determination  of  the  succession  of  belts.  They 
are  without  foldmg  so  marked  that  the  belts  do  not  follow  in  regular  order  from  south  to  north. 
The  series  is  terminated  on  -the  east  by  the  unconformably  overlying  horizontal  Cambrian 
sandstone  and  on  the  west  by  areas  in  which  it  has  been  entirely  swept  away  by  erosion,  the 
Keweenawan  series  coming  tlirectly  against  the  southern  complex.  It  is  marked  off  from  the 
underlying  granitic  ami  gneissic  rocks  on  the  south  and  the  Keweenawan  series  on  the  north 
by  great  unconformities. 

The  major  features  of  the  topography  of  the  district  are  dependent  upon  the  relative 
resistance  of  the  formations.  The  strike  of  the  harder  fonnations  largely  controls  the  direction 
of  the  ridges.  Extending  along  the  southern  border  of  the  Huronian  rocks  is  a  prominent 
ridge,  the  crest  of  which  in  the  western  and  eastern  parts  of  the  district  is  fonned  by  the  iron- 
bearing  formation  and  in  the  central  part  of  the  district  by  the  granitic  rocks  of  the  Archean. 
The  Keweenawan  igneous  rocks  north  of  the  Huronian  mark  a  second  distinct  ridge,  the  so-called 
Trap  Range.     Between  these  ridges,  in  the  central  two-thirds  of  the  district,  the  soft  Tyler 

<•  For  further  detailed  description  of  the  geology  of  this  district  see  Mon.  U.  S.  Geol.  Survey,  vol.  19,  and  references  there  given. 
47517°— VOL  52—11 15  225 


226  GEOLOGY  OK  THE  LAlvE  SUPERIOR  REGION. 

slate,  constitutps  level  tracts  and  swampy  areas  between  the  more  resistant  rocks  to  the  south 
and  north. 

The  major  lines  of  drainage  are  almost  directly  transverse  to  the  ridges.  All  the  important 
streams  of  the  district  rise  in  the  basement  complex,  traverse  the  entire  Iluronian  series,  and 
break  through  the  Keweenawan  Trap  Range  to  the  north  on  their  wa\'  to  Lake  Superior.  Tiius 
there  are  many  notches  in  the  east-west  ridges.  The  elevation  of  the  major  portion  of  the  dis- 
trict is  between  1,400  and  1,600  feet,  but  a  few  points  reach  an  altitude  of  1,700  or  1,800  feet. 

ARCHEAN   SYSTEM. 

GENERAL  STATEMENT. 

The  Archean  rocks  comprise  the  Keewatin  series  (greenstones  and  green  schists)  and  the 
Laurentian  series  (granites  and  gneisses),  the  latter  being  intrusive  in  the  fonner.  WTien  the 
relations  were  first  appreciated  for  the  Gogebic  district  the  term  "Mareniscan"  was  applied  to 
the  greenstones  and  green  scliist  series."  At  that  time  it  was  not  known  that  the  rocks  named 
"Mareniscan"  are  equivalent  to  the  Keewatua  series  of  the  Lake  of  the  Woods  district.  Inas- 
much as  the  relations  between  the  Keewatin  and  the  Laurentian  were  worked  out  by  Lawson 
for  the  two  series  of  the  Lake  of  the  Woods  before  the  tenn  "Mareniscan"  was  proposed,  Kee- 
watin has  precedence  over  "Mareniscan"  as  a  general  term. 

KEEWATIN  SERIES. 

The  Keewatin  rocks  are  found  in  two  principal  areas,  one  in  the  central  and  the  other  in 
the  eastern  part  of  the  district.  They  are  mainly  scliistose  basalts,  for  the  most  part  fme 
grained  and  compact.  The  strikes  and  dips  of  the  scliistosity  vary  greatly,  in  tliis  respect 
contrasting  strongly  with  the  strikes  and  dips  of  the  beds  of  the  Iluronian  sediments.  The 
chief  mineral  constituents  of  the  Keewatin  are  quartz,  a  variet}^  of  feldspar,  hornblende,  and 
biotite,  with  chlorite,  magnetite,  sericite,  and  epidote  as  subordinate  constituents,  although 
locally  any  one  of  these  latter  minerals  may  be  very  abundant.  In  places  the  schists  have  a 
banded  appearance  and  are  true  gneisses.  For  the  most  part  the  Keewatm  scliists  are  com- 
pletely crystalline  and  are  allied  to  igneous  rather  than  sedimentary  rocks.  Indeed,  when  the 
Gogebic  district  was  mapped  no  material  was  anywhere  found  which  could  be  asserted  to  be 
sedimentary,  although  patiently  searched  for.  However,  west  of  Sunday  Lake  a  biotite  schist 
was  found  which  was  stated  to  present  in  thin  section  a  "strong  fragmental  appearance." 
Later  work  has  showTi  that  south  and  east  of  this  lake  some  of  the  material  is  banded,  weathers 
white,  and  appears  to  be  true  slate.  It  seems  clear  that  here  there  is  sedimentary  material, 
but  it  is  difficult  to  draw  a  line  between  the  sediments  and  the  greenstones.  It  is  to  be  noted 
that  the  area  in  which  the  sediments  are  foimd  is  2  miles  from  the  Laurentian  granite. 

The  existence  of  iron  fomiation  is  reported  in  the  Keewatin  area  near  ^larenisco.  This, 
presumably,  is  analogous  to  the  iron  formation  belts  so  common  in  the  Keewatin  in  other  parts 
of  the  Lake  Superior  region.     It  has  not  been  examined  l)y  the  authors. 

LAURENTIAN  SERIES. 

The  Laurentian  granite  occurs  m  three  large  areas — in  the  western,  central,  and  eastern 
parts  of  the  district.  The  granites  of  these  areas,  like  all  the  other  granites  of  the  Laurentian 
of  the  Lake  Superior  region,  vary  greatly  in  chemical  composition,  mineral  content,  and  struc- 
ture. In  general,  in  the  district  under  discussion  the  granites  are  of  a  somewhat  acidic  type. 
However,  in  the  central  area,  besides  the  granites  there  are  syenites  and  even  gabbros,  and  the 
three  rocks  seem  to  grade  into  one  another.  Structurally  the  granites  range  from  rocks  which 
have  comparatively  little  schistosity  to  those  which  in  general  are  strongly  gneissoid.     Aside 

a  Bull.  U.  S.  Geol.  Survey  No.  80, 1892,  p.  490. 


CAMBRIAN 


KEWEENAWAN  SERIES 


LEGEN  D 

ALGONKIAN 


HURONIAN     SERIES 


ARCHEAN 

LAUftENTIAN  SERIES  KEEWATm  SERIES 


PENOKEE-GOGEBIO  IKON  DISTRICT.  227 

from  the  various  feldspars  and  quartz,  the  most  abundant  minerals  are  the  micas  and  horn- 
blende. There  are  other  subordinate  minerals,  of  which  magnetite  and  chlorite  are  important. 
In  the  dominant,  more  acidic  phases  of  the  rocks  the  alkaline  feldspars,  comprising  orthoclase, 
microclme,  and  acidic  plagiociase,  are  invariably  the  cliief  constituents  and  in  many  places  com- 
pose as  much  as  three-fourths  of  the  rock.  The  gneissoid  varieties  of  the  Laurentian  may  be  in 
part  metamorphosed  forms  of  granite.  Correlative  -with  the  structural  changes  are  important 
mincralogical  changes.  The  most  mteresting  is  that  by  which  the  feldspars  alter  into  biotite 
and  quartz.  Wliere  this  process  has  gone  far  little  or  no  feldspar  remains,  this  mineral  being 
replaced  by  a  fuiely  crystallme  interlockmg  mass  of  quartz  and  biotite.  This  results  in  a 
somewhat  coarsely  crystallme  feldspathic  rock  (normal  granite),  changing  into  a  finely  crystal- 
line gneissoid  biotite-quartz  rock.  It  is  interesting  to  note  that  identical  changes  of  a  feld- 
spathic fragmental  rock  in  the  Tyler  slate  have  formed  a  mica  schist. 

RELATIONS  OF  KEEWATIN  AND  LAURENTIAN  SERIES. 

The  fact  has  already  been  mentioned  that  the  Laurentian  granites  intrude  the  Keewatin 
schists.  It  is  characteristic  for  the  district  that  with  approach  from  the  Keewatin  rocks  to 
the  contact  of  the  Keewatm  with  the  Laurentian  granite  the  former  rocks  become  coarser  and 
finally  grade  into  coarse  gneisses,  not  very  different  from  granitoid  gneisses.  In  many  jDlaces 
the  granites  are  foxmd  to  cut  through  the  schists  in  dikes  and  stocks.  Indeed,  there  is  between 
the  two  series  usually  a  zone  of  considerable  breadth  in  wliich  the  two  rocks  are  in  approximately 
equal  proportions.  In  placing  the  boundary  line  between  the  series  on  the  maps  the  plan  has 
been  to  mclude  m  the  Keewatin  all  those  rocks  the  hand  specimens  of  wliich  do  not  have  a 
strong  granitic  appearance.  The  relations  between  the  two  are  plaiiily  those  which  so  charac- 
'  teristically  obtaui  between  the  Laurentian  and  Keewatin.  The  former  rocks  are  batholithic 
intrusions  in  the  latter  and  have  cut  them  intricately.  Along  the  border  the  granites  have  pro- 
foundly metamorphosed  the  Keewatin,  producing  marked  exomorphic  effects,  so  that  the  most 
altered  varieties  of  schists  approximate  the  character  of  the  granite. 

ALGONKIAN   SYSTEM. 

HURONIAN  SERIES. 
LOWER    HURON  IAN. 

The  lower  Huronian  in  the  Penokee-Gogebic  district  is  represented  only  by  the  Smuday 
quartzite  and  the  Bad  Iliver  limestone. 

SUNDAY  QUARTZITE. 

Liihology  and  distribution. — The  Smiday  quartzite  is  so  named  because  of  its  exposures 
east  of  Sunday  Lake.  It  may  prove  to  be  the  same  as  the  Mesnard  quartzite  of  the  Marquette 
district,  but  in  the  absence  of  defuiite  proof  that  it  is  the  same  formation  the  new  name  Sunday 
is  here  introduced  for  it.  The  only  known  exposures  of  the  formation  are  those  a  short  distance 
east  of  Little  Presque  Isle  Iliver  and  those  near  the  Newport  mine.  The  former  are  rather 
extensive  and  the  latter  are  small.  Probably  this  quartzite  is  coextensive  with  the  Bad  River 
limestone,  although  it  is  not  usually  exposetl.  Wherever  the  Bad  River  limestone  occurs  there 
is  room  between  it  antl  the  underlying  Archean  for  the  Smiday  quartzite  to  be  present.  East 
of  Presque  Isle  River  the  formation  is  mainly  quartzite,  with  a  thickness  of  at  least  150  feet. 
Below  the  quartzite  is  a  basal  conglomerate,  the  fragments  of  wliich  are  largely  derived  fi-om  the 
immediately  underlying  Keewatin  schists.  This  conglomerate  for  the  most  part  is  but  a  few 
inches  thick,  but  in  places  it  has  a  thickness  of  10  feet.  The  dip  of  the  quartzite  is  about  30°  N. 
Near  the  Newport  mine  the  Sunday  quartzite  is  represented  by  a  thm  belt  of  conglomerate 
clinging  to  the  face  of  the  granite.     This  conglomerate  contains  different  kinds  of  granite, 


228  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

porphyry,  and  various  basic  rocks.  From  tlio  relations  of  tliis  conglonacratc  to  tlie  Palms 
formatiojT  it  is  believed  to  be  tlic  equivalent  of  the  fonfjloiuerate  east  of  Presquc  Isle  River. 
Relations  to  adjacent  formations. — The  relati(jns  of  llie  Sunday  quartzite  to  the  underlj^ing 
formations,  and  especially  to  the  Keewatin  east  of  Presque  Isle  River,  show  that  there  is  a  great 
unconformity  between  them.  The  actual  contact  between  the  two  is  beautifully  exposed  for 
some  distance.  The  scliistosity  of  the  Keewatin  abuts  against  the  bedding  of  the  quartzite 
at  various  angles  up  to  perpendicular.  The  Keewatin  had  been  formed,  metamorphosed,  and 
denuded  before  the  deposition  of  the  conglomerate.  The  Sunda}'  quartzite  grades  upward  into 
the  Bad  Kiver  hmestone. 

BAD  RIVER  LIMESTONE. 

Distribution. — The  Bad  River  hmestone  is  so  named  because  of  its  occurrence  at  Bad 
River  in  the  Penokee  Gap  section.  The  formation  is  present  at  several  localities  in  the  western 
part  of  the  district,  at  one  place  iii  the  central  part,  and  in  one  area  in  the  eastern  part.  The 
eastern  area  shows  the  most  extensive  exposures  of  the  district,  the  formation  here  being 
continuous  for  several  miles.  Wlierever  the  formation  is  found  it  strikes  approximately  par- 
allel to  the  formations  of  the  upper  Huronian,  and  the  dip  is  always  to  the  north,  being  as  high 
as  70°  or  80°  in  the  western  part  of  the  district  and  as  low  as  30°  in  the  eastern  part. 

Lithology. — The  formation  is  called  a  limestone  because  that  is  the  predominant  rock. 
The  limestone  is  heavily  magnesian  and  in  places  approaches  a  dolomite.  It  commonly  bears 
sUicates,  of  wlxich  tremolite  is  the  most  abimdant,  but  chlorite  and  sericite  are  not  rmcommon. 
The  rock  is  very  sihceous.  The  coarsest  varieties  of  the  silica  are  quartz,  but  chert  is  more 
common.  In  many  places  the  silica  is  closely  intermingled  with  the  dolomite.  In  other  places 
it  occurs  in  bands  varying  from  a  fraction  of  an  inch  to  a  much  greater  width,  antl  in  one  place 
a  band  of  siliceous  material  45  or  50  feet  wide  was  observed.  Thus  the  chert  and  limestone  are 
intermingled  and  iiiterstratified.  The  cherty  limestone  is  a  water-deposited  sediment.  Whether 
the  original  carbonate  was  of  chemical  or  organic  origin  we  have  no  definite  evidence,  but  there 
is  no  more  reason  to  suppose  that  life  was  not  concerned  in  the  deposition  of  tliis  cherty  hme- 
stone than  of  those  of  later  age. 

MetamorpMsm. — The  Bad  River  limestone  has  been  much  metamorphosed  since  its  deposi- 
tion. During  its  metamorphism  the  silica  recrystalhzed.  It  was  concentrated  into  bands. 
It  was  rearranged  into  veinlike  forms.  During  these  changes  a  part  of  the  silica  may  have 
been  introduced  from  an  extraneous  source  or  at  least  from  parts  of  the  formation  now  removed 
by  erosion.  The  abundant  tremolite  is  evidence  that  the  metamorphism  took  place  under  deep- 
seated  contlitions  when  the  silica  united  with  the  calcium  and  magnesium  to  form  sihcates, 
the  carbon  dioxide  being  released  at  the  same  time.  This  is  an  anamorphic  change  which  took 
place  with  decrease  of  volume. 

Relations  to  adjacent  formations. — The  relations  of  the  Bad  River  hmestone  to  the  Simday 
quartzite  have  already  been  considered.  It  is  probable  that  everywhere  it  grades  down  into 
this  formation,  but  whether  it  does  so  or  not  the  distribution  of  the  limestone  at  various  phices 
along  the  southern  border  of  the  Huronian,  with  a  strike  parallel  to  the  upper  Huronian,  thus 
contrasting  strongly  with  the  varying  strikes  and  dips  of  the  green  scliist  and  gneisses,  leaves  no 
doubt  that  between  the  Archean  and  the  Bad  River  limestone  there  is  a  great  imconformity. 
Indeed,  as  chemical  sedimentation  at  several  points  for  a  distance  of  60  or  70  miles  followed  so 
promptly  after  the  burial  of  the  southern  complex  below  the  sea,  it  appears  probable  that  when 
the  limestone  was  laid  down  the  Archean  was  reduced  to  an  approximate  plane.  The  lack  of 
continuity  of  the  limestone  formation  is  due  to  the  erosion  which  took  place  after  its  deposition 
before  the  lowest  member  of  the  upper  Huronian  was  laid  down.  Evidences  of  this  erosion 
are  given  under  the  desciiption  of  the  relations  of  tlie  Palms  formation  to  adjacent  formations. 
If  formations  later  than  tiie  Bad  River  limestone  belonging  to  the  lower  Huronian  were  depos- 
ited, they  were  removed  by  erosion  before  the  deposition  of  the  upper  Huronian,  as  was  the 
larger  part  of  the  Bad  River  hmestone  itself.  The  limestone  above  the  quartzite  in  the  western 
area  has  a  thickness  of  at  least  200  feet,  and  to  the  west  the  tluckuess  is  not  less  than  300  feet. 


PENOKEE-GOGEBIC  IRON  DISTRICT.  229 

UPPER    HURONIAN   (aNIMIKIE    GROUP). 
GENEKAL  STATEMEITT. 

The  upper  Huronian  comprises  the  Pahiis,  Ironwood,  and  Tyler  formations.  These 
formations  extend  continuously  from  Presque  Isle  River,  east  of  Sunday  Lake,  several  miles 
west  of  Bad  River.  They  constitute  a  northward-dipping  monocUne.  Tliis  monochne  has 
various  minor  pUcations  which  give  local  variations  to  the  strikes  and  dips,  but  they  are  neither 
abrupt  nor  large,  the  extreme  variations  in  strike  usually  being  between  N.  60°  W.  and  N. 
60°  E.  At  various  places  there  are  cross  faults,  the  most  notable  of  which  are  those  at  Penokee 
Gap,  with  a  throw  of  at  least  900  feet,  at  Potato  River,  with  a  throw  of  280  feet,  and  west  of 
Sunday  Lake.  Detailed  studies  of  the  iron-bearing  formation,  made  in  connection  with  the 
exploitation  of  the  iron  ore,  show  the  presence  of  very  numerous  small  transverse  faults  as 
well  as  numerous  longitudinal  faults,  with  hades  parallel  to  the  bedding,  or  nearly  so.  The 
latter  were  detected  by  the  displaced  dikes.  Part  of  the  faulting  was  prior  to  Keweenawan 
extrusions  because  it  does  not  displace  the  Keweenawan.  A  notable  instance  of  tlxis  appears 
in  the  great  transverse  fault  just  west  of  Sunday  Lake.  Other  faults  are  clearly  post-Kewee- 
nawan,  for  they  affect  both  Huronian  and  Keweenawan  beds. 

PALMS  FORMATION. 

Distribution. — The  Palms  formation  is  given  tliis  name  because  it  occurs  in  typical  develop- 
ment south  of  the  Palms  mine.  It  comprises  the  lowest  of  the  upper  Huronian  rocks  of  the 
Penokee-Gogebic  district. 

It  constitutes  a  well-marked  zone  traceable  tlirough  its  entire  extent,  except  in  the  volcanic 
area  at  the  east  end.  It  strikes  on  the  average  about  N.  70°  E.  Its  dip  is  everywhere  north, 
varjang  from  about  40°  to  75°,  the  usual  dips  being  between  55°  and  65°.  For  the  larger 
portion  of  the  district  the  formation  is  400  to  500  feet  thick,  but  east  of  Sunday  Lake  it  is 
tliicker,  the  maximum  being  800  feet. 

Lithology. — The  Palms  formation  consists  of  three  members,  of  which  the  lowest  is  a  thin 
layer  of  conglomerate,  the  central  and  dominant  mass  of  the  formation  is  a  clayey  slate,  and 
the  uppermost  is  a  quartzite.  The  conglomerate  is  generally  less  than  10  feet  thick  and  in 
many  places  is  not  more  than  1  to  3  feet.  The  quartzite  layer  at  the  top  is  about  50  feet  tliick. 
The  conglomerate  varies  with  the  character  of  the  rock  with  which  the  Pahns  formation  is 
in  contact.  Where  it  is  next  to  the  Bad  River  limestone,  as  would  be  expected,  there  are  in  it 
very  abundant  fragments  of  chert  and  hmestone,  but  with  these  are  also  granite,  gneiss,  and 
schist  from  the  Archean.  Where  the  contacts  are  with  the  Keewatin,  as  at  Potato  River 
and  the  west  branch  of  Montreal  River,  the  dominant  fragments  of  the  conglomerate  are 
derived  from  the  schist.  W^here  the  formation  is  in  contact  with  the  granite,  as  in  the  central 
part  of  the  area,  the  dominant  fragments  are  from  tlus  formation,  but  in  places — as,  for  instance, 
south  of  the  Palms  mine — with  these  fragments  there  are  also  pebbles  of  jasper,  chert,  and  quartz. 

The  central  part  of  the  formation  is  a  pelite.  It  has  many  facies,  varying  from  fuie- 
grained  clayey  slates  through  novacuUtes  to  graywackes.  For  the  most  part  the  alterations 
through  wliich  the  pelite  has  gone  are  mainly  metasomatic  ones,  such  as  quartz  enlargement 
and  the  alteration  of  the  feldspar  to  other  minerals,  especially  biotite,  chlorite,  and  quartz. 
In  the  western  part  of  the  district  the  feldspathic  alteration  and  recrystallization  are  sufficiently 
important  so  that  in  places  the  rocks  have  become  chloritic  and  biotitic  slates.  This  greater 
metamorphism  is  doubtless  connected  with  the  intrusions  so  characteristic  of  this  part  of  the 
district.  For  the  most  part  there  seems  to  be  Hthologic  correspondence  of  the  main  mass  of 
the  slate  with  the  immediately  underlying  Archean  rocks,  the  slate  being  substantially  the 
same  whether  north  of  the  Keewatin  schists  or  north  of  the  Laurentian  granite. 

The  upper  part  of  the  formation  is  a  psammite  which  has  been  indurated  by  the  process 
of  cementation  to  a  clean,  typical,  vitreous  quartzite.  As  this  quartzite  approaches  the  over- 
lying iron-bearing  formation  it  becomes  stained  with  oxide  of  iron  and  at  the  contact  it  is 
commonly  of  a  deep  brownish-red  color. 


230  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Relations  to  adjacent Jmrnations. — In  giviiit;  (he  relations  of  tlie  Palms  to  the  inferior  forma- 
tions it  is  necessary  to  consider  separately  its  relations  with  the  Bad  River  Jinicstone  of  the 
lower  Huronian  and  -with  the  Archean. 

The  fact  that  where  the  belt  of  conglomerate  at  the  base  of  the  Palms  formation  lies 
above  the  Bad  River  hmestone  it  bears  much  detritus  from  that  hmestone  sliows  that  the 
limestone  after  deposition  became  indurated  and  was  eroded  before  Palms  time.  In  general, 
the  strikes  and  the  dips  of  the  two  formations  are  approximately  parallel,  as  are  tho.sc  of  corre- 
lated formations  in  the  Menonunee  district,  but  it  is  plain  that  the  erosion  was  sulhcient  to 
remove  the  major  portion  of  the  Bad  River  limestone  and  also  any  later  formations  that  may 
have  been  deposited  in  the  lower  Huronian.  The  lack  of  marked  discordance  in  the  bedding 
of  the  Bad  River  limestone  and  the  Pahns  formation  is  no  evidence  that  the  time  gap  between 
the  two  was  not  long  enough  to  have  produced  a  pronounced  discordance  elsewhere,  for  the 
Penokee  district  at  tliis  time  may  have  been  distant  from  areas  of  important  folding  and 
thrusting  wliich  elsewhere  may  have  occurred. 

Between  the  Palms  formation  and  the  Archean  there  is  a  great  unconformity.  The  proofs 
of  tliis  unconformity  may  be  summarized  as  follows:  First,  the  Palms  formation  and  the  other 
sedimentary  formations  of  the  upper  Huronian  strike  with  considerable  uniformity  across  the 
country,  being  here  in  contact  with  one  variety  of  the  Archean,  there  with  another,  everywhere 
keeping  their  course,  nowhere  being  penetrated  or  interfered  with  by  any  of  the  Keewatin 
or  Laurentian  rocks,  whether  scliists,  gneisses,  or  granites.  Second,  the  Archean  rocks  are 
either  massive  ones  wloich  are  presumably  igneous  or  schists  and  gneisses  in  which  the  extreme 
of  foliation  and  crystalline  character  is  found,  whereas  the  overlying  upper  Huronian  rocks 
are  plainly  water-deposited  sediments.  Third,  in  a  dozen  places  or  more  above  the  Archean 
are  basal  conglomerates  or  recomposed  rocks  which  show  the  unconformable  contacts.  The 
detritus  in  each  place  is  dominantly  the  same  in  character  as  the  rock  on  w4iich  it  rests.  Where 
the  inferior  rock  is  granite  it  must  be  inferred  that  deep  erosion  must  have  exposed  it  at  the 
surface  prior  to  the  deposition  of  the  conglomerate.  \^Tiere  the  basement  rocks  are  Keewatin 
green  scliists  their  foliation  had  been  tleveloped  and  has  been  truncated  before  Pahns  time. 
This  is  well  illustrated  at  Potato  River,  where  the  conglomerate  contains  large*  flat  fragments 
of  green  schists  which  have  their  scliistosity  lying  parallel  to  the  bedding  of  the  Pahns,  which 
is  at  right  angles  to  the  scliistosity  of  the  Keewatin  below.  Fourth,  the  horizons  of  the  upper 
Huronian  with  wliich  the  Archean  is  in  contact  are  witliin  a  zone  not  more  than  300  or  400 
feet  thick  at  most.  Tliis  is  the  clearest  sort  of  evidence  that  the  underlying  rocks  were  reduced 
to  a  peneplain  before  the  beginning  of  the  deposition  of  the  Palms  formation.  From  the 
foregoing  fact  it  is  clear  that  the  break  between  the  Palms  formation  and  the  Archean  is  profound. 
It  included  the  time  represented  by  the  unconformity  between  the  lower  Huronian  and  the 
Archean,  the  time  retjuired  for  the  (lep<3sition  of  the  lower  Huronian,  and  the  time  between  the 
lower  Huronian  and  the  Palms  formation. 

IRONWOOD    FORMATION. 

Distribution. — The  Ironwood  formation  was  given  tliis  name  from  the  fact  that  near  the 
town  of  Ironwood  it  is  well  developed,  and  in  this  vicinity  occur  the  more  important  mines. 
The  formation  is  coextensive  in  its  distribution  with  the  underlying  Palms  formation.  Its 
strike  and  dip  are  conformable  with  those  of  the  Palms.  The  belt  lor  the  greater  part  of  the 
district  has  a  breadth  of  800  to  1,000  feet.  West  of  Sunday  Lake  the  surface  width  of  the 
formation  is  greater"  and  north  and  east  of  Sunday  Lake  the  belt  is  narrower.  Faults  cross  and 
follow  the  bods.  These  aflFect  the  distribution  of  the  ores  and  the  iron-bearing  formation, 
as  described  on  page  237.  The  average  tliickness  of  the  formation  is  about  850  feet.  In  the 
extreme  eastern  part  of  the  district,  where  volcanic  action  prevailed  through  much  or  all  of 
upper  Huronian  time,  the  Ironwood  formation  is  broken  into  tliin  and  impure  belts.  West 
of  Sunday  Lake  it  is  divided  into  two  or  more  belts  by  intercalated  (luartzito  and  fjuartz  slate 
beds.  In  other  parts  of  the  district,  notably  near  Upson,  the  formation  is  divided  b}*  slate 
layers.     In  the  main,  in  the  western  part  of  the  district,  except  for  the  gaps  whore  the  streams 

o  Recent  work  seems  to  show  this  widening  to  be  due  to  pre-Keweenawan  overthrust  folding  and  faulting  from  the  west. 


PENOKEE-GOGEBIC  IRON  DISTRICT.  231 

break  through  it,  the  Ironvvootl  formation  is  a  continuous  ridgo,  and  it  was  tliis  range  which 
first  attracted  the  attention  of  explorers  at  Penokee  Gap  and  vicinity.  In  the  central  part  of 
the  district  the  formation  is  softer  and  the  prominent  features  are  made  by  the  Archean  rocks 
to  the  south.  Still  farther  east,  beyond  Sunday  Lake,  the  Ironwood  formation  again  consti- 
tutes prominent  bluffs. 

LitJiology. — The  Ironwood  is  the  iron-bearing  formation  of  the  district.  In  the  memoir 
on  the  Penokee  iron-bearing  series  (Monograph  XIX)  it  w-as  simply  called  the  iron-bearing 
formation,  without  a  geographic  name.  The  greater  portion  of  the  formation  contains  more 
than  25  per  cent  metallic  iron  and  there  are  considerable  thicknesses  in  which  the  amount 
of  iron  averages  37  per  cent.  (See  p.  238.)  The  ore  bodies  contain  a  higher  percentage  of 
iron. 

The  Ironwood  formation  consists  of  four  main  varieties  of  rock — (1)  slaty  and  commonly 
cherty  iron  carbonate  and  ferrodolomite,  (2)  ferruginous  slates  and  ferruginous  cherts,  (3) 
actinolitic  and  magnetitic  slates,  and  (4)  black  slates. 

Tlie  iron-bearing  carbonates  are  usually  found  only  near  the  upper  part  of  the  formation, 
where  they  have  been  protected  by  the  Tyler  slate.  The  ferruginous  slates  and  ferruginous 
cherts  are  characteristic  of  the  central  iron-producing  part  of  the  district,  and  the  actinolitic 
and  magnetitic  slates  are  characteristic  of  the  western  and  eastern  parts  of  the  district.  The 
latter  also  form  a  belt  20  to  300  feet  wide  bordering  the  Keweenawan  rocks  on  the  north.  In 
the  intermediate  areas  there  are  of  course  gradations  between  the  ferruginous  slates  and  ferru- 
ginous cherts  and  the  actinohtic  and  magnetitic  slates,  as  there  are  also  gradations  between 
the  cherty  iron  carbonates  and  the  ferruginous  slates  and  ferruginous  cherts.  Black  slates 
form  thin  intercalated  layers  in  the  iron-bearing  formation.  Quartzite  is  also  found  in  layers 
up  to  100  feet  thick  well  up  from  the  base  of  the  formation  near  Sunday  Lake. 

The  slaty  and  cherty  iron-bearing  carbonates  are  composed  largely  of  iron  carbonate  and 
chert,  but  with  these  materials  are  various  amounts  of  calcium  carbonate  and  magnesium  car- 
bonate. Recent  reexamination  has  shown  that  in  these  rocks  there  are  also  subordinate  amounts 
of  greenalite.  With  these  important  constituents  are  other  minor  constituents,  largely  second- 
ary, such  as  limonite,  magnetite,  carbonaceous  and  graphitic  matter,  iron  pyrites,  and  rarely 
fragmental  quartz.  The  carbonate  is  both  fine  and  coarse  grained  and  both  origmal  and 
secondary.  Coarse-grained  recrystallized  carbonate  is  especiallj'  abundant  near  the  contact  of 
the  Keweenawan  in  the  Sunday  Lake  area. 

The  cherty  iron-bearing  carbonate  was  the  original  rock  of  the  iron  formation.  The  origin 
of  tliis  class  of  rock  is  fully  discussed  in  another  place  (pp.  499  et  seq.)  and  therefore  the  subject 
will  not  be  considered  here.  From  it  the  ferruginous  cherts  and  actinolitic  cherts  have  been  pro- 
duced. The  actinolitic  and  magnetitic  cherts  were  formed  under  deep-seated  conditions  largely 
through  the  influence  of  the  Keweenawan  intrusive  rocks,  and  especially  of  the  great  western 
laccolith.  These  changes  are  anamoi'phic  ones,  which  mainly  took  place  in  Keweenawan  time. 
Tlie  ferruginous  slates  and  ferruginous  cherts  formed  from  the  cherty  iron  carbonates  by 
katamorphic  changes  largely  in  the  belt  of  weathering  and  also  in  part  in  the  belt  of  cementa- 
tion. These  changes  were  mainly  post-Keweenawan,  after  erosion  brought  the  iron-bearing 
formation  to  the  surface,  and  they  have  continued  to  the  present  day.  Previously  formed 
actinolitic  and  magnetitic  rocks  were  in  a  much  more  refractory  condition  than  the  unaltered 
cherty  iron  carbonates  and  have  been  little  affected  by  the  alterations  of  the  zone  of  katamor- 
phism. 

The  ferruginous  slates  and  ferruginous  cherts  have  silica  as  their  predominant  constituent 
in  various  forms  of  crystallization,  from  amorphous  through  partly  crystalline  and  chalcedonic 
material  to  finely  crystalline  quartz.  With  the  silica  are  the  various  oxides  of  iron.  Hematite 
and  brown  hydrated  hematite  are  especially  prevalent.  Limonite  is  common  and  some  mag- 
netite occurs.  Where  the  hematite  is  in  large  quantity,  to  the  exclusion  of  the  hydrous  oxides, 
the  rocks  are  genuine  jaspers;  but  this  variety  is  rather  unusual  in  the  district.  The  rocks 
vary  in  their  stratification  from  the  regular  lamination  of  a  slate  to  irregularity.  In  many 
places  the  laminae  have  the  appearance  of  having  been  ilisrupted  and  recemented. 


232  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

The  actinolitic,  griineritic,  and  magnetitic  cherts  and  slates,  like  the  rocks  of  the  second 
variety,  have  quartz  as  their  (h)ininant  constituent.  Tliis  f|iiartz  is  crystalline  throughout 
and  clearly  nonclastic.  The  actinolite  varies  in  amount  from  a  verj'  little  to  a  constituent 
of  great  prominence.     The  iron  oxides  are  mainly  in  the  form  of  hematite  and  magnetite. 

The  black  slates  are  carbonaceous  fragmental  slates  in  la3'ers  in  the  iron-bearing  formation. 
These  exceptionally  form  the  foot  wall  of  the  ore  deposits.     (See  p.  242.) 

Relations  to  adjacent  formations. — The  Ironwood  formation  rests  conformably  upon  the 
Palms  formation.  The  change  from  the  clastic  quartzite  to  the  nonclastic  iron-beaiing 
formation  is  astonisliingly  abrupt.  Generally  it  can  not  be  said  that  there  is  any  evidence  of 
the  transition  between  them.  Locally  a  thin  conglomerate  marks  the  contact.  For  some 
reason  the  clastic  deposits  of  the  quartzite  ceased  and  the  nonclastic  deposits  of  the  Ironwood 
formation  began.     Above,  the  Ironwood  formation  passes  gradually  into  the  Tyler  slate. 

TYLER  SLATE. 

Distribution. — The  Tyler  slate  was  given  its  name  from  the  t3'pical  occurrence  of  the 
formation  along  Tylers  Fork.  It  extends  from  a  point  about  6  miles  west  of  Bad  River 
nearly  to  Sunday  Lake — that  is,  it  is  confined  to  the  central  two-thirds  of  the  district.  In 
breadth  the  formation  varies  up  to  2^  miles  at  Tylers  Fork.  The  strike  of  the  formation  is 
parallel  to  that  of  the  iron-bearing  formation  below.  Its  dip  is  also  similar  to  that  of  the  iron 
formation.  At  this  wider  part  its  dip  is  from  70°  to  75°.  It  apparently  follows,  therefore, 
that  for  the  central  part  of  the  district — that  is,  from  Bad  River  to  Montreal  River — this  forma- 
tion has  a  tliickness  ranging  from  7,000  to  11,000  feet.  It  is  plainly  the  great  formation  of  the 
district.     It  is  probable  that  minor  plications  partly  explain  this  apparent  tliickness. 

Lithology. — Study  of  the  formation  as  a  whole  shows  that  it  is  dominantly  a  pelite  but 
locally  it  is  a  psammite,  including  both  arkoses  and  feldspathic  sandstones.  There  is  a  general 
connection  between  the  character  of  the  rocks  to  the  south  and  those  of  the  slate  belt  adjacent. 
The  greater  part  of  the  belt  has  received  its  material  in  part  from  the  granitic  and  in  part  from 
the  schistose  areas;  the  part  of  the  belt  west  of  Penokee  Gap  has  received  nearly  all  its  material 
from  the  syenitic  granite  to  the  south  and  west.  The  different  varieties  of  rocks  of  the  Tyler 
slate  may  be  grouped  under  three  heads — (1)  mica  schists  and  mica  slates;  (2)  graywackes  and 
graywacke  slates;  (3)  clay  slates  or  phyllites.  Each  of  these  main  types  has  the  various  phases 
shown  by  the  follo\\ang  tabulation: 

Mica  schist  and  mica  slate: 

iMuscovitic. 
Biotitic. 
Musco\'itic  and  biotitic. 

Micaceous  and  chloritic fChloritic  and  biotitic. 

IChloritic  and  sericitic  or  muscovitio. 
Graywacke  and  graywacke  slate: 

Micaceous jBiotitic. 

iBiotitic  and  muscovitic. 
Micaceous  and  chloritic Chloritic  and  biotitic. 

fChloritic. 
Chloritic JMagnetitic  and  chloritic. 

[Ferruginous  and  chloritic. 

Clay  slate fChloritic. 

\Chloritic  and  magnetitic. 

It  is  not  necessary  to  describe  in  detail  the  different  varieties  of  these  rocks,  except  as  to 
their  alterations. 

Metamorphism. — In  the  monograph  on  the  Penokee  iron-bearing  series  the  alterations  of 
this  slate  are  discussed. "^  It  is  there  shown  that  each  of  the  varieties  of  rocks  mentioned  above 
has  developed  from  pclitcs  and  psammites  almost  wholly  by  mctasomatic  changes  witliin  the 
formation  itself,  without  the  addition  or  subtraction  of  material  from  an  extraneous  source. 

a  Hon.  U.  S.  Geo!.  Survey,  vol.  19, 1892,  pp.  332-345. 


PENOKEE-GOGEBIC  IRON  DISTRICT.  233 

In  general,  the  eastern  part  of  tlie  formation  is  less  altered  than  the  western  part.  Here 
the  prevailing  rocks  are  clay  slates,  graywackes,  and  graywacke  slates.  From  tlie  central  to 
the  western  part  of  the  district  the  rocks  become  more  crystalline,  and  at  the  extreme  west  end, 
especially  west  of  Penokee  Gap,  only  mica  slates  and  mica  schists  are  found.  Where  the  rocks 
are  much  metamorphosed  conlierite  is  sparingly  developed. 

The  parts  of  the  Tyler  slate  wluch  contain  large  fragmental  particles  of  quartz  are  those 
in  which  the  clastic  character  is  easiest  to  recognize,  for  the  grains  of  quartz  everywhere 
remain  in  their  entirety.  It  may  be  and  indeed  it  is  usually  true  that  they  have  undergone  a 
second  growth  and  have  thus  become  angular;  but  generally  the  original  cores  are  easily  dis- 
covered. In  the  nearly  pure  feldspar  sediments,  on  the  other  hand,  where  the  feldspar  has 
changed  to  other  minerals,  it  is  more  diilicult  and  in  specimens  of  the  most  crystalline  mica 
schist  impossible  to  make  out  the  original  fragmental  character  of  tlie  rock. 

On  the  whole,  the  major  modifications  of  the  formation  are  those  of  the  zone  of  anamorphism 
rather  than  the  zone  of  katamorphism.  This  is  what  would  naturally  be  expected,  for  at  the  time 
these  alterations  took  place  the  rocks  were  buried  to  an  unknown  deptli  below  the  overlying 
Keweenawan  rocks. 

As  the  processes  by  which  a  clastic  rock  alters  into  a  fine-grained  crystalline  mica  schist  were 
first  described  in  detail  with  regard  to  the  Pcnokee-Gogebic  district,"  the  principles  involved 
in  the  development  of  this  particular  rock  will  be  summarized  here.  As  already  indicated,  the 
setliments  from  which  the  mica  scliists  were  derived  were  very  feldspatliic.  Without  going 
into  details,  the  process  which  has  resulted  in  the  development  of  mica  schists  has  been  the 
alteration  of  the  feldspar  into  mica,  both  muscovite  and  biotite,  with  the  simultaneous  separa- 
tion of  quartz.  For  the  change  into  muscovite  the  feldspar  itself  contains  all  the  necessary 
constituents.  For  the  change  into  biotite  a  certain  amount  of  iron  and  magnesium  are  neces- 
^ry.  For  the  iron  it  is  not  so  clifRcult  to  account,  as  the  sediments  are  ferruginous.  In  some 
f^aces  also  the  sediments  contain  more  or  less  carbonate,  and  doubtless  from  tliis  source  has 
been  derived  at  least  a  part  of  tlie  necessary  magnesium.  At  the  tune  of  the  recrystallization 
tlie  newly  formmg  mica  flakes  developed  with  a  parallel  arrangement.  At  the  same  time  the 
quartz  recrystallized.  The  total  result  was  to  produce  from  a  somewhat  coarsely  crystalline 
arkose  a  finely  laminated  mica  slate  or  mica  schist. 

The  Penokee-Gogebic  district  is  an  exceptionally  good  one  in  which  to  work  out  the  changes 
from  the  little-altered  pelite  to  a  mica  schist,  because  of  the  very  gradiial  change  in  the  amount 
of  alteration  in  passing  from  the  central  to  the  western  part  of  the  district. 

At  the  time  the  Penokee-Gogebic  monograph  was  written  no  reason  was  assigned  for  the 
crystalline  character  of  the  rocks  at  the  west  end  of  the  district.  Later  studies  on  meta- 
morphism  have  led  us  to  connect  tliis  alteration  with  the  great  laccoHth  of  the  Keweenawan 
gabbro,  wliich,  in  the  western  part  of  the  district,  occurs  m  contact  with  and  cutting  the 
Huronian  rocks.  The  intrusion  of  tliis  rock  essentially  parallel  to  the  bedding  would  result  in 
great  pressure,  as  well  as  in  raising  the  temperature,  and  it  was  under  these  conditions  that  the 
recrystallization  took  place.  The  absence  of  similar  alterations  m  the  central  and  eastern 
parts  of  the  district  is  explained  by  the  fact  that  there  immediately  overlying  the  slate  are  the 
surface  Keweenawan  lavas,  which  are  locally  interstratified  with  sandstones.  It  is  plain  that 
the  alterations  of  the  pelites  to  mica  slates  and  mica  schists  took  place  in  Keweenawan  time. 

Relations  to  adjacent  formations. — The  Tyler  slate  rests  conformably  on  the  iron-bearing 
Ironwood  formation.     It  is  overlain  unconformably  by  the  rocks  of  the  Keweenawan  series. 

UPPER  HTJRONIAN  (ANIMIKIE  GROUP)  OF  THE  EASTERN  AREA. 

In  the  eastern  part  of  the  district — that  is,  from  about  6  miles  east  of  Sunday  Lake  to 
Gogebic  Lake — the  upper  Huronian  rocks  have  an  exceptional  character.  In  the  larger  part 
of  the  district  the    conditions  were    those  of  quiet  sedimentation,  but  in  this  eastern    area 

o  Van  Hise,  C.  R.,  Upon  the  origin  of  the  mica  scliists  and  black  mica  slates  otthe  Penokee-Gogebic  iron-bearing  series:  Am.  Jour.  Sci.,3d  ser., 
TOl.  31,  1886,  pp.  453-459. 


t> 


234  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

throiipjliout  the  greater  part  of  the  upper  Iluroiiiaii  there  was  eontinuous  volcanic  action.  In 
conse()uence  the  rocks  are  hiva  flows,  volcanic  tull's,  conglomerates,  agglomerates,  and  slates, 
with  all  sorts  of  gradations,  just  such  as  one  would  expect  if  a  volcano  arose  in  a  sea  and 
volcanic  action  continued  for  a  great  period.  Naturally  in  this  area  it  is  not  possible  to  map 
any  continuous  sedimentary  belts.  The  dominant  rocks  are  greenstone  conglomerates  and 
lavas  and  massive  eruptives.  The  uppermost  formation  for  the  extreme  eastern  part  of  the 
area  is  a  ferruginous  slate.  This  ferruginous  slate,  though  dominantly  clastic,  contains  narrow 
bands  of  nonclastic  sediments,  such  as  chert,  chertj^  ferrodolomite,  ferrodolomitic  chert.  It  is 
believed  that  the  ferruginovis  slate  is  probably  at  the  same  horizons  as  the  Ironwood  formation 
to  the  west  and  that  its  dominant  fragmental  character  is  due  to  the  presence  in  tliis  area  of 
one  or  more  volcanic  mountains  which  rose  above  the  water  and  upon  wliich  the  waves  were 
at  work  after  the  close  of  the  period  of  active  volcanic  outbreaks. 

KEWEENAW  AN  SERIES. 
GENERAL    DESCRIPTION. 

Rocks  of  the  Keweenawan  series  lie  north  of  and  are  coextensive  with  the  upper  Huronian 
rocks;  indeed,  to  the  west  they  extend  far  beyond  tlie  westernmost  kno\\'n  outcrop  of  the 
Huronian.  It  is  not  the  purpose  here  to  describe  this  series  more  than  is  sufficient  to  show 
its  relations  to  the  Huronian.  It  has  already  been  indicated  that  for  most  of  the  district  the 
appearance  of  the  Keweenawan  is  marked  by  a  distinct  range  Ivnown  as  the  Trap  Range.  For 
the  eastern  part  the  Keweenawan  rocks  first  encountered  in  traveling  north  are  ordinary 
basic,  amygdaloidal  lava  flows  characteristic  of  that  series.  One  bed  follows  upon  another 
and  it  is  easy  to  ascertam  then  strike  and  dip.  These  bedded  lava  flows  may  be  very  conven- 
iently seen  adjacent  to  Sunday  Lake.  Their  strike  and  dip  are  easily  determinable  as  almost 
exactly  parallel  to  the  beds  of  the  underlying  Huronian. 

In  the  central  part  of  the  district  the  Keweenawan  rocks  immediately  above  the  Tvler 
slate  are  sandstones  and  conglomerates.  These  are  seen  in  Micliigan  north  of  Bessemer  and  in 
Wisconsin  a  few  miles  west  of  the  State  boundary.  Above  the  sedimentary  beds  of  the  lower 
Keweenawan  follow  lavas  similar  to  those  which  occur  farther  east. 

In  the  western  part  of  the  district  the  sediments  and  bedded  lavas  of  the  Keweenawan  are 
replaced  by  the  great  plutonic  basal  gabbro  laccolitli  of  Wisconsm,  analogous  to  the  laccolith 
of  the  north  shore  of  Lake  Superior. 

RELATIONS    TO    ADJACENT    SERIES. 

The  Keweenawan  series  reposes  upon  tlie  upjier  Huronian  (^Vnimikie  group)  imconformably. 
As  the  two  series  are  nearly  conformable  in  strike  and  dip,  this  fact  was  only  slowlj^  appreciated. 
The  proof  of  the  unconformity  rests  entirely  upon  broad  field  relations.  In  the  central  part  of 
the  district  the  Keweenawan  is  upon  a  great  slate  formation  (the  Tyler  slate)  wMch  has  a 
maximum  tliickness  of  at  least  several  thousand  feet.  At  the  east  and  west  ends  of  the  ilistrict 
the  Keweenawan  cuts  diagonally  across  these  slates  and  comes  into  contact  with  the  iron- 
bearing  Ironwood  formation.  In  the  west  end  of  the  district  this  relation  might  be  supposed 
to  be  explained  by  the  intrusion  of  the  Keweenawan  laccolith,  but  this  can  not  apply  to  the 
eastern  part  of  the  district,  for  there  the  lower  beds  of  the  Keweenawan  are  the  surface  lava 
flows.  The  time  gap  between  the  Huronian  series  and  the  Keweenawan  series  must  have  been 
sufficient  for  a  widespread  orographic  movement  and  tleep  denutlation. 

As  the  Keweenawan  series  is  largely  composed  of  igneous  rocks  and  rests  upon  the  Huronian 
series,  naturally  the  latter  has  been  extensively  intruded  by  the  former.  The  intrusives  in  the 
Huronian  series,  so  far  as  known,  are  mainly  doleritcs.  ('onsideral)le  masses  of  them  in  tlu' 
eastern  and  western  ends  of  tlic  district  appear  to  follow  rouglily  parallel  to  the  range  and 


PENOKEE-GOGEBIC  IRON  DISTRICT.  235 

seem  to  be  intruded  sheets  or  laecoliths.  Some  of  them  may  be  surface  flows  contemporaneous 
in  origin  with  adjacent  Iluronian  sediments.  In  addition  to  those  intercalated  masses,  numer- 
ous dikes  cut  the  Huronian  formations.  These  dikes  are  found  in  all  formations,  but  they  have 
an  especial  significance  and  importance  in  connection  with  the  iron  ore.  (See  pp.  235-238.) 
In  that  part  of  the  district  which  has  been  the  seat  of  mining  operations  a  large  number 
of  these  dikes  cut  the  containing  formations  perpendicularly  to  the  bedding.  That  these 
dolerite  ilikes  are  the  avenues  tlu'ough  wliich  have  passed  from  deep  witliin  the  earth  the  vast 
amount  of  material  which  formed  the  overlying  basic  volcanic  flows  of  the  Keweenawan  series 
of  the  Trap  Range  to  the  north  can  hardly  be  doubted,  for  in  chemical  composition  the  lavas  of 
this  range  are  practically  identical  with  the  dikes.  (See  pp.  404-405.)  In  general,  the  dolerite 
dikes  are  very  fresh,  except  in  the  lower  parts  of  the  Ironwood  formation,  where  they  have 
been  subject  for  a  long  time  to  the  action  of  percolating  waters.  Analyses  of  the  latter  rocks 
show  that  they  have  undergone  extensive  changes,  which  have  been  referred  to  in  cormection 
with  the  origin  of  the  iron  ores. 

By  far  the  greatest  of  the  intrusive  masses  is  the  great  gabbro  laccolith  at  the  west  of 
Bad  River.  This  was  at  first  supposed  to  be  a  great  basal  flow,  but  all  their  later  studies  lead 
the  writers  to  believe  that  it  is  a  plutonic  intrusive  introduced  comparatively  late  in  Keweenawan 
time,  the  major  dimensions  of  the  mtrusion  being  nearly  parallel  to  the  beds  of  the  Huronian 
and  the  lava  flows  of  the  Keweenawan,  which  were  separated  by  the  inwelling  mass  of  gabbro. 

CAMBRIAN   SANDSTONE. 

The  Cambrian  sandstone  is  found  only  in  the  northeastern  part  of  the  district,  near  Gogebic 
Lake.  It  is  there  found  as  a  flat-lymg  reddish  sandstone,  known  as  the  Lake  Superior  sand- 
stone. It  rests  in  horizontal  position  against  the  Keweenawan,  the  Huronian,  and  the  base- 
ment complex.  In  one  place  a  basal  conglomerate  bears  detritus  from  all  the  lower  forma- 
tions. It  is  plain  that  during  and  after  Keweenawan  time  the  Huronian  and  Keweenawan 
series  were  turned  up  steeply.  Lofty  ranges,  which  must  have  been  formed  then,  were  removed 
by  denudation,  and  the  Cambrian  sandstone  was  deposited.  Therefore  a  great  unconformity 
separates  the  Cambrian  sandstone  and  all  the  earlier  series. 

THE  IRON   ORES    OF   THE  PENOKEE-GOGEBIC   DISTRICT. 

By  the  authors  and  \V.  J.  Mead, 
DISTRIBUTION,  STRTJCTUBE,  AND  RELATIONS. 

The  iron-ore  deposits  occupy  part  of  the  district,  extending  from  a  point  about  2  miles 
east  of  Sunday  Lake  in  Michigan  to  within  4  miles  of  Potato  River  in  Wisconsin,  a  distance 
of  about  26  miles.  Ore  has  recently  been  developed  in  sections  15  and  21,  T.  47  N.,  R.  43  W., 
near  Gogebic  Lake,  far  to  the  east  of  the  previously  kno\vn  deposits. 

The  iron-ore  deposits  constitute  about  1  per  cent  of  the  area  of  the  iron  formation.  This 
percentage  is  less  than  that  in  the  Mesabi,  which  is  8  per  cent.  However,  the  vertical  dimensions 
are  much  greater  than  in  the  Mesabi  district.  Ores  have  already  been  found  to  extend  to  a 
depth  of  2,500  feet,  one  of  the  largest  ore  bodies  in  the  district  now  being  known  at  that  depth. 

In  both  the  east  and  west  ends  of  the  Gogebic  district  the  character  of  the  formation  has 
been  influenced  by  intrusives,  with  the  result  that  the  iron  oxides  are  largely  magnetite  dis- 
seminated tlirough  the  formation  and  not  concentrated  to  a  commercial  extent. 

The  ore  deposits  come  to  the  surface  most  largely  along  the  north-middle  slopes,  locally 
on  the  lower  slopes,  of  the  topogi'aphic  feature  known  as  the  Gogebic  Range. 

In  the  Gogebic  district  the  u'on-bearmg  Ironwood  formation  dips  with  the  other  forma- 
tions of  the  upper  Huronian  toward  the  north  at  angles  averaging  about  65°.  The  under- 
lying rock  is  the  quartzite,  at  the  top  of  the  Palms  formation,  and  it  thus  forms  the  foot  wall  of 
the  iron-bearing  formation;    the  overlying  formation  is  the  Tyler  slate.     Slate  and  quartzite 


236 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


also  form  Lnterbedded  layers  in  the  iron-bearing  formation.  Numerous  greenstone  dikes  oi 
Keweenawan  age  cut  the  entire  series  in  such  a  manner  that  the  intersection  of  the  dikes  with 
the  bedding  usually  pitches  to  the  east.  The  intersections  of  the  dikes  with  imjxTvious  layers, 
principally  cjuartzite  of  the  foot  wall,  but  also  slate  layers  within  the  iron-bearing  formation, 
constitute  eastward-pitching  troughs  at  angles  from  15°  to  30°,  in  wliicli  most  of  the  deposits 
are  found  (fig.  2S). 

Westward-pitching  dikes  intersecting  with  eastward-pitching  dikes  or  continuous  with 
them,  and  both  intersecting  foot-wall  quartzite,  form  canoe-shaped  basins  for  the  ore,  as  illus- 
trated by  the  Aurora,  Pabst,  and  Newport  deposits.     East  of  the  Bessemer  the  main  dike  in  the 

Tilden  mine  has  an  eastward  pitch 
and  the  main  dike  in  the  Pahnsmine 
has  a  westward  pitch. 

On  the  south  the  foot  wall  of  the 
ore  is  therefore  generally  quartzite, 
locally  slate,  and  on  the  north  the 
ore  rests  against  greenstone  dikes. 
Slate  foot  walls  are  seen  at  the  Iron 
Belt,  Mikado,  Brotherton,  and  Sun- 
day Lake  mines,  from  250  to  2,000 
feet  north  of  the  quartzite  foot  wall. 
The  ore  deposits  are  generally 
sharply  defined  along  the  foot  walls 
and  the  dike  rocks,  but  in  many 
places  vary  upward  by  imperceptible 
stages  into  the  ferruginous  cherts  of 
the  iron-bearing  Ironwood  forma- 
tion, ^^liere  there  are  a  number  of 
parallel  dikes,  one  below  the  other, 
there  may  be  several  ore  bodies  one 
below  another — as,  for  instance,  at 
the  Asldanil  and  Norrie  mines.  (See 
fig.  29,  a  and  h.)  After  many  years 
of  mining  on  upper  dikes  in  the  New- 
port mine  one  of  the  largest  deposits 
of  the  district  has  been  found  on  a 
lower  dike.  The  main  Norrie  dike 
is  over  .30  feet  thick.  The  main 
Aurora-Pabst-Newport  dike  is  from. 
20  to  25  feet  thick.  The  mam  Colby 
tlike  is  over  90  feet  tliick.  "\Miere  a 
strong  dike  breaks  intomany  stringers 
at  a  depth,  as  in  the  Colby  mine,  the 
ore  body  is  also  likely  to  be  broken 
up  and  become  small  and  perhaps 
worthless.  The  dike  rocks  are  altered 
to  soapstone  or  paint  rock  along  tlieir 
contacts  with  the  ore  l)y  tlie  leaching 
of  the  bases. 

An  ore  deposit  is  likely  to  have  its  maximum  depth  m  the  apex  of  a  trougli,  and  from  tlus 
apex  a  belt  of  ore  may  extend  to  the  north  along  the  dike  and  to  the  south  along  the  foot 
wall.  In  many  instances  the  ore  bodies  follow  the  foot  walls  almost  exclusively,  as  at  the 
Norrie  mine.  (See  fig.  29,  c.)  Usually  where  the  deposits  follow  both  the  quartzite  and  tUkes 
the  former  is  larger  and  more  continuous  than  the  latter.  Where  an  ore  deposit  follows  both 
it  may  divide  before  reaching  the  surface  into  two  parts  separated  by  rock,  called  the  south  and 


ifc 


0         100       JOO  400  Feet 


FlGUKE  28.— Cross  section  showing  the  occurrence  of  ore  In  pitching  troughs  formed  by 
(lilies  and  quartzite  foot  wall,  in  the  Gogebic  district.  Made  up  from  mine  plats  and 
slightly  generalized. 


PENOKEE-GOGEBIC  IRON  DISTRICT. 


237 


north  veins  of  the  mines,  but  where  such  deposits  are  traced  below  the  surface  they  unite  into  a 

sino-le  body.     The  ore  grades  above  or  laterally  into  the  ferruginous  chert  or  ferruginous  slat«. 

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tion  of  the  ores  with  pitclung 
troughs  formed  by  the  inter- 
section of  dikes  and  foot-wall 
quartzite  for  a  considerable 
time  obscured  the  importance 
of  fracturing  in  localizing  the 
ore  deposits.  From  evidence 
now  available  it  seems  likely 
that  tlus  factor  also  is  of 
great  consequence.  An  ex- 
amination of  nune  sections 
taken  almost  at  random  in 
the  district  shows  ore  cutting 
tlu-ough  the  ferruginous  cherts 
and  dikes  in  a  most  h-regular 
way  and  quite  independent  of 
the  pitching  troughs.  Much 
of  it  may  be  directly  con- 
nected with  brecciation,  fis- 
suring,  or  faulting  to  be  seen 
in  the  ore  and  adjacent  rocks. 
It  is  altogether  likely  that 
more  fractures  exist  than  are 
known,  because  the  concen- 
tration of  the  ores  is  of  a 
nature  to  obscure  evidences 
of  them.  There  are  fault 
planes  both  parallel  with  and 
intersecting  the  bedding.  The 
displacements  have  both  hori- 
zontal and  vertical  compo- 
nents. The  faults  intersecting 
the  bedding  were  first  recog- 
nized because  of  the  ease  of 
detecting  the  displacements  in 
the  bedding.  Those  parallel 
to  the  bedding  were  for  a  long 
time  not  observed,  and  proba- 
bly there  are  still  many  to  be 
detected  because  of  the  diffi- 
culty of  distinguisliing  the 
evidence.  They  may  be  deter- 
mined only  from  the  displace- 
ments of  the  cUkes,  and  as  the 
dikes  are  numerous  and  of 
varying  tliickness  a  consider- 
able amount  of  ground  must 
be  opened  up  before  the  va- 
rious fractured  dikes  may  be  correlated.  The  ore  in  many  places  lies  between  the  displaced 
edges  of  the  dike.  At  the  Pabst  mine  the  ore  follows  down  over  the  broken  and  displaced  ends 
of  the  faulted  dike  toward  another  dike  below,  where  it  again  develops  into  a  large  body. 


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Tyler  Fork 
Annie 

Shores 

iron  Belt 

Atlantic 

Laura 

Caledonia 
Imion 
Emma  and 
Daylight 


Nome 

East  Norrie 

Aurora 

Aurora  and  Pabst 

Newport 

Wisconsin  and 
Geneva 

Royal 

Puritan 

Ironton 
Winona 

Jackpot  and 
Yale 

Colby  g 


Tiiden 


Palms 


> 

2 


EureKa 
Black  River 


Section  8 

Chicago 

Pike 

Brotherton 

Sunday  Lake 
Iron  Cnief 

Castile 


238 


GEOLOGY  OF  THE  LAKE  SUPERIOIl  REGION. 


Another  important  factor  governing  the  location  of  tlie  ore  deposits  has  only  recently  been 
clearly  recognized.  Certain  of  the  iron  formation  layers  were  originally  ricJKir  in  iron  than 
others,  and  the  ores  show  a  distinct  tendency  to  follow  these  rich  original  beds.  In  some 
d;>posits,  like  those  of  tlic  Mikado,  Brothcrton,  and  Rvmday  Lake  mines,  this  seems  to  be  a  con- 
trolling factor,  though  the  ores  of  the  Brotherton  and  Sunday  Lake  mines  and  less  certainly  of 
tJic  ^Vlikado  min?  have  suffered  more  or  less  secondary  concentration  along  intersections  of  dike 
and  foot  wall  and  along  fissures. 


CHEMICAL  COMPOSITION  OF  THE  FERRUGINOUS  CHERTS  AND  ORES. 

The  following  analj'ses  represent  two  completL'  sections  through  tlie  iron-bearmg  forma- 
tion. In  the  Norrie  mine  a  crosscut,  extending  from  foot-wall  quartzite  to  the  hanging-wall 
slate,  entirely  in  ferruginous  chert,  was  sampled  in  five  samples,  each  sample  representing 
approximately  120  feet  of  crosscut.  In  the  Atlantic  mine  a  crosscut  m  feiTuginous  chert  extend- 
ing for  several  hundred  feet  across  the  formation  was  sampled.  Anaylses  are  by  Lerch  Brothers, 
Hibbing,  Minn. 

Partial  analysis  of  fcrruijinous  chert,  Gogebic  range. 

[Samples  dried  at  212°  F.) 


Fe. 

SiO". 

1'. 

AljOa. 

Volatile 
matter. 

35.33 
23.39 
30.03 
27.02 
26.81 
29.20 

43.78 
61.22 
51.80 
54.57 
54. 02 
52.07 

0.143 
.034 
.037 
.040 
.074 
.037 

1.54 
.71 
1.09 
1.78 
1.94 
.88 

1.42 

.85 

Norrie  mine  No.  3  . . .                                       .                                 

1.48 

1.67 

Norrie  mine  No.  5 .          .       .   .       .          

1.64 

2.89 

28.74 

53.11 

.062 

1.32 

1.66 

It  is  believed  that  this  average  represents  closely  the  true  average  composition  of  the 
unaltered  ferruginous  cherts. 

A  large  part  of  the  ferruginous  cherts  shows  partial  alterations  to  ore.  An  average  of  490 
analyses,  representing  5,S90  feet  of  drilling  in  tliis  phase  of  the  formation,  wliich  is  probably 
nearer  to  the  true  average  of  the  formation,  is  36.65  per  cent. 

The  average  composition  of  the  Gogebic  ores  for  the  years  lOOfi  and  1909,  calculated  from 
average  cargo  analyses  for  each  grade,  each  analysis  being  weighted  in  proportion  for  the  tonnage 
represented,  is  given  in  the  following  table : 

Average  composition  of  ore  mined  on  the  Gogebic  range  in  1906  and  1909. 


Moisture  (loss  on  drying  at  212°  F.). 
Analysis  of  dried  ore: 

Iron 

Phosphorus 

Silica 

Alumina 

Manganese 

Lime 

Magnesia 

Sulphur 

Loss  by  ignition 


PENOKEE-GOGEBIC  IRON  DISTRICT. 


239 


Range  in  percentage  of  each  constituent  in  Gogebic  ores  mined  in  1909,  as  shown  hij  average  cargo  analyses. 
Moisture  (loss  on  drying  at  212°  F.) 4.  51    to  15.  75 


Analysis  of  di'icd  ore: 

Iron 43.  70  to  G3.  40 

Phosphorus 027  to     .206 

Silica 4.07  to  23.  52 

Alumina 58  to    3.  29 

Mangane.se 20  to    7.20 

Lime 0  to      .87 

Magnesia 01  to      .79 


Sulphur ■. OOG  to 


Loss  on  ignition  . 


FERRIC  OXIDE 


.022 


.56    to    5.80 


MINOR  SILICA 

CONSTITUENTS 

Principally  alumina  and 
water  of  hydration 

Figure  30.— Triangular  diagram  showing  cliemical  composition  of  various  phases  of  Gogebic  ores  and  ferruginous  cherts  in  terms  ot  ferric  oxide, 
silica,  and  minor  constituents  (essentially  alumina  and  combined  water).  These  analyses  include  all  of  the  ores  and  cherts  shown  in  figure  32 
and  also  a  number  of  additional  analyses. 

In  figure  30  the  triangular  method  of  j)httting  is  employed  to  show  the  chemical  composition 
of  the  various  phases  of  the  chert  and  ore  studied.     (See  p.  182  for  explanation  of  diagram.) 


240 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


MINERALOGICAL  COMPOSITION  OF  THE  FEKKUGINOUS  CHEKTS  AND  ORES. 

The  approximate  miiu'iiil  comijositioii  of   the  ores  and  clicrts  wa.s  cak'idatcd  from  tlie 
chemical  analyses,  as  follows: 

Approriinate  mineral  composition  of  average  ferruginous  chert  and  average  ore  of  the  Gogebic  range. 


Average 
chert. 


Average  ore. 


1909 


Hematite  (i  lu-luding  a  small  amount  ot  magnetite) 

Limonite  (other  hydrated  iron  oxides  calcnlated  as  llnionite) 

Quartz 

Kaolin 

Other  minerals 


34.00 
8.20 

51.63 
3.3S 
2.82 


73.  SO 
14.70 
4.31 
4.89 
2.60 


100.00 


100.00 


77.25 
9.30 
5.81 
4.70 
2.94 


100.00 


PHYSICAL  CHARACTERISTICS. 


GENERAL    APPEARANCE. 


The  iron  ore  of  the  Gogebic  district  is  a  soft  red,  somewhat  hydrated  hematite.  Much  of 
it  is  so  friable  that  it  can  be  broken  down  with  a  pick,  although  as  taken  from  the  mines  a  large 
portion  of  it  is  compact  enough  to  hold  together  in  moderately  large  lumps.  These  lumps 
are  porous,  many  of  them  more  or  less  nodular,  and  many  also  roughly  stratiform.  The  strata 
conform  in  a  general  way  to  the  strike  and  dip  of  the  iron  formation.  Mingled  ^\'ith  this  soft 
hematite  in  a  few  mines  is  a  small  quantity  of  aphanitic  hard  steel-blue  hematite,  which  breaks 
with  conchoidal  fracture  and  is  of  remarkable  purity.  In  general,  this  exceptionally  hard 
material  is  found  in  contact  with  or  close  to  the  diorite  dikes  of  the  mines. 


DENSITY. 

The  specific  gravities  of  the  ores  and  cherts  were  determined  by  the  two  general  methods 
already  discussed  (see  p.  184) — (a)  calculated  specific  gravity  obtained  by  properh*  combining 
the  specific  gravities  of  constituent  minerals;  (6)  actual  determinations  by  gravitj"  methods. 
The  specific  gravities  of  the  minerals  as  used  in  determinmg  the  mmeral  specific  gra^Tit}'  of  the 
ore  or  chert  are  as  follows:  Hematite,  4.5  for  earthy  ores  and  chert,  5.1  for  crystalline  and  hard 
ores;  limonite,  3.6;  kaolin,  2.62;  quartz,  2.65. 

The  average  mineral  density  of  all  ore  mined  in  1906,  calculated  from  the  above  mineral 
analysis,  is  4.33. 

Following  are  six  analyses  of  ferruginous  cherts  and  ores  with  specific  gravities  determined 
by  both  methods,  as  a  check  on  tlie  accuracy  of  determination: 

Density  of  individual  cherts  and  ores  determined  by  calculation  and  by  measurement. 


Chemical  composition. 

Mineral  composition. 

Specific  gravity. 

Fe. 

SiOa. 

P. 

AljOa. 

Volatile 
matter. 

Mn. 

Moisture 
of  satu- 
ration. 

Hema- 
tite. 

Limonite. 

Quartz. 

Kaolin. 

Calcu- 
lated 
from 
analyses. 

Deter- 
mined 
bypyc- 
nometer. 

3.30 
30.30 
43.  20 
40.  80 
52.00 
63.40 

«7.91 
48.34 
32.88 
29.83 
12.59 
4.00 

0.007 
.027 

.022 
.013 
.029 
.078 

6.81 
6.30 

4.40 
1.32 
7.40 
2.94 

0.41 
1.S3 
1.55 
1.08 
5.43 
1.78 

0.15 
.50 
.45 

.85 
.25 
.70 

3.70 
9.30 
3.35 

10.50 
7.15 

10.50 

4.71 
43.30 

61.70 
63.30 
57.70 
86.20 

79.91          17.25 

2.73 
3.38 
3.80 
3.83 
3.89 
4.79 

2.68 

3.38 

3.809 

3.89 

3.90 

4.74 

43.24 
27.70 
28.28 
3.89 
.54 

10.95 
11.13 

3.34 
18.72 

7.45 

4.21 
19.38 
5.24 

PENOKEE-GOGEBIC  IRON  DISTRICT.  241 

POROSITY. 

Porosity  was  determined  on  hand  s])ecimens  by  the  usual  method  of  saturation  in  water 
described  on  page  185.  An  average  of  ten  determinations  on  typical  specimens  of  ferruginous 
cliert  gave  4.1  per  cent  pore  space.  The  average  of  the  porosity  of  all  the  ores  examined  was 
approximately  34  per  cent. 

CUBIC    CONTENTS. 

The  ores  vary  in  cubic  content  from  7. .5  cubic  feet  to  the  ton  in  the  small  masses  of  pure 
steel  ore  to  14  cubic  feet  in  the  softest  yellow  ores.  The  average  calculated  for  the  1906  output 
is  approximately  10.75  cubic  feet  to  the  ton. 

TEXTURE. 

The  average  texture  of  the  Gogebic  ores  is  shown  by  the  following  table  of  screening  tests. 
Thes3  were  made  by  the  Oliver  Iron  Mining  Company  and  represent  all  of  the  ores  mined  by 
that  company  in  the  Gogebic  district  in  1909.  Samples  of  the  different  ores  were  taken  twice  a 
week,  cpiartered  down  each  month  according  to  the  tonnage  shipped,  and  at  the  end  of  the 
shipping  season  quartered  to  100  pounds  of  dry  ore,  on  which  the  tests  were  made.  The  fol- 
lowing table  represents  10  grades  of  ore,  totaling  1,256,557  tons.  The  texture  of  the  ore  is 
seen  to  be  similar  to  tliat  of  the  ores  of  the  Marquette  district.  A  comparison  of  the  textures  of 
the  ores  of  the  several  Lake  Superior  districts  is  shown  in  figure  72,  page  481. 

Textures  of  Gogebic  ores  as  shown  by  screening  tests. 

Per  cent. 

Held  on  J-inch  sieve 28.  97 

|-inch  sieve 32.  30 

No.  20  sieve. 16.  08 

No.  40  sieve 8.  32 

No.  GO  sieve 4.  03 

No.  80  sieve 2.  56 

No.  100  sieve 1.  89 

Passed  through  No.  100  sieve 5.  92 

MAGNETITIC  ORES. 

At  the  extreme  east  and  west  ends  of  the  Gogebic  range  the  iron-bearing  formation  consists 
of  dark-gray,  green,  or  black  dense  crystalline  banded  rocks,  consisting  of  magnetite,  cpiartz, 
amphiboles,  and  other  silicates  in  varying  proportions  in  different  bands  and  different  localities. 
Ore  deposits  are  rare  or  altogether  lackmg.  For  a  discussion  of  i-easons  for  this  condition  see 
pages  552-553.     The  average  chemical  composition  of  these  rocks  is  as  follows: 

Analyses  of  mngnetitic  rocks  fi 

Fe,03 44.  606 

FeO 13.811 

SiO„ 34.  616 

ALO3 588 

CaO 1.  802 

MgO 2. 166 

MnO 1.158 

P2O5 018 

S 083 

HoO 997 

99.  845 
Metallic  iron 41.  95 

a  Mon.  U.  S.  Geol.  Survey,  vol.  19,  1892,  p.  197. 
47517°— VOL  52—11 16 


242  GEOLOGY  OF  THE  hAKE  SUPERIOR  REGION. 

Wlien  this  composition  is  compared  witli  that  of  tlic  fcrruLciiious  cherts  of  the  Gogebic 
district  it  is  apparent  that  there  is  ])ut  link'  differenco  between  the  two. 

SECONDARY  CONCENTRATION  OF  GOGEBIC  ORES. 

STRUCTURAL    CONDITIONS. 

The  ores  of  tliis  district  are  probably  localized  in  bands  of  the  iron  formation  wliich  were 
originality  rich  in  iron,  but  for  most  of  the  district  secondary  concentrations  have  so  masked 
the  primary  distribution  in  bands  that  the  evidence  for  it  is  not  clear.  Probabh'  the  clearest 
case  is  in  the  Mikado,  Brotherton,  and  Sunday  Lake  mines,  where  the  ores  seem  to  follow 
certain  originally  rich  horizons  in  the  iron  formation,  the  later  concentration  apparently  not 
having  seriously  modified  their  distribution. 

The  secondary  alterations  of  the  iron-bearing  beds  are  accomplished  (1)  by  waters  follow- 
ing the  pitching  trough  formed  by  the  intersection  of  the  dikes  with  impervious  quartzite  or 
slate  beds  below  the  iron  formation  layers,  and  (2)  by  following  fissures  or  beddhig  planes 
independent  of  the  dikes.  The  control  by  the  dikes  is  by  far  the  most  conspicuous  one  for  the 
district  as  a  whole.  The  movement  of  the  concentrating  waters  is  in  general  eastward  toward 
lower  levels,  following  the  eastward  pitch  of  the  trouglis  fomied  by  the  intersection  of  the 
dikes  with  foot-waU  quartzite  or  exceptionally  foot-wall  slate.  The  waters  may  thus  be  brought 
beneath  other  dikes.  Tliis  explains  the  common  occurrence  of  ores  on  several  dikes  one  below 
the  other.  The  movement  of  the  water  is  controlled  to  an  important  degree  by  bedtling  planes, 
by  faults,  and  by  joints,  and  where  so  controlled  the  ores  are  more  or  less  independent  of  the 
dikes  and  foot  wall.  The  control  by  faults  is  especially  well  shown  in  one  locahty  where  faulting 
parallel  to  the  bedding  has  displaced  tlie  ends  of  a  dike  and  the  ore  follo^vs  over  the  broken  end 
of  the  dike  along  the  fault  plane,  obviously  a  zone  followed  by  percolating  waters.  Faults  and 
joints  may  give  an  eastward  pitch  of  the  ore  bodies,  for  many  of  the  fissures  along  wliich  altera- 
tion takes  place  pitch  in  the  same  direction  as  the  dikes;  in  fact,  the  dikes  have  been  intruded 
along  fissures  of  this  kind.  That  some  fissures  were  there  before  the  intrusion  of  the  dikes  is 
shown  further  by  the  fact  that  the  iron  formation  near  Sunday  Lake  has  been  displaced  by 
faultmg,  whereas  the  Keweenawan  igneous  beds  to  the  north,  with  wliich  the  dikes  are  genetically 
connected,  have  not  been  displaced.  These  early  fissures  also  preceded  the  Keweenawan  folding. 
If  fissures  were  present  in  the  rocks  before  the  dikes,  there  is  no  reason  win"  some  concentration 
should  not  have  been  prior  to  the  intrusion  of  the  dikes  in  the  east  and  west  ends  of  the  district, 
where  the  cover  of  slate  was  not  too  great  to  prevent  ingress  of  water;  but  evidence  of  tliis 
would  be  extremely  difficult  to  detect  because  of  later  alterations  since  the  dikes  were  intruded. 

The  greatest  depth  to  which  the  w^aters,  and  therefore  the  ore  concentration,  may  be  car- 
ried by  the  eastward-pitching  trouglis  or  by  the  fault  and  joint  planes  is  yet  unknown.  Large 
ore  bodies  have  been  found  to  a  depth  of  more  than  2,500  feet;  one  of  the  largest  deposits  thus 
far  found  in  the  district  was  recently  developed  at  tliis  depth.  Theoretically  the  de])tli  of  con- 
centration is  a  function  of  the  head  detemiuied  by  the  height  of  the  erosion  edge  of  the  iron 
formation  and  the  lowest  point  of  escape;  but  the  difficulty  is  to  determine  where  the  latter 
point  is,  for  reasons  stated  above.  Even  if  the  head  were  known,  there  would  be  difficulty  in 
calculating  the  effective  depth  of  the  circulation  because  the  medium  tlirough  which  it  is  flowing 
is  not  homogeneous.  Further,  if  the  depth  of  the  active  circulation  could  be  worked  out  witliin 
reasonable  limits,  this  would  give  us  only  the  maximum  depth  of  the  ore  deposits,  for  it  might 
well  be  that  the  waters  do  not  carry  oxygen  abundantly  to  the  maximum  dejitli  to  wliich  they 
penetrate. 

Theoretically  the  concentration  of  the  ore  should  be  more  effective  on  the  middle  slopes 
of  the  hills,  because  these  would  be  places  where  descending  waters  are  efl'ective,  whereas 
valleys  are  places  where  the  waters  are  ascending  unless  prevented  by  other  structural  condi- 
tions, and  not  so  effective  for  the  purposes  of  ore  concentration.  It  is  unlikel_y  that  each  of  the 
cross  valleys  should  have  the  same  control  of  the  circulation,  and  it  is  difficult  to  tell  which  of 
the  valleys  has  been  most  effective.     Also  it  is  to  be  remembered  that  the  pitch  of  the  dikes  to 


PENOKEE-GOGEBIC  IRON  DISTRICT.  243 

the  east  is  greater  than  the  surface  slope  and  that  tlierefore  the  underground  waters,  where 
passing  under  a  valley,  would  be  prevented  from  escaping  b,y  the  overlying  impervioiis  dikes, 
except  where  faulting  would  allow  the  waters  to  come  tlu'ougli.  Mining  operations  actually 
disclose  artesian  flows  through  dikes,  as  at  the  Germania  niiue.  Also,  ascending  waters  are 
actually  observed  to  follow  faults  across  the  dikes,  as  in  the  Newport  mine.  From  anything 
that  is  now  known  to  the  contrary,  the  faults  in  the  tlikes  may  be  sufficiently  numerous  to  allow 
upward  escape  of  the  water  somewhat  freely  along  the  cross  valleys  at  the  surface.  This  is 
especially  likely  in  view  of  the  fact  that  the  cross  valleys  are  observed  to  have  developed  along 
fault  planes.  These  planes  must  cut  the  dikes,  though  some  of  them  are  not  observed  to  do 
so.  The  cross  valley  under  such  conditions  is  simply  the  surface  expression  of  the  weak  faulted 
zone.  It  is  therefore  not  to  be  expected  that  there  is  a  close  relation  to  be  observed  between 
the  topography  and  the  distribution  of  the  ores.  The  ore  deposits  extend  below  both  eleva- 
tions and  minor  valleys,  but  at  some  of  the  principal  cross  valleys  ore  deposits  are  small  or 
lacking.  For  illilstration,  ores  extend  abundantly  under  Montreal  River  at  Ironwood,  but 
east  of  the  Newport  mine  these  ores  seem  to  end  at  a  pronounced  cross  depression  northwest 
of  Bessemer,  through  which  Black  River  flows.  It  is  thought  by  James  R.Thompson,  formerly 
manager  of  the  Newport  mine,  that  the  drainage  for  the  Ironwood-Newport  group  of  mines  is 
probably  carried  eastward  and  escapes  through  tliis  channel. 

ORIGINAL    CHARACTER    OF    THE    IROX-BEARING    FORMATION. 

Originally  the  iron-bearing  formation  consisted  largely  of  cherty  iron  carbonate  inter- 
layered  with  sideritic  slates  and  possibly  also  with  banded  chert  and  ferric  hydrates.  (See 
p.  2.31.)  Some  layers  were  probably  richer  than  others.  The  alteration  of  the  cherty  iron 
carbonates  to  ore  has  been  accomplished  in  the  general  manner  already  described  as  ty|:)ical 
for  the  region — (1)  oxidation  and  hydration  of  the  iron  minerals  in  ])lace,  (2)  leaching  of  silica, 
and  (3)  introduction  of  secondary  iron  oxide  and  iron  carbonate  from  other  parts  of  the  forma- 
tion. These  changes  may  start  simultaneously,  but  change  1  is  usually  far  advanced  or  com- 
plete before  changes  2  and  3  are  conspicuous.  The  early  products  of  alteration,  therefore,  are 
ferruginous  cherts — that  is,  rocks  in  wliich  the  iron  is  oxidized  and  hydrated  and  the  silica  is 
not  removed.     The  later  removal  of  silica  is  necessary  to  produce  the  ore. 

ALTERATION    OF    CHERTY    IRON    CARBONATE    TO    FERRUGINOUS    CHERT. 

Chemical  change. — The  alteration  of  cherty  iron  carbonate  to  ferruginous  chert  involves 
the  oxidation  of  iron  according  to  the  following  reaction: 

2FeC03  +  nH,0  +  O  =  Fe.Og.nH^O  +  200^. 

Mineral  change. — The  cherty  iron  carbonate  is  practically  identical  mineralogically  with 
the  sideritic  cherts  of  the  Mesabi  range.  The  constituent  minerals  are  segregated  into  alternate 
layers  of  siderite  and  chert.  The  oxidation  of  the  siderite  involves  a  change  to  a  heavier 
mineral.     Either  introduction  or  removal  of  silica  may  accompany  this  change. 

Volume  change. — The  volume  involved  in  the  alteration  indicated  in  the  above  ecpiation 
is  a  loss  of  49.25  per  cent,  considering  the  resulting  iron  oxide  to  be  anhydrous.  If  hydration 
of  the  iron  oxide  takes  \Aace,  the  volume  reduction  is  smaller  in  proportion  to  the  degree  of 
hydration,  being  only  18.3  per  cent  when  the  product  is  limonite.  Approximately  60  per  cent 
of  the  volume  of  the  cherty  iron  carbonate  is  silica;  therefore  the  reduction  in  volume  caused 
by  the  oxidation  of  the  iron  is  efl'ective  on  approximately  only  40  per  cent  of  the  volume  of  the 
rock.  The  loss  in  volume,  then,  for  tiie  entire  rock,  taking  into  account  both  iron  and  silica, 
ranges  from  17.2  per  cent  to  6.4  per  cent,  depending  on  the  degree  of  hydration  of  the  resulting 
iron  oxide. 

Development  of  ijorvsity.— The  decrease  in  volume,  due  to  the  alteration  of  the  iron  minerals, 
develops  pore  space  in  the  resulting  ferruginous  chert.  Determinations  of  porosity  on  several 
typical  specimens  of  cherty  iron  carbonate  showed  an  average  of  less  than  1  per  cent  pore  space. 


244  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

A  series  of  ten  determinations  on  typical  specimens  of  ferruginous  chert  gave  a  range  of  0.9  to 
8  per  cent  pore  space,  with  an  average  of  4.1  per  cent.  Evidently  the  actual  porosity  is  not 
sufficient  to  iK'count  for  the  tlieoretical  volume  change.  This  may  be  explained  in  the  following 
ways:  (a)  Part  of  the  iron  oxide  in  the  ferruginous  chert  may  have  been  original  and  not  altered 
from  siderite.  As  the  calculated  pore  space  is  based  on  the  assumption  that  all  of  the  iron 
oxide  in  the  ferruginous  chert  is  the  result  of  the  oxidation  of  siderite,  original  ferric  oxide  in 
the  chert  would  decrease  the  resulting  pore  space,  (h)  Infiltration  of  iron  oxide  or  silica 
subserjuent  to  or  accompanying  the  alteration  may  have  closed  part  of  the  openings  formed. 
This  is  certainly  true  to  at  least  a  small  extent,  as  shown  by  microscopic  examination  of  thin 
sections,  (c)  The  difficulty  of  obtaining  saturation  and  perfect  drying  in  the  determination  of 
porosity  in  the  specimens  of  ferruginous  chert  may  have  made  the  results  too  low.  (J)  In 
the  rocks  under  discussion,  both  original  and  secondary,  the  iron  minerals  tend  to  be  seg- 
regated in  parallel  layers  separated  by  comparatively  barren  chert.  The  volume  changes 
in  the  alteration  of  the  iron  minerals  would  then  be  largely  confined  to  the  ferrugmous  la3-ers. 
If  these  are  assumed  to  be  practically  pure  iron  mineral,  the  cubical  slmnkage  should  vary 
between  49.25  per  cent  and  18.3  per  cent  (as  previously  calculated)  for  the  different  original 
and  secondary  minerals  noted  above,  the  linear  shrinkage  between  6.5  and  20.3  per  cent.  The 
shrinkage  normal  to  the  layers  would  probably  not  result  in  openings  to  any  large  extent,  as 
slumping  of  the  flat  layers  would  close  any  cavities  formed,  and  as  a  matter  of  fact  such  openings 
are  not  observed.  On  the  other  hand,  slmnkage  parallel  to  the  beds  is  taken  to  explain  the 
common  intersecting  sets  of  cracks  confined  to  the  ore  layers  and  breaking  them  into  small 
parallelepiped  blocks  when  the  ore  has  not  suffered  general  deformation.  These  by  actual 
measurement  give  a  volume  of  openings  ranging  from  12^  to  36  per  cent  of  the  volume  of  the 
iron  layers,  wliich  would  be  approximately  5  to  14^  per  cent  of  the  volume  of  the  rock. 

It  is  believed,  then,  that  the  increase  in  porosity  and  development  of  cracks  in  the  ferruginous 
chert,  together  with  the  slump  which  has  obliterated  a  part  of  these  openings  and  the  infiltration 
of  iron  salts,  fully  accounts  for  the  change  in  volume  which  accompanies  the  production  of 
these  cherts  from  the  cherty  iron  carbonate. 

ALTERATION    OF    FERRUGINOUS    CHERT    TO    ORE. 

The  alteration  of  ferruginous  chert  to  ore  is  almost  identical  with  the  secondary  concen- 
tration of  the  ores  of  the  Mesabi  district.  As  in  the  Mesabi  concentration,  the  essential  change 
is  the  leacliing  of  silica.  The  several  possibilities  resulting  in  the  leaching  of  silica  are  dis- 
cussed on  pages  537-538.  It  is  seen  that  the  space  left  by  the  removal  of  silica  may  remain  as 
pore  space  and  may  be  partly  or  entirely  closed  by  slump  or  may  be  filled  partly  or  entirely 
with  infiltrated  iron  oxide.  To  determine  the  relative  importance  of  these  possibilities,  quanti- 
tative methods  similar  to  those  employed  in  investigation  of  the  Mesabi  ore  were  used. 

In  order  to  include  the  factor  of  porosity  in  a  comparison  of  ores  and  cherts,  it  is  necessary 
to  consider  their  composition  in  terms  of  volume  rather  than  of  weight.  The  volume  composi- 
tion of  any  chert  or  ore  is  readily  calculated  from  the  mineral  composition  and  the  porosity. 

The  volume  composition  of  the  average  ores  anil  ferruginous  cherts  is  as  follows: 

Average  volume  composition  of  ores  and  cherts  of  Gogebic  range. 


Femigi- 

IIUUS 

cherts. 


Hematite.. 
Limonite... 

Quartz 

Kaolin 

Pore  space . 


37.30 
U.9S 
10.43 
3.25 
3-1.00 

99.% 


19.60 
7.23 

ti.OO 
4.03 
4.10 

99.96 


PENOKEE-GOGEBIC  IRON  DISTRICT. 


245 


The  above  volume  composition  is  expressed  diagrammatically  in  figure  31.      The  most 
important  factor  in  forming  ore  from  the  cherts,  as  sliown  by  tlie  diagram,  is  tlie  removal  of 

silica. 


I 


QUARTZ 
Finely  crystalline  quartz  grading  into 

'  amorphous  forms  in  Iwth  the  cherty  iron  carbonat. 
'  and  the  ferruginous  cherts 

1 


Reduction  of  pore         SLUMP 

PORE  SPACE  -~_^rneehanical  packing  of  ore  by  weight 

Porosity  is  first  due  to  the  decrease  — ..^matenal  above 

in  volume  accompanying  the  oxidation 

of  iron  carbonate  and  later  to  the  removal 
of  silica  in  solution 


Partially  replacing  volume  occupied 
by  iron  carbonate 
alterine         to        clay] 


Secondary  hydrous  iron  oxide 


Deposited  by  iron-bearing  solutions 
from  above 


HYDROUS    IRON    OXIDE 

The  degrees  of  hydration  of  the  iron  o.vide  in  the 

■  ferruginous  cherts  and  ores  may  be  expressed  by  ratios 

of  hematite  to  limonite  of  -1 .1  and  5 :1  respectively 


Average  ferruginous  chert 


Average  ore 


v^      Average  cherty  iron  carbonate 

FiGtJEE  31.— Diagrammatic  representation  of  the  changes  involved  in  the  alteration  of  cherty  iron  carbonate  to  ferruginous  chert  and  ore,  Gogebic 
dfstrict.  The  mineral  compositions  of  the  various  phases  are  indicated  in  terms  of  volume  by  vertical  distances.  The  compositions  of  the  cherty 
iron  carbonate,  ferruginous  chert,  and  ore  represented  are  averages  of  a  large  number  of  analyses. 


IRON    MINERALS 


Average  ore 
(Cargo  analysis  for  1906) 


Average 
ferruginous  chert 


SILICA  PORE    SPACE 

Figure  32.— Triangular  diagram  showing  volume  composition  in  terms  of  pore  space,  iron  minerals,  silica,  and  minor  constituents  (clay,  etc.)  of 
the  ferruginous  cherts  and  iron  ores  of  the  Gogebic  range.    See  page  189  for  discussion  of  method  of  nlatting. 


246 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


TRIANGULAR    DIAGRAM    ILLUSTRATING    SECONDARY    CONCENTRATION    Or    GOGEBIC    ORES. 

In  figure  32  tlie  trianfjular  method  (described  on  p.  IS!))  of  i('j)resenting  tlie  volume 
relation  of  ferruginous  cherts  and  ores  and  intermediate  phases  is  applied  to  tiie  Gogebic  ores. 
As  already  explained,  each  small  triangle  within  the  large  one  represents  an  individual  specimen 
and  by  its  size  and  position  indicates  composition  in  terms  of  the  volume  of  ])ore  space,  silica, 
iron  minerals,  and  minor  constituents.  The  average  ferruginous  chert,  as  indicated,  is  repre- 
sented by  a  small  triangle  in  tiie  lower  left-hand  side  of  the  diagram,  with  low  pore  space  and 
a  large  content  of  silica.  The  average  ore  is  represented  in  the  upper  right-hand  j)art  of  the 
diagram,  and  has  more  pore  space,  less  silica,  and  more  iron  than  the  average  ferruginous  chert. 
Scattered  about  in  the  area  between  these  two  points  are  intermediate  phases  between  ferrugi- 
nous chert  and  ore. 

In  the  alteration  of  ferruginous  chert  to  ore,  as  represented  in  the  triangle,  the  following 
changes  have  evidently  taken  place:  («)  Decrease  in  silica,  (b)  increase  in  pore  space,  and  (c) 
increase  in  iron.  Obviously  the  dominant  process  has  been  the  removal  of  silica,  as  this  is 
necessary  to  an  increase  of  pore  space  and  iron.  Removal  of  silica  alone  without  introducticm 
of  iron  or  mechanical  slump  would  increase  the  porosity  in  proportion  to  the  amount  of  silica 
removed.  Such  a  process  would  be  represented  on  the  triangle  by  a  series  of  small  triangles 
in  a  line  parallel  to  the  base,  as  the  relative  volume  of  iron  would  remain  constant.  In  the 
actual  case  kno\\Ti  the  relative  volume  of  the  iron  mineral  increases  from  26.83  per  cent  in  the 
cherts  to  52. IS  per  cent  in  the  ores.  This  could  be  accomplished  in  two  ways — by  mechanical 
slumping  or  packing  of  the  material,  weakened  by  too  great  a  porosity,  or  by  infiltration  of 
iron.  From  the  diagram  it  is  impossible  to  tell  wliich  of  these  processes,  slumping  or  infiltra- 
tion, is  more  important.  Observation  shows,  however,  that  slumping  has  been  important,  but 
that  introduction  of  iron  has  taken  place  to  a  much  greater  extent  than  it  did  in  the  con- 
centration of  the  Mesabi  ores. 

ALTERATION  OF  ROCKS  ASSOCIATED  WITH  ORES  DURING  THEIR   SECONDARY  CONCENTRATION. 

The  various  conditions  and  agencies  which  were  effective  in  the  concentration  of  the  ore 
from  the  cherty  iron  carbonates  and  ferruginous  cherts  caused  alterations  of  a  similar  nature 
in  the  various  rocks  associated  with  the  iron-bearing  formation — namely,  the  interbcdded 
slates,  the  basic  intrusive  rocks,  and  the  slates  immediatelj-  overlying  the  iron-bearing  forma- 
tion. The  alteration  of  the  slates  produced  paint  rock  or  ferruginous  slate  similar  to  that  of 
the  Mesabi  range.  The  alteration  of  the  basic  dikes  by  oxidation  of  the  iron,  breaking  do^\•n 
of  feldspars,  and  leaching  of  soluble  constituents  formed  a  soft  kaolinic  product,  locally  termed 
soap  rock  or,  if  iron  stained,  paint  rock.  The  following  anal3'ses  of  fresh  and  altered  dike 
rock  are  typical  of  this  alteration: 

.     Analysts  of  fresh  and  allcnd  dikes  associated  vilh  ore. 


1  (fresh). 


!  (altered). 


Assuming 

-Vl.Oj 
constant. 


SiOj... 
AI2O3.. 
FeaOs.. 
FeO.... 
MgO... 
CaO.... 
NajO... 
KsQ.... 
HjO-.. 
H20+. 
TiOj... 
PzO:, . . . 
CO2.... 


47.90 

1,1.(0 

3.f0 

8.41 

8.11 

9.99 

2.C5 

.23 

.15 

2.34 

.82 

.13 

.38 


4B.8S 
22.  f>2 
5.12 


2.01 
1.25 
.80 

2.cr. 

3.12 
8.25 
1.12 

.ir. 

1.89 


32.20 
l.i.  CO 
3.53 


1.39 
.86 
.55 
1.83 
2.15 
5.(8 
.77 
.11 
1.30 


1.  Specimen  12880.    Unaltered  diabase  dike  rock  in  iron-bearing  formation,  from  southeast  part  of  sec.  13,  T.  47  N.,  R.  46  W.,  Uichigan. 

2.  Specimen  12S78.    Altered  diabase  dike.    Same  locality  aa  No.  1. 


PENOKEE-GOGEBIC  IRON  DISTRICT. 


247 


A  comparison  of  the  two  analyses  on  the  assumption  that  alumina  has  remained  constant 
(see  third  column  in  the  table)  shows  a  loss  of  silica,  iron,  magnesia,  lime,  soda,  phosphorus, 
and  titanium  and  a  gain  in  potassa,  water,  and  carbon  dioxide.  Except  for  the  behavior 
of  potassn,  the  alteration  is  typical  of  weathering  under  conditions  of  oxidation,  carbonation, 
and  hydration. 

Specific  gravity  and  porosity  determinations  on  the  specimens  analyzed  resulted  as  follows: 

Sptxiftc  (jraritij  and  porosilij  of  inialtcrtd  and  allcrcd  phases  of  diabase. 


Specific 
gravity. 


Porosity. 


Unaltered  diabase 

Altered  phase  of  diabase. 


2.92 
2.76 


n.so 

28.40 


On  the  basis  of  the  specific  gravities  ami  the  assumption  that  alumina  is  constant,  the 

calculated  porosity  due  to  leaching  of  soluble  constituents  is  27.1   per  cent  of  the   volume 

/  15.60     2.92  \ 

(  1.00  — <jy-^X.Y^  =  0. 271  ),  which  agrees  very  well  with  the  actual  determinations  of  porosity, 

and  also  denotes  the  approximate  correctness  of  the  assumption  that  alumina  is  constant. 

The  approximate  mineral  composition  of  the  fresh  and  altered  diabase,  calculated  from 
the  analyses,  is  as  follows: 

Mineral  composiiion  of  fresh  and  altered  diabase. 


Unaltered 
diabase. 


Altered 
pha-se  of 
diabase. 


Feldspars 

Fercoraagnesian  minerals 

Quartz 

Calcium-magnesia  carbonates. 

Apatite 

Magnetite 

Ilmenite 

Kaolin,  chlorite,  sericite,  etc. . 
Limonite 


Specific  gravity.. 
Porosity 


51.00 

40.00 

1.00 

.90 

.31 

5.34 

1.50 


2. 92 
.50 


6.82 


17.00 

4.00 

.38 


2.13 
63.00 
6.10 


2.76 
28.40 


OCCURRENCE    OF    PHOSPHORUS    IN    THE    IRON-BEARING    FORMATION. 
PHOSPHORUS  CONTENT. 

The  phosphorus  content  of  the  jirincijial  phases  of  the  iron-bearing  formation  is  as  follows : 

Phosphorus  content  of  the  iron-hearinr;  Irontrood formation. 


Iron. 


Phos- 
phorus. 


Ratio  of 

phos- 
phorus to 

iron. 


Cherty  iron  carbonate 

Ferruginous  chert 

Iron  ore 


Per  cent. 
24.51 
28. 76 
68.15 


Per  cent. 

0.020 

.  046 

.  0C2 


Per  cent. 

0.001000 

.001600 

.001067 


The  range  in  phosphorus. content  in  the  various  commercial  grades  of  ore  produced  in  the 
district  in  1906  was  from  0.028  to  0.275  per  cent.  In  the  Mesabi  ores  the  jihosphorus  content 
was  found  to  depend  to  a  large  extent  on  the  chemical  composition  of  the  ore,  high  pliosphorus 
occurring  as  a  rule  in  the  more  hydrous  ore  and  in  ore  high  in  alumina.  In  the  Gogebic  ores 
the  increase  of  phosphorus  with  the  degree  of  hydration  is  not  apparent,  as  is  sho^\^l  in  figure  33, 
where  the  relation  of  phosphorus  content  to  water  of  hydration  is  represented  graphically. 


248 


GEOLOGY  OF  THE  LAKE  SUPERIOR  liEGTON. 


The  average  degree  of  liydration  of  the  Gogebic  ores  is  coiisiderahl}-  lower  liiaii  tliat  of  the 
Mesabi  ores,  and  tlie  liigh  phosphorus  ores  of  the  Mesabi  range  contain  more  water  of  hydration 
tlian  the  most  liydrous  of  the  GogeI)ic  ores. 

As  in  tlie  ilcsabi  range,  the  phosphorus  content  of  tlie  altered  slates  or  paint  rocks  and 
slaty  ores  of  the  Gogebic  range  is  high.  These  phases  are  high  in  alumina  and  comprise  a 
complete  gradation  from  high-grade  ore  to  ferruginous  clay.  The  unaltered  interbedilcd  slates 
as  a  rule  have  a  higher  phosjihorus  content  than  the  iron-bearing  rocks  proper  and  their  altera- 
tion j)roducts  are  correspondingly  high  in  phosphorus.  Hence  an  examination  of  analyses 
shows,  in  a  general  way,  an  increase  of  phosphorus  with  an  increase  in  the  alumina  content. 
The  altered  dikes,  locally  termed  soap  rock  or  ])aint  rock,  are  characteristically  high  in  phos- 
phorus, evidently  owing  to  the  original  phosphorus  content  of  the  diabase.  (See  analyses, 
p.  246.) 


,120 


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FiGDEE  33.  — Diagram  showing  relation  of  phosphorus  to  degree  of  hydration  in  Gogebic  ores. 

High-phosphorus  ores  are  sometimes  found  immediately  above  the  dikes  and  in  the  angle 
of  the  trough  formed  b}'  a  dike  and  the  foot  wall.  It  is  not  true,  however,  that  all  ore  imme- 
diately overlying  dikes  is  high  in  phosphorus,  the  opposite  being  true  in  many  places. 

MINERALS    CONTAINING   PHOSPHORUS. 

The  discussion  of  the  occurrence  of  phosphorus  on  the  Mesabi  range  (pp.  192-196)  applies 
practically  verbatim  to  the  Gogebic  range.  No  phosphorus-bearing  minerals  have  been  iden- 
tified in  the  ores  or  cherts;  hence  possible  occurrence  of  ])hosph(U-us  must  be  inferred  fi-om 
chemical  evidence.  Figure  34  is  similar  to  figure  23,  showing  the  relation  of  phosphorus  to  lime 
and  the  possibility  of  phos])horus  occurring  as  apatite  (calcium  phosphate).  The  diagonal 
dotted  line  indicates  the  ratio  of  the  two  elements  in  apatite.  Points  falling  above  the  line 
indicate  an  excess  of  calcium  and  points  below  the  line  an  excess  of  phosphorus.  From  the 
fact  that  a  number  of  analyses  show'  an  excess  of  phosphorus  it  is  to  be  inferred  that  phosphorus- 


PENOKEE-GOGEBIC  IRON  DISTRICT. 


249 


bearing  minerals  other  than  apatite  are  present.  It  is  highly  probable  that  at  least  pait  of 
the  phosphorus  occurs  in  combination  with  the  hydrates  of  iron  and  alumina.  The  extremely 
small  percentages  present  make  determination  of  these  minerals  practically  impossible. 

BEHAVIOR  OF  PHOSPHORUS  DTJRING  SECONDARY   CONCENTRATION. 

Examination  of  the  average  analyses  of  the  cherty  iion  carbonates,  ferruginous  chert,  and 
ore  shows  that  the  ratio  of  phosphorus  to  iron  has  remained  practically  constant  during  the 
concentration  of  the  ores;  in  other  words,  both  have  been  concentrated  to  essentially  the  same 
degree. 

It  was  found  that  in  the  secondary  concentration  of  the  ores  of  the  Mesabi  district  phos- 
phorus was  actually  introduced  into  the  ores  from  the  overlying  Cretaceous  rocks  known  to  be 


FiGUKE  34 — Diagram  showing  relative  amounts  of  phosphorus  and  lime  in  Gogebic  ores. 

higli  in  phosphorus.  The  absence  of  a  source  of  phosphorus  such  as  the  Cretaceous  rocks  of 
the  ]\Iesabi  district  may  explain  why  phosphorus  has  not  been  concentrated  to  a  greater  degree 
in  the  iron  in  the  Gogebic  ores. 

It  was  suggested  that  the  hydrated  portions  of  the  Mesabi  may  have  had  some  effect  in 
causing  this  increase  in  phosphorus.  The  Gogebic  ores  are  much  less  hydrous  than  the  Mesabi 
ores,  and  this  fact  suggests  a  further  possible  explanation  for  the  increase  in  phosphorus  in 
one  case  and  not  in  the  other. 

High-phosphorus  ores  commonly  occur  immediately  above  or  below  dikes.  The  dikes 
themselves  are  characteiistically  high  in  phosphorus,  and,  fuithermore,  the  alteration  of  the 
dike  is  accompanied  by  a  loss  of  phosphorus.  (See  analyses,  p.  246.)  It  is  possible  that  the 
high  phosphorus  in  the  neighboring  ore  may  be  directly  contributed  by  the  altering  dike  rock. 


250  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

SEQUENCE  OF  OKE  CONCENTRATION  IN  THE  GOGEBIC  DISTRICT. 

Before  the  deposition  of  llie  Keweenawuu  series  there  had  been  a  sligiit  f(jlding  of  the 
upper  Iluronian  (Animikie  group)  containing  tlie  productive  iron-bearing  Ironwood  forma- 
tion. A  gentle  synchne  was  developed  along  the  present  productive  area  ■w'ith  its  limbs  at  the 
two  ends  of  the  district.  Erosion  then  exposed  the  iron-bearing  formation  only  at  the  two 
ends  of  the  district,  leaving  the  central  and  nonproductive  part  still  covered  by  a  considerable 
tliiclaiess  of  slate,  and  soft  ores  and  ferruginous  cherts  were  developed  along  erosion  surface 
and  fissures  at  the  east  and  west  ends  of  the  district  to  a  minor  extent.  The  Keweenawan 
igneous  and  sedimentary  rocks,  laid  down  upon  this  gently  bowed  surface,  affected  the  underly- 
ing ferruginous  cherts  and  soft  ores  at  the  east  and  west  ends  of  the  district,  perhaps  tlehydrating 
them,  and  developed  red  jaspers  and  hard  ores.  The  iron  carbonates  at  these  places  were 
changed  to  amphibole-magnetite  rocks  by  contact  metamorphism.  Igneous  intrusives  of 
Keweenawan  age  are  more  abundant  at  these  localities  than  elsewhere  in  the  district.  They 
had  no  contact  effect  upon  the  iron-bearing  formation  in  the  central  part  of  the  district  because 
it  was  covered  by  slate.  Then  came  the  great  post-Keweenawan  folding,  resulting  in  the  tilting 
of  the  upper  Huronian  iron-bearing  formation  (Ironwood)  and  Keweenawan  beds  in  the  Gogebic 
district  to  angles  of  60°  and  70°  N.  The  iron-bearing  formation  underwent  dynamic  metamor- 
pliism  at  the  east  and  west  ends,  where  it  constituted  a  comparative!}-  tliin  layer  between  the 
hard  rocks  of  its  basement  and  those  of  the  covering  Keweenawan.  The  follo\\'ing  erosion 
exposed  not  only  amphibole-magnetite  rocks,  hard  ores,  and  jaspers  previously  formed  at  the 
east  and  west  ends  of  the  district  but  exposed  for  the  first  time  from  beneath  the  Tyler  slate 
the  unaltered  iron-bearing  formation,  consisting  principally  of  carbonate,  in  the  central  and 
present  productive  portion  of  the  region.  The  concentration  of  the  ore  for  this  district  began 
at  tliis  time.  It  was  well  advanced  before  Cambrian  time  and  has  continued  intermittently 
since.     (See  pp.  557-560.) 


CHAPTER  XI.  THE  MARQUETTE  IRON  DISTRICT  OF  MICHIGAN, 

INCLUDING  THE  SWANZY,  DEAD  RIVER,  AND 

PERCH  LAKE  AREAS. 

MARQUETTE  DISTRICT." 

INTRODUCTION. 

Although  the  following  account  of  the  geology  of  the  Marquette  district  is  based  mainly 
on  the  work  of  the  United  States  Geological  Survey,  we  would  express  our^mdebtedness  to 
Prof.  A.  E.  Seaman,  of  the  Michigan  College  of  Mines,  for  important  modifications  in  our  ideas 
of  the  structure  and  distribution  of  the  rocks.  Prof.  Seaman  was  the  first  to  prove  the 
existence  of  an  luiconformity  between  what  is  here  called  middle  Huronian  and  the  lower 
Huronian,  botli  of  which  had  been  treated  together  as  lower  Iliu'onian  in  the  United  States  Geo- 
logical Survey  monograph  on  tlie  district.  He  has  also  contributed  a  considerable  number  of 
corrections  to  the  geologic  map  and  a  detailed  plat  of  the  extensive  faulting  near  Teal  Lake 
(PL  XIX).  The  Cascade  area  shows  considerable  changes  in  mapping,  due  to  the  large  amount 
of  carefid  exploration  work  accompanied  by  geologic  mapping  that  has  been  done  by  mming 
companies.  We  are  especially  indebted  to  Messrs.  Oscar  Rohn,  O.  B.  Warren,  and  W.  0. 
llotchkiss  and  to  the  Oliver  Iron  Mining  Company  for  changes  in  this  area.  Other  important 
corrections  on  the  Marquette  map  have  been  furnished  by  the  work  of  the  Cleveland-Cliffs 
Iron  Company,  Longyear  &  Hodge,  and  others. 

LOCATION,    SUCCESSION,   AND    GENERAL   STRUCTURE. 

The  Marquette  district  extends  from  Marquette,  on  Lake  Superior,  in  longitude  87°  20', 
west  to  Lake  Michigamme,  in  longitude  88°,  a  distance  of  somewhat  less  than  40  miles.  The 
district  roughly  follows  parallel  46°  30'.  (See  PI.  XVII,  in  pocket.)  It  lies  wholly  in  Michi- 
gan and  derives  its  name  from  the  city  of  Marquette.  The  more  important  towns  besides 
Marquette  are  Ishpeming,  Negaunee,  Champion,  'and  Rep\d)lic.  The  breadth  of  Algonkian 
rocks,  which  are  the  special  subject  of  this  chapter,  varies  from  about  1  mile  to  more  tlian  6 
miles.  From  the  western  part  of  the  main  Algonkian  area  two  arms  project  for  several  miles, 
one  to  the  southeast,  the  Republic  trough,  andone  to  the  soutli,  the  Western  trough. 

The  succession  of  the  formations  for  the  district,  from  the  top  downward,  is  as  follows: 

Quaternary  system: 

Pleistocene  series. 
Cambrian  system: 

Upper  Cambrian  sandstone  (Potsdam  sandstone). 
Unconformity. 
Algonkian  system : 

Keweenawan  series Not  identified  but  probably  represented  by   part  of 

intrusives  in  upper  Huronian. 


Huronian  series: 


Upper  Huronian  (Animikio  group). . 


Greenstone  intrusives  and  exirusives. 

Michigi;mme    slate    (slate    and    mica    schist),    locally 

largely  replaced  by  volcanic  Clarksburg  formation. 
P.ijiki  schist  (iron  bearing). 
Goodrich  quartzite. 


"  For  further  detailed  description  of  the  geology  of  this  district  see  Men.  U.  S.  Geol.  Survey,  vol.  28, 1897,  and  references  there  given. 

251 


252 


GEOLOGY  OF  THE  LAKE  SLTERIOR  REGION. 


Algonkian  system — Contibued. 
Huronian  series — Continued. 
Unconformilv. 


Middle  Iluronian. 

Unconformity. 
Lower  Huronian.. 


Unconformity. 
Archean  system: 

Laurentian  series. 


Keewatin  series . 


Nej^aunee    formation    (chief   productive    iron-bearing 

formation). 
Siamo  slate. 
Ajibik  quartzite. 

We  we  slate. 
Kona  dolomite. 
Mesnard  quartzite. 


Granite,  syenite,  peridotite. 

Palmer  gneiss. 

Kitchi  schist  and  Mona  schist,  the  latter  banded  and  in 
a  few  places  containing  narrow  bands  of  nonproduc- 
tive iron-bearing  formation. 


In  addition  to  the  rocks  tabulated  above,  basic  igneous  rocks  in  many  dikes  and  bosses, 
large  and  small,  intrude  all  the  Archean  and  Huronian  formations. 

The  central  and  western  parts  of  the  district  are  bounded  on  the  north  and  south  by  more 
or  less  continuous  east-west  linear  ridges  of  Algonkian  rocks.  The  area  between  these  ridges 
is  relatively  low  laying,  mth  minor  elevations.  Also  these  ridges  on  the  whole  stand  above  the 
country  north  and  south  of  the  district.  The  major  portion  of  the  district  is  a  bluffy  jilateau,  for 
the  most  part  lying  between  altitudes  of  1,400  and  1,600  feet,  but  it  has  points  that  rise  higher 
and  a  few  points  that  reach  an  altitude  of  1,800  feet.  The  eastern  part  of  tlie  plateau  slopes 
rather  steeply  toward  Lake  Superior,  and  for  this  part  of  the  area  the  altitudes  of  the  higher 
points  are  between  800  feet  and  1,000  feet.  Each  of  the  formations  is  locally  resistant,  and 
where  in  this  condition  constitutes  bluffs.  One  traversing  the  district  from  north  to  south  is 
almost  constantly  either  climbing  or  descending  a  steep  slope. 

The  drainage  is  largely  transverse  to  the  longer  dimension  of  the  district.  Branches  of 
Escanaba  River,  in  the  Lake  Michigan  drainage  basin,  cross  the  central  part  of  the  range  at 
two  places.  In  tlie  eastern  part  of  the  district  Carp  River  flows  to  Lake  Superior  in  a  direction 
roughly  parallel  to  the  strike  of  the  rocks.  In  the  western  part  of  the  district  Michigamme 
Lake,  the  one  large  lake  in  the  district,  and  Michigamme  River  are  purely  structural  in  their 
locations,  the  main  arm  of  the  lake  lying  east  and  west  parallel  to  the  strike  of  the  district  and 
a  north  arm  swdnging  toward  the  south  with  the  cross  fold,  which  appears  at  this  locality. 
Mi(;higamme  River,  the  outlet  of  the  lake,  follows  the  axis  of  the  Rej)ubUc  trough  and  connects 
with  Lake  Michigan  waters. 

The  Archean  rocks  occur  in  two  areas,  one  north  and  one  south  of  the  Huronian  series. 
The  northern  one  is  called  the  northern  complex  and  the  southern  one  the  southern  complex. 
In  a  broad  way  the  Huronian  rocks  constitute  a  great  synclinorium  between  the  two  areas  of 
Archean.  Superimposed  upon  the  larger  folds  are  folds  of  lesser  orders  down  to  minute  plica- 
tions. Though  it  is  therefore  clear  that  the  folding  is  extremely  complex  from  Lake  Superior 
to  Michigamme  Lake,  it  maybe  said  that  the  Algonkian  constitutes  a  great  canoe-shaped  basin, 
wliich  comes  to  a  point  at  the  east  end  of  the  district  but  does  not  at  ilichigamme.  As  a  result 
of  this,  in  passing  from  Lake  Superior  to  the  west,  one  comes  to  higher  and  higher  formations 
and  only  reaches  tlie  highest  formation  of  the  district  west  of  Ishpeming. 

This  synclinorium  is  of  peculiar  and  complicated  character.  For  much  of  the  district  the 
rocks  in  the  outer  borders  of  the  iVlgonkian  belt  are  in  a  series  of  sharply  overturned  folds. 
The  Algonkian  rocks  on  either  side  of  the  trough  have  moved  up  and  outward  over  the  more 
rigid  Archean  granite,  and  as  a  consequence  on  each  side  of  the  Algonkian  trough  a  series  of 
overfolds,  esj)ecially  in  the  softer  slates,  plunge  steeply  toward  its  axis,  producing  a  structure 
resem])ling  in  tliis  respect  the  composed  fan  structure  of  the  Alps.  There  is,  lu)we\er,  this 
great  dilference  between  the  structure  of  the  Marquette  district  am!  that  of  the  Alps — that  newer 


MARQUETTE  IRON  DISTRICT. 


253 


^  jW 


rocks  appear  near  the  axis  of  tlie  trough  rather  than  older  ones,  as  if  composed  fan  folds  of 
Alpine  type  were  sagged  downward  into  a  synclinorinm.  Tlie  stnicture  also  differs  from  the 
inverted  intermont  trough  of  Lapworth.  It  may  be  called  an  abnormal  synclinorium.'^  (See 
fig.  35.)  This  structure  prevails  in  the  central  part  of  the  area  from  Ishpeming  and  Negaimee 
westward  to  Clarksburg,  but  it  does  not  extend  to  Lake  Superior  on  the  east  nor  to  Lake  Michi- 
gamme  on  the  west. 

Although  the  more  conspicuous  folds  of  the  district  have  in  general  an  east-west  axis,  the 
rocks  have  also  been  imder  strong  east-west  compression,  as  a  consequence  of  which  the  folds 
are  buckled  so  that  many  of  them  show  a  steep  pitch.  In  places  the  north-south  folds  become 
more  prominent  than  the  east-west  folds  and  control  the  prevalent  strikes  and  dips.  This  is 
illustrated  l)y  the  western  trough,  at  the  west  end  of  the  district.  In  certain  areas  in  the 
southeastern  part  of  the  district  the  compression  has  been  about  equally  great  in  both  directions, 
producing  most  irregular  strikes  and  dips. 

Minor  fracturing  in  the  district  has  been  pervasive,  as  will  be  exj)lained  in  succeeding  pages, 
but  only  at  a  few  localities  are  there  faults  so  extensive  that  they  have  been  detected  in  the 
mapping  of  the  formations. 
Of  these  major  faults,  three 
at  least  are  of  very  consider- 
able displacement,  all  in  the 
eastern  part  of  the  district, 
one  of  these  being  the  Carp 
River  fault  (PI.  XVIII)  anil 
the  other  two  in  the  Cascade 
area.  A  number  of  less  im- 
portant faults  occur  in  the 
quartzite  east  of  Teal  Lake 
(PL  XIX). 

The  lower  Huronian, 
comprising  the  Mesnard 
quartzite,  the  Kona  dolo- 
mite, and  the  Wewe  slate, 
is  confined  to  the  eastern 
third  of  the  district.  At  the 
time  these  rocks  were  depos- 
ited either  the  western  part  of  the  district  was  not  submerged  or  else  the  erosion  following  this 
period  removed  the  rocks  before  the  deposition  of  the  succeeding  midtlle  Huronian  beds. 

The  middle  and  upper  Huronian  rocks  west  of  the  central  part  of  the  district  are  in  linear 
belts,  one  following  the  other  in  regular  order,  but  east  of  the  central  part  of  the  district  the 
distribution  is  less  uniform  and,  because  of  the  somewhat  equal  closeness  of  the  north-south  and 
east-west  folds,  some  of  the  formations  lose  their  linear  character  and  cover  considerable  areas. 

The  Marquette  district  is  the  type  district  for  the  Lake  Superior  Huronian  in  that  it  is  the 
only  district  in  which  the  upper,  middle,  and  lower  Huronian  are  well  represented.  Moreover, 
the  unconformable  relations  between  the  middle  and  lower  Huronian  and  between  the  middle 
and  upper  Huronian  are  demonstrated  by  the  clearest  evidence,  as  is  also  the  unconformity 
between  the  Huronian  and  the  Archean. 


Figure  35.— Idealized  north-south  section  tlirough  the  Marquette  district,  showing  abnormal 
type  of  synclinorium.  The  axial  planes  of  the  minor  folds  converge  downward.  The  atti- 
tudes of  the  minor  folds  are  detonninod  by  the  differential  movements  in  the  more  competent 
strata  indicated  by  arrows.  In  general  the  soft  slate  layers  of  the  district  are  the  ones  best 
illustrating  the  minor  folds.  The  quartzite  layers  are  more  competent  and  therefore  more 
simple  in  outline. 


ARCHEAN   SYSTEM. 


The  rocks  of  the  Archean  system  are  so  different  in  character  from  the  Huronian  sediments 
that  there  is  really  no  difficulty  in  distinguishing  between  them.  This  discrimination  was 
made  by  Brooks  and  Irving  before  it  was  known  that  an  unconformity  separated  them.     The 


»  Van  Hise,  C.  B.,  Principles  of  North  American  pre-Cambrian  geology:  Sixteenth  Ann.  Rept.  U.  S.  Geol.  Sorvey,  pt.  1, 1896,  pp.  612,  615-621. 


254  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Arclioan  rocks  arc  all  crystalline,  comprising  both  massive  and  schistose  varieties.  The  diflFerent 
phases  have  very  intricate  relations  to  one  another  as  compared  with  the  Huronian,  and  this 
led  Irviiif^  to  designate  the  whole  mass  as  the  "Basement  Complex."  The  rocks  of  the  Archean 
are  divisible  mto  two  series,  Keewatin  and  Laurentian.  This  was  recognized  before  the  Inter- 
national Committee  had  agreed  on  the  definitions  of  these  terms  and  to  the  two  divisions  were 
given  the  names  "Mareniscan"  and  Laurentian,  as  m  the  Gogebic  district. 

The  northern  area  and  southern  area  of  Archean  will  be  se])arately  described. 

NORTHERN  AREA. 

KEEWATIN    SEKIES. 

The  Keewatin  rocks  of  the  northern  area  were  described  in  the  Marquette  monograph 
under  two  divisions — the  Mona  schist  and  the  Kitchi  schist.  The  Mona  schist  comprises  both 
basic  and  acidic  varieties,  the  former  being  dominant.  The  basic  schists  comprise  both  dense 
and  l)an(]e(l  forms.  In  color  all  are  various  shades  of  green.  They  Vjelong  to  the  general  class 
whicli  lias  been  described  by  G.  H.  Williams  "  as  greenstone  schists.  The  mineral  constituents 
are  mainly  epidote,  chlorite,  hornblende,  plagioclase  (largely  albite),  leucoxene,  quartz,  and 
usually  calcite.  The  chloritic  and  calcitic  character  of  these  schists  is  very  widespread,  per- 
sistent, and  characteristic.  In  general  the  composition  of  the  schists  is  very  similar  to  that 
of  basalts.  The  banded  character  of  these  rocks  early  led  to  the  belief  that  they  were  water- 
arranged  sediments,  but  the  later  studies  have  shown  that  while  this  is  true  in  part  they  are 
largely,  though  not  altogether,  schistose  basic  flows  and  tuffs. 

The  basic  Kitchi  schist  differs  from  the  basic  Mona  schist  mainly  in  that  it  clearly  shows 
an  agglomeratic  and  in  some  places  a  conglomeratic  character.  This  appearance  is  typically 
shown  at  the  old  Deer  Lake  furnace.  A  close  study  of  the  rocks  of  this  area  shows  beyond 
all  question  that,  while  they  are  largely  volcanic  conglomerates,  some  of  the  material  of  these 
conglomerates  has  been  worked  over  by  water  into  greenstone  conglomerates.  Where  the 
material  is  comparatively  fine  they  approach  in  appearance  the  banded  Mona  schist. 

The  change  from  Mona  to  Kitchi  schist  takes  place  by  the  appearance  of  conglomeratic 
and  agglomeratic  bands  in  the  Mona.  There  is  reason  to  believe  that  the  main  mass  of  basic 
schists  composing  the  Kitchi  and  Mona  schists  is  the  same  formation,  the  main  difference 
being  that  the  Mona  schist  is  more  metamorphosed  and  probably  contained  a  larger  proportion 
of  finer  material. 

In  the  areas  of  both  the  Mona  and  Kitchi  schists  are  subordinate  areas  of  acidic  rocks 
which  are  largely  sericite  schists.  WTiether  these  are  contemporaneous  with  the  basic  schists 
or  are  later  intrusives  is  not  altogether  clear. 

In  the  area  of  Mona  schist  are  small  masses  of  ferruginous  slate,  ferruginous  chert,  and 
magnetite-griinerite  schists  which  are  identical  in  hand  specimen  and  in  microscopic  character 
with  similar  rocks  of  the  iron-bearing  Negaunee  formation.  These  appear  in  their  best  develop- 
ment within  the  banded  Mona  schist  adjacent  to  Lighthouse  Point,  but  are  found  also  in  other 
locahties,  especially  north  of  the  old  Holyoke  mine  in  sec.  2,  T.  48  N.,  R.  27  W.  If  these  rocks 
are  supposed  to  be  of  the  same  origin  as  the  similar  rocks  of  the  Negaunee  formation,  and  there 
is  no  evidence  that  they  are  not,  they  indicate  the  presence  locally  of  conditions  of  nonclastic 
subaqueous  sedimentation,  and  if  this  is  so  it  is  probable  that  much  of  the  banded  Mona  schist 
of  this  area  has  been  extensively  rearranged  by  water.  Therefore,  while  these  schists  have 
the  composition  of  an  igneous  rock,  it  is  probable  that  they  partly  represent  fine  volcanic  ash 
which  has  been  deposited  in  water  and  arranged  by  it  without  much  assortmg. 

Near  Mud  Lake  a  series  of  green  schists,  graywackes,  and  slates  intervenes  between  the 
typical  Mona  schist  on  the  north  and  the  Iluronian  beds  on  the  south.  The  intervening  series 
is  conglomeratic  near  its  contact  with  the  Mona  schist  and  in  turn  is  overlain  imconformably 
by  the  Huronian  scries,  with  basal  conglomerate.     These  green  schists  and  slates  look  not 

<•  Bull.  U.  S.  Gcol.  Survey  No.  6?,  1890. 


MONOGRAPH  Ul       PLATE  X 


DETAILED  MAP  OF  QUARTZITE  RIDGES  OF  TEAL  LAKE,  MICfflGAN 

SHOWING  FAULTING  AND  UNCONFORMITY  OF  AJIBIK  QUARTZITE 
AND  MESNARD  QUARTZITE 

by  A.K.  Sea 


MARQUETTE  IRON  DISTRICT.  255 

unlike  some  of  the  phases  of  Kitchi  schist  farther  west,  suggesting  the  possibility  that  the  Kitchi 
schist  may  be  partly  younger  than  the  Mona  schist  and  may  be  locally  more  largely  sedimentary 
than  is  apparent  in  the  typical  Mona  schist  area.  Indeed,  if  mapped  independently  of  other 
parts  of  the  district,  the  green  schists  and  slates  between  the  Mona  schist  and  the  Mesnard 
quartzite  of  the  Mud  Lake  area  would  be  mapped  as  sedimentary,  probably  lying  imconformably 
below  the  Huronian  and  unconformubly  upon  the  Mona  schist. 

In  conclusion  it  may  be  said  that  both  the  Mona  and  the  Kitclii  schists  are  dominantly 
igneous  in  origin,  being  mainly  a  set  of  lava  flows  and  volcanic  fragments  which  fell  upon 
water  and  were  more  or  less  arranged  by  it;  locally  subordinate  amounts  of  material  from 
other  sources  have  been  contributed. 

LAUKENTIAN    SERIES. 

The  Laurentian  rocks  of  the  northern  area  comprise  principally  granites  and  grieissoid 
granites  which  include  both  biotite  and  muscovite  granites.  In  general  these  rocks  show  a 
considerable  amount  of  djmamic  action  and  alteration,  the  schistose  phases  passing  into  rocks 
which  may  be  called  granitoid  gneiss.  In  the  western  part  of  the  district  the  Laurentian  rocks 
are  adjacent  to  the  Huronian;  in  the  eastern  part  between  them  and  the  Huronian  are  inter- 
posed the  Kitclu  and  Mona  scldsts,  into  which  the  Laurentian  rocks  are  batholithic  intrusions. 
The  boundary  between  the  two  sets  of  rocks  is  not  sharp  ami  defined.  Numerous  dikes  and 
bosses  of  granite  are  found  in  the  schists  along  the  border,  and  schist  masses  are  included  in 
the  granite. 

Another  important  variety  of  Laurentian  rock  is  hornblende  syenite,  which  is  found  in 
the  eastern  part  of  the  area.  This  rock  has  the  same  relations  to  the  schists  as  the  granite. 
It  differs  from  the  granite  in  the  absence  of  quartz,  the  primary  constituents  being  ortho- 
olase,  plagioclase,  sphene,  magnetite,  and  biotite.  The  secondary  products,  plagioclase,  micro- 
cline,  chlorite,  quartz,  epidote,  muscovite,  and  leucoxene,  have  developed  to  some  extent. 
The  structure  of  the  gneissoid  syenites  is  the  same  as  that  of  the  gneissoid  granites. 

A  third  class  of  intrusive  rocks  m  the  Keewatin  schists  is  peridotite.  One  well-known 
area  of  peridotite  occurs  at  Pi'esque  Isle,  but  by  far  the  largest  area,  between  4  and  .5  miles 
hi  length,  is  in  the  central  part  of  the  district  within  the  Kitchi  schist.  These  peridotites  are 
very  much  altered,  the  olivine  and  diallage  both  being  extensively  serpentinized  and  magnetite, 
dolomite,  and  other  usual  products  developing;  also  uralite  and  chlorite  have  formed  from  the 
diallage.  Indeed,  in  most  of  the  specimens  the  olivine  and  diallage  have  entirely  disappeared 
and  secondary  products  have  taken  their  place. 

SOUTHERN  AKEA. 

The  southern  area  is  composed  dominantly  of  granites,  granitoid  gneiss,  and  gneissoid 
granites  which  are  in  most  respects  not  different  from  the  granites  of  the  northern  area.  Schists 
are  subordinate,  but  are  found  at  several  places.  They  include  micaceous  schists,  clilorite 
schists,  and  ampliibole  schists  similar  to  those  of  the  northern  area.  The  micaceous  scliists 
include  muscovite  schists,  biotite  schists,  feldspathic  biotite  schists,  and  hornblende-biotite 
schists.  They  have  nowhere  sufficient  extent  to  be  mapped  as  formations  separate  from  the 
granites.  The  origin  of  these  schists  is  not  clear.  Their  foliation  is  secondary,  due  to  masliing 
and  recrystalhzation.  In  places  they  have  a  clastic  appearance  and  may  be,  m  jjart  at  least, 
sedimentary  in  origin.  Between  the  different  varieties  of  schists  there  are  of  course  gratlations. 
There  is  every  reason  to  suppose  that  the  clilorite  schists  and  hornblende  schists  are  similar  in 
origin  to  like  rocks  of  the  northern  area. 

In  the  eastern  part  of  the  district  south  of  the  Cascade  range  and  bordering  the  Huronian 
is  a  narrow  and  distinct  belt  of  Laurentian  rocks  which  has  been  called  the  Palmer  gneiss.  It  is 
a  gneiss  consisting  dominantly  of  quartz  with  minor  quantities  of  feldspar  and  mica,  the  origin 
of  which  is  in  doubt.     Phases  of  it  look  like  metamorphosed  sediments;  other  parts  seem  to  be 


256  GEOLOGY  OF  THE  LAlvE  SUPERIOR  REGION. 

tlie  result  of  motamorphism  of  granitic  and  pegmatitir  rocks.  On  the  earlier  map  of  the  district " 
there  were  included  in  the  west  end  of  tiie  Palmer  gneiss  belt  certain  metanKirjihic  schists  wliich 
have  since  been  found  to  re])resent  metamorphosed  phases  of  the  Siamo  slate. 

ISOLATED  AREAS  OF  ARCHEAN  ROCKS. 

In  the  eastern  part  of  the  district,  witliin  the  Algonkian,  are  small  isolated  areas  of  Archean 
rocks.  Tliese  comprise  granites,  gneissoid  granites,  and  greensto7ie  schists  in  no  respect 
differing  fi'om  the  corresponding  rocks  of  the  main  northern  and  southern  areas. 

Intrusive  in  all  the  previously  described  rocks  are  dikes  and  bosses  of  diabase  and  diorite 
which  are  similar  to  those  which  intrude  the  Iluronian  rocks,  and  therefore  are  much  later  in 
age.     They  do  not  properly  belong  with  the  Archean. 

ALGONKIAN    SYSTEM. 

HTJRONIAN  SERIES. 

As  already  stated,  the  Huronian  series  in  the  Marquette  district  is  divided  into  upper 
Huronian  (Animikie  group),  middle  Huronian,  and  lower  Huronian. 

LOWER  HXTEONIAN. 

The  lower  Huronian  consists,  from  the  base  upward,  of  the  Mesnard  quartzite,  the  Kona 
dolomite,  and  the  Wewe  slate.  It  has  been  pointed  out  that  these  formations  appear  onlj'  in 
the  northeastern  part  of  the  district. 

MESNARD  QTTARTZITE. 

Name  and  distribution. — The  Mesnard  quartzite  is  so  named  because  it  composes  the 
larger  part  of  the  mass  of  Mount  Mesnard,  south  of  Marquette.  The  quartzite  borders  the 
Huronian  I'ocks  from  a  locality  a  short  distance  east  of  Teal  Lake'  east  to  Lake  Superior,  thence 
south  and  west  to  a  point  2  miles  west  of  Goose  Lake.  Also  patches  of  Mesnard  are  found  on 
the  north  margin  of  the  Huronian  rocks  as  far  west  as  2  miles  west  of  Teal  Lake.  In  the  eastern 
part  of  the  district  the  Mesnard  cjuartzite  is  repeated  by  the  appearance  of  a  central  anticUne, 
so  that  in  making  a  section  north  and  south  just  west  of  Mount  Mesnard  four  belts  of  the 
formation  are  found.  The  nature  of  the  structure  at  this  locality  is  shown  by  the  section  on 
Plate  XVII  (in  pocket). 

LitJiology. — The  Mesnard  quartzite  has  three  distinct  membei's — a  lower  conglomerate, 
a  central  quartzite,  and  an  upper  slate.  These  members  are  not  separately  mapped  because 
exposures  as  a  whole  are  not  suflicient  to  make  this  possible. 

The  lower  conglomerate  member  comprises  conglomerates  with  subordinate  amounts  of 
graywacke,  graywacke  slate,  and  quartzites,  with  all  gradations  between  the  different  phases. 
Naturally  the  iiner-grained  varieties  are  more  prevalent  near  the  top  of  this  member,  and 
locally  a  slate  appears  between  the  conglomerate  and  the  quartzite. 

The  conglomerate  adjacent  to  the  southern  granite  has  two  different  phases.  The  common 
phase  is  a  coarse  granite  conglomerate,  but  locally  the  granite  of  the  Archean  seems  to  have 
been  disintegrated  so  that  it  yielded  individual  grains  of  quartz,  feklspar,  and  mica,  ^liere 
this  w^as  the  case  the  recomposed  rock  very  closely  resembles  the  original  granite.  Tiiis  is 
especially  true  where  the  two  together  have  been  anamorphosed  to  schists.  Indeed,  at  such 
places  it  is  difTicult  to  place  the  exact  line  between  the  two  formations. 

The  conglomerates  adjacent  to  the  northern  Archean  bear  detritus  both  from  the  granite 
and  from  the  Mona  schist  and  therefore  carry  pebbles  and  bowlders  from  both  of  these  forma- 
tions. The  granite  pebbles  compri.se  coarse-grained  niuscovite  granite  anil  fine-grained  granite. 
The  pebbles  from  the  Mona  schist  include  various  kinds  of  greenstone  schists  and  cliloritic 
scliists  identical  with  the  phases  of  the  Mona  scliist,  so  that  there  can  be  no  doubt  as  to  the 
source  of  the  material. 


o  Mon.  U.  S.  0«oI.  Survey,  vol.  28, 1897,  atlas  sheet  IV. 


MARQUETTE  IRON  DISTRICT. 


257 


The  second  member,  constituting  the  great  mass  of  the  formation,  is  dominantly  a  pure 
vitreous  quartzite,  although  locally  there  are  feldspathic  quartzites  and  fine-grained  conglom- 
erates. In  this  belt  is  one  laj^er  of  conglomerate  in  which  cherty  jasper,  quartz,  and  ferru- 
ginous schist  pebbles  are  characteristic.  For  the  most  part  the  quartzite  is  indurated  by  cemen- 
tation. Toward  the  top  the  quartzite  member  becomes  slaty  and  finally  passes  into  a  gray- 
w^acke  slate.  Tliis  rock  is  from  less  than  30  to  about  100  feet  thick  and  is  in  fact  a  transition 
pelite  member  between  the  quartzite  and  the  Kona  dolomite. 

Metamorphism. — The  Mesnard  quartzite  as  a  whole  has  been  much  mashed,  and  the  result 
is  that  the  conglomerates,  quartzites,  and  graywackes  include  rocks  varying  from  those  which 
are  indurated  mainly  by  siliceous  cementation  to  those  which  are  crystalline  schists.  Some  of 
the  rocks  have  been  much  shattered,  the  shattering  extending  to  the  individual  grains.  The  open- 
ings which  have  been  formed  by  the  shattering  have  been  cemented  mainly  by  quartz  and  by  iron 
oxide.  So  pervasive  have  been  the  dynamic  effects  that  not  a  single  clastic  grain  has  escaped. 
Wliei-e  the  pressure  has  been  the  least  undulatory  extinction  is  shown  by  the  quartz  grains. 
A  large  portion  of  the  quartz  grains  have  been  sliced  by  parallel  fractures,  some  in  one  direction, 
some  in  two  directions  at  right  angles  to  each  other.  Where  the  formation  is  feldspathic  the 
feldspars  have  very  extensively  altered  into  sericite  and  quartz.  In  places  where  the  meta- 
morphism is  extreme  the  formation  is  transformed  into  a  sericite  schist  by  grairulation  and 
recrystallization.  The  scliistose  varieties  of  the  rocks  are  especially  prevalent  along  the  south- 
ern border  of  the  southern  conglomerate  adjacent  to  the  granite. 

Partial  analyses  of  the  massive  Mesnard  quartzite  and  the  schistose  phase  along  its  contact 
with  underlying  formations  are  given  below: 

Analyses  ofmassire  and  schistose  Mesnard  quartzite. 
[Analyst,  R.  D.  Hall,  University  of  Wisconsin.] 


Specimen 

24096 

(quartzite). 


Specimen 
24123  (seri- 
cite schist). 


SiOs... 
AljOs- 
FsjOs.. 
FeO... 
MgO... 
Na20.. 
K3O... 
H20-\ 
H,0+/ 


58.85 

26.22 

3.01 

.17 

.63 

.05 

8.44 

2.31 


Apparently  the  jnincipal  result  of  the  development  of  schistose  structure  has  been  the 
loss  of  soda  and  sUica  and  ferrous  iron.  Alumina,  potassa,  ferric  iron,  water,  and  magnesia 
have  remained  in  nearly  constant  and  mutual  proportions.  This  change  is  similar  in  all  respects 
to  one  shown  by  the  Waterloo  quartzite  of  southern  Wisconsin.  It  is  believed  to  be  one  due 
to  metamorphism,  but  the  possibihty  can  not  be  excluded  that  the  differences  are  partly  those 
of  original  composition. 

Bdations  to  adjacent  formations. — The  conglomerates  at  the  base  of  the  Mesnard  quartzite, 
here  adjacent  to  the  Laurentian  granite,  there  next  to  the  scliists  of  the  Keewatin,  show  that 
between  the  Archean  and  the  Mesnard  there  is  a  very  great  unconformity.  It  is  clear  that 
the  complex  history  of  the  Archean  was  practicalh'  complete  before  the  Mesnard  quartzite 
was  deposited.  The  Keewatin  schists  had  been  intruded  and  metamorphosed  by  the  granites 
and  the  two  together  had  been  deeply  truncated  before  the  Mesnard  was  laid  down.  One  of  the 
conglomerates  at  the  base  of  the  Mesnard  is  especially  interesting,  in  that  it  was  the  first  in 
wliicli  the  clear  evidence  of  unconformity  was  found.  This  contact  is  north  of  Mud  Lake  and 
along  an  old  road  known  as  the  State  road,  and  the  conglomerate  has  sometimes  been  called  the 
"State  Road"  conglomerate.  Since  the  discovery  of  this  contact  other  contacts  have  been 
found  along  the  southern  belt  of  Mesnard  at  a  score  of  places.  The  conglomerates  adjacent 
•17517°— VOL  52—11 17 


258  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

to  them  are  splendid  granite  conglomerates,  many  of  which  contain  great  well-waterwom 
bowlders  of  granite. 

On  the  knobs  northeast  of  the  southeast  end  of  Goose  Lake  quartzite  mapped  as  Mesnard 
is  found  to  lie  directly  upon  the  Kona  dolomite.  The  quartzite  with  this  relation  may  be  an 
interstratified  layer  m  the  Kona  dolomite  similar  to  quartzite  laj'ers  seen  in  this  formation  in 
the  Mount  Chocolay  section.  The  boundary  between  the  quartzite  overlying  the  Kona  dolo- 
mite in  tliis  locality  and  the  true  Mesnard  quartzite  is  not  known. 

TJiickness. — As  tlie  Mesnard  quartzite  was  the  first  formation  of  a  transgressing  sea,  it 
naturally  varies  in  tliickness  owing  to  the  irregularities  of  tlie  basement  upon  which  it  was 
deposited.     The  thickness  ranges  from  150  feet  to  nearly  700  feet. 

KONA  DOLOMITE. 

Name  and  distribution. — The  Kona  dolomite  is  given  the  name  Kona  from  the  prominent 
hills  of  that  name  east  of  Goose  Lake,  where  it  is  exposed. 

This  formation,  like  the  Mesnard  quartzite,  is  confined  to  the  eastern  part  of  the  district. 
In  distribution  it  constitutes  a  westward-facing  U,  the  arms  of  which  terminate  a  short  dis- 
tance east  of  Teal  Lake  on  the  north  and  at  the  east  shore  of  Goose  I^ake  on  the  south. 

The  exposures  commonly  constitute  a  set  of  sharp  and  abrupt  cliffs  cut  by  ravines  or 
separated  by  drift-hlled  valleys.  The  formation  very  well  illustrates  the  complex  folding  of 
the  district.  In  some  places  the  north-south  folds  are  the  more  prominent,  but  more  generally 
the  east-west  folds  are  dominant. 

Lithology. — The  Kona  formation  is  dominantly  a  dolomite,  fjut  interstratified  %\-itli  this  are 
layers  of  slate,  graywacke,  and  quartzite  with  all  gradations  between  the  mechanical  sedi- 
ments and  tiie  pure  dolomites.  Thus  there  are  finely  crystalline  dolomite,  chertj'  dolomite, 
quartzose  dolomite,  argillaceous  dolomite,  dolomitic  quartzites,  dolomitic  slates,  dolomitic 
cherty  cpiartzites,  and  dolomitic  chert.  The  dolomite  beds  range  in  thickness  from  a  few  inches 
to  man}-  feet,  but  even  the  most  dolomitic  beds  contain  thin  chert}-  layers,  mingled  with  which 
in  some  places  is  clastic  material.  In  color  the  rocks  vary  from  pink  and  red  to  dark  brown. 
Because  of  the  imjixuities  of  the  dolomite  the  weathered  surface  has  very  characteristically  a 
jagged  appearance,  due  to  the  solution  of  the  dolomite  and  the  consequent  protrusion  of  siliceous 
phases. 

MetamorpMsm. — The  dolomite  has  usually  jrielded  to  the  folding  without  prominent  frac- 
tures or  cleavage,  but  it  has  suffered  a  minute  shattering  and  is  cemented  by  finely  crystalline 
quartz  or  coarsely  crystalline  dolomite,  or  the  two  combmed.  The  slate  layers  usually  have 
a  slaty  cleavage  and  many  of  the  graywacke,  quartz,  and  cherty  quartz  layers  are  brecciated. 
These  breccias  where  scliistose  are  difficult  to  distinguish  from  conglomerates.  The  com- 
pleteness of  tliis  shattering  and  brecciation  was  appreciated  only  by  a  study  of  the  thin  sections, 
where  eveiy  one  of  the  numerous  slides  shows  the  phenomena  mentioned  to  a  greater  or  less 
extent.     Not  a  half-mcli  cube  has  escaped. 

Relations  to  adjacent  formations. — The  Kona  dolomite  grades  into  the  Mesnard  quartzite 
below.  Above,  by  a  lessening  of  the  calcareous  constituent,  it  gradually  passes  into  theWewe 
slate. 

Thickness. — ^Because  of  the  complicated  folding  of  the  Kona  dolomite  it  is  difTicult  to  give 
an  accurate  estimate  of  its  thickness,  which  probably  varies  greatly.  In  some  places  it  seems 
to  be  a  comparatively  tliin  formation,  not  more  than  200  to  250  feet  thick.  In  other  places  where 
the  wdiole  formation  is  well  exposed  it  appears  to  be  650  or  700  feet  thick,  and  it  may  be  thicker 
than  this. 

WEWE  SLATE. 

Distribution. — The  Wewe  slate,  like  the  Mesnard  quartzite  and  Kona  dolomite,  is  confined 
to  the  east  end  of  the  district,  making  a  westward-facing  U.  The  slate,  bcmg  a  less  resistant 
fornuxtion  than  the  Kona  dolomite  below  or  the  Ajibik  quartzite  above,  is  in  general  marked 
by  valleys,  and  consequently  the  exposures  are  few. 


MARQUETTE  IKON  DISTRICT.  259 

Liihologij. — The  Wewe  slate  was  a  pelite  formation  evidently  varying  in  its  character  from 
a  fine  mud  to  a  coarse  sandy  mud  with  numerous  alteration  phases.  As  a  result  of  the  com- 
pacting and  modification  .of  these  beds  the  formation  is  now  a  slate,  shale,  novaculite,  and 
graywacke.  The  color  of  these  rocks  varies  from  red  to  black,  depending  on  the  quantity  and 
conditions  of  the  iron  oxide. 

Metamorphism. — In  consequence  of  the  folding  and  metamorphism  the  slates  have  devel- 
oped a  cleavage.  The  rock  locally  has  been  sufficiently  metamorphosed  to  become  a  mica  slate 
and  even  to  approach  a  mica  schist,  but  usually  tlie  alteration  has  not  gone  sufficiently  far  to 
obliterate  the  bedcUng. 

The  rocks  have  been  commonly  fractured  parallel  to  the  bedding  or  to  the  secondary  struc- 
tures whicli  intereect  the  bedding.  At  some  localities  fi-acturing  has  been  sufficiently  powerful 
to  shatter  the  rocks  throughout,  or  even  to  produce  friction  breccias.  Wliere  further  move- 
ments have  rounded  the  fi'agments  of  the  breccia  the  rock  becomes  a  pseudoconglomerate. 
The  openings  wliich  have  been  produced  by  the  fracturing  have  been  cemented  by  (juartz,  by 
hematite,  and  by  a  jaspery  mixture  of  the  two.  In  some  places  these  varieties  of  material 
follow  one  another  and  locally  the  amount  of  hematite  in  the  breccia  is  so  great  as  to  have  led 
to  prospectmg  of  the  formation  for  iron  ore. 

Relations  to  adjacent  formations. — It  has  been  pointed  out  that  the  Kona  dolomite  grades 
into  the  Wewe  slate  by  a  disappearance  of  the  calcareous  material.  The  Wewe  slate  is  overlain 
imconformably  by  the  Ajibik  quartzite.  The  evidence  of  this  unconformity  will  be  given  under 
the  description  of  the  latter  formation. 

Thichness. — In  one  place,  where  there  is  an  almost  continuous  exposure  of  slate,  the  thick- 
ness is  calculated  at  1,050  feet,  but  it  is  entirely  probable  that  there  are  here  subordinate  rolls. 
The  real  thickness  of  the  slate  is  doubtless  much  less  than  this.  At  one  place,  indeed,  the 
thickness  of  the  formation  does  not  appear  to  be  more  than  100  feet. 

MIDDLE    HURONIAN. 

The  middle  Huronian  of  the  Marquette  district  comprises  the  Ajibik  quartzite,  the  Siamo 
slate,  and  the  ii'on-bearing  Negaunee  formation. 

AJIBIK  QUARTZITE. 

Name  and  distribution. — The  Ajibik  quartzite  is  so  named  because  the  predominant  rock 
is  quartzite  and  because  typical  exposures  of  it  occur  on  the  bold  Ajibik  Hills  northeast  of 
Palmer. 

The  distribution  of  the  Ajibik  quartzite  is  practically  coextensive  with  the  outlines  of  the 
Marquette  district.  For  all  of  the  area  west  of  Negaunee  it  is  the  Huronian  formation  wliich 
rests  against  the  Archean.  For  the  area  east  of  Negaimee  it  is  separated  from  the  Archean  by 
the  lower  Huronian  rocks  already  described.  Along  the  south  side  of  the  cfistrict  the  formation 
is  very  thin,  locally  not  more  than  a  few  feet.  The  Ajibik  ci[uartzite,  being  a  resistant  formation, 
is  for  the  most  part  well  exposed  and  at  various  places  it  constitutes  prominent  bluffs — as, 
for  instance,  east  of  Teal  Lake. 

Deformation. — In  general  the  folding  of  the  Ajibik  is  that  of  a  great  synclinorium,  the  dips 
being  south  fi'om  the  great  northern  belt  and  north  from  the  southern  belt.  The  Cascade 
trough,  the  Republic  trough,  and  southwestern  arms  at  the  west  end  of  the  district  constitute 
subordinate  synclinoria.  In  detail,  as  at  Broken  Bluffs,  there  is  secondary  infolding  of  the 
formation  with  i.soclinal  dips. 

The  formation  is  displaced  by  at  least  three  great  faults — that  of  Carp  River,  the  east-west 
tlu-ow  of  which  apparently  amounts  to  as  much  as  3,000  feet;  the  fault  along  the  south  side 
of  the  Ajibik  Hills,  the  tlu"ow  of  wliich  is  apparently  several  thousand  feet;  and  the  fault  at 
the  Volunteer  mine,  which  again  apparently  has  a  horizontal  throw  of  2,000  feet  or  more.  In 
addition  to  these  there  are  a  number  of  minor  faults  east  of  Teal  Lake,  the  character  of  which 
is  indicated  by  Plate  XIX  (p.  254). 


260  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Lithology. — Petrograpliically  tlie  Ajil)ik  formation  has  two  facics — conglomerate,  ■which  is 
in  t~uhordinate  amount  and  is  at  the  bottom  of  the  formation,  and  quartzite,  whicii  constitutes 
tlie  major  portion  of  tlie  formation.  Associated  \\itli  llie  conglomerates  are  interstratified 
shites  and  graj'wackes. 

The  conglomeratic  phase  has  two  main  areas — a  western  one  in  wiiicli  it  rests  directly 
upon  the  Archean  and  an  eastern  one  in  which  it  is  underlain  hy  lower  Huronian  rocks.  Where 
the  formation  rests  chrectly  upon  the  Archean  its  basal  part  is  a  conglomerate  or  recomposed 
rock,  the  material  of  which  is  derived  mainly  from  the  rocks  immeiliately  subjacent.  This 
conglomerate  varies  from  place  to  place  as  the  subjacent  rock  varies.  In  general  it  is  a 
granite  conglomerate.  In  the  Cascade  range  such  material  is  derived  from  the  Palmer  gneiss 
and  other  igneous  formations,  and  near  the  Ivitchi  schist  the  material  is  mainly  derived  from 
that  formation.  In  the  eastern  part  of  the  district  the  Ajibik  cpiartzite  rests  upon  the  Wewe 
slate,  somewhat  farther  west  upon  the  Kona  dolomite,  and  still  farther  west  upon  the  ^lesnard 
cpiartzite — that  is,  it  cuts  diagonally  across  these  tlvree  formations.  This  ap{)lies  to  both  its 
northern  and  its  southern  arms.  As  Mould  be  expected  from  this  relation,  the  conglomerate 
at  the  bottom  of  the  formation  in  tliis  area  contains  dominantly  debris  from  the  loAver  Huronian, 
but  includes  also  material  from  the  Archean. 

The  basal  conglomerates,  slates,  and  gra3*wackes  are  usually  of  only  moderate  thickness, 
although  the  conglomeratic  beds  are  persistent.     These  rocks  grade  up  mto  c[uartzite. 

Metamorphism. — The  major  portion  of  the  formation  was  a  quartz  sand.  By  cementation, 
d\-namic  action,  and  recrystallization  it  has  now  been  transformed  to  many  varieties,  of  wliich 
normal  quartzite,  cherty  quartzite,  ferruginous  quartzite,  ferruginous  cherty  quartzite,  quartz 
rocks,  quartzite  breccia,  and  quartz  scliists,  in  places  sericitic,  are  the  more  prominent. 
The  predominant  phase  is  a  typical  rather  pure  vitreous  cpiartzite,  which  locally  is  conglom- 
eratic. This  least-altered  variety  of  the  quartzite  is  composed  almost  wholly  of  rounded 
grains  of  quartz  of  somewhat  uniform  size,  which  are  beautifully  enlarged,  the  enlargements 
filling  the  interspaces.  The  grains  uniformly  show  undulatory  extinction,  and  some  of  them 
are  distinctly  fractured.  From  tliis  variety  there  are  all  gradations  to  the  other  forms 
mentioned. 

Locally  interstratified  with  the  quartzite  were  mud  beds  wliich  now  have  become  gray- 
wacke  slate,  mica  slate,  or  mica  scliist.  The  micaceous  varieties  of  the  rock  are  especially 
abundant  where  the  psammite  w-as  feldspatliic,  the  feldspar  altering  into  mica  and  quartz  or 
into  chlorite  and  quartz.  At  the  west  end  of  the  district,  especially  in  the  Republic  and  West- 
em  tongues,  the  masliing  has  been  so  great  as  to  transform  the  rock  to  a  quartz  schist,  and 
where  the  psammite  was  impure  there  were  developed  t^'pical  biotite  schists,  muscovite  scliists, 
and  chlorite  schists,  which  are  in  ])laces  gametiferous. 

In  their  very  general  brecciation,  with  consecjuent  abundance  of  pseudoconglomerates;  in 
the  secondar}'  veining,  both  with  coarsely  and  finely  crystalline  cjuartz;  and  in  the  large  cjuan- 
tity  of  secondary  hematite  and  magnetite,  these  quartzites  differ  from  the  Goodrich  quartzite 
of  the  upper  Huronian. 

Relations  to  adjacent  formations. — The  Ajibik  quartzite  rests  upon  both  the  Archean  and 
the  lower  Huronian  unconformably.  The  unconformity  bet^veen  the  Ajibik  cjuartzite  and  the 
Archean  is  conspicuous  and  was  early  recognized,  but  the  unconformable  relation  between  the 
Ajibik  and  the  lower  Huronian  was  overlooked  at  the  time  the  Marquette  monograph  was 
MTitten.  The  careful  mapping  and  studies  of  Seaman  in  the  eastern  part  of  the  chstrict  showed 
the  true  relation. 

It  has  already  been  pointed  out  that  in  going  from  the  east  end  of  the  Ajibik  westward  it 
is  found  at  first  in" contact  with  the  Wewe  slate,  next  with  the  Kona  dolomite,  next  with  the 
Mesnard  quartzite,  which  thins  westward  to  an  edge.  West  of  the  last  outcrops  of  Mesnard 
quartzite  the  Ajibik  is  in  contact  with  the  Keewatia,  Kitchi  schist,  and  with  Laurentian  rocks. 
Thus  it  cuts  diagonallj'  across  the  beveled  edges  of  formations  varying  in  age  from  Wewe  to 
Keewatin.  These  relations,  together  with  the  presence  of  conglomerates  at  the  base  whicli 
bear  debris  from  the  lower  Huronian,  show  that  the  low-er  Huronian  was  sufficientlv  indurated 


MARQUETTE  IRON  DISTRICT.  261 

to  yield  fragments  to  the  Ajibik  before  the  deposition  of  that  formation.  The  absence  of  the 
lower  Huronian  in  the  western  part  of  the  district  is  douljtk'ss  largely  if  not  wholly  due  to  its 
removal  by  erosion  between  the  time  of  the  Wewe  slate  and  the  deposition  of  tiie  Ajibik 
quartzite. 

Wliere  mashing  and  metamorphism  have  been  sufficient  to  transform  the  conglomerate 
into  a  schist  the  Archean  has  been  similarly  metamorphosed;  conseciuently  the  Ajibik  quartzite 
apparently  grades  down  into  the  Archean. 

The  Ajil^ik  (juartzite  in  the  northern  belt  and  in  the  eastern  part  of  the  district  grades 
upward  into  the  Siamo  slate.  This  change  takes  place  by  a  gradual  transition  of  the  psammite 
into  a  peUte  formation.  In  the  southern  belt  the  Siamo  slate  is  absent  and  the  Ajibik  cjuartzite 
grades  into  the  Negaunee  formation.  This  gradation  may  be  particularly  w'ell  seen  in  the 
Cascade  area. 

Thickness. — The  best  opportunity  to  determine  the  thickness  of  the  formation  is  at  the  east 
end  of  the  U,  where  the  apparently  secondary  folding  is  absent.  Here  the  thickness  appears  to 
be  about  700  to  750  feet.     Along  the  south  side  of  the  district  the  formation  tliins  to  a  few  feet. 

SIAMO  SLATE. 

Name  and  distribution. — The  Siamo  slate  is  so  called  because  abundant  and  typical  exposures 
occur  on  the  Siamo  Hills  southwest  of  Teal  Lake.  The  formation  appears  at  the  northwestern 
part  of  the  district,  north  of  the  Michigamme  mine,  and  extends  in  a  continuous  belt  of  varying 
width  to  a  point  nortlieast  of  Negaunee.  Here,  owing  to  the  canoe  shape  of  the  eastern  part 
of  tlie  district,  it  widens  out  to  broad  irregular  areas  with  several  arms  between  the  Ajibik 
quartzite  and  the  Negaunee  formation.  There  is  no  southern  belt  of  Siamo  slate  corresponding 
to  the  northern  belt.  The  slate,  being  a  soft  formation,  is  not  well  exposed,  but  where  it  is 
metamorphosed  into  a  mica  slate  or  where  it  is  a  coarse  graywacke  ledges  are  munerous. 

Deformation. — The  folding  of  the  formation  as  a  whole  corresponds  to  that  of  the  district. 
In  detail  it  is  more  complex  than  that  of  the  associated  quartzites.  The  northern  belt,  with 
southern  dip,  has  superimposed  upon  it  isoclinal  folds  of  the  second  order.  In  the  eastern 
part  of  the  district,  where  the  broad  area  of  Siamo  slate  is  situated,  the  formation  is  folded  into 
a  series  of  rolls,  indicated  by  the  sinuous  contact  between  the  Siamo  and  Negaunee  forma- 
tions. The  .westward-projecting  salients  of  the  Siamo  constitute  the  crests  of  anticlines  and 
the  reentrants  are  the  synclines. 

Lifhology. — The  rocks  of  the  Siamo  formation  are  dark  gray  or  greenish  gray  and  some 
of  the  coarser  are  light  gray.  They  vary  from  a  coarse-grained  graywacke,  approaching  a 
quartzite,  through  massive  graywacke  to  a  very  fine  grained  slate.  The  slates  and  fine-grained 
graywackes  are  the  predominant  phases.  The  finer-grained  varieties  are  in  many  places 
affected  by  slaty  cleavage,  which  is  rather  uniform  in  tlirection  for  a  given  area  ami  thus  trav- 
erses the  bedding.  Locally  movements  later  than  the  development  of  the  cleavage  have 
resulted  in  many  partings  along  this  secondary  structure,  giving  the  rock  a  fissility. 

The  less-altered  Siamo  rocks  are  composed  mainly  of  well-rounded  grains  of  quartz,  a  few  of 
them  finely  complex  and  cherty  looking,  and  of  grains  of  feldspar,  between  which  is  a  sparse 
matrix  cojisisting  of  chlorite,  biotite,  muscovite,  finely  crystalline  quartz,  and  more  or  less 
iron  oxide.  Usually  the  chlorite  predominates  over  tlie  muscovite  and  biotite,  but  in  some  of 
the  rocks  the  micas  are  as  abundant  as  the  chlorite.  Some  of  the  quartz  grains  are  distinctly 
enlarged.  Most  of  them  show  pressure  effects  by  imdulatory  extinction  and  fracturing,  the 
fractures  being  locally  arranged  in  a  rectangular  system.  The  feldspars  comprise  orthoclase, 
microcline,  and  plagioclase,  in  places  changed  into  chlorite  and  quartz,  biotite  and  quartz,  or 
muscovite. 

Metamorpliism. — The  mineral  alterations  have  been  noted.  In  proportion  as  there  is 
dynamic  action  there  is  a  tendency  for  secondary  leaflets  of  the  chlorite,  biotite,  and  mus- 
covite to  have  a  parallel  arrangement.  Where  this  is  well  advanced  there  is  also  granulation 
of  the  larger  quartz  grains,  and  tlie  secondary  quartz  may  become  as  coarsely  crystalline  as  the 
original  quartz.     Wliere  all  these  changes  have  gone  far  the  rock  becomes  a  mica  slate  or  a  mica 


262  GEOLOGY  OF  THE  LAKE  SUPEKIOK  REGION. 

schist.  The  process  of  development  thus  Ijiii'fl}'  outhned  is  the  same  as  for  the  Tj'ler  slate  of 
the  Penokee-Gogebic  district,  described  in  another  place.     (See  pp.  2.32-233.) 

Other  phases  of  the  Siamo  1-ocks  exhibit  very  well  a  fractuic  or  slip  cleavage  which  may 
be  in  only  a  single  direction  parallel  l<>  the  bedding,  oi-  in  two  directions  intei-secting  at  angles 
varying  from  nearly  right  angles  where  t-lie  pressure  has  been  least  to  acute  angles  where  it  has 
been  strong.     In  thin  section  tlie  latter  rock  has  an  a])i)earanc('  like  tjiat  of  a  drawn-out  net. 

The  largest  areas  of  mica  scliist,  representing  the  most  advanced  phase  of  metamorphism 
of  the  formation,  lie  nortli  of  Michigamme.  The  greater  nictamorphism  of  this  part  of  the 
formation  is  attributed  to  tlie  large  masses  of  intnisive  greenstone  which  have  been  introduced 
roughly  parallel  to  the  contact  of  the  vSiamo  slate  and  the  Negaunee  formation.  Other  consid- 
erable masses  of  greenstone  are  also  found  within  the  area  of  the  Si;imo.  Evidence  of  the 
metamorphic  effect  of  the  greenstone  is  afforded  by  numerous  large  secondary  crystals  of  horn- 
blende in  the  slate  adjacent  to  the  larger  masses  of  greenstone. 

Relations  to  adjacent  formations. — At  the  upper  and  lower  horizons  the  slates  tend  to  becom,e 
ferruginous.  In  these  phases  there  is  present  a  considerable  cpiantity  of  iron  oxide,  generally 
hematite  but  in  many  places  magnetite.  In  the  upper  part  of  the  formation  especially  these 
ferruginous  slates  have  interlaminated  layers  of  material  similar  to  the  ferruginous  and  sider- 
itic  slates  and  cherts  and  giiinerite-magnetite  schists  of  the  Negaunee  formation.  The  Ajibik 
quartzite  grades  up  into  tlie  Siamo  slate.  It  is  apparent  from  the  appearance  of  interlaminated 
layers  of  material  like  the  Negaunee  formation  in  the  upper  parts  of  the  Siamo  slate  that  the 
transition  into  the  Negaunee  is  a  gradation  by  interstratification.  The  fragmental  sediments 
gradually  die  out  and  nonfragmental  sediments  become  dominant;  this  change  takes  place 
irregularly,  producing  interstratification  of  the  two  forms  of  sediments. 

Thiclcness. — The  area  perhaps  most  favorable  for  detemiining  the  thickness  of  the  Siamo 
slate  is  that  adjacent  to  Teal  Lake.  If  the  formation  were  there  assumed  to  be  monoclinal,  the 
tliickness  would  be  from  1,250  feet  to  1,300  feet,  but  as  there  are  an  unknown  number  of  sub- 
ordinate rolls  at  this  locality,  and  slat}'  cleavage  has  developed,  it  is  probable  that  the  real 
thickness  of  the  formation  is  not  more  than  half  of  this  amount. 

NEGAUNEE  FORMATION. 

Name  aiul  distribution. — The  principal  iron-bearing  formation  of  the  Marquette  district  is 
named  Negaunee  because  in  the  town  of  that  name  and  to  the  south  are  typical  exposures  of 
the  formation. 

The  Negaunee  formation  extends  from  the  northwest  end  of  the  district  along  the  north 
side  of  the  Huronian  to  the  north  side  of  Michigamme  Lake.  From  this  place  eastward  for  a 
distance  of  5  miles  the  formation  is  cut  out  by  the  unconformity  at  the  base  of  the  Upper 
Huronian.  Near  Ishpeming  it  widens  out  into  a  broad  area  and  occupies  a  large  portion  of  the 
famous  T.  47  N.,  R.  27  W.,  and  also  a  considerable  portion  of  T.  47  N.,  R.  26  W.  From  this 
broad  area  a  short  southern  arm,  known  as  the  Cascade  range,  extends  to  the  east  and  a  long  arm 
to  the  west  along  the  south  side  of  the  Algonkian;  the  formation  is  found  also  on  both  sides  of 
the  Republic  and  southwestern  arms.  In  the  western  part  of  tlie  main  southern  belt  and  in 
the  Republic  and  southwestern  arms  the  formation  is  apparently  al)scnt  for  distancps  varying 
from  a  fraction  of  a  mile  to  several  miles.  It  is  believed  tliis  lack  of  continuity  is  due  to  the 
fact  that  the  Negaunee  formation  was  completely  removed  by  erosion  liefore  the  deposition  of 
tlie  upper  Huronian  (Animikie  group). 

Deformation. — The  two  long  arms  of  the  iron-bearing  formation  of  the  main  belt,  as  well 
as  the  two  belts  of  iron-bearing  formation  in  the  Republic  and  southwestern  belts,  are  the  two 
sides  of  a  synclinorium.  The  two  main  arms  join  in  the  large  area  of  Negaunee  at  Ishpeming, 
showing  that  it  also  is  in  a  broad  way  an  east-west  synclinorium.  This  trough  pitches  to  the 
west.  Thus  the  lower  members  of  the  Negaunee  formation  outcroj)  on  the  east  adjacent  to  the 
Siamo  slate  and  the  higher  inembers  outcrop  on  the  west  adjacent  to  the  Goodrich  quartzite.  The 
suiuous  contacts  between  tlie  Negaunee  and  the  formations  above  and  below  express  its  folding. 


]\IARQUETTE  IRON  DISTRICT.  263 

The  salients  to  the  east  into  the  Siamo  slate  represent  synclines  and  the  reentrants  anticlines;  the 
salients  to  the  west  into  the  Goodrich  quartzite  represent  anticlines  and  the  reentrants  synclines. 
The  Palmer  belt  of  the  Negaunee  formation,  extentling  from  the  main  area  as  a  southeastern 
arm,  is  also  a  synclinal  fold,  which  ends  to  the  east  in  a  canoe  with  a  westward  pitch.  The 
structure  of  this  syncline  is  modified  by  a  great  fault  along  the  south  side  of  the  Ajibik  Hills 
and  by  faulting  at  the  Volunteer  mine. 

Lithology,  including  metamorpJiism . — Petrographically  the  iron-bearing  formation  com- 
prises sideritic  slates,  which  may  be  griineritic,  magnetitic,  hematitic,  or  limonitic;  griinerite- 
magnetite  schists;  ferruginous  slates;  ferruginous  cherts;  jaspilite,  and  iron  ores.  The  ferru- 
ginous cherts  and  jaspilite  are  commonly  brecciated,  the  other  kinds  less  commonly. 

The  sideritic  slates  are  most  abuntlant  in  the  valleys  between  the  greenstone  masses  in  the 
large  area  south  of  Ishpeming  and  Negaunee.  These  rocks  are  regularly  laminated,  are  fine 
grained,  and  when  unaltered  are  of  a  dull-gray  color.  The  purest  phases  of  them  are  approxi- 
mately cherty  iron  carbonate,  as  sliown  by  two  analyses  made  by  George  Steiger  in  the  laboratory 
of  the  Survey.  It  is  unusual  to  find  exposures  of  the  cherty  siderite  slates  which  have  not  been 
more  or  less  affected  by  deep-seated  alteration  or  by  weathering  processes.  The  iron  car- 
bonates pass  by  gradations,  on  the  one  hand  into  griinerite-magnetite  schists  and  on  the  other 
into  ferruginous  slates,  ferruginous  chert,  jasper,  or  iron  ore. 

The  grunerite-magnetite  schists  consist  of  alternating  bands  composed  of  varying  pro- 
portions of  the  minerals  griinerite  and  magnetite  and  (|uartz.  Where  least  modified  they  have 
a  structure  precisely  Hke  the  sideritic  slates  from  which  they  grade,  the  grunerite-magnetite  belts 
having  taken  the  place  of  the  carbonate  bands.  In  some  places  the  grunerite-magnetite  schists 
are  minutely  banded,  the  alternate  bantls  consisting  of  dense  green  griinerite  and  white  or  gray 
chert,  with  but  a  small  quantity  of  magnetite.  Certain  important  kinds  appear  to  be  com- 
posed almost  altogether  of  griinerite,  with  a  little  magnetite.  In  general  the  griinerite- 
magnetite  schists  are  found  at  low  horizons,  below  the  ferruginous  chert  and  jaspilite — that  is, 
at  or  near  the  same  horizon  as  the  sideritic  slates.  In  many  places  also  they  are  below  intrusive 
masses  of  greenstone. 

By  oxidation  of  the  iron  carbonate  the  sideritic  slates  pass  into  the  ferruginous  slates,  the 
iron  oxide  being  hematite  or  limonite,  or  both.  These  rocks,  in  regularity  of  lamination  and  in 
structure,  are  similar  to  the  sideritic  slates,  differing  fi'om  them  mainly  in  the  fact  that  the  iron 
is  present  as  oxide.  In  the  different  ledges  may  be  seen  every  possible  stage  of  change  from 
the  sideritic  slates  to  the  ferruginous  slates.  The  only  necessary  change  is  a  loss  of  carbon 
dioxide  and  oxidation  of  tlie  iron.  On  Meathered  surfaces,  along  veins,  and  along  some  of  the 
bedding  planes  the  transfcjrmation  may  be  complete,  and  between  this  material  and  the  original 
rock  there  are  numerous  gradations. 

From  the  oxidation  of  the  less  slaty  phases  of  the  sideritic  rocks  result  tlie  ferruginous 
cherts,  consisting  mainly  of  alternating  layers  of  chert  and  iron  oxitle,  although  the  iron  oxide 
bands  contain  chert  and  the  chert  bands  contain  iron  oxide  (PI.  XXXIII,  B,  p.  466).  This  iron 
oxide  is  mainly  hematite,  but  both  limonite  and  magnetite  are  locally  present.  Rarely  mag- 
netite is  tlie  predominant  oxide  of  iron.  In  such  places  the  silica  is  usually  coai-sely  crystalline. 
The  rocks  are  folded  in  a  complicated  fashion,  as  a  result  of  which  the  layers  present  an  extremely 
contorted  appearance.  Many  of  the  folded  layers  show  minor  faulting.  On  account  of  the 
exceedingly  brittle  character  of  these  rocks,  they  are  very  commonly  broken  through  and  through, 
and  some  of  them  pass  into  friction  breccias.  In  places  the  shearing  of  the  fragments  over  one 
another  has  been  so  severe  as  to  produce  a  conglomeratic  aspect.  The  ferruginous  cherts  are 
particularly  abundant  in  the  middle  and  lower  parts  of  the  iron-bearing  formation,  just  above 
or  in  contact  with  the  greenstone  masses.  In  a  number  of  places  they  are  between  the  griinerite- 
magnetite  schists  or  sideritic  slates  below  and  the  jaspilite  above.  The  rocks  here  named 
ferruginous  chert  are  called  by  the  miners  "soft-ore  jasper"  to  discriminate  them  from  the 
"hard-ore  jasper,"  or  jaspilite,  because  within  or  associated  with  them  are  found  the  soft  ores 
of  the  district. 


264  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

The  jaspilites  consist  of  alternate  bands  composed  mainly  of  finely  crystalline,  iron-stained 
quartz  iiiid  iron  oxide  (PI.  XXX  FT,  ]).  4CA) .  Tiu^  exposures  jiresent  a  brilliant  appearance,  due  to 
the  interlaniinationof  llie  brij,d it-red  jasper  and  tlie  dark-red  or  black  iron  oxides.  Tiie  iron  oxide 
is  mainly  hematite  and  includes  both  red  and  specular  varieties,  but  magnetite  is  commonly 
present.  Many  of  the  jasper  bands  have  oval  terminations  or  die  out  in  an  irregidar  manner. 
The  folding,  faulting,  and  brecciation  of  the  jaspilites  are  precisely  like  those  of  the  feniiginous 
chert,  except  that  in  the  jaspilite  they  are  more  severe.  The  interstices  produced  by  the  dynamic 
action  are  largely  cemented  with  crystalline  hematite,  but  magnetite  is  present  in  subordinate 
quantity.  In  the  foldmg  of  the  rock  the  readjustment  has  occurred  mainly  m  the  iron  oxide 
between  the  jasper  bands.  As  a  result  of  this  the  iron  oxide  has  been  sheared,  and  when  a 
specimen  is  cleaved  along  a  layer  it  presents  a  brilliant  micaceous  appearance;  such  ore  has 
been  called  micaceous  hematite.  This  sheared  lustrous  hematite,  present  as  some  form  of  iron 
oxide  before  the  dynamic  movement,  is  discriminated  with  the  naked  eye  or  with  the  lens  from 
the  later  crystal-outlined  hematite  and  magnetite  which  fill  the  cracks  in  the  jasper  bands 
and  the  spaces  between  the  sheared  laminse  of  liematite.  The  jaspilite  differs  mainly  from  the 
ferruginous  chert,  with  which  it  is  closely  associated,  in  that  the  siliceous  bands  of  the  jaspilite 
are  stained  a  bright  red  by  hematite,  and  the  bands  of  ore  between  them  are  mainly  specular 
hematite,  whereas  in  the  cherts  the  iron  oxide  is  earthy  hematite.  The  jaspilite  in  its  typical 
form,  whenever  present,  usually  occupies  one  horizon — the  present  stratigrapliic  top  of  the 
iron-bearing  formation,  just  below  the  Goodrich  quartzite.  In  different  parts  of  the  district 
it  has  a  varying  thickness.  With  this  jasper,  or  just  above  it,  are  the  hard  iron  ores  of  the 
district;  hence  it  has  been  called  "hard-ore  jasper"  by  the  miners  to  discriminate  it  from 
the  ferruginous  chert,  or  "soft-ore  jasper." 

Relations  to  adjacent  formations. — The  iron-bearing  formation  rests  conformably  upon  the 
Siamo  slate  or  upon  the  Ajibik  quartzite  and  grades  downward  into  one  or  the  other  of  these 
formations  through  the  increase  of  clastic  material  and  a  lessening  of  the  ferruginous  constitu- 
ents. The  gradation  may  occur  within  a  few  feet  or  may  require  100  feet  or  more.  The  transi- 
tion is  accomplished  by  interlaminations  of  material  which  are  alternatively  chiefly  fragmental 
and  chiefly  nonfragmental. 

The  overlymg  formation,  the  Goodrich  quartzite,  rests  unconformably  u]:)on  the  Negaunee 
formation.  The  amount  of  foldmg  and  erosion  of  the  Negaunee  formation  accomplished  before 
the  Goodrich  quartzite  was  deposited  ditt'ers  in  diflferent  parts  of  the  district.  In  some  places 
the  erosion  has  gone  so  far  as  to  have  removed  the  iron  formation  entirely.  It  therefore  follows 
that  the  contact  between  the  two  formations  is  here  at  one  horizon  of  the  iron-bearing  formation 
and  there  at  another,  ranging  from  the  highest  known  horizon  to  the  lowest. 

TliicJcness. — It  is  evident  from  these  relations  that  the  thiclcness  of  the  formation  varies 
from  practically  nothing  to  its  maximum.  It  is,  however,  difficult  to  estimate  this  maximum 
because  of  the  pervasiveness  of  the  intrusive  rocks  in  the  Negaunee.  It  is  rouglily  estimated 
that  in  the  Ijroad  area  to  the  east  of  Ishpeming  and  Negaunee  the  thickness  may  be  con- 
siderably above  1,000  feet,  although  it  is  entirely  probable  that  the  maximum  thickness  is 
less  than  this  amount. 

Intrusive  and  eruptive  rocJcs. — Within  the  iron-bearing  formation  there  are  numerous 
intrusive  masses  of  "greenstone,"  really  diabase  and  its  altered  equivalents.  Tliese  occur 
in  the  form  of  both  dikes  and  bosses,  and  many  of  the  latter  are  of  large  size,  running  up  to 
masses  2  miles  or  more  in  extent.  These  rocks  are  especiaUy  prevalent  in  the  broad  area  of  the 
iron-bearing  formation  near  Ishpeming,  where  they  occupy  between  one-third  and  one-lialf 
of  the  area.  In  many  places  the  greenstones  intrude  the  sedimentary  series  in  a  roughly 
laccolithic  fasliion.  In  consequence  of  this,  where  the  two  have  been  folded  together  their 
relations  are  roughly  similar  to  those  of  sedimentary  formations,  but  when  examined  closely  the 
greenstones  are  always  found  to  cut  the  Negaunee  formation  to  a  lesser  or  greater  degree. 

Surface  eruptive  rocks  also  appear  in  the  formation  in  the  vicinity  of  Clarksburg.  (See 
p.  268.) 


MARQUETTE  IRON  DISTRICT.  265 

UI'PER   HURONIAN    (aNIMIKIE    GKOXJP). 

The  upper  Huronian  is  structurally  divisible  into  a  lower  belt  of  conglomerate  and  quartz- 
ite,  called  the  Goodrich  quartzite,  a  belt  of  ferruginous  rocks  called  the  Bijiki  schist,  a  belt  of 
slate  and  schist  kno\vii  as  the  Michigamme  slate,  and,  to  the  south,  a  mass  of  volcanic  rocks 
called  the  Clarksburg  formation.  The  Animikie  group  as  a  whole  occupies  the  center  of  the  main 
Algonkian  synclinorium  from  Ishpeming  to  the  west  end  of  the  district.  In  this  part  of  the 
region  it  is  the  chief  surface  rock,  occupying  all  the  area  between  the  belts  of  the  Negaunee 
formation. 

GOODRICH  QUARTZITE. 

Distribution  and  structure. — The  belt  of  Goodrich  quartzite  forms  a  westward-opening  U, 
bordered  on  the  outside  principally  by  the  Negaunee  formation,  with  its  eastern  margin  near 
the  city  of  Ishpeming.  The  folding  is  similar  to  that  of  the  Negaunee  formation,  though  some- 
what less  complex.  The  sinuous  contact  of  the  two  formations  in  the  vicinity  of  Ishpeming 
expresses  the  complexity  of  folding  at  this  end  of  the  synclinorium. 

Litliology,  including  metamorphism. — Petrographically  the  Goodrich  is  dominantly  a  quartz- 
ite, although  usiuxlly  there  is  a  conglomerate  at  the  base.  As  the  underlying  rock  is  in  most 
places  the  Negaunee  formation  this  conglomerate  is  an  ore,  chert,  jasper,  and  quartz  con- 
glomerate. Wliere  the  conglomerate  is  near  the  Archean  this  system  may  furnish  material 
for  it — as,  for  instance,  at  Palmer,  where  there  are  numerous  granite,  greenstone,  and  schist 
bowlders  derived  from  the  Archean. 

Wliere  the  conglomerate  is  ore,  chert,  and  jasper  conglomerate  immediately  in  contact 
with  the  Negaunee  formation,  the  jiarticles  have  been  flattened  and  schistosity  has  developed 
in  both  the  conglomerate  and  the  original  basement  rock,  making  it  difficult  to  place  the  exact 
line  between  the  two  formations.  This  is  illustrated  at  Ilumljoldt.  At  several  localities  the 
conglomerate  resting  upon  the  Negaunee  formation  has  had  quartz  leached  out  and  hematite 
and  magnetite  deposited,  developing  a  material  rich  enough  in  iron  to  be  an  ore.  This  is  illus- 
trated at  the  Goodrich  and  Volunteer  mines.  The  quartzite  is  mahily  quartz  but  contains  many 
particles  of  chert  and  jasper  and  usually  considerable  amounts  of  feldspar.  Cementation  by 
enlargement  is  an  important  process  in  the  induration  of  the  rock.  In  the  eastern  part  of  the 
district  dynamic  action  has  not  usiuxlly  been  great  enough  to  give  the  particles  more  than 
undulatory  extinction,  or  at  most  fracturing.  However,  these  effects  are  pervasive,  not  a  single 
clastic  particle  escaping.  The  mashing  in  the  central  and  western  parts  of  the  district  has  been 
severe  and  the  formation  has  been  transformed  to  a  schist.  In  the  western  part  of  the  district, 
especially  in  the  Republic  trough,  the  alterations  have  been  so  great  as  to  transform  the  fekU 
spathic  quartz  rocks  into  micaceous  quartz  schists,  or  locally,  where  the  mica  is  sufficiently 
abundant,  into  muscovite-biotite  schists  or  biotite  schists.  In  this  change  the  feldspar  has 
usually  altered  into  quartz  and  mica,  including  both  muscovite  and  biotite,  especially  muscovite. 

Relations  to  adjacent  formations. — The  Goodrich  quartzite  rests  unconformably  upon  the 
Negaunee  formation.  The  evidence  of  this  unconformity  consists  both  in  the  discordance  of 
strike  and  dip,  varjdng  from  a  few  degrees  up  to  perpendicularity,  as  at  the  Goodrich  mine, 
and  in  the  existence  of  conglomerates  derived  from  the  Negaunee  formation  at  scores  of  locali- 
ties along  the  contact.  At  many  places,  as  has  already  been  pointed  out,  the  erosion  between 
Negaunee  and  Goodrich  time  cut  through  the  Negaimee  formation.  In  these  places  the  mate- 
rial of  the  Goodrich  quartzite  comes  from  the  underlying  formations,  the  Ajibik  quartzite  or 
the  rocks  of  the  Ai-chean.  There  are  few  Lake  Superior  formations  that  have  a  more  complete 
set  of  conglomerates  at  the  base  or  that  have  clearer  proof  of  unconformity  with  the  rocks 
upon  which  they  rest.  The  Goodrich  quartzite,  by  the  diminution  of  coarse  fragmental  quartz, 
grades  above- into  the  Michigamme  slate,  the  Bijiki  schist,  or  the  Clarksburg  formation.  The 
nature  of  each  gradation  will  be  mentioned  in  connection  with  these  formations. 

TliicJcness. — The  thickness  of  the  Goodrich  quartzite  varies  greatly  from  place  to  ])lace. 
At  the  Goodrich  mine  it  is  calculated  to  be  as  great  as  1,500  feet,  but  this  is  probably  much 
beyond  tlie  average  for  the  district. 


266  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

BIJIEI  SCHIST. 

•  Name  and  distribution. — The  Bijiki  schist  is  given  this  name  l)ecaiisc  typical  exposures 
occur  near  the  mouth  of  Bijiki  River.  It  is  confined  to  tliree  narrow  l)elts  in  tlie  nortli western 
part  of  the  district.  North  of  tlie  northernmost  of  tliese  behs  is  tlie  Goodricii  quartzite  and 
between  the  north  and  middle  belts  is  the  Michigamme  slate.  These  two  belts  make  a  synclinal 
structure.  Tlie  middle  and  southern  belts  unite  at  (lie  east  and  rej)resent  the  outcrop  of  an 
eroded  anticline. 

Lithology,  including  metamorphism. — ^Lithologically  tlie  Bijiki  schist  comprises  two  main 
varieties,  one  of  which  is  characteristic  of  the  eastern  ])art  of  tlie  lielts  and  tlie  otlicr  of  the 
western  ])art. 

In  the  eastern  jiart  tlie  least-altered  phases  consist  of  a  sideritic  chert  interbedded  \rith 
the  Michigamme  slate  and  jirobably  representing  a  slightly  higher  horizon  than  the  phase  of 
the  Bijiki  schist  described  in  the  following  paragrajiii.  Not  imcommonly  the  siderite  is  the 
predominating. constituent.  This  slate  has  been  extensively  altered  by  weathering  and  meta- 
soniatic  changes  into  ferruginous  slates  and  ferruginous  cherts,  with  .subordinate  amounts  of 
griinerite-magnetite  scliist.  In  a  few  localities,  where  the  ferruginous  material  is  very  abun- 
dant and  the  conditions  of  deposition  are  favorable,  small  ore  bodies  have  been  found.  These 
are  illustrated  by  the  North  Phenix,  Pascoe,  Hortense,  Northampton,  Marine,  PhenLx,  and 
Bessie  deposits.  These  ores  differ  from  the  soft  ores  of  the  Negaunee  formation  in  that  the 
iron  oxide  is  largely  limonite  and  the  associated  slates  are  carbonaceous  and  graphitic. 

In  the  western  area,  which  contains  the  chief  exposures  of  the  formation,  the  Bijiki  is 
dominantly  a  banded  griinerite-magnetite  schist.  This  rock  consists  mamly  of  three  miner- 
als— -rjuartz,  griinerite,  and  magnetite.  Here  and  there  a  small  amount  of  residual  siderite  is 
seen.  The  rock  is  discriminated  from  the  griinerite-magnetite  schists  of  the  Negaunee  foimation 
chiefly  by  its  exceeding  toughness  and  the  dilliculty  with  which  it  is  broken  jiarallel  to  the 
stratification. 

One  of  the  most  conspicuous  mineralogical  features  of  the  iron-bearing  Bijiki  formation 
near  Michigamme  is  its  content  of  large  garnets,  up  to  2  inches  in  diameter,  developed  late  in 
tlie  metamorphism.  These  have  been  apparently  altered  to  clilorite  and  amphibole,  early 
described  by  Pumpelly  as  chlorite  pseudomorphs  after  garnet."  Microscopic  examination  .shows 
that  although  much  of  the  matrix  material  is  chlorite,  the  garnet  is  largely  replaced  bv  green 
amphibole  and  magnetite.  Porphyritic  biotite  in  a  chloritic  matrix  is  also  a  very  conspicuous 
mineralogical  feature  of  these  rocks,  giving  them  in  the  hand  specimen  a  brilliantly  spangled 
appearance.     The  garnet  may  be  really  a  poikilitic  development  later  than  clilorite. 

The  two  chief  phases  of  the  Bijiki  schist  may  be  in  part  at  separate  horizons,  but  there 
seem  also  to  be  gradations  between  the  ferruginous  slates  and  cherts  and  tlie  griinerite-magnetite 
schist.  As  the  schists  are  largely  confined  to  the  western  ])arts  of  the  belts,  where  there  are 
important  masses  of  intrusive  igneous  rocks,  and  occur  in  the  part  of  the  district  where  the 
Negaunee  formation  is  also  changed  to  a  griinerite-magnetite  schist,  it  is  believed  that  the 
schist  represents  the  original  sideritic  formation  altered  under  the  influence  of  igneous  rocks 
while  deeply  buried  and  largely  by  the  process  of  sihcation,  whereas  the  eastern  part  of  the 
formation,  consisting  tif  ferruginous  slates  and  cherts  and  containing  ore  bodies,  was  altered 
after  the  formation  was  exposed  at  the  surface,  later  than  upper  Iluronian  time,  by  the  proc- 
esses of  weathering. 

Relations  to  adjacent  I'ocks. — ,Uong  the  northern  belt  where  tlie  base  of  the  Bijiki  schist 
is  exposed,  roundetl  fragmental  quartz  appears  near  the  bottom  of  the  formation,  and  with 
an  increase  of  this  material  the  member  grades  downward  into  the  Goodrich  quartzite.  The 
Bijiki  schist  grades  above  into  the  Michigamme  slate. 

In  the  central  and  eastern  parts  of  the  ilarquette  district  the  Bijiki  has  not  lieen  detected. 
Apparently  in  the  greater  portion  of  the  district  between  the  time  of  the  Goodrich  quartzite 


a  I'liiiipelly,  Riiplmel,  On  pseudomorphs  of  chlorite  after  garnet  at  the  Spurr  Mountain  iron  mine,  Lake  Superior:  .\m.  Jour.  Sii.,  lid  ser., 
vol.  10,  July,  isrs,  pp.  17-21. 


MARQUETTE  IRON  DISTRICT.  267 

and  tlie  Michigamme  slate  the  conditions  were  not  favorable  for  the  deposition  of  the  iron- 
bearing  formation. 

The  u-on-bearing  Bijiki  schist,  though  not  tliick  or  economically  of  as  great  consequence 
as  the  Negaunee,  is  of  considerable  significance  in  the  matter  of  correlation,  for  it  occurs 
at  tlie  same  horizon  as  an  important  u'on-])oaring  formation  in  other  districts — notably  the 
Menominee,  Gogebic,  and  Mesabi. 

Thickness. — The  Bijiki  schist  apparently  has  a  maximum  thickness  of  about  520  feet  and 
from  this  it  ranges  down  to  the  disappearing  point. 

MICHIGAMME  SLATE. 

Name,  disfrihution,  and  correlation. — The  name  Michigamme  is  given  to  the  upper  slate 
and  mica  schist  formation  because  extensive  exposures  of  it  occur  on  the  islands  of  Lake  ilichi- 
gamme  and  on  the  mainland  adjacent  to  the  shore. 

The  ^lichigamme  slate  is  mainly  in  a  single  great  area,  which  extends  from  a  ]>oint  about 
a  mile  west  of  Ishpeming  along  the  axis  of  the  Marquette  synclinorium  to  the  west  end  of  the 
district.  To  Lake  Michigamme  the  breadth  of  this  belt  is  for  the  most  part  less  than  2  miles, 
but  at  Lake  Michigamme  it  broadens  out  into  an  area  5  miles  or  more  in  width,  from  which 
extend  the  Republic  and  southwestern  arms.  Beyond  the  limits  of  the  Marquette  district 
proper  the  formation  continues  to  widen  and  covers  a  great  expanse  of  country,  extending  to 
the  Crystal  Falls  district  on  the  south  and  well  toward  the  Gogebic  district  on  the  west.  It  is 
the  ecpiivalent  of  and  is  contmuous  with  the  slate  to  which  the  name  "Hanbury"  has  been 
given  in  previous  reports.  It  is  also  probably  the  equivalent  of  the  Tyler  slate  of  the  Penokee- 
Gogebic  district,  to  judge  from  its  relations  with  associated  formations  and  from  the  probability 
(indicated  by  known  outcrops)  of  direct  areal  connection,  though  outcrops  are  not  sufficient h' 
numerous  to  establish  this  connection  absolutely. 

Deformation. — The  Michigamme  slate  in  most  of  the  district  forms  a  great  synclinorium, 
the  secondary  folds  of  which  are,  however,  not  sufficiently  large  to  bring  up  the  lower  rocks 
to  the  erosion  surface  except  in  a  central  anticline  at  the  east  end  of  Lake  Michigamme, 
where  the  Bijiki  schist  and  Goodrich  quartzite  appear  at  the  surface. 

Litlwlogii. — The  formation  is  a  pelite,  which  now  comprises  two  main  ^•arieties — slates 
and  graywackes  and  mica  schists  and  mica  gneisses — each  of  which  includes  both  ferruginous 
and  nonferruginous  kinds.  The  slates  and  graywackes  occur  east  of  Lake  Michigamme  and 
the  mica  schists  and  mica  gneisses  at  Lake  Michigamme  and  to  the  west,  including  the  Repubhc 
and  southwestern  arms.  The  slates  and  graywackes  differ  from  each  other  chiefly  in  coarse- 
ness of  grain,  the  two  being  interlaminated  in  many  exposures.  There  are  all  gradations  from 
aphanitic  black  shales  or  slates  to  a  graywacke  so  coarse  as  to  approach  a  cpiartzite  or  even  a 
conglomerate.  In  color  the  rocks  vary  from  gray  to  black.  Where  fine  grained  they  have 
a  well-developed  slaty  cleavage.  In  places  they  are  graphitic,  pyritic,  and  ferruginous.  Two 
specimens  showing  the  maximum  amount  of  graphite  analyzed  15.69  and  18.92  per  cent  of 
carbon. 

The  slates  and  graywackes  differ  in  no  essential  respect  fi-om  the  similar  rocks  of  the  Siamo 
slate  (see  pp.  261-262)  or  from  the  Tyler  slate  of  the  Gogebic  district  (see  pp.  232-233),  there- 
fore they  will  not  again  be  described. 

MetamorpMsm. — The  slates  and  graywackes  by  increase  in  metamorphism  pass  into  chlo- 
rite schists,  mica  schists,  and  even  mto  mica  gneisses.  The  process  of  alteration  for  the  mica 
schists  is  identical  with  that  already  described  in  connection  with  the  development  of  similar 
rocks  for  the  Siamo  slate  and  the  Tyler  slate.  (See  pp.  232-233,  261-262.)  In  many  places 
where  the  rocks  are  completely  crystalline  garnet,  staurolite,  chloritoid,  and  andalusite  are  plenti- 
frdly  present.  In  the  more  coarsely  crystalline  rocks  much  feldspar  has  developed,  and  the  rock 
thus  becomes  a  gneiss.  This  material  appears  in  bands  which  seem  to  be  altered  beds  of  the 
formation  but  which  resemble  granitic  material.  The  appearance  is  that  of  a  rock  pegmatized 
throughout.     These  bands  grade  into  ordmary  mica  schists.     No  independent  granites  have 


268  GEOLOCxY  OF  THE  LAKE  SUPERIOR  REGION. 

boon  discovered  in  oonnoction  with  tliis  extremely  metamorphosed  variety  of  rock,  but  it  can 
not  be  asserted  that  such  rocks  are  not  somewhere  present.  Where  the  rocks  have  become 
schists  the  ferruginous  constituents  have  been  largely  transformed  to  magnetite. 

Relations  to  adjacent  formations. — The  Michigamme  slate  grades  downward  into  the  Bijiki 
schist  or  the  Goodrich  quartzite. 

TliicTcness. — Tlie  thickness  of  the  Michigamme  slate  is  considerable,  as  is  shown  by  the 
wide  area  which  it  covers.  There  are,  however,  so  many  subordinate  folds  and  the  mota- 
morphism  is  so  extreme  that  it  is  ini|)ossible  to  make  even  an  approximate  estimate  of  its 
thickness.  Within  the  area  described  the  thickness  of  the  formation  may  not  be  more  than 
1,000  or  2,000  feet,  or  may  be  greatly  in  excess  of  this. 

CLARESBXTIIG  FORMATION. 

Distribution. — ^The  Clarksburg  formation  differs  from  the  other  Algonkian  formations  of 
the  Marquette  district  in  that  it  is  dominantly  a  volcanic  formation.  It  is  confined  to  the 
south  side  of  the  Iluronian  area,  extending  from  the  region  north  of  Stoneville  to  a  point  some- 
what west  of  Champion,  the  largest  and  most  typical  areas  being  east  of  Clarksl)urg.  It  is 
clearly  a  local  formation,  not  only  in  its  eastern  and  western  extent  but  in  being  confined  tip 
one  side  of  the  district.  This  is  explained  by  its  volcanic  character,  the  vents  being  on  the 
south  border  of  the  Algonkian  area. 

Lithology. — Petrographically  the  formation  comprises  massive  greenstones  of  the  general 
character  of  diorites;  lavas  that  are  interbedded  with  sediments  and  tuffs;  tuffs  that  grade  off 
imperceptibly  into  sediments,  the  material  of  which  is  mainly  of  volcanic  origin;  and,  finally, 
greenstone  conglomerates  and  fine-grained  sediments,  the  material  of  which  is  mainly  volcanic 
but  has  evidently  been  arranged  by  water.  All  these  rocks  are  extremely  altered  and  in  places 
so  much  so  that  they  are  now  schistose.  The  pyroclastic  material  may  have  been  partly  sub- 
aerial,  but  doubtless  a  large  part  of  it  fell  upon  the  water.  The  volcanoes  of  Clarksburg  time 
were  very  plainly  of  explosive  type.  The  center  of  volcanic  activit}^  was  east  of  Clarksburg, 
and  m  this  vicinity  are  found  the  largest  amounts  of  massive  and  coarse  material,  lavas,  breccias, 
and  conglomerates.  Toward  the  east  and  west  the  formation  becomes  thinner  and  its  material 
finer,  imtil  it  dies  ovit  in  both  directions  into  the  Michigamme  slate. 

It  is  not  the  purpose  here  to  describe  in  detail  the  many  different  varieties  of  rocks  of 
this  volcanic  formation.  These  are  discussed  in  Monograph  XXVIII  of  the  United  States 
Geological  Survey."  This  volcanic  formation  is  similar  to  that  of  the  volcanic  formation  at 
tlie  east  end  of  the  Gogebic  district,  the  chief  difference  being  that  the  latter  is  much  less  meta- 
morphosed. It  is  notable  that  both  occur  in  the  upper  Iluronian  and  mainl}-  take  the  place 
of  the  great  upper  slate  formation  (Michigamme  slate),  although  the  beginning  of  the  volcanic 
outbreak  was  early  in  upper  Huronian  time  or  earlier.  In  the  eastern  part  of  the  district 
a  small  amount  of  volcanic  material  appears  also  to  be  associated  with  some  of  the  earlier 
formations,  especially  with  the  Siamo  slate. 

Eelations  to  adjacent  formations. — The  volcanic  outbreaks  of  the  Clarksburg  began  early  in 
Goodrich  time^  or  perhaps  even  in  late  Negaunee  time,  but  the  main  volcanic  deposits  were 
in  Michigamme  time.  Later  in  Michigamme  time,  by  the  dying  out  of  volcanic  activity,  the 
sediments  became  more  largety  ordmary  material,  and  thus  the  Clarksburg  grades  above  into 
the  Michigamme. 

Thicl-ncss. — There  is  no  way  to  ascertain  the  maximum  thickness  of  tiie  formation,  but 
east  of  Clarksburg  it  must  be  several  thousand  feet  thick.  From  this  maximum  it  ranges 
down  to  a  knife-edge. 

INTRUSIVE    IGNEOUS    UOCKS. 

Into  all  the  formations  of  the  Huronian  series  igneous  rocks  are  intruded.  These  are  of 
at  least  two  ages;  the  older  probably  belong  to  the  Huronian  and  the  later  to  the  Keweenawan 
period.     Much  the  larger  number  of  intrusive  masses  are  distinctly  of  post-Hin*onian  and 

1  Van  Hise,  C.  R.,  and  Bayley,  W.  S.,  The  Marquette  Iron-bearing  district  ot  Michigan:  Mon.  U.  S.  Geol.  Survey,  vol.  28, 1897,  pp.  4(»-186. 


MARQUETTE  IRON  DISTRICT.  269 

probably  Keweenawan  age.  Many  of  them  are  distinctly  bo.sses,  laccoliths,  and  sills  which 
in  their  upward  movement  have  been  stopped  by  the  massive  competent  layers  of  the  Negaunee 
or  Goodrich  qoartzite,  and  therefore  on  the  present  erosion  surface  are  likely  to  show  close 
areal  relations  with  the  Negaunee  formation.  This  is  especially  conspicuous  in  the  vicinity  of 
Ishpeming,  Negaunee,  and  Spurr. 

The  intrusive  rocks  have  been  described  by  various  authors  under  the  terms  diorite,  diorite 
schist,  chlorite  schist,  magnesian  schist,  soapstone,  and  paint  rock.  Part  of  them  have  been 
regarded  by  some  geologists  as  metamorphosed  sediments,  but  microscopical  study  of  all  the 
varieties  shows  that  they  were  originally  basic  rocks  of  the  composition  of  diabases.  The  great 
bosses  of  greenstone,  commonly  known  as  diorite,  are  a  prominent  feature  of  the  topography 
in  the  general  area  covered  by  the  iron-bearing  Negaunee  formation,  and  the  relations  of  these 
greenstones  to  the  genesis  of  the  ores  has  already  been  described.  During  the  folding  there 
was  much  differential  movement  between  the  greenstone  masses  and  the  surrounding  forma- 
tions, and  also  the  contact  plane  is  one  favorable  to  the  action  of  percolating  waters.  As  a 
result  of  this  it  is  a  common  thing  for  the  periphery  of  the  greenstone  knobs  to  be  schistose. 

In  the  area  around  Ishpeming  and  Negaunee  the  schistosity  has  obviously  been  the  result 
of  differential  movement  between  the  greenstones  and  the  overlying  Goodrich  quartzite.  The 
Goodrich  quartzite  has  moved  in  the  usual  direction  upward  along  the  limbs  of  the  folds, 
developing  cleavage  dipping  more  steeplj-  than  the  contact  of  the  greenstone  and  the  quartzite. 
Wliere  not  heavily  stained  by  iron  these  rocks  are  commonly  called  chloritic  schists.  Adjacent 
to  the  iron-bearing  formation  the  rocks,  besides  having  a  schistosity,  have  been  much  leached 
and  modified  m  composition  and  are  commonly  known  as  soapstones  because  of  their  greasy 
feel.  The  much-altered  greenstones  that  have  a  strongly  developed  schistosity  and  have  been 
stained  by  iron  oxide  are  called  paint  rock  by  the  miners.  Even  in  the  massive  varieties 
of  dikes,  laccoliths,  and  bosses  the  original  augite  has  extensively  changed  to  hornblende  and 
consequently  the  rock  in  the  district  has  generally  been  called  diorite. 

In  the  western  part  of  the  district,  both  within  the  intrusive  greenstone  masses  and  in 
the  adjacent  formations,  there  have  been  important  contact  effects.  This  is  shown  by  the 
extensive  development  of  garnet  m  both  the  intrusive  and  intruded  formations,  by  the  less 
common  development  of  biotite,  and  by  the  metamorphism  of  the  iron-bearing  formation 
mto  griinerite-magnetite  schist  and  of  the  Michigamme  slate  into  a  mica  schist.  Griinerite 
has  formed  to  some  extent  within  the  intrusive  rocks  also. 

The  intrusive  character  of  these  igneous  rocks  of  Huronian  age  is  shown  not  only  by  con- 
tact effects  but  by  the  manner  in  wliich  they  cut  across  the  bedding  of  adjacent  rocks  and 
project  dikes  into  them.  However,  evidence  of  this  kind  is  not  available  for  all  the  igneous 
masses,  especially  those  of  laccolithic  and  sheet  form,  and  it  is  regarded  as  not  at  all  unlikely 
that  some  of  them  may  be  really  extrusive  rocks  put  down  contemporaneously  with  the 
adjacent  sediments. 

The  latest  intrusive  rocks  are  fresh  diabase  dikes  which  are  probably  of  Keweenawan 
age.  They  cut  all  the  other  formations  of  the  district,  including  the  older  greenstones  wliich 
have  just  been  described.  These  rocks  include  diabase,  quartz  diabase,  olivine  diabase, 
porphyrites,  and  basalts. 

CAMBRIAN   SANDSTONE. 

Upper  Cambrian  or  Potsdam  sandstone  is  exposed  in  an  east-west  belt  along  Carp  River 
to  the  south  of  the  city  of  Marquette  and  Mount  Mesnard,  where  it  rests  unconformably  upon 
the  Kona  dolomite. 

QUATERNARY  DEPOSITS. 

The  district  is  more  or  less  covered  by  Pleistocene  deposits.  On  the  southeast  it  is  so 
thoroughly  covered  that  the  bed-rock  geology  is  not  well  known.  The  Pleistocene  is  discussed 
in  Chapter  XVI  (pp.  427-459). 


270 


GEOLOGY  OF  TPIE  LAKE  SUPERIOR  REGION. 


THE  IRON   ORES   OF  THE  MARQUETTE  DISTRICT. 

By  the  authnr-i  and  W.  J.  Mead. 
DISTRIBUTION,    STRUCTURE,    AND   RELATIONS    OF    ORE    DEPOSITS. 

The  cliief  iron-bearing  formation  ul  the  Marquette  district  is  the  Negaunee.  It  bears  ore 
at  various  horizons.  Ores  also  occur  at  tJio  basal  horizon  of  the  Goodricli  quartzitc,  where  it 
rests  upon  and  h:is  derived  debris  from  tlie  Negaunoe  formation.  Snitdl  cjuantities  of  oi-e  are 
found  in  tlio  iron  beds  of  the  Bijiki  schist,  associated  witli  the  Micliigiumue  slate.  Workable 
iron-oie  deposits  have  been  found  at  many  places  from  a  point  east  of  Negaimee  to  Michi- 
gamnie  and  Spurr.  The  Marquette  district  differs  from  the  ilesabi  and  Gogebic  districts  in  not 
having  long  stretches  of  nonproducing  iron-bearing  rocks. 

The  maximum  depth  of  concentration  of  ores  in  the  Marquette  district  is  still  imknown. 
On  the  Teal  Lake  range  the  depth  is  not  more  than  700  feet;  in  the  Ishpeming  and  Nogaunee 
areas  depths  as  great  as  1,500  feet  are  known.     In  the  Champion  area  ore  has  been  foUowed 


'ii  ore/i 

Figure  36,— Ore  deposits  of  the  Marqiiettfi  district.    (Both  ore  exploited  and  ore  now  in  mine  are  represented  as  ore,  as  the  purpose  of  this  figure 
is  to  show  themanner  of  the  development  of  the  ore  rather  than  the  present  stage  of  exploitation.) 

a,  Generalized  section  in  Marquette  district,  showing  relations  of  all  classes  of  ore  deposits  to  associated  formations.  On  the  right  is  soft  ore 
resl  ing  in  a  V-shaped  trough  between  the  Siamo  slate  and  a  dike  of  soapstone.  In  the  lower  central  part  of  the  figure  the  more  common  relations 
of  soft  ore  to  vertical  and  inclined  dikes  cutting  the  ja,sper  are  shown.  The  ore  may  rest  upon  an  inclined  dike,  between  two  inclined  dikes, 
and  upon  the  upper  of  the  two,  or  be  on  bothsidesof  a  nearly  vertical  dike.  In  the  upper  central  part  of  the  figure  are  seen  the  relations  of  the 
hard  ore  to  the  Negaunee  formation  and  the  Goodricjiquartzite.  .\t  the  left  is  soft  ore  resting  in  a  trough  of  soapstone  which  grades  downward 
into  greenstone.    (From  Mon.  U.  S.  Geol.  Survey,  vol.  28,  1S97,  PI.  XXVIII,  fig.  1.) 

6,  Cross  section  of  Section  lij  mine.  Lake  Superior  mines,  in  the  Marquette  district.  On  the  right  is  a  V-shaped  trough  made  by  the  junction 
of  a  greenstone  mass  and  a  dike.  The  hard  ore  is  between  theseand  below  the  Goodrich  quartzite.  On  the  left  the  hardoreagain  rests  upon  a 
soapstone  which  is  upon  and  contains  bands  of  ore-bearing  formation.  The  ore  is  overlain  by  the  Goodrich  quartzite.  Scale:  1  inch=220  feet. 
(From  Mod.  U.  S,  Geol.  Survey,  vol.  28, 1897,  PI.  XXIX,  fig.  1.) 

down  2,000  feet  and  is  laiown  to  extend  fartlier.  The  Negaunee  formation  constitutes  a  part 
of  the  westward-pitchmg  Marquette  trough,  and  west  of  Ishpeming  and  Xegaunee  the  central 
part  of  the  trough  goes  beneath  a  considerable  thickness  of  upper  Huronian  sediments.  Bectiuse 
of  this  dee])  burial  ))ut  little  drilling  has  been  done  to  ascertain  whether  or  not  the  ores  go  down 
here,  but  the  discovery  of  a  large  ore  deposit  at  the  very  bottom  of  the  Xegaunee  formation  near 
Negaunee  has  led  to  deep  drilling  west  of  Ishpeming  and  Negaunee  with  such  results  as  to  indi- 
cate that  the  ores  extend  to  unlooked-for  deptlis  in  this  direction. 

In  general  the  ores  come  to  the  rock  surface  along  the  middle  slopes  of  the  hill.s,  l)ut  they 


In  general  tlie  ores  come  to 
also  go  under  the  lowest  ground. 


MARQUETTE  IRON  DISTRICT.  271 

The  ore  deposits  of  the  Xegaunee  formation  and  the  associated  ores  may  be  divided,  accord- 
ing to  stratigraphic  position,  into  three  chisses — (1)  ore  deposits  at  the  bottom  of  the  iron- 
bearing  formation;  (2)  ore  deposits  within  the  iron-bearing  formation  (these  ores  in  many 
places  reach  the  surface  but  are  not  at  the  uppermost  horizon  of  the  formation);  (3)  ore 
deposits  m  the  top  layers  of  the  Negaunee  formation  and  the  bottom  layers  of  the  Goodrich 
quartzite.  (See  fig.  36.)  This  last  class  of  deposits  runs  past  an  unconformity.  Some  of 
these  ore  bodies  are  almost  wholly  in  the  Goodrich  quartzite.  Stratigraphically  these  deposits 
ought  to  be  separately  considered,  but  they  are  so  closely  connected  genetically  and  in  position 
with  the  Negaunee  ore  deposits  that  they  are  treated  with  the  deposits  of  that  formation.  The 
first  two  classes  of  ore  are  generally  soft,  and  the  adjacent  rock  is  ferruginous  chert  or  "soft- 
ore  jasper;"  the  deposits  at  the  top  of  the  iron-bearing  formation  are  hard,  specular  ores  and 
magnetite  and  the  adjacent  rock  is  jaspilite,  also  called  "  sjiecular  jasper"  and  "hard-ore 
jasper." 

Although  the  larger  number  of  ore  bodies  can  be  referred  to  one  or  another  of  the  three 
classes  above  given,  it  not  infrequently  happens  that  the  same  ore  deposit  belongs  partly 
in  one  and  partly  in  another.  Also  the  upper  part  of  an  ore  deposit  may  be  at  the  topmost 
horizon  of  the  iron-bearing  formation  and  be  a  specular  ore,  whereas  the  lower  part  may  lie 
wholly  within  the  iron-bearing  formation  and  may  be  soft  ore.  In  some  places  there  is  a  grada- 
tion between  the  two  phases  of  such  a  deposit,  but  more  commonly  the  two  bodies  are  sepa- 
I'ated  by  dikes,  now  changed  to  soapstone  or  paint  rock. 

1.  The  ore  deposits  at  the  bottom  horizon  of  the  Negaimee  formation  have  been  mined 
principally  where  the  lowest  horizon  of  the  formation  outcrops — that  is,  they  are  confined  to 
that  part  of  the  formation  resting  upon  the  Siamo  slate  or  the  Ajibik  quartzite,  along  the  outer 
borders  of  the  Negaimee  formation.  The  best  examples  of  these  deposits  are  those  occurring 
at  the  Teal  Lake  range  and  east  of  Negaunee.  East  of  Negaunee  the  ore  bodies  occur  at  places 
where  the  slate  is  folded  into  synclinal  troughs  which  pitch  sharply  to  the  west.  Here  the  iron- 
bearing  formation  is  in  places  cut  by  a  set  of  steep  vertical  dikes,  and  the  conjunction  of  these 
dikes  with  the  foot-wall  slate  forms  sharp  V-shaped  troughs,  as  in  the  Cleveland  Hematite  mine, 
where  the  ore  bodies  are  found  between  a  series  of  vertical  dikes  and  the  Siamo  slate.  By  com- 
paring this  occurrence  with  the  ore  deposits  of  the  Penokee-Gogebic  district,  it  will  be  seen  that 
they  are  almost  identical,  there  being  on  one  side  of  each  of  the  ore  bodies  an  impervious  dike, 
the  two  uniting  to  form  a  pitching  trough.  The  ore  deposits  of  this  horizon  are  being  found 
by  deep  drilling  to  be  extensive.  The  opening  of  the  Maas  mine  at  the  east  end  of  Teal  Lake 
and  the  discovery  of  ore  by  deep  drillmg  at  this  horizon  in  the  western  part  of  the  Ishpeming 
area  suggest  that  the  beds  of  this  horizon  at  gi'eat  depth  may  ultimately  be  foimd  to  carry  a 
larger  tonnage  of  ore  than  those  of  any  of  the  other  horizons. 

2.  The  typical  area  for  the  soft-ore  bodies  within  the  Negaunee  formation  is  that  of  Ish- 
peming and  Negaunee.  Here  are  the  Cleveland  Lake,  the  Lake  Angeline,  the  Lake  Superior 
Hematite,  the  Salisbury,  and  many  others.  The  large  deposits  rest  upon  a  pitching  trough 
composed  wholly  of  a  single  mass  of  greenstone  or  on  a  pitching  trough  one  side  of  which  is  a 
mass  of  greenstone  and  the  other  side  a  dike  joining  the  greenstone  mass.  The  underlying  rock 
is  called  greenstone  where  unaltered;  that  immediately  in  contact  with  the  ore  is  known  by  the 
miners  as  paint  rock  or  soap  rock  or  soapstone.  The  greenstone  changes  by  minute  gradations 
into  the  schistose  soapstone,  and  this  into  the  paint  rock.  Many  of  the  thinner  dikes  are  wholly 
changed  to  paint  rock  or  to  soapstone,  or  to  the  two  combinetl.  The  larger  number  of  these 
troughs  are  found  along  the  western  third  of  the  Ishpeming-Negaunee  area.  Plate  XVII  (in 
pocket)  shows  several  westward-opening  bays  occupied  by  the  iron-bearing  formation  in  the 
masses  of  greenstone.  Conspicuous  among  these  are  the  Ishpeming  basin,  the  northern  Lake 
Angeline  basin,  the  southern  Lake  Angeline  basin,  and  the  Salisbury  basin.  The  iron-formation 
embayments  open  out  and  pitch  to  the  west.  At  Lake  Angehne  an  eastward  dike  cuts  across 
the  basin  south  of  the  center,  and  this  combined  with  the  gi-eenstone  bluffs  to  the  north  and  to 
the  south  forms  two  westward-pitching  troughs,  the  northern  of  which  has  the  greatest  ore 
deposits  of  the  Marquette  district,  containing  many  millions  of  tons  of  ore. 


272  GEOLOGY  OF  THE  LAKE  SUPEKIOK  REGION. 

3.  The  hard-ore  bodies,  mainly  specular  hematite  but  in  some  deposits  including  nuuli 
magnetite,  are  at  the  top  horizons  of  the  iron-bearing  formation,  immediately  below  and  in  the 
basal  members  of  the  Goodrich  quartzite.  Examples  of  this  class  are  the  Jackson  mine,  the 
Lake  Superior  Specular,  the  ^'ohulteer,  the  Michigamme,  the  Kiverside,  the  Champion,  the 
Republic,  and  the  Barnum.  Also,  as  interesting  deposits,  giving  the  history  of  the  ore,  may 
be  mentioned  the  Klomau  and  the  Goodrich.  In  all  these  deposits  the  associated  rocks  of  the 
iron-bearing  formation  are  jaspilite  or  griinerite-magnetite  schist,  usually  the  former.  Many 
of  these  ore  deposits  weld  together  the  Goodrich  quartzite  and  the  Negaunee  formation  and 
can  not  be  separated  in  description.  As  in  classes  1  and  2,  all  the  large  ore  deposits  belonging 
to  this  third  class  have  at  their  bases  soapstone  or  paint  rock.  Wliere  the  soapstone  is  within 
the  Negaunee  formation  it  is  a  modified  greenstone  mass  or  this  in  conjunction  wdth  a  dike  or 
dikes.  Where  the  ore  deposits  are  largely  or  mainly  in  the  Goodrich  cjuartzite  the  basement 
rock  may  likewise  be  a  greenstone  or  it  may  be  a  layer  of  sedimentary  slate  belonging  to  tlie 
Goodrich  quartzite.  These  different  classes  of  rocks  are,  however,  not  discriminated  by  the 
miners,  but  are  lumped  together  as  soapstone  and  paint  rock.  Wlierever  the  deposits  are  of 
any  considerable  size  the  basement  rock  is  folded  into  a  pitching  trough,  or  else  an  impervious 
pitching  trough  is  formed  by  the  union  of  a  mass  of  greenstone  with  a  dike,  or  by  the  union 
of  either  one  of  these  with  a  sedimentary  slate.  Perhaps  the  most  conspicuous  example  of 
this  is  at  the  Repubhc  mine,  but  it  is  scarcely  less  evident  in  the  other  large  deposits.  A  few 
small  deposits  of  ore  (chimneys  and  shoots)  occur  at  the  contact  of  the  Negaunee  and  Goodrich 
formations,  where  no  basement  soapstone  has  been  found. 

As  examples  of  ore  deposits  which  are  largely  or  wholly  witliin  the  Goodrich  quartzite 
may  be  mentioned  the  Volunteer,  Michigamme,  Champion,  and  Riverside.  These  are  partly 
recomposed  ores  and  differ  in  appearance  from  the  specular  hematite  or  magnetite  of  the 
Negaunee  formation  in  having  a  peculiar  gray  color  anil  in  containing  small  fragmental  particles 
of  quartz  and  complex  pieces  of  jasper;  in  many  of  them  also  sericite  and  chlorite  are  discovered 
with  the  microscope. 

Ore  deposits  in  the  Bijiki  schist,  associated  with  the  Michigamme  slate,  have  slate  as  foot 
and  hanging  walls.     They  are  illustrated  by  the  Beaufort,  Bessie,  Ohio,  and  Imperial  mines. 

Although  these  different  classes  of  ore  bodies  have  the  distinctive  features  indicated  above, 
they  have  important  features  in  couunon.  They  are  confined  to  the  iron-bearing  formations. 
They  occur  upon  impervious  basements  in  pitching  troughs.  The  impervious  basement  may 
be  a  sedimentary  or  an  igneous  rock,  or  a  combination  of  the  two.  Wliere  the  ore  deposits 
are  of  considerable  size  the  plication  and  brecciation  of  the  chert  and  jasper  are  usual  phe- 
nomena. In  many  places  this  shattering  was  concomitant  wth  the  folding  into  troughs  or 
with  the  intrusion  of  the  igneous  rocks. 

In  any  of  these  classes  the  deposits  may  be  cut  into  a  number  of  bodies  by  a  combination 
of  greenstone  dikes  and  masses.  A  deposit  which  in  one  part  of  the  mine  is  continuous  may 
in  another  part  of  the  mine  be  cut  into  two  deposits  by  a  gradually  projecting  mass  of  green- 
stone which  passes  into  a  dike,  and  each  of  these  may  be  again  dissevered,  so  that  the  deposit 
may  be  cut  up  into  a  numl)(>r  of  ore  bodies  separated  by  soapstone  and  paint  rock.  Some  of 
the  ore  deposits  have  a  somewhat  regular  form  from  level  to  level,  but  the  shape  of  the  deposits 
at  the  next  lower  level  can  never  be  certainly  predicted  from  that  of  the  level  above.  Horses 
of  "jasper"  may  appear  along  the  dikes  or  within  an  ore  body  at  almost  any  place.  The  ore 
bodies  grade  al)()ve  and  at  the  sides  into  the  jasper  in  a  variable  manner.  As  a  result  of  the 
comljination  of  these  uncertain  factors,  most  of  the  ore  bodies  have  extraordinarily  irregular 
and  curious  forms  when  examined  in  detail,  although  in  general  shape  they  conform  to  the 
above  descriptions. 


MARQUETTE  IRON  DISTRICT. 


273 


CHEMICAL  COMPOSITION  OF  MARQUETTE  ORES. 

The  following  average  partial  analyses  were  calculated  from  cargo  analyses  in  shipments 
for  1906  and   1909: 

Arcrar/c  partial  analyses  of  Marquette  orcsfi  calculated  from  cargo  analyses  for  1906  and  lS09.b 


Composition  of  ore  dried  at  212°  F. 

Loss  on 

of  total 
pro- 

Fe. 

P. 

SiOj. 

-\IjO3. 

ignition. 

duction. 

100.0 

59.55 

0. 107 

8.21 

2.28 

1.66 

100.0 

57.05 

.  105 

10.16 

2.18 

2.31 

21.  S 

59.60 

.078 

8.47 

2.13 

.57 

37.0 

61.40 

.094 

6.40 

2.34 

2.61 

39.5 

59.20 

.082 

S.U 

2.54 

2.20 

2.0 

53.70 

.290 

11.30 

1.17 

7.05 

Moisture 
(loss  on 
drvinsat 
212°  F.). 


Average  of  entire  district: 

1906 

1909 

Upper  horizon.  1906 

Middle  horizon,  1906 

Lower  horizon,  1906 

Bijiki  formation,  1906- . . . 


9.04 
9.  .52 
1.24 
11.75 
11.32 
8.30 


a  Including  ores  of  Swanzy  district.  ^  Calculated  from  analyses  from  I^ake  Superior  Iron  Ore  Association  booklet. 

In  addition  to  the  constituents  listed  above  the  ores  contain  small  amounts  of  manganese, 
lime,  magnesia,  sulphur,  soda,  and  jjotassa.  The  range  for  the  various  constituents  of  the  ores 
as  shown  by  average  cargo  analyses  for  1906  and  1909  is  as  follows: 

Range  of  percentage  of  each  constituent  in  the  Marquette  ores  for  1906  and  1909.'^ 


Moisture  (loss  on  drying  at  212°  F.) . 
Analysis  of  dried  ore  : 

Iron 

Phosphorus 

Silica 

Alumina 

Manganese 

Lime 

Magnesia 

Sulphur 

Loss  by  ignition 


0.51  to  14. 33 


0.50  to  15.  75 


90  to  64. 61 
029  to   .402 
21  to  34.  20 


.09 
.04 
.18  to 
.09  to 
.004  to 
.18  to 


6.26 
2.72 
2.00 
1.18 
.062 
7.07 


40. 20  to  05.  69 
.018  to   .387 
3.25  to  40.  77 
to 
to 
to 
to 


.42 
.00 
.00 
.00 


.003  to 


4.32 
2.78 
2.09 
.039 


.10    to  11.40 


a  Calculated  from  analyses  from  Late  Superior  Iron  Ore  Association  booklet. 

The  magnetites  do  not  differ  essentially  m  composition  from  the  dommant  hematites  and 
limonites  except  m  having  less  water. 

CHEMICAL   COMPOSITION  OF   IRON-BEARING   NEGAUNEE  FORMATION. 

An  average  of  1,727  analyses  representing  11,025  feet  of  drilling  from  the  district  away  from 
the  available  ores  gives  35.12  per  cent  of  iron.  This  includes  both  the  lean  jaspers  and  the  partly 
altered  jaspers,  but  not  the  ores.  Because  of  their  great  mass  compared  with  the  ores,  this 
figure  represents  nearly  the  general  average  composition  of  the  entire  formation.  If  the  unal- 
tered jaspers  alone  are  taken,  the  average  is  somewhat  lower. 

The  composition  of  a  typical  amphibole-magnetite-quartz  rock  is  as  follows: 

Average  analysis  of  griinerite-magnetite  schist." 


Loss 1.  03 

SiO, 50.02 

AUOj 97 

Fe^Oj 10. 05 

Feb 28.  29 

MnO 74 

CaO 2.63 

MgO 4.13 


CuO Trace. 

Na,0 

P265 

CO2 

H.^6  (above  110°) 


0 

0,S 

09 

1 

55 

42 

100.  00 
Total  Fe 29.  20 


It  mil  be  noted  that  this  differs  but  little  from  the  average  composition  of  the  jaspers. 


TrMT" 


4751 


a  Calculated  from  analyses  given  in  Mon.  U.  S.  Geol.  Survey,  vol.  28, 1897,  p.  338. 
-VOL  52—11 IS 


274  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

MINERAL   COMPOSITION   OF   MAKQUETTE   ORES. 

The  ores  of  the  Marquette  district  are  doiiiinantly  hydrous  lieniatites  and  sulxirdinately 
anhydrous  specular  hematites  and  magnetites.  Owing  to  the  presence  of  magnetite,  tlie  mineral 
conijjosition  can  not  be  calculated  fi-om  analyses  in  which  ferrous  and  ferric  iion  are  not 
separated. 

The  coarse  specular  hematites  are  made  up  mainly  of  large,  closely  fitting  flakes  of  hematite, 
most  of  which  take  an  imperfect  polish  and  have,  therefore,  a  gray,  sheeny,  sj)otted  appearance. 
The  flakes,  which  are  parted  along  the  cleavage,  reflect  the  light  hke  a  mirror.  The  large 
number  of  individuals  of  this  kind  is  appreciated  only  by  rotating  the  sections  under  the  micro- 
scojje.  This  In'ings  successively  different  flakes  of  hematite  into  favoi-able  positions  to  reflect 
the  light  into  the  microscope  tube.  In  some  sections  cut  transverse  to  the  cleavage  the  scliistose 
character  of  the  rock  is  apparent  in  reflected  light,  innumerable  laminae  of  hematite  giving  fine, 
narrow,  parallel  dark  and  light  bands,  which  are  comparable  in  appearance  to  the  ])oIysynthetic 
twummg  bands  of  feldspar.  As  both  the  magnetite  and  the  hematite  are  usuafly  opaque,  the 
two  minerals  in  general  can  not  be  discriminated,  although  in  some  sections  the  crystal  forms  of 
magnetite  are  seen  and  a  small  part  of  the  hematite,  much  of  it  in  little  crystals,  shows  the 
characteristic  blood-red  color.  The  important  accessory  minerals  are  quartz,  griinerite,  feldspar, 
and  muscovite.  Some  of  the  small,  detached  areas  of  cjuartz  and  feldspar  appear  to  be  frag- 
mental.  The  muscovite  occurs  mainly  in  small,  independent  flakes,  but  some  of  it  is  apparently 
secondary  to  the  feldspar. 

The  fine-grained  specular  hematites  dift'er  from  the  so-caUed  micaceous  hematites  chiefly 
in  that  much  more  of  the  hematite  is  translucent  and  hence  at  the  edges  and  in  sjiots  in  the 
slides  is  of  a  l)rilliant  red  color.  The  "slate  ores"  m  reflected  light  show  the  laminated  character 
of  the  rock,  while  the  massive  ores  give  the  peculiar  spotty  reflections,  exactly  the  same  as 
magnetite. 

The  mottled  red  and  black  specular  ores  in  reflected  hght  present  a  pecuhar  appearance,  the 
true  specular  material  giving  the  usual  briUiant,  spotty  reflections,  whereas  the  soft  hematite 
has  a  brownish-red  color. 

The  soft  hematites  in  transmitted  light  show  in  many  slides  the  characteristic  blood-red 
color  of  hematite,  although  for  the  most  part  the  sections  are  so  thick  as  to  give  a  brownish 
appearance  or  are  ojjaque.  In  the  softest  ores  m  reflected  light  a  dark  brownish-red  color  is 
every^vhere  seen,  which  is  much  less  lirilliant  than  that  presented  bj'  the  same  mmeral  in  trans- 
mitted light.  In  some  of  the  soft  hematites,  however,  within  the  mass  of  red  material  are 
many  small  areas  which  reflect  the  light  m  the  same  maimer  as  the  specular  ores.  The  limonitic 
hematites  differ  from  the  pure  hematites  onlj^  in  that,  m  both  transmitted  antl  reflected  hght, 
in  many  places  the  reddish  colors  are  not  so  bright. 

Lender  the  microscope  the  magnetites  are  opaque  m  transmitted  light ;  in  reflected  Ught 
they  give  the  characteristic  spotty  appearance  of  that  mineral.  Where  not  pure  the  usual 
mmerals  contamed  in  the  iron  formation  appear  with  their  ordmary  relations.  Those  most 
plentifully  seen  are  quartz,  griinerite,  muscovite,  and  biotite.  Here  and  there  garnet  and 
chlorite  as  an  alteration  pi'oduct  are  alnmdant.  On  the  borders  of  the  mcluded  material  the 
magnetite  invariably  shows  crystal  outlmes.  As  a  result  each  area  of  included  mmerals  has  a 
serrated  form.  With  the  magnetite  there  is  always  more  or  less  of  hematite,  a  large  part  of 
wiiicii  in  many  places  results  from  the  alteration  of  the  magnetite.  The  liematite  ranges  from  a 
subordinate  to  an  important  amount.  Also  at  many  places  with  the  magnetite  are  varying 
cjuantities  of  pyrite  and  garnet  and  alteration  jiroducts  of  the  latter,  chlorite  and  amphibole. 
The  magnetites  range  in  color  from  ])lack  to  gray. 

PHYSICAL  CHARACTERISTICS  OF  MARQUETTE  ORES. 

The  magnetites  and  specular  hematites  are  called  hard  ores  b}'  the  miners,  and  tiie  iivtirous 
red  hematites  are  called  soft  ores.  The  magnetites  range  from  very  coarsely  granular  to  finely 
granular  magnetite. 


MAKQUETTE  IRON  DISTRICT.  275 

As  the  ores  are  made  up  essentially  of  ii'on  minerals  and  quartz,  the  mineral  density  varies 
directly  with  the  iron  content,  ranging;;  from  as  high  as  5.1  in  some  of  the  dense  hard  ores  to  as 
low  as  3.5  m  some  of  the  low-grade  limonitic  ores.  Owing  to  the  witle  variation  iji  the  mineral 
composition  of  the  ores,  an  average  figure  for  the  district  would  have  no  significance.  The 
avei'age  density  of  fhe  soft  hematites,  calculated  from  the  1906  cargo  analyses,  is  4.14. 

The  porosity  varies  from  less  than  1  per  cent  m  the  hard  specular  ores  to  over  40  per  cent 
in  the  limonitic  ores.  The  average  moisture  content  of  the  ores  of  the  middle  horizon  indicates 
a  porosity  of  approximately  35  per  cent,  assuming  the  mmeral  density  to  be  4.14.  This  is 
probal)ly  n(jt  far  from  the  true  figure. 

The  number  of  cubic  feet  per  ton  varies  from  7  in  the  pure  hard  hematites  to  as  high  as 
14.5  m  the  limonitic  ores.  The  average  for  the  soft  red  hematites  is  approximately  11.9  cubic 
feet  per  ton,  calculated  from  a  mineral  density  of  4.14,  a  porosity  of  35  per  cent,  and  a  moisture 
content  of  11.75  per  cent. 

The  following  table,  showing  an  average  of  a  number  of  screening  tests  on  the  soft  ores  of 
the  Marquette  district,  gives  a  good  idea  of  the  average  texture  of  these  ores.  A  comparison 
of  the  textures  of  the  ores  of  the  several  Lake  Superior  districts  is  shown  in  figure  72,  page 
481.  The  screening  tests,  of  which  the  following  is  an  average,  were  made  by  the  Oliver 
Iron  Mining  Company  on  11  typical  grades  of  ore  mined  in  the  Marc|uette  district  in  1909  and 
aggregating  a  total  of  746,779  tons.  For  each  grade  of  ore  tested  a  sample  was  taken  biweekly, 
quartered  down  monthly  in  proportion  to  the  number  of  tons  mined,  and  at  the  end  of  the  year 
quartered  down  to  100  pounds,  dried,  and  tested.  The  average  was  obtained  by  combining 
the  results  of  the  11  screening  tests  in  proportion  to  the  number  of  tons  represented  by  each 
of  the  11  grades. 

Composite  of  screening  tests  on  typical  soft  ores  of  the  Marquette  district. 

Per  cent. 

Held  on  J-inch  sieve 28. 15 

^-inch  sieve 42.  22 

No.  20  sieve 10.  98 

No.  40  sieve 4.  90 

No.  60  sieve 2.  90 

No.  SO  sieve 1.  23 

No.  100  sieve 1.  15 

Passed  through  No.  100  sieve 7. 19 

SECONDARY  CONCENTRATION  OF  MARQUETTE  ORES. 

Structural  conditions. — The  structural  conditions  controlling  the  circulation  of  water  in 
the  Marquette  district  are  various.  At  the  lower  horizons  of  the  Negaunee  formation  the 
impervious  basement  is  formed  by  the  pitching  folds  of  the  Sianio  slate,  as  on  the  Teal  I^ake 
range.  At  the  middle  and  upper  horizons  of  the  Negaunee  formation  the  irregular  bosses  and 
intnisive  masses  of  greenstone  constitute  impervious  basements  in  the  reentrants  of  which  the 
ores  are  found.  The  greenstone  and  its  altered  form,  soapstone,  accommodated  themselves 
to  folding  without  extensive  fractures  and,  while  probabh"  allowing  more  or  less  water  to  pass 
through,  acted  as  practically  impervious  masses  along  which  water  was  deflected  when  it  came 
into  contact  with  them.  It  is  a  common  opinion  among  miners  that  a  few  inches  of  soap  rock 
is  more  effective  in  keeping  out  water  than  many  feet  of  the  iron-bearing  formation.  On  the 
other  hand  the  brittle  siliceous  ore-bearing  formation  was  fractured  by  the  folding  to  which 
it  was  subjected,  so  that  where  this  process  was  extreme  water  passes  through  it  as  through  a 
sieve.  It  is  evident  that  the  tilted  bodies  of  greenstone,  or  soap  rock,  especially  those  that  occur 
in  pitching  S3'nclines  or  that  form  pitching  troughs  bj^  the  union  of  dikes  and  masses  of  green- 
stone, must  have  converged  downward-flowing  waters.  It  is  also  clear  that  the  weak  contact 
plane  between  the  Goodrich  quartzite  and  the  Negaunee  formation  was  one  of  accommodation 
and  shattering,  favorable  for  the  free  movement  of  waters.  Finally,  the  ores  in  the  Bijiki  schist 
of  the  upper  Iluronian  have  been  developed  by  the  percolation  of  waters  along  impervious 
slate  basements  with  which  the  Bijiki  schist  has  been  folded. 


276 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


Chemical  and  mineralogical  changes  in  secondary  concentration  of  Marquette  ores. — The  soft 
ores  and  the  associated  ferruginous  cherts  of  the  middle  and  lower  horizons  of  tlie  Negaunee 
formation  arc  similar  physically,  chemically,  and  mineralogically  to  the  ores  of  the  Penokec- 
Gogebic  district.  They  are  derived  by  the  same  processes,  under  similar  conditions,  from 
cherty  iron  carbonate  rocks  which  arc  practically  identical  with  those  of  that  district. 

The  hard  ores  have  undergone  not  only  this  change  but  the  additional  anamorphic  changes 
of  deep  bu-rial  antl  igneous  intrusion,  the  result  being  that  the  hard  ores  differ  from  the  soft 
ores  chemically  only  in  that  they  have  less  water  and  a  little  less  oxygen,  mincralogicallv  in 
that  they  have  developed  in  them  certain  anhydrous  silicates  and  some  magnetite,  and  tex- 
turally  in  that  they  are  coarsely  crystalline  and  in  places  schistose.  To  some  slight  extent 
also  similar  hard  ores  may  have  been  developed  directly  from  the  original  cherty  iron  carbon- 
ates by  deep  burial  or  igneous  contact  action,  but  it  is  shown  elsewhere  that  such  action  usually 
results  in  lean  silicated  iron-bearing  rock  rather  than  in  rich  ore  bodies.     The  associated  ferru- 


Quartz 
(chert) 

V 

N 

S 

.     s 

V        N 
S        N 

\ 

Pore  space, 

slump, 

secondary  iron 

oxide  and  silica 

{relative 

proportions  not 

known  ) 

I>or.^.  [>,-.<■. 

Ro^uit,I,^■fr..n, 

SoIUtl'jn  -.'f  ;lllL.i 

and  reduction 

in  volume  of  iron 

mineral 

\ 
\ 

\ 
s 

Quartz 
(chert) 

\ 

N 

Aluminum  silicate 

Quartz 

JS Fore  space 

QuarU 

Iron 

carbonate 

Kaolin 

Secondary  iron 
oxide  replacing 
iron  carbonate- 

Amphibole 

Hematite 
dehydrated 
by  pressure 

Aluminum  silicate 

Hydrated 

iron  oxide 

from  oxidation 

of  iron  carbonate 

Hydra  ted 

iron  oxide 

from  oxidation 

of  iron  carbonate 

Sideritic 
chert 


Ferruginous 
chert 


Soft  ore 


Hard  ore 


FiGUKE  37.— Graphic  representation  of  the  volume  composition  of  the  principal  phases  of  the  iron-bearing  Negaimee  formation,  showing  the 
changes  in  volume  and  mineral  composition  involved  in  the  concentration  of  the  ores  from  the  cherty  siderite  and  the  production  of  hard  ore 
from  soft  ore  by  dynamic  agencies. 

ginous  cherts  or  soft-ore  jaspers  umlergo  similar  changes  so  far  as  the  iron  oxide  laj-ers  are 
concerned.  The  chert  beds  are  recrystallized,  but  not  othermse  changed.  The  result  is  a 
hard-ore  "jasper  or  jaspilite  differing  from  the  ferruginous  cherts  in  being  more  crystalline, 
having  less  pore  space,  and  being  less  hj'drated,  and  accorduigly  having  red  rather  than  yellow 
or  brown  colors. 

Volume  changes  in  secondary  concentration  of  Marquette  ores. — The  volume  changes  in  the 
concentration  of  the  ores  and  the  development  of  the  hard  ores  are  shown  in  figure  37.  The 
volume  composition  of  the  four  phases  of  the  iron-bearing  formation  is  represented,  thus 
permitting  a  consideration  of  porosity  as  well  as  mineral  composition.  The  mineral  compo- 
sition of  the  sideritic  chert  is  calculated  from  a  typical  analysis."  The  mineral  composition  of 
the  ferruginous  chert  is  calculated  from  the  sideritic  chert  analysis,  allowing  for  oxidation  of 
the  iron  mineral.  The  result  is  about  an  average  for  ferruginous  cherts,  as  shown  by  analyses. 
The  indicated  volume  compositions  of  the  soft  and  hard  ores  represent  actual  average  partial 
analyses  of  all  ore  as  mined  and  averages  of  porosity  determinations. 

\^^len  subjected  to  oxidizing  solutions,  the  siderite  of  the  chert}''  siderite  is  oxidized  to  a 
more  or  less  hydrated  iron  oxide,  involving  a  considerable  reduction  in  volume  (see  Gogebic 
discussion,  pp.  242  et  seq.)  ranging  from  49.25  per  cent  when  the  product  is  hematite  to  18.3 
per  cent  when  limonite  is  produced.  If  no  iron  were  introduced,  the  actual  amount  of  oxide 
resulting  would  be  intermediate  between  these  two  figures  and  j)robably  would  not  tlitl'er  greatly 

o  Mon.  U.  S.  Geol.  Survey,  vol.  28, 1897,  p.  337,  second  analysis. 


MARQUETTE  IRON  DISTRICT. 


277 


from  the  hyclrated  oxide  of  the  soft  ores,  which  is  represented  by  a  ratio  of  hematite  to  Umonite 
of  7  to  1.  Even  if  a  considei'able  amount  of  iron  were  introduced,  the  resulting  rock  would  be 
banded  ferruginous  chert  having  a  larger  pore  space  than  the  original  cherty  siderite.  The 
reduction  in  volume  of  iron  mineral  accompanying  the  alteration  of  the  carbonate  is  partly 
compensated  by  several  factors,  the  relative  importance  of  which  is  not  known — by  mechanical 
slump  and  by  the  introduction  of  secondary  iron  oxide  and  quai'tz. 


IRON    MINERALS 


High-grade  hard  oie 


Low-grade  hard  ore 


Soft  ore 


SILICA 


PORE   SPACE 


Figure  38. —Triangular  diagram  showing  the  volume  composition  of  the  several  grades  of  ore  mined  in  the  Marquette  district  in  1900,  in  terms  of 
pore  space,  iron  minerals,  and  silica.  The  altitudes  of  the  small  triangles  show  in  each  case  the  amount  of  minor  constituents  (amphibole, 
clay,  etc.) 

The  development  of  ore  from  the  sideritic  chert  involves,  in  atldition  to  the  oxidation  of 
the  iron  in  place,  the  removal  in  solution  of  a  considerable  amount  of  quartz.  This  gives  a 
still  larger  pore  space,  which  agam  is  partly  compensated  by  slump  and  by  infiltration  of  iron. 
Observation  shows  that  the  oxidation  of  the  iron  carbonate  in  place,  producing  ferruginous 
chert,  mainly  precedes  the  removal  of  the  larger  amount  of  silica.  The  oxidation  of  the  iron 
is  chemically  more  readily  accomplished  than. the  solution  of  silica;  and,  further,  the  conse- 
quent development  of  pore  space  affords  opportunity  for  more  abundant  flow  of  solution  to 
accomplish  the  solution  of  silica.  When  the  passage  of  the  ore  bodies  into  the  chert  or  jasper 
is  examined  in  detail  it  is  found  that  a  siliceous  band,  if  followed  toward  the  ore,  instead  of 
remaining  solid  becomes  porous  and  may  contain  considerable  cavities.     These  places  in  the 


278  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

transition  zone  are  lined  with  iron  oxide.  In  passing  toward  the  ore  deposit  more  and  more  of 
the  silica  is  found  to  liavc  been  removed,  and  iron  oxide  has  partly  replaced  it.  An  exami- 
nation at  many  of  the  localities  shows  this  transition  from  the  l)andcd  ore  and  jasper  to  take 
place  as  a  consequence  of  the  removal  of  the  silica  and  the  partial  substitution  of  iron  oxide. 
In  many  such  instances  the  fine-grained  part  of  the  ore  is  that  of  the  original  rock,  and  the 
coarser  crystalline  material  is  a  secondary  infiltration.  It  is  not  uncommon,  however,  for  the 
ore  deposits  to  terminate  abruptly  along  joint  cracks  or  fractures. 

The  solution  of  quartz  and  the  introduction  of  iron  oxide  ultimately  produce  the  soft  ores 
from  the  ferruginous  ciierts.  These  soft  ores,  as  the  diagram  shows,  have  an  average  porosity 
of  about  36  per  cent  and  are  made  up  essentially  of  hydrated  iron  oxide,  quartz,  and  cla\-.  The 
iron  oxide  largely  represents  siderite  oxidized  in  place,  but  partly  represents  iron  secondarily 
introduced. 

The  development  of  the  hard  ores  is  accomplished  by  pressure  or  igneous  contact  action  on 
the  soft  ores,  causing  a  reduction  in  volume  of  approximately  40  per  cent  or  less,  by  decreasing 
the  porosit}^,  dehydrating  the  iron  oxide,  and  developing  some  magnetite  and  certain  meta- 
morphic  ferromagnesian,  aluminum-bearing  minerals,  such  as  amphil)ole  and  garnet. 

Representation  of  ores  and  jaspers  on  triangular  diagram. — The  volume  compositions  of  the 
various  phases  of  the  iron-bearing  formation  are  represented  in  the  triangular  diagram,  figure  38. 
(For  explanation  see  p.  189.)  The  lines  of  demarcation  between  the  hard  and  soft  ores  and 
between  the  low  and  high  grade  hard  ores  are  not  as  sharp  as  the  grouping  of  the  small  triangles 
would  indicate.  Tyj)ical  specimens  of  each  grade  were  selected  and  intermediate  phases  were 
neglected.  If  all  phases  were  represented  the  entire  upper  corner  of  the  large  triangle  would 
be  covered  with  small  ones,  indicating  complete  gradation  between  the  various  classes  of  ore. 

SEQUENCE  OF  ORE  CONCENTRATION  IN  THE  MARQUETTE  DISTRICT. 

1.  The  alteration  of  the  Negaunee  formation  began  before  upper  Huronian  time,  when 
the  formation  had  been  slightly  folded,  eroded,  and  intruded  by  igneous  rocks.  Prior  to  upper 
Huronian  time  all  the  phases  of  the  iron-bearing  formation  now  known,  except  the  specular 
hematites,  had  been  developed,  for  all  of  them  appear  as  pebbles  in  the  basal  conglomerate  of 
the  upper  Huronian,  and  it  is  unhkely  that  such  closely  intermingled  diversity  of  pebbles  could 
have  been  developed  from  a  single  type  of  iron-bearing  material  after  it  had  been  deposited  as 
pebbles  in  the  conglomerate  at  the  base  of  the  upper  Huronian.  Erosion  was  not  deep,  and 
ores  seem  to  have  been  developed'  only  near  the  erosion  surface  which  bevels  at  a  low  angle  the 
upper  beds  of  the  Negaunee  formation  and  now  constitutes  the  horizon  exposed  nearest  to  the 
overlying  upper  Huronian  conglomerate.  That  ores  M'ere  formed  at  this  time  and  place  is 
indicated  by  the  fact  that  at  this  horizon  occur  specular  hematites  having  a  secondary  cleavage 
developed  during  the  folding  which  followed  the  deposition  of  the  upper  Huronian  and  which 
preceded  the  second  great  period  of  ore  concentration. 

2.  Inter-Huronian  alteration  of  the  formation  was  inten-upted  b}'  the  deposition  of  the 
upper  Huronian  (Animikie  group),  the  base  of  which  was  made  up  of  conglomerate  carrving 
fragments  of  ferruginous  chert  and  iron  ore  derived  from  the  Negaunee  formation.  A  higher 
formation  (Bijiki  schist)  contained  iron  carbonate. 

3.  The  deposition  of  the  upper  Huronian  was  followed  by  severe  folding  and  both  intrusion 
and  extrusion  of  the  basic  igneous  rocks.  Much  of  the  intrusion  preceded  the  folding,  for  the 
cleavage  in  the  sedimentary  beds  developed  during  the  fokling,  and,  having  an  attitude  deter- 
mined by  the  differential  movement  between  the  folds,  affects  also  the  intrusive  rocks.  Many 
of  these  post-upper  Huronian  (Keweenawan)  intrusive  rocks  are  now  found  in  the  area  of  the 
Negaunee  formation.  It  is  certain  that  some  of  them — as,  for  instance,  those  in  the  vicinity  of 
Michigamme — represent  laccolithic  masses  which  were  unable  to  penetrate  above  the  massive 
Goodrich  quartzite  and  spread  out  in  the  upper  portion  of  the  Negaunee  formation.  The 
intrusion  and  folding,  with  varying  relative  efi'ectiveness  in  ilifferent  parts  of  the  range,  anamor- 


MARQUETTE  IRON  DISTRICT. 


279 


pliosed  the  iron-bearing  formations,  but  witli  widely  differing  results,  depending  on  the  condi- 
tions of  the  iron  formation  before  the  anamorphism.  The  ferruginous  cherts  and  ores  of  the 
upper  horizons  of  the  Negaunee  formation  were  changed  to  hard  hematites  and  jaspers,  becom- 
ing specular  when  folded.  The  iron-bearing  conglomerate  at  the  base  of  the  Goodrich  quartzite 
was  similarly  affected.  The  iron  carbonate  of  the  Bijiki  schist  of  the  upper  Huronian  was 
changed  into  a  coarsely  crystalline  amphibole-magnetite  rock.  Portions  of  the  formations 
farther  removed  from  the  intrusive  rocks  were  less  anamorphosed.  These  would  include  the 
part  of  the  Bijiki  schist  near  the  Bessie  mine  and  the  lower  part  of  the  Negaunee  formation, 
both  of  which  up  to  this  time  still  remained  as  iron  carbonate. 

Post-Keweenawan  erosion  exposed  all  phases  of  the  iron-bearing  Negaunee  formation, 
together  with  the  ferruginous  detrital  base  of  the  upper  Huronian  and  the  still  unaltered  car- 
bonates higher  in  the  upper  Huronian.  The  iron  carbonates,  both  of  the  lower  parts  of  the 
Negaunee  formation  and  of  the  Bijiki  schist,  now  for  the  first  time  exposed,  became  altered  in 
the  ordinary  manner,  producing  soft  ores  associated  with  soft  ferruginous  cherts,  now  found 
typically  along  the  Teal  Lake  range  and  in  the  Bessie  mine  of  the  western  Marquette  district. 
The  other  phases  of  the  Negaunee  formation,  which  had  been  previously  altered  to  chert,  jasper 
or  iron  ore,  or  amphibole-magnetite  rocks,  were  also  attacked  to  some  extent,  principally  by  the 
leaching  of  siHca,  which  can  be  conspicuously  observed  in  the  loss  of  chert  pebbles  from  the 
conglomerate  at  the  base  of  the  upper  Huronian,  and  by  alteration  of  garnets  and  amphibole  to 
chlorite.  The  total  effect  of  the  alteration  at  this  time  on  these  harder  phases,  however,  was 
probably  not  so  essential  in  the  concentration  of  the  ore  deposits  as  that  which  had  gone  on 
before. 

The  great  varieties  of  phases  of  the  iron-bearing  rocks  of  the  Marquette  district  are  therefore 
the  results  of  katamorphic  and  anamorphic  processes  described  in  earlier  pages,  acting  alone  or 
successively  on  different  parts  of  the  iron-bearing  formations. 

OCCURRENCE  OF  PHOSPHORUS  IN  THE  MARQUETTE  ORES. 

DISTRIBUTION    OF    PHOSPHOKUS. 

The  ores  of  the  Marquette  range  are  as  a  whole  higher  in  phosphorus  than  those  of  the 
Vermilion,  Mesabi,  or  Gogebic  districts.  They  also  show  a  greater  range  in  phosphorus  content 
than  the  ores  of  any  of  these  three  districts.  Of  the  total  shipments  of  ore  from  the  Marquette 
range  in  1906  approximately  18  per  cent  was  of  Bessemer  grade.  The  lowest  phosphorus  grade 
was  Sheffield  (Fe  =  64.61,  P  =  0.029,  P/Fe  =  0.000448),  and  the  highest  phosphorus  grade  was 
Cambridge  (Fe  =  59.60,  P  =  0.570,  P/Fe  =  0.00957). 

The  phosphorus  and  iron  contents  of  the  ores  of  the  Marquette  range  are  shown  in  the 

following  table :     . 

Phosphorus  and  iron  content  of  Marquette  ores. 


Iron. 


Phospho- 
rus. 


Ratio  of 
phospho- 
rus to 


Average  total  sliipraents  for  1906 ._ 

Average  ore  from  Ijottom  horizon  of  the  Negaunee  formation  — 
Average  ore  from  the  middle  horizon  of  the  Negaunee  formation 
Average  of  ore  from  upper  horizon  of  the  Negaunee  formation. . . 
Average  ores  from  upper  Huronian  Bijiki  schist 


69.55 
58.38 
57.22 
59.00 
65.91 


0. 1072 
.103 


.063 
.369 


0.00180 
.00176 
.OOlliS 
.  00107 
.00642 


Six  hundred  partial  analy.ses  of  jasper  carrying  between  20  and  50  per  cent  of  iron,  repre- 
senting 10,450  feet  of  drill  holes  in  the  area  south  of  Negaunee,  showed  an  average  of  35  per 
cent  of  iron  and  0.050  per  cent  of  phosphonis. 


280 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


The  local  distribution  of  phosphorus  in  the  ores  is  extremely  irregular.  In  many  ore  bodies 
fho  pliospliorus  coiitont  is  found  to  increase  as  the  greenstone  or  soap  roek  (altered  greenstone) 
walls  are  approached.  This  is  shown  by  the  foUowmg  analyses  of  ore  and  greenstone  collected 
from  the  Cliicago  shaft  of  the  Lake  Superior  Iron  Company: 

Partial  analyses  of  ore  and  greenstone  from  Chicago  shaftM 


P. 


AljO^         CaO. 


Ore  2  feet  from  foot  wall 

Paint  rock  (altered  greenstone)  2  feet  from  contact 

(Jreen.stune  foot  wall,  soft,  S  feet  from  contact 

Greenstone  foot  wall,  hard.  S  feet  from  contact 

Greenstone  foot  wail,  hard,  3;i  feet  from  contact 

Greenstone  foot  wall,  hard,  70  feet  from  contact. . . 

Altered  greenstone  (soap  reel;)  at  contact" 

Fresh  greenstone  80  feet  from  contact " 


0.112 
.192 
.132 
.134 
.064 
.106 
.181 
.090 


1.12 
4.67 


6.41 
15.30 


0.19 

.15 


.22 
.14 


a  The  last  two  samples  were  from  another  part  of  the  deposit. 


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Figure  39. — Diagram  showing  relation  of  phosphorus  to  degree  of  hydration  in  Marquette  ores. 

Local  variations  in  ])hosiihorus  also  occur  apparently  intle])endent  of  relations  to  green- 
stone walls  or  channels  of  flow,  being  due,  perhaps,  to  origmal  difl'erences  in  the  iron-bearing 
formation.  Typical  of  this  is  an  occurrence  in  the  Volunteer  mine,  where  high-phosphorus 
ore  is  found  against  hangmg-wall  jasper  and  low-])hosphorus  ore  against  tlie  jasper  foot  wall. 

The  increase  of  phosphorus  with  degree  of  h3'dration  is  showTi  in  figure  39. 

Two  wasliing  tests  similar  to  those  made  on  Mesabi  ores  (see  p.  193)  were  made  on 
sam])les  of  soft  red  hematite  ore  from  the  Lake  Angehne  mine  and  the  Hartford  mine.  The 
results  of  these  tests  are  shown  in  the  following  table : 


MARQUETTE  IRON  DISTRICT. 

Partial  analyses  from  washing  tests  on  Marquette  ores. 


281 


Fe. 

P. 

AI2O3. 

HjO. 

Lake  AnReline  mine: 

Heavy  residue 

65.00 
02.53 
61.20 

02. 23 
00.  :i9 
60.07 

0.078 
.100 
.100 

.120 
.100 
.080 

0.89 
1.56 
2.20 

2.30 
2.27 
2.64 

2  12 

2.43 

Finest  material 

3  40 

Hartford  mine: 

1  82 

Medium 

2  i6 

Finest  material 

2  32 

The  test  on  the  Lake  Angeline  ore  gave  results  similar  to  those  obtained  from  the  tests  on 
Mesabi  ore,  showing  the  association  of  phosphonis  with  the  more  hydrated  parts  of  the  ore. 
The  washing  test  on  the  Hartford  ore,  however,  does  not  show  this  relation. 

MINERALOGICAL  OCCURRENCE  OF  PHOSPHORUS. 

Phosphoi-us  is  known  to  occur  as  apatite,  dufrenite,  and  as  aluminum  ])hosphate.  It 
probably  occurs  in  a  variety  of  combinations  with  iron,  magnesium,  calcium,  and  aluminum 
and  in  forms  too  minute  to  be  identified.  Apatite  has  been  identified  by  Prof.  Seaman " 
and  others  at  a  number  of  localities  in  the  Negaunec  formation  and  in  the  upper  Huronian 
iron-ore  deposits.  In  the  chemical  determination  of  phosjihoiiis  it  is  found  that  only  a  part  of 
it  is  soluble  in  hydrocliloric  acid,  the  insoluble  portion  remaining  with  the  sihceous  residue. 
Tliis  seems  to  indicate  that  phosphonis  is  present  in  at  least  two  combinations.  The  soluble 
phosphorus  may  be  present  in  a  variety  of  combmations,  as  iron  phosphate,  calcium  phosphate, 
alid  some  aluminum  phosphates  are  soluble  in  hydrochloric  acid.  Charles  T.  Mixer  and  II.  W. 
Dubois  *  analyzed  the  insoluble  residue  remaining  after  treating  ore  with  hydrocldoric  acid 
(1.10  specific  gravity)  and  found  its  composition  in  percentages  of  original  residue  to  be  AljOj 
9;55,  CaO  0.92,  P2O5  4.10,  from  wliich  they  concluded  that  the  insoluble  phosphorus  is  to  a 
large  extent  combined  with  alumina.  What  this  aluminum  phosphate  is  it  is  impossible  to  say. 
It  is  of  interest  to  note  that  the  relative  amounts  of  soluble  and  insoluble  phosphonis  are  not 
uniform  in  the  various  ores;  in  some  the  insoluble  form  is  entirely  absent,  but  in  others  it  makes 
up  the  greater  part  of  the  phosphorus  present.  It  is  believed  by  some  of  the  chemists  of  the  iron 
range  that  the  insoluble  phosphorus  is  highest  in  ores  liigh  in  alumina.  In  order  to  ascertain  the 
possibility  of  the  phosphorus  being  present  as  apatite,  the  percentages  of  calcium  oxide  and  phos- 
phoras,  in  the  difl'ercnt  grades  of  ore  produced  in  1906,  were  platted  as  ordinates  and  abscissas  in 
figure  40.  The  diagonal  line  indicates  the  relative  amounts  of  the  two  constituents  in  apatite. 
It  may  be  seen  that  most  of  the  points  fall  below  the  liuje,  indicating  an  excess  of  iime  over 
the  amount  requii'ed  to  combine  with  the  phosphorus  present  as  apatite.  It  is  of  interest  to 
note  that  the  high  phosphorus  ores  are  correspondingly  high  in  lime,  indicating  rather  strongly 
the  possibility  of  at  least  a  large  part  of  the  phosphorus  being  present  in  apatite. 

PHOSPHORUS  IN  RELATION  TO  SECONDARY  CONCENTRATION. 

As  shown  in  the  table  on  page  279,  there  is  apparently  a  gradation  in  the  phosphorus  content 
of  the  ores  of  the  Negaunee  formation,  from  comparatively  low  pliosphorus  in  those  of  the 
upper  horizon  to  high  phosphorus  in  those  of  the  bottom  horizon.  The  difference  is  most 
marked  between  the  hard  ores  of  the  upper  horizon  and  the  soft  ores  of  the  middle  horizon. 
The  difference  between  the  ores  of  the  two  lower  horizons  is  very  small  and  may  be  apparent 
rather  than  real.  In  explanation  of  the  difference  in  phosphorus  content  between  the  hard 
and  soft  ores  may  be  cited  the  opportunity  for  leacliing  of  pliosphorus  from  the  upper  strata 
during  the  erosion  interval  previous  to  the  deposition  of  the  Goodrich  quartzite.  Another 
possibility  may  be  an  original  difference  in  the  phosphorus  contents  of  the  ores  at  the  two 
horizons. 


a  Personal  communication. 


i>  Jour.  Am.  Chem.  Soc,  vol.  19,  No.  8,  p.  619. 


282 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


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SWANZY  DISTRICT.  283 

The  abundant  slaty  phases  of  the  Michigamme  may  have  some  bearing  on  the  high  plios- 
phorus  of  the  ores,  as  in  all  the  iron  districts  the  slates  are  higher  in  phosphorus  than  the  iron- 
bearing  formation  proper.. 

The  local  occurrence  of  high-phosphorus  ore  near  greenstone  contacts  is  believed  to  be  due 
to  dh-ect  transfer  of  tliat  constituent,  leaclied  from  the  greenstone  during  its  alteration  to  soap 
rock  or  paint  rock  and  deposited  in  the  neighboring  oi-es.  The  analyses  on  page  280  show  tJiat 
there  is  actually  a  loss  of  pliosphorus  in  the  alteration  of  the  greenstone  if  alumina  is  assumed 
to  have  remained  constant,  although  the  actual  percentage  of  phosphorus  increases. 

Local  variations,  apparently  not  related  to  greenstone  contacts,  are  probably  due  to  origi- 
nal differences  in  the  phosphorus  content  of  the  formation  and  not  to  secondary  transfer  or 
infiltration. 

SWANZY  DISTRICT. 

GEOGRAPHY  AND  TOPOGRAPHY. 

The  Swanzy  iron  district  lies  about  16  miles  south  of  the  city  of  Marquette,  in  T.  45  N., 
R.  25  W.  (fig.  41).  In  1908  the  productive  area  was  less  than  2  miles  long  and  about  half  a 
mile  wide  and  contained  five  producing  mines.  Future  exploration  and  development  will 
undoubtedly  extend  the  district  to  the  south  and  east,  but  northward  and  westward  extensions 
are  apparently  cut  off  by  the  granite  area  that  bounds  the  district  on  these  sides.  The  towns 
within  the  producing  area  are  Gwinn  and  Princeton,  both  reached  by  the  Munising  Railway. 
The  district  occupies  a  range  of  hills  typical  of  the  granite  area,  and  slopes  on  the  south  and 
east  to  a  flat  sand-covered  phxin  above  which  stand  a  few  monadnocks  of  pre-Cambrian  rocks. 

GENERAL   SUCCESSION  AND   STRUCTURE. 

The  succession  is  as  follows: 

Quaternary  system: 

Pleistocene Glacial  deposits. 

Cambrian  sandstone.    ■ 
Ordovician  limestone. 
Unconformity. 
Algonkian  system: 

Huronian  series: 

"Michigamme  slate. 


Upper  Huronian  (Animikie  group) . . 


Bijiki  iron-bearing  member.     In  lenses  and  layers 
near  ba^^  of  Michigamme  slate. 
Goodrich  quartzite.     Quartz  slate  and  quartzite,  grad- 
ing down  into  arkose  or  recomposed  granite. 


Unconformity. 
Archean  system; 

Laurentian  series Granite. 

The  Swanzy  district  consists  of  a  southeastward-pitching  synclinorium  of  upper  Huronian 
rocks,  bounded  on  all  but  the  southeast  side  by  Archean  granite.  It  is  about  2  miles  long;  its 
width  is  for  the  most  part  not  more  than  three-quarters  of  a  mile,  and  at  the  narrowest  point, 
near  the  Stevenson  mine,  is  only  half  a  mile.  To  the  southeast  it  widens,  but  in  this  du-ection 
the  structure  is  not  known  because  of  the  deep  overburden.  The  pitches  of  the  minor  folds  at 
the  Stegmiller,  Princeton,  and  Swanzy  mines  are  toward  the  northwest.  The  slates  have 
developed  a  good  cleavage,  usually  crossing  the  bedding.  This  structure  does  not  affect  the 
quartzite  and  the  iron-bearing  member. 

ARCHEAN   SYSTEM. 

The  Archean  forms  the  basement  upon  which  the  Huronian  sediments  lie.  It  is  repre- 
sented by  granites  similar  to  tlie  basal  granites  of  the  neighboring  iron  ranges. 

The  Archean  bounds  the  district  on  the  north,  west,  and  southwest  sides.  Isolated  expos- 
ures stand  as  monadnocks  above  the  flat  sand  plains  of  the  district. 


284 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


NVIWNOOIV 


NV3HDyV 


1 

■s 

Si" 

•  * 

4 

SWANZY  DISTRICT.  285 

ALGONKIAN   SYSTEM. 

HXJRONIAN  SERIES. 

UPPER   HURONIAN    (aNIMIKIE    GROUP). 
GOODRICH  QTJARTZITE. 

The  Goodrich  sediments  lie  unconformably  upon  the  Archean  rocks.  They  consist  of  a 
coarse  arkose  or  recomposed  granite  at  the  base,  which  grades  upward  tlirough  quartzite  and 
quartz  slate  to  the  Brjiki  iron-bearing  member  of  the  Michigamme  slate.  The  arkose  horizon 
represents  a  shore  phase  of  sedimentation  where  disintegration  was  very  active  and  rapid 
transportation  of  the  disintegrated  material  prevented  decomposition.  In  places  the  arkose  is 
distinguished  from  the  granite  with  difficulty.  The  quartzite  is  petrographically  very  similar 
to  the  Goodrich  quartzite  of  the  Alarquette  range  and  exhibits  all  phases  of  gradation  between 
the  arkose  below  and  a  thin-bedded  quartz  slate  above.  Both  the  quartzite  and  the  quartz 
slate  are  locally  iron  stained,  and  in  places  the  impregnation  is  so  strong  as  to  have  attracted 
prospecting  operations.  The  arkose  phase  is  best  exhibited  in  drill  cores.  The  quartzite  and 
quartz  slate  phases  are  well  exposed  in  abundant  outcrops  on  the  north  slope  of  the  range  of 
hills  wliich  crosses  sec.  19,  T.  45  N.,  R.  24  W.  The  quartzite  also  outcrops  in  a  small  hill  near 
the  northeast  corner  of  sec.  18,  T.  45  N.,  R.  24  W. 

The  thickness  of  the  quartzite  and  the  quartz  slate  varies  and  locally  the  slate  and  jasper 
lie  directly  oh  the  recomposed  granite  or  on  the  granite  itself. 

MICHIGAMME  SLATE. 

The  Michigamme  slate  is  best  exposed  at  the  old  Swanzy  open  pit,  near  the  center  of  sec. 
18,  T.  45  N.,  R.  25  W.,  where  it  is  found  in  contact  with  the  Bijiki  iron-bearing  member.  It 
both  underlies  and  overlies  the  iron-bearing  beds,  which  are  therefore  treated  as  a  member  of 
the  slate.     The  Michigamme  forms  much  the  larger  part  of  the  upper  Huronian. 

The  iron-bearing  member  is  a  banded  ferruginous  chert  or  "soft-ore  jasper"  similar  in 
appearance  to  part  of  the  Bijiki  schist  of  the  Marquette  range.  Locally  it  grades  into  a  ferru- 
ginous slate.  It  apparently  occurs  in  lens-shaped  beds  in  and  near  the  base  of  the  Michigamme 
slate,  and  therefore  it  is  treated  as  a  member  of  that  formation.  Drilling  and  mining  operations 
have  shown  jasper  with  slate  above  and  below,  or  slate  above  and  quartzite  below,  or  in  places 
the  iron-bearing  member  is  found  directly  above  the  arkose  and  overlain  by  slate,  the  quartzite 
and  quartz  slate  being  absent.  The  iron-bearing  member  is  exposed  at  several  places  in  the 
vicinity  of  the  Princeton,  Stegmiller,  and  Austin  mines  and  also  in  the  old  Swanzy  open  pit. 
An  exposure  near  the  center  of  the  SE.  J  sec.  18,  T.  45  N.,  R.  25  W.,  shows  typical  banded 
soft-ore  jasper  with  a  nearly  vertical  dip.  Near  the  southeast  corner  of  the  same  section,  just 
west  of  tlie  Stegmiller  mine,  is  a  similar  exposure.  An  exposure  about  600  feet  west  of  Princeton 
station  shows  the  member  folded  and  contorted. 

PALEOZOIC    SEDIMENTS. 

On  the  east  side  of  the  district  flat-lying  sandstones  antl  limestones  belonging  to  the  Cam- 
brian and  Ordovician  overlap  the  pre-Cambrian  formations  unconformabh".  The  nearest 
exposure  of  limestone  is  in  the  northeast  corner  of  sec.  18,  T.  45  N.,  R.  24  W.,  where  a  small 
hill  of  quartzite  has  a  few  remnants  of  a  limestone  capping. 

QUATERNARY  DEPOSITS. 

Pleistocene  sand  flats  of  glacial  origin  cover  most  of  the  district.  (See  Chapter  XVI, 
pp.  427-459.) 


286  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

CORRELATION. 

Tlie  upper  Iliironian  (Animikie  f^roup)  is  very  similar,  both  in  stratigraphy  and  in  litliolojjy, 
to  the  upper  Huronian  of  the  Marquette  district  on  tlie  north  and  tlie  Crystal  Falls  and  Menom- 
inee districts  on  the  south. 

THE  IRON   ORES   OF  THE   SWANZY  DISTRICT. 

By  the  authors  and  W.  J.  Mead. 
GENERAL  DESCRIPTION. 

The  ores  of  the  Swanzy  tlistrict  are  in  the  Bijiki  iron-bearing  memljer,  which  is  interbcdded 
with  the  lower  part  of  the  Michigamme  slate  of  tlie  upper  Huronian  and  rests  upcm  the  Arcliean 
o-ranite  with  only  a  comparatively  thin  intervening  zone  of  quartzites,  quartz  slate,  or  recom- 
posed  granite,  constituting  the  Goodrich  quartzite.  The  upper  Huronian  constitutes  'a  south- 
eastward-pitching synclinorium,  ])ut  some  of  the  minor  folds  on  its  limbs  pitch  to  tlie  northwest. 
They  are  of  the  drag  type  so  common  to  the  Lake  Superior  region.  (See  fig.  12,  p.  123.)  The 
iron-bearing  member  takes  part  in  this  general  structure.  The  ores  therefore  appear  as 
much-folded  deposits  with  foot  wall  of  slate,  quartzite,  recomposed  granite,  or  granite  and 
with  hanging  wall  of  black  slate.  All  the  ore  deposits  reach  the  erosion  surface  either  at  the 
border  of  the  s3'nclinorium  or  on  the  eroded  minor  anticlines  in  the  main  synclinoriiun. 

Five  mines  are  in  operation  and  several  additional  ore  deposits  are  known.  (See  map, 
fig.  41.) 

The  ore  is  a  soft  hydrated  non-Bessemer  hematite  containing  a  rather  high  percentage  of 
moisture.     The  following  is  the  average  composition  of  ore  sluppetl  in  1906: 

Average  composition  of  ore  shipped  from  Swanzy  district  in  1906. 

Moisture  (loss  on  drying  at  212°) 13.  50 

Analysis  of  dried  ore: 

Iron 58.60 

Phosphorus 211 

Silica 10.  20 

Manganese 71 

Alumina 1.  05 

Lime 1. 15 

Magnesia 46 

Sulphur 012 

Loss  by  ignition 1. 25 

SECONDARY  CONCENTRATION  OF  SWANZY  ORE. 

The  structural  conditions  governing  the  concentration  of  the  ores  in  the  Swanzy  district 
are  a  foot  wall  of  granite,  quartzite,  or  slate  and  a  hanging  wall  of  slate,  conforming  to  the 
structure  of  a  sj'nclinorium  that  has  a  gentle  southeastward  pitch  with  many  minor  variations. 
Erosion  has  exposed  the  iron-bearing  member  near  the  borders  of  the  synclinorium  and  along 
the  arches  of  the  minor  anticlines.  The  circidation  of  the  iron-bearing  solutions  has  obviously 
been  controlled  n(jt  only  by  the  impervious  basement  but  by  the  overlapping  impervious  forma- 
tions which  determined  tlieir  points  of  escape. 

The  ores  and  ferruginous  cherts  have  been  derived  from  the  alteration  of  sidcritic  cherts 
and  slates,  accompanied  by  the  removal  of  silica  and  the  development  of  pore  space. 


MONOGRAPH  Lii  Plate  «» 


GEOLOGIC  MAP  OF  DEAD  RIVER  AREA,  MICHIGAN 


H^  A.K.SKAMAN 
ScalR  aiA>(i 


s//,r//       I    I 


u. 


^k^lii^ 


LEGEND 
ALGONKIAN    (Huronlan  sei-ies) 


UPPER    HURONIAN    TANIMIKtE      GROUP* 


OLE    HUFtONIA 


Aus 


N«Ratiiii»«  fbniinlii.in 


Siamo  tilatr- 


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witk  Httfltc 


"H;. 


,3S  ~*.       33*4.   *     -^-^.^* 


'-^.  ^ 


v_  ^-■ 


-'1' . 


'■V-.J.~:- 


-:^: 


L 


s^-^^;;>^r 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION.  287 

DEAD  RIVER  AREA." 

The  Dead  River  area  lies  north  of  the  Marquette  district  along  the  Dead  River.  Its  {greatest 
extent  is  18  miles  west-northwest  and  east-southeast.  Its  maximum  width  is  6  miles. 
(See  PI.  XX.)  The  basin  is  largely  a  low,  flat  sand-covered  plain  with  an  amphitheater  of 
rock-exposed  hills  about  it. 

GENERAL   SUCCESSION. 

The  general  succession  is  as  follows : 

Quaternary  system: 

Pleistocene  deposits. 
Unconformity. 
Algonkian  system: 
Huronian  series: 

Upper  Huronian  (Animikie  group) Slates  and  conglomerate. 

Unconformity. 

{Negaunee  formation  (iron  bearing). 
Siamo  slate. 
.\iibik  quartzite. 
Unconformity. 
Archean  system: 

Laurentian  series Graiiite  intrusive  into  Keewatin  series. 

Keewatia  series,  including  Kitchi  and  Mona  schifits. 

The  Laurentian  and  Keewatin  rocks  occupy  the  liills  siu-rounding  the  basin;  the  middle 
Huronian  rocks  outcrop  along  the  margin  of  the  basin,  anci  the  upper  Huronian  (xVnimikie 
group)  occupies  nearly  all  of  the  basin  itself. 

ARCHEAN  SYSTEM. 

KEEWATIN  SERIES. 

The  Keewatin  series  forms  hills  along  the  northeast  and  southeast  sides  of  the  basin.  The 
series  includes  on  the  south  side  the  Kitchi  and  Mona  schists,  already  described  for  the  Mar- 
quette district,  and  on  the  north  side  schists  entirely  similar  in  aspect,  even  to  their  content  of 
iron-bearing  sediments  consisting  of  jasper,  cherty  siderite,  and  cherty  slate.  Slate  and  con- 
glomerate are  weU  exposed  at  the  German  exploration  in  sec.  35,  T.  49  N.-,  R.  27  W.,  and  in  the 
Holyoke  mine  on  the  south  side  of  the  hiU. 

LATJBENTIAN  SERIES. 

Laurentian  granites  and  gneisses  bound  the  Dead  River  district  on  the  southwest,  west, 
and  northwest  and  also  for  a  short  distance  along  the  southeast  end.  They  are  not  different 
from  the  rocks  of  the  northern  complex  of  the  Marquette  district. 

ALGONKIAN   SYSTEM. 

HURONIAN  SERIES. 

Middle  Huronian. — The  middle  Huronian  is  exposed  along  the  south  and  southeast  sides 
of  the  district  and  also  at  the  extreme  west  end  bordering  the  amphitheater  of  Keewatin  and 
Laurentian  rocks.  The  best  exposed  of  these  rocks  is  the  Ajibik  quartzite,  forming  the  base 
of  the  middle  Huronij^n,  and  showing  unconformity  between  Laurentian  and  middle  Huronian 
by  discordance  in  structure  and  by  conglomerates. 

The  Siamo  slate  outcrops  in  a  narrow  belt  along  the  north  side  of  the  Ajibik  quartzite 
where  it  follows  the  south  boundary  of  the  district. 

The  Negaunee  formation  (iron  bearing)  is  exposed  in  only  one  area,  in  sec.  15,  T.  48  N., 
R.  26  W.,  along  the  railway  track  and  in  pits.     Here  the  iron-bearing  formation,  mth  the  under- 

o  Mapped  by  A.  E.  Seaman,  Michigan  College  of  Mines. 


288  GEOLOGY  OF  THE  LAKE  SLIPERIOR  REGION. 

lying  Siamo  slate  and  Ajil)ik  quartzite  is  much  folded.  Oveilyint;  tlie  iron-bearin<;  formation 
(in  direct  contact  in  pits)  is  the  basal  conglomerate  of  the  ui)per  lluronian,  containing  fragments 
both  of  middle  lluronian  and  Keewatin. 

Upper  lluronian  (Animikie  group). — The  upper  lluronian  consists  princijjall}-  of  slates, 
similar  in  all  respects  to  the  Michigamme  slate  of  the  Marquette  district.  They  outcrop  in 
isolated  exposures  over  the  area  and  their  presence  is  further  indicated  by  the  prevailing  low 
relief  of  the  basin.  The  base  of  these  rocks  is  probabh'  marked  l)y  the  conglomerate  resting 
unconformably  on  the  Keewatin  series  at  the  Holyoke  mine  and  eastward  at  intervals  to  the 
east  end  of  the  basin;  also  by  the  conglomerate  covering  the  Negaunee  formation,  alreadj' 
referred  to.  The  slates  have  not  been  connected  directly  with  tlie  conglomerate,  l)ut  tlie  fact 
that  the  conglomerate  contains  fragments  not  only  of  Keewatin  but  of  middle  Hurunian  rocks 
seems  to  require  its  correlation'  with  the  upper  Huronian. 

Greenstone  dikes  cut  the  slates.  One  of  them  constitutes  the  falls  of  Dead  River  where  it 
cuts  through  the  slates  in  sec.  9,  T.  48  N.,  R.  26  W. 

PERCH  LAKE   DISTRICT  (IXCLUDIXG  WESTERN   IMARQl'ETTE). 

GEOGRAPHY  AND  TOPOGRAPHY. 

The  Perch  Lake  district  includes  territory  extentlLng  west  from  the  Marquette  district 
and  north  from  the  Ciystal  Falls  and  Iron  River  districts  to  a  line  extending  from  L'^Vnse  Bay 
on  the  northeast  to  the  south  end  of  Lake  Gogebic  on  the  southwest.  The  area  thus  defined 
includes  roughly  1,200  square  miles.  (See  fig.  42;  PI.  XXI,  in  pocket.)  A  topographic  map 
has  been  prepared  of  the  area  around  Perch  Lake,  extending  from  SS°  30'  to  88°  45'  west  and 
46°  15'  to  46°  30'  north.  The  remainder  of  the  country  has  not  been  surveyed  topographically. 
As  a  whole  the  country  is  characterized  by  morainal  topography  with  much  local  irregularity, 
but  has  no  consj^icuous  ranges  characteristic  of  the  principal  ore-pi'oducing  districts. 

GENERAL,   SUCCESSION. 

The  succession  is  as  follows,  from  the  top  downward: 

Quaternary  system: 

Pleistocene  or  glacial  deposits. 
Cambrian  sandstone. 
Algonkian  system: 

Huronian  series: 


Upper  Huronian  (Animikie  group). . 


Middle  Huronian . 


Michigamme  slate  (slates  and  graywackes  with  pos- 
sible iron-bearing  lenses).  Equivalent  and  areally 
continuous  with  the  Michigamme  slate  of  the  Crys- 
tal Falls,  Iron  River,  and  Menominee  districts. 

Goodrich  quartzite  (quartzites  and  conglomerates). 

Intrusive  diorite. 

Xegaunee  formation  (iron  bearing). 

Siamo  slate. 

.\jibik  quartzite. 
Unconformity. 
Archean  system: 

Laurentian  series Granite  and  syenite. 

ARCHEAN   SYSTEM. 

LAURENTIAN  SEKIES. 

The  Laurentian  granite  and  syenite  bound  the  district  on  the  northeast.  They  show  no 
features  different  from  the  Laurentian  of  the  contiguous  Marquette  district.  The  rocks  are 
abimdantly  exposed.  The  topograph}'  of  the  Archean  area  is  as  a  whole  rougiier  and  more 
irregular  than  that  of  the  Algonkian  on  its  southwestern  margin,  affording  a  very  satisfactoiy 
guide  for  discrimination  in  the  field  mapping.  The  Archean  underlies  the  Huronian  uncon- 
formably. 


PERCH  LAKE  DISTRICT. 


289 


ALGONKIAN   SYSTEM. 

HUBONIAN  SERIES. 

Middle  Huronian. — Between  the  upper  Huronian  slates  and  graywackes  (Michigamme 
slate)  and  the  Archean  granite  on  the  northeast  there  appears  a  belt  about  5  miles  long  extend- 
ing from  the  Marquette  district  northwest,  in  which  are  exposed  middle  Huronian  sediments 
and  upper  Huronian  Goodrich  quartzite.  (See  fig.  42.)  The  middle  Huronian  Ajibik  quartz- 
ite  and  Siamo  slate  show  no  features  different  from  those  of  the  Marquette  district.  They 
rest  unconformably  against  the  Archean.  On  the  northwest  and  along  their  trend  they  become 
covered  by  glacial  materials  until  they  can  no  longer  be  followed.     Presumably  they  extend 


R.31  W. 


Eruptive  diaba-se 
aiid  diorite 


///a 


Tvliclii^amme  slate 
and  Bijiki  scliist 

(iron  tjf-'oririg) 


Goodrich  quartziie 


Negaunee  formation 

{iron  hearing) 


Siamo  slate 


Ajibik  qiiartzite 


1# 


Granite 


3  Miles 


Figure  42.— Geologic  map  of  west  end  of  Marquette  district,  Michigan.    By  W.  N.  Merriara  and  M.  H.  Newman. 

considerably  farther  than  the  map  indicates.  The  Negaunee  formation  also  is  similar  to  the 
Negaunee  formation  of  the  Marquette  district.  It  is  followed,  however,  principally  by  mag- 
netic observations  to  the  point  indicated  on  the  map,  where  it  is  lost  beneath  the  covering 
of  later  drift.  Wliether  it  extends  farther  or  whether  this  represents  the  end  of  the  originally 
deposited  iron-bearing  lens  is  not  known. 

Upinr  Huronian  {AnimiJcie  gi'oup). — The  district  is  underlain  principally  by  upper  Hui-o- 
nian  slates  and  graywackes,  known  as  the  Michigamme  slate.  On  the  northeast  they  rest 
unconformably  against  the  Archean  granite  and  middle  Huronian  rocks.  On  the  northwest 
they  are  overlain  by  Cambrian  sandstone,  the  relations  of  the  two  locally  being  obscured  by 
faulting.  The  Goodrich  quartzite  of  the  upper  Huronian  is  exposed  only  in  the  northeastern 
part  of  the  area  bordering  the  middle  Huronian  and  at  the  northwest  end,  presumably  over- 
47517°— VOL  52—11 19 


290  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

lapping  on  the  Archean.  Its  characteristics  are  similar  to  those  of  the  Goodrich  quartzite 
of  the  Marquette  district.  The  Michigamme  slate  covers  much  the  larger  part  of  the  Perch 
Lake  area.  Exposures  are  fairly  abundant,  especially  in  the  Perch  Lake  district.  Presum- 
ably contemporaneous  basic  volcanic  rocks  are  associated  with  these  slates,  to  judge  from 
the  facts  observed  to  the  south,  but  their  detailed  (hstribution  is  not  known.  There  Ls  difli- 
culty  in  identil'ymg  horizons  in  the  slate  and  graywacke,  and  thei'efore  in  working  out  the 
structure  of  this  area.  From  the  abundance  of  exposures,  however,  it  is  probable  that  this 
may  be  accomplished  in  the  future.  The  locations  of  most  of  the  exposures  have  been  noted 
in  commercial  surveys,  but  the  Geological  Survey  has  not  examined  this  area  in  detail  to  work 
out  the  structure.  From  the  promising  development  in  similar  series  in  the  adjacent  Iron 
River  district,  it  would  seem  that  this  area  would  warrant  careful  examination  for  iron-bearing 
lenses. 

QUATERNARY   DEPOSITS. 

Pleistocene  glacial  deposits  cover  all  of  this  area.     (See  Chapter  XVI,  pp.  427-459.) 


CHAPTER  XII.  THE  CRYSTAL  FALLS,  STURGEON,  FELCH  MOUN- 
TAIN, CALUMET,  AND  IRON  RIVER  IRON  DISTRICTS  OF  MICHI- 
GAN AND  THE  FLORENCE  IRON  DISTRICT  OF  WISCONSIN. 

The  Crystal  Falls,  Sturgeon,  Felch  Mountain,  Calumet,  and  Iron  River  iron  districts  of 
Michigan  and  tlie  Florence  iron  district  of  Wisconsin  together  form  the  ore-producing  area 
between  the  Marquette  district  on  the  north  and  the  Menominee  district  on  the  south.  (See 
fig.  43.)  The  ores  of  all  these  districts  occur  in  the  upper  Huronian  (Animikie  group)  and  have 
many  similarities  in  kind  and  relations,  and  the  limits  of  the  several  districts  are  poorly  defined. 
They  are  accordingly  grouped  together  in  one  cliapter. 

CRYSTAL  FALLS  IRON  DISTRICT."^ 
LOCATION  AND   AREA. 

The  Cr\'stal  Falls  district  is  centered  in  the  town  of  that  name  in  the  Northern  Peninsula 
of  Michigan.  (See  PL  XXII,  in  pocket.)  As  the  term  is  here  used  it  includes  an  area  of  about 
540  square  miles,  covering  all  the  territory  between  the  Marquette  and  Menominee  districts  as  • 
these  have  been  limited  on  the  maps  of  the  United  States  Geological  Survey.  In  commercial 
parlance  the  Menominee  district  includes  the  Crystal  Falls  and  southwestward  extensions,  and 
reports  of  shipments  for  the  Menominee  district  include  these  districts.  However,  they  are 
geologically  and  structurally  more  or  less  independent  and  have  been  treated  in  two  reports,' 
hence  here  the  Crystal  Falls  district  will  be  treated  independently  of  the  Menominee  district. 
The  Felch,  Sturgeon,  and  Calumet  troughs  bordering  the  Crystal  Falls  district  on  the  southeast 
are  also  discussed  in  this  chapter,  as  well  as  the  Iron  River  and  Florence  districts,  which  lie  to 
the  south  and  southwest. 

GENERAL,   SUCCESSION  AND    STRUCTURE. 

The  succession  is  as  follows: 

Quaternary  system: 

Pleistocene  drift. 
Cambrian  sandstone  (in  southern  and  eastern  parts  of  district). 
Algonkian  system: 

Huronian  series: 


Upper  Huronian  (Animikie  group). 
Unconformity  (?). 

Middle  Huronian  (?) 


Volcanic  rocks  interbedded  with  slates. 
Michigamme  slate.    Thickness  unknown,  but  proba- 
bly several  thousand  feet. 

Vulcan  iron-bearing  member,  300  feet. 

Negaunee  (?)  formation  (iron  bearing). 

Ajibik  quartzite. 

Hemlock   formation   (volcanic),    1,000   to   10,000  feet. 

Includes  at  top  iron-bearing  slate  member,  1  to  1,900 

feet  thick,  formerly  called  "Mansfield  slate." 

Unconformity  (?). 

T  Tr         •  (Randville dolomite,  500  to  1,. 500  feet. 

Lower  Huronian i  .  '      , 

[Sturgeon  quartzite,  100  to  1,000  feet. 

Unconformity. 

Archean  system : 

Laurentian  series Granites  and  gneisses. 

o  For  further  detailed  description  of  the  geology  of  this  district  see  Mon.  U.  S.  Geol.  Survey,  vol.  36, 1899,  and  references  there  given. 
6  Clements,  I.  M.,  and  Smyth,  H.  L.,  The  Crystal  Falls  iron-bearing  district  of  Michigan:  Mon.  U.  S.  Geol.  Survey,  vol.  30,  1S99.    Bayley, 
W.  8.,  The  Menominee  iron-bearing  district  of  Michigan:  Mon.  U.  S.  Geol.  Survey,  vol.  46,  1904. 

291 


292 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


The  northeastern  part  of  the  area  is  underlain  by  Archean  granites.     Bordering  tliis  main 
Archean  area  on  the  southwest,  with  longer  axes  parallel  and  striking  north-northwest  an,d 


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south-southeast,  are  two  minor  oval  areas  of  Archean  granite.  Iluronian  sediments  and  basic 
igneous  rocks,  exposed  ]irinci])ally  in  the  western  part  of  the  district,  lap  around  the  Archean 
ovals  and  against;  the  main  Archean  area  to  the  northeast,  and  their  general  structure  is  deter- 


CRYSTAL  FALLS  IRON  DISTRICT.  293 

mined  by  their  relations  to  the  Archean  ovals.  The  Crystal  Falls  antl  Amasa  districts  are  on 
the  southwest  side  of  one  of  tlicse  Archean  ovals.  Therefore  both  tlie  di])  and  (he  pitch  of  the 
minor  folds  of  the  upper  Iluronian  occui)ying  these  areas  are  in  southwesterly  directions. 

ARCHEAN  SYSTEM. 

LAUBENTIAN  SERIES. 

The  Archean  or  basement  rocks  occupy  the  northeastern  part  of  the  district,  filling  the 
angle  between  the  Crystal  Falls  antl  Marquette  districts.  To  the  west  of  this  they  also  appear 
in  two  elliptical  cores  with  longer  axes  north-northwest  and  south-southeast,  approximately 
parallel  to  the  axes  of  the  major  folds  of  the  district. 

The  Archean  rocks  consist  mainly  of  massive  and  schistose  granites  and  of  gneisses.  No- 
where in  them  have  any  rocks  of  sedimentary  origin  been  discovered.  They  have  been  cut  by 
igneous  rocks,  both  basic  and  acidic,  at  diflerent  epochs.  These  occur  in  the  form  both  of 
bosses  and  of  dikes,  the  latter  in  places  cutting  but  more  ordinarily  showing  a  parallelism  to  the 
foliation  of  the  schistose  granites.  The  Ai-chean  granites  and  gneisses  and  the  earlier  intrusive 
rocks  alike  have  been  profoundly  metamorphosed,  and  at  several  places  have  been  completely 
recrystallized.  In  the  westernmost  oval  there  is  to  be  observed  a  distinct  arrangement  of 
feldspar  crystals  with  their  longer  dimensions  parallel  to  the  contact  with  the  Huronian  rocks. 

ALGONKIAN  SYSTEM. 

HURONIAN  SERIES. 

LOWER    HXJEONIAN. 
STURGEON  QtTARTZITE. 

In  the  central  part  of  the  district  the  Sturgeon  quartzite  is  represented  only  by  thin  frag- 
mental  layers  at  the  base  of  the  overlying  Randville  dolomite.  These  are  too  thin  to  be  mapped. 
Its  principal  outcrops  are  to  the  southeast  in  the  Felcli  Mountain  and  Sturgeon  districts,  de- 
scribed later  in  this  chapter. 

RANDVILLE  DOLOMITE. 

The  Randville  dolomite  completely  surroimds  the  Ai-chean  oval  northeast  of  the  town  of 
Crystal  Falls.  Here  it  constitutes  the  base  of  the  sedimentary  series  and  rests  directly  upon 
the  Archean  with  only  thin  intervening  layers  of  fragmental  quartzose  dolomite  and  recom- 
posed  granite,  all  more  or  less  altered  to  cpiartz  schist  and  in  many  places  difficult  to  distin- 
guish from  schistose  phases  of  the  granite  itself. 

On  the  west  side  of  the  western  Archean  oval  the  dolomite  is  poorly  exposed  and  its  thick- 
ness is  not  estimated.  On  the  east  side  the  belt  is  about  half  a  mile  wide  and  the  thickness 
about  1,500  feet.  The  formation  constitutes  here  an  eastward-dipping  monocline  with  mmor 
plications.  In  the  scattered  outcrops  of  the  Michigamme  Mountain  area  the  dolomite  strikes 
and  dips  toward  all  points  of  the  compass  as  a  result  of  the  gentle  arching  from  the  general 
northwest-southeast  axis,  combined  with  sharp  local  folds  which  run  nearly  east  and  west. 

Petrographically  the  formation  ranges  from  coarse  saccharoidal  marbles,  in  places  very  pure 
but  usually  filled  with  secondary  silicates,  to  fine-grained,  little-altered  limestones,  which  are 
here  and  there  so  impure  as  to  be  calcareous  or  dolomitic  sandstones  and  shales.  The  prevalent 
colors  are  white,  but  various  shades  of  pink,  liglit  and  deep  blue,  anil  pale  green  occur. 
Some  of  the  varieties  are  oolitic.  This  structure  does  not  seem  to  have  been  previously  noted 
in  limestones  of  pre-Cambrian  age  in  the  Lake  Superior  region. 


294  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

MIDDLE    HURONIAN    (?). 
HEMLOCK  FORMATION. 

Distribution  and  general  character. — The  volcanic  Hemlock  formation  occupies  a  large  area 
in  the  Crystal  Falls  district.  It  is  believed  almost  to  surround  the  westernmost  Archean  oval 
and  also  to  occur  in  a  great  area  northwest  of  Crystal  Falls  and  in  one  isolated  area  near  the 
Mastodon  mine,  south  of  Crystal  Falls.  Its  general  stratigraphic  position  is  conformably 
above  the  Randville  dolomite  and  beneath  the  upper  Huronian  slates,  but  like  most  volcanic 
formations  its  relations  differ  in  different  parts  of  the  district  in  ways  wliich  will  appear  below. 
Well-bedded  cherty  slates,  iron-bearing  lenses,  and  limestone  are  mterbedded  with  tlie  Hem- 
lock foi'iiiation  and  also  both  underlie  and  overlie  it.  The  volcanic  extrusions  may  be 
regarded  as  interruptions  of  otherwise  continuous  deposition  of  sediments.  The  lack  of  con- 
tinuity of  the  volcanic  flows  antl  of  the  interbedded  sediments,  and  the  difhculty  of  correlating 
the  beds  of  either  in  different  parts  of  the  district,  make  it  practically  impossible  to  use  geologic 
names  for  these  sediments  which  will  have  anything  more  than  very  local  significance.  One 
of  the  prmcipal  local  sedimentary  units  within  the  Hemlock  formation  has  been  described 
and  mapped  in  the  United  States  Geological  Survey  monograph  on  the  Crystal  Falls  district" 
as  the  "Mansfield  slate."  Limestone  and  slate  layers  appear  abimdantly  in  the  Hemlock 
formation  near  Hemlock  River  immediately  northeast  of  the  town  of  Amasa  and  in  several 
other  localities. 

Area  south  and  west  of  the  westemrnost  Archean  oval. — Exposures  of  the  Hemlock  formation 
are  numerous  west  and  south  of  the  western  Archean  oval,  and  where  erosion  has  removed  the 
di'ift  the  formation  has  a  marked  influence  on  the  topography.  The  thickness  is  estimated 
from  the  dip  to  reach  23,000  feet,  but  this  is  probably  illusory  because  of  reduplication  due  to  fold- 
ing. The  formation  here  consists  mainly  of  bedded  surface  basic  extrusive  rocks  and  crystalline 
scliists  derived  fi'om  them.  Sedimentary  rocks  play  a  subordinate  part.  The  Hemlock  rocks 
are  similar  in  all  respects  to  the  Keewatin  volcanic  rocks  and  to  the  volcanic  Clarksburg  formation 
of  the  Marcjuette  district.  The  formation  is  cut  by  a  few  acicUc  chkes  and  by  numerous  dikes 
and  enormous  bosses  of  basic  rock.  On  the  former  Survey  map  of  the  district ''  certain  of 
these  were  discriminated,  but  they  are  not  discriminated  on  the  accompanying  map  (PI.  XXII, 
in  pocket),  because  more  study  has  sho^\^l  a  most  intimate  association  of  extrusive  and  intru- 
sive phases  of  the  formation  tliroughout  the  area.  The  acidic  intrusive  rocks  mclude  rhyohte 
porphyry  and  aporhyolite  porphyry.  The  rhyolite  porphyry  shows  interesting  micropoikihtic 
textural  characters.  Acithc  pyroclastic  rocks  are  scarce  and  were  derived  from  the  aporh3'olite. 
The  basic  lavas  correspond  to  the  modem  basalts.  They  are  much  altered  and  are  called 
"  metabasalts."  The  basic  lavas  include  nonporphyritic,  porphyritic,  and  variolitic  and  eUip- 
soidal  types.  Clements "  has  described  the  ellipsoidal  textures  and  concludes  that  basalts 
possessing  this  structure  were  origmally  very  viscous  and  correspond  to  the  modern  aa  lavas, 
probably  of  submarine  origin.  The  pyroclastic  rocks  comprise  eruptive  breccia,  includiug 
friction  breccias  and  flow  breccias,  and  volcanic  sedimentary  rocks.  The  colian  deposits, 
which  are  described  as  tuffs,  grade  from  fine  dust  up  to  those  in  which  the  fragments  are  bowl- 
ders. The  water-deposited  volcanic  fragmental  rocks  are  known  as  volcanic  conglomerates, 
and  likewise  range  from  those  of  which  the  particles  are  minute  to  those  of  which  the  fragments 
are  very  large.  At  many  places  occur  clastic  rocks  which  are  now  schistose  and  whose  exact 
mode  of  origin — that  is,  whether  eolian  or  water-deposited — could  not  be  determined. 

The  crystalline  schists  of  Bone  Lake  include  rocks  of  completely  crystalline  character, 
which  by  field  and  microscopic  study  have  been  connected  Math  the  volcanic  rocks  and  are 
considered  to  have  been  derived  from  rocks  similar  in  nature  to  them. 

In  general  some  of  the  volcanic  rocks  are  submarine.  The  greater  proportion,  however, 
were  derived  from  volcanic  vents,  which  could  not  be  located,  but  were  probably  situated  near 
the  Huronian  shore  line.     Clements  suggested  that  volcanic  activity  began  in  the  north  and 

a  Mon.  U.  S.  Geol.  Survey,  vol.  36, 1899,  pp.  54-73.  b  Idem,  PI.  III.  « Idem,  pp.  112-124. 


CRYSTAL  FALLS  lEON  DISTRICT.  295 

moved  to  the  south,  and  that  some  of  the  volcanic  deposits  to  the  north  are  contemporaneous 
with  the  so-called  "Mansfield  slate." 

Fence  River  area. — In  the  Fence  River  area  the  Hemlock  formation  occupies  a  belt  between 
2,000  and  3,000  feet  in  width,  between  the  Randville  dolomite  on  the  west  and  the  Negaunee 
formation  on  the  east.  The  best  exposures  occur  on  the  sections  made  by  Fence  River.  No 
folds  have  been  observed  within  the  formation.  The  tliickness  probably  ranges  up  to  2,300 
feet  as  a  maximum.  The  rocks  of  the  formation  in  tliis  area  are  cliiefly  chlorite  and  ophitic 
schists,  with  which  are  associated  schists  bearing  biotite,  ilmenite,  and  ottrelite,  greenstone, 
conglomerates  or  agglomerates,  and  amygdaloids.  As  evidence  of  the  origm  of  these  schists 
several  facts  may  be  cited.  First,  they  include  no  rocks  possessmg  any  sedimentary  characters; 
next,  lavas  and  also  greenstone  conglomerates  or  agglomerates  are  undoubtedly  present  in  the 
series;  furtherinore,  the  minerals  wliich  compose  the  schist  are  those  wliich  would  result  from 
the  alteration  in  connection  with  dynamic  metamorphism  of  igneous  rocks  of  basic  or  inter- 
mediate chemical  composition;  and  finally,  the  grain  and  character  of  the  groundmass  and  in 
some  sUdes  the  presence  of  plagioclase  microlites  disposed  in  oval  lines  point  dii-ectly  to  an 
igneous  origin  and  to  consohdation  at  the  surface.  The  conclusion  is  reached  that  the  Hemlock 
formation  of  the  Fence  River  area  is  composed  of  a  series  of  old  lava  flows  varying  in  compo- 
sition from  acichc  to  basic. 

Other  areas  of  the  Hemloclc  formation. — Other  areas  of  volcanic  rocks  similar  to  those  of  the 
Hemlock  formation  appear  to  the  north  and  west  of  the  town  of  Crystal  Falls,  near  the  Mastodon 
mine,  and  elsewhere,  as  shown  on  the  accompanymg  general  map  of  the  Crystal  Falls  district 
(PL  XXII).  Wliether  these  are  of  the  same  age  as  the  main  mass  of  the  Hemlock  formation 
and  owe  their  distribution  to  folding,  or  whether  they  are  later  extrusions,  is  not  yet  known. 
Ironr-hearing  slate  member  {"Mansfield  slate")  of  the  Hemlock  formation. — The  so-called 
"Mansfield  slate,"  wliich  is  interbedded  near  the  top  of  the  Hemlock  formation,  is  best  exposed 
in  the  vicmity  of  the  town  of  Mansfield.  It  here  occupies  a  valley  through  wliich  flows  ^lichi- 
gamme  River.  Petrograplucally  the  member  includes  graywackes,  clay  slate,  phyllite,  siderite 
slate,  chert,  ferruginous  chert,  and  iron  ores,  with  several  metamorphic  products  derived  from 
them.  The  strike  is  north  and  south  and  the  dip  on  an  average  80°  W.  The  maximum  thick- 
ness of  the  belt  is  1,900  feet.  Southward  from  the  point  of  maximum  tliickness  it  rapidly 
thins  out  and  disappears. 

The  iron-bearing  beds  form  a  belt  32  feet  wide  or  less  between  black  slate  walls.  The 
strike  and  dip  are  the  same  as  those  of  the  slate.  A  single  ore  body  of  commercial  importance 
has  been  mined.     (See  p.  324.) 

The  Hemlock  formation  both  east  and  west  of  the  main  belt  of  this  slate  carries  thm  bands 
of  slate  with  similar  strike  and  dip.  In  general,  in  this  vicuiity,  there  is  a  monoclinical  west- 
ward-dipping succession  of  volcanic  rocks  extending  2  miles  or  more  east  of  Mansfield  and  about 
the  same  distance  west,  containing  interbedded  layers  of  slate,  which  in  the  vicinity  of  Mansfield 
are  in  considerable  abundance  and  include  also  iron-bearing  beds.  These  rocks  may  be  best 
seen  on  the  hill  just  east  of  Michigamme  River,  southeast  of  the  Mansfield  mine,  where  eight  or 
ten  layers  of  cherty  slate  from  a  few  inches  to  10  feet  or  more  in  width  are  interlay ered  with 
westward-dipping  ellipsoidal  basalt  flows.  The  centei-s  of  the  flows  are  usually  homogeneous 
and  coarse  grained,  and  the  ellipsoidal  structures  appear  only  within  a  few  feet  of  the  top  or 
bottom  of  the  flow  immediately  next  to  the  slate.  As  a  whole  the  contact  between  the  basalt 
and  the  slate  is  a  plane  surface,  making  it  possible  to  follow  a  bed  of  slate  even  2  feet  thick  for 
hundreds  of  feet.  In  detafl,  however,  the  contact  may  be  very  irregular,  following  interstices 
in  the  ellipsoidal  surface  as  if  deposited  upon  an  initially  irregular  surface. 

Slates  mapi)ed  as  "Mansfield"  by  Smyth  also  outcrop  on  Michigamme  Mountain  and  thence 
at  intervals  for  6  miles  to  the  northwest.  The  area  northwest  of  Michigamme  Mountain,  mapped 
as  Pleistocene  on  Plate  XXII,  is  believed  to  be  largely  underlain  by  slate  from  its  appearance 
in  a  few  pits  and  exposures.  The  information  is  so  meager,  however,  that  it  is  not  thought 
desirable  to  map  this  area  as  slate.     On  Michigamme  Mountain  the  geologic  position  of  the 


296  GEOLOGY  OF  THE  LAKE  SUPEIUOR  REGION. 

so-called  "Mansfield"  rocks  is  free  from  doubt.  In  the  principal  synclinc  of  sec.  32,  T.  44  N., 
R.  31 W.,  they  overhe  tlio  dolomites  and  j)ass  downward  hitcj  them  hy  a  relatively  slow  gradation ; 
on  the  borders  of  the  MicJugamme  Mountain  syncline  they  underlie  the  iron-bearing  Negaunee 
("Groveland")  formation.  The  j)assagc  to  the  higher  formation  likewise  is  graded,  though 
rapidly,  and  is  marked  m  certaui  bands  by  an  increase  m  clastic  grains  and  by  changes  in  the 
character  of  the  matrix  in  whicii  these  are  set.  The  average  thickness  of  the  formation  in  tliis 
mountain  is  not  less  tliau  400  feet. 

NEGAUNEE   0)   FORMATION. 

Magnetic  helts  northeast  of  Fence  River. — By  reference  to  tlie  map  (PI.  XXII,  in  pocket)  it 
will  be  noted  that  there  is  a  magnetic  line  marked  "A"  along  the  west  side  of  the  mam  north- 
eastern area  of  Ai'chean  rocks.  That  tliis  magnetic  line  is  caused  bj'  and  marks  the  position 
of  the  ii-on-bearing  Negaunee  formation  there  can  not  be  much  doubt,  according  to  Snij-th," 
for  that  rock  outcrops  in  a  few  scattered  localities,  occurs  abundantly  in  the  drift,  and  has 
been  found  in  test  pits  and  drill  holes  here  and  there  along  this  Une.  The  underlying  quartzite 
outcrops  beneath  tlie  non-bearing  formation  near  the  north  end  of  the  Une,  but  farther  south 
it  is  entirely  covered  by  the  drift,  so  far  as  the  territory  has  been  examined.  The  overlying 
upper  Huronian  rocks  ai"e  also  known  to  be  present  just  west  of  the  Negaunee  formation  as 
far  south  as  sec.  19,  T.  46  N.,  R.  30  W.  The  dip  along  the  "A"  line  is  probably  therefore, 
on  the  whole,  toward  the  west,  although  the  observed  dips  at  the  few  locahties  where  deter- 
muiations  have  been  made  are  either  vertical  or  slighth'  mchned  fi-om  the  vertical  toward 
the  east.  In  an  east-west  section  of  driU  lioles  in  sees.  IS,  19,  29,  and  30,  T.  46  N.,  R.  30  W., 
cutting  the  magnetic  belt  "A,"  the  iron-bearing  formation  is  found  to  be  amphibole-magnetite 
rock  cut  by  intrusives. 

Ai-ound  the  immecUately  adjacent  Archean  oval  on  the  west  the  magnetic  line  "B"  has 
been  traced  for  25  miles  without  a  single  exposure.  The  known  facts  with  reference  to  the 
"B"  line,  according  to  Smyth,'' are  these:  (1)  It  represents  a  magnetic  rock;  (2)  this  magnetic 
rock  completely  eucncles  an  Archean  core.  It  may  further  be  inferred  with  practical  cer- 
tainty that  this  formation,  winch  carries  such  constant  magnetic  properties  for  25  miles,  must 
be  sedimentary.  With  regard  to  its  structure  the  foregomg  considerations  would  necessarily' 
involve  the  conclusion  that  it  dips  away  from  the  Archean  core  on  all  sides,  ami  this  conclusion 
is  fortified  by  the  unsymmetrical  separation  of  the  horizontal  maxima  on  tlie  magnetic  cross 
sections. 

East  of  the  "B"  line,  between  it  and  the  "A"  line,  is  found  the  basal  member  of  the  upper 
Huronian.  The  rock  which  is  manifest  m  the  "B"  Ihie  must,  therefore,  be  older  than  any 
member  of  the  upper  Huronian.  The  Negaunee  formation,  represented  in  the  "A"  fine,  dips 
west,  but  the  rock  of  the  "B"  hne  dips  east.  They  are  both  older  tlian  the  basal  member 
of  the  upper  Huroruan  and  are  both  younger  than  the  Archean.  They  are  l)oth  strongly  and 
persistently  magnetic.  For  8  or  10  miles  the}'  run  parallel  to  each  other  less  than  half  a  mile 
apart.  Their  broad  structural  relations  to  the  Archean  basement  of  the  region  are  precisely 
similar.  Therefore,  although  the  rock  that  gives  rise  to  the  "B"  line  has  never  yet  been  seen, 
it  may  be  concluded  with  confitience  that  it  is  the  Negaunee  formation,  and  that  the  "A"  and 
"B"  lines  represent  this  rock  brought  up  in  the  two  limbs  of  a  narrow  and  probably  deep 
synclmal  fold. 

Negaunee  (?)  formation  at  MicMgamme  Mountain  and  in  tlie  Fence  Jiiver  area. — The  known 
outcrops  of  u'on-l)earmg  formation  (previously  mapped  as  "Groveland"  formation)  in  tlus 
belt  are  limited  to  three  localities — the  vicinity  of  Michigamme  Mountain,  in  sec.  33,  T.  44  N., 
R.  31  W.,  and  sec.  3,  T.  43  N.,  R.  31  W.;  the  exposures  and  test  pits  at  the  Sholdice  explora- 
tion, in  sec.  21,  T.  45  N.,  R.  31  W.;  and  the  test  pits  at  the  Doane  exploration,  in  sec.  16, 
T.  45  N.,  R.  31  W.  The  last  two  localities  are  1  mile  apart,  and  the  more  southern  is  8  miles 
north  of  Michigamme  Mountain. 

<■  Van  Hise,  C.  U.,  Clements,  J.  M.,  and  Smyth,  H.  L.,  The  Crystal  Falls  iron-bearing  district  of  Michigan:  Jlon.  U.  S.  Geol.  Survey,  vol.  3fi, 
1899,  p.  453. 

b  Idem,  p.  454. 


CRYSTAL  FALLS  IRON  DISTRICT.  297 

Magnetic  lines  connect  the  outcrops  on  Micliigamme  Mountain  witi:  tliose  to  the  north. 
The  magnetic  line  also  extends  beyond  the  outcrops  around  the  north  side  of  the  western 
Ai-chean  oval.  The  eastern  belt  was  not  traced  farther  than  a  mile  southeast  of  Micliigamme 
Mountain.  In  the  central  and  southeastern  portions  of  T.  43  N.,  R.  31  W.,  however,  in  the 
direct  prolongation  of  the  anticlinal  axis,  is  a  broad  belt  of  slight  magnetic  disturbance,  along 
the  western  margin  of  wliich  he  volcanic  rocks,  dipping  west.  In  sec.  26,  T.  43  N.,  R.  31  W., 
this  magnetic  belt  splits  into  two  branches,  one  of  which  runs  directly  east  for  a  mile  and  then 
southeast  mdefinitely,  while  the  other  maintains  a  general  southerly  coiu-se  to  the  south  luie 
of  the  townsliip.  In  sec.  26  large  angular  bowlders,  evidently  tierived  fi"om  the  iron-bearing 
formation,  are  found  in  the  zone  of  magnetic  disturbance,  but  no  outcrops  have  been  discovered. 
There  can  be  little  doubt  that  these  disturbances  roughly  outline  the  position  of  the  Vulcan 
formation  m  the  axial  region. 

Except  in  Micliigamme  Mountain,  the  most  elevated  pomt  of  the  district,  the  n-on-bearing 
formation  is  not  topographically  proimnent.  In  the  Fence  River  area  it  produces  a  more 
subdued  and  somewhat  lower-lying  surface  than  the  underlj-mg  formation,  but  the  difference 
is  slight  and  is  of  Uttle  moment  in  comparison  with  the  confusing  effects  of  glaciation. 

At  Micliigamme  Mountam  the  iron-bearuig  formation  caps  the  hill  hi  a  well-marked 
syncluie,  the  axis  of  which  runs  northwest  and  southeast.  The  structure  is  distmctly  shown 
by  the  attitude  both  of  the  ferruginous  rocks  and  of  the  underlyuig  phjdUtes  ("Mansfield  slate"). 
At  the  Interrange  exploration,  half  a  mile  to  the  south,  is  found  a  secondary  but  more  open 
embayment  of  the  same  syncluie.  These  are  the  only  folds  of  the  Micliigamme  Mountain  area 
sufficiently  deep  to  include  the  iron-bearmg  rocks.  The  thickness  of  the  formation  can  only 
be  guessed  at,  as  no  complete  section  is  exposed,  and  the  data  for  determmmg  its  upper  limit 
are  decidedly  shadowy.  The  magnetic  observations  mdicate  a  breadth  of  400  to  600  feet, 
aind  as  in  the  Fence  River  area  it  is  certainly  much  thinner  than  the  two  lower  formations  its 
thickness  may  be  approximately  500  feet. 

The  rocks  are  interbanded  ferruginous  quartzite  and  actinolite  and  griinerite  scliists, 
which  still  contain  evidence  of  detrital  origin.  The  formation  contains  less  iron  than  the  Vulcan 
formation  of  the  Felch  district,  and  consecjuently  the  Ughter-colored  varieties  are  niore  abundant, 
it  contains  more  detrital  material,  and  in  the  Michigamme  Mountain  area  the  texture  is  generally 
closer  and  less  granular.  Moreover,  in  passing  north  from  the  Micliigamme  Mountain  area  to 
the  Fence  River  area  we  find  at  the  Sholdice  and  Doane  explorations  that  the  lower  portion  of 
the  formation  is  composed  of  ferruginous  quartzite,  which  is  succeeded  higher  up  by  actinolite 
schists  and  griinerite  scliists  similar  in  all  respects  to  the  characteristic  rocks  of  the  Negaunee 
formation  in  the  western  Marquette  district. 

The  stratigrapliic  position  of  the  iron-bearing  formation  is  above  the  Hemlock  formation 
on  Micliigamme  Mountain ;  to  the  west  of  the  mountain  the  formation  is  apparently  below  the 
Hemlock  formation;  to  the  north  of  the  mountain,  in  the  Fence  River  area,  it  is  above  the 
Hemlock  formation.  In  the  last-named  area  nothing  is  known  of  the  nature  of  the  overljang 
rocks. 

Tliis  iron-beaiing  formation  is  doubtfully  called  Negaunee  because  of  its  lithologic  character 
and  because  it  comes  ^\•ithin  2  miles  of  the  ''B"  line  of  attraction,  regarded  by  Smyth  as  Negau- 
nee, suggesting  that  it  is  the  same  belt  brought  up  again  on  the  west  side  of  this  intervening 
gap  of  2  miles  by  synchnal  structure.  On  the  other  hand,  it  is  nearly  connected  by  a  magnetic 
belt  around  the  north  side  of  the  oval  with  the  Vulcan  formation  and  for  this  reason  its  correla- 
tion has  been  regarded  as  doubtful.  However,  by  reference  to  the  map  (PI.  XXII,  in  pocket), 
it  \\"ill  be  noted  that  this  belt  of  supposed  Negaunee,  extending  around  the  north  side  of  the 
oval  and  south  as  far  as  the  north  line  of  T.  45  N.,  R.  33  W.,  fails  to  connect  b}^  nearly  2  miles 
with  the  known  Vulcan  formation,  which  is  represented  by  a  magnetic  line  running  as  far  north 
as  sec.  16,  T.  45  N.,  R.  33  W.  Moreover,  at  the  north  end,  near  the  Red  Rock  mine,  the  Vulcan 
is  associated  with  conglomerate  carrying  fragments  of  an  earlier  iron-bearing  formation  very 
suggestive  of  unconformity.     Still  further,  the  iron-bearing  Vulcan  formation  where  last  seen. 


298  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGIOTs^. 

is  associated  with  red  slates  and  apparently  unaltered,  while  the  rooks  associated  with  the 
magnetic  line  to  the  north,  supposed  to  represent  the  Negaunee,  are  micaceous  and  amphibolitic 
slates  and  scliists  showing  a  mucii  higher  degree  of  metamorphism. 

Ferruginous  quartzite  associated  with  irorir-b earing formution  north  of  Michigamme  Mountain. — 
Ferruginous  quartzite  is  found  in  isolated  exposures  in  sees.  27  and  34,  T.  44  N.,  R.  31  W., 
Michigan,  lying  inune<liutely  east  of  the  eastward-dipping  Randville  dolomite  and  west  of  a 
belt  of  attraction  forming  the  southward  extension  of  a  belt  tliat  is  taken  to  represent  the 
iron-bearing  Negaunee  ( ? )  formation. 

UPPER    HURONIAN    (aNIMIKIE    GROUP). 

The  upper  Huronian  occupies  a  large  part  of  the  western  half  of  the  district,  lapping  around 
the  oval  areas  of  older  rocks  and  coming  into  contact  for  the  most  part  with  tlie  Ilendock 
formation.  It  is  directly  continuous  with  the  upper  Huronian  rocks  of  the  Marquette  district 
on  the  north  and  with  those  of  the  Menominee  district  on  the  south  and  also  extends  far  west 
of  the  boundaries  of  the  area  mapped  into  the  Iron  River  and  Florence  districts.  The  exposures 
are  scanty.  The  formation  influences  the  topography  very  shghtly,  being  for  the  most  part 
heavily  drift  covered. 

Tlie  upper  Huronian  in  this  district  is  essentially  a  great  slate  formation  interbedded  with 
small  quantities  of  graywacke  and  chert,  called  the  Micliigamme  slate,  and  near  its  base  vnth. 
iron-bearing  lenses  called  Vulcan  iron-bearing  member. 

MICHIGAMME  SLATE. 

General  character. — The  formation  known  as  the  Michigamme  slate  consists  principally  of 
slates  wdth  some  graywacke,  Hke  that  of  the  Menominee  district.  In  previous  reports  on  this 
distiict  it  has  been  called  upper  Huronian  slate,  but  as  the  formation  seems  to  be  equivalent  to 
and  continuous  wdth  the  Michigamme  slate  of  the  Marquette  district,  the  name  ilichigamme 
will  be  appUed  to  it  in  this  monograph.  True  water-deposited  conglomerates  are  usually 
absent  in  this  formation,  being  known  in  only  two  places  in  this  district,  and  in  these  places 
their  stratigraphic  position  is  unknown.  For  the  most  part  the  slates  have  good  cleavage  and 
locally  they  are  highly  graphitic,  chloritic,  sericitic,  and  micaceous,  and  rarely  staurohtiferous 
and  garnetiferous.  The  cleavage  usually  stands  nearly  vertical,  but  the  bedding  may  have 
gentle  dips.  In  general  the  rocks  may  be  said  to  lap  in  broad  folds  around  the  lower 
Huronian,  but  everywhere  with  minor  phcations.  The  result  is  that  in  the  ore-producing 
parts  of  the  district,  in  the  Crystal  Falls  and  Amasa  areas,  the  dip  and  pitch  of  the  minor 
folds  are  in  general  westerly  and  southwesterly  directions.  Away  from  the  base  of  the  forma- 
tion it  is  difficult  to  identify  horizons  in  the  slates,  and  tliis  fact,  together  with  lack  of  expo- 
sures, has  thus  far  prevented  the  working  out  of  the  structure  satisfactorily.  In  general, 
■however,  the  strikes  and  dips  at  these  horizons  away  from  the  base  of  the  formation  are  similar 
to  those  near  the  base;  that  is,  the  strikes  are  in  northerly  and  northwesterly  directions,  and 
the  dips  and  pitches  of  the  minor  folds  are  westerly  and  southwesterly.  The  exposures  imme- 
diately above  Crystal  Falls  seem  to  be  part  of  a  much  crenulated  southwestward  pitching 
S3mchne.  It  has  been  assumed  further  that  the  area  of  volcanic  rocks  associated  with  the  upper 
Huronian  slates  northwest  of  the  town  of  Crystal  Falls  is  of  Hemlock  age,  and  hence  that  it 
represents  an  antichne  brought  up  by  folding.  If  these  volcanic  rocks  should  be  really  later 
in  age. than  the  Hemlock,  as  is  entirely  possible  (see  p.  299),  then  there  is  left  no  evidence 
for  this  antichnal  structure  in  this  locality.  As  mining  explorations  furnish  more  data  it 
should  be  possible  to  work  out  the  structure. 

Vtilcan  iron-hearing  member. — The  Vulcan  iron-bearing  member  is  similar  to  and  is  correlated 
wdth  the  Vulcan  formation  of  the  ]\Ienominee  district.  In  the  Crystal  Falls  tlistrict  it  consists 
principally  of  ferruginous  chert,  ferruginous  slate,  iron  ore,  and  iron  carbonate,  interbediled 
in  layers  and  lenses  in  the  Michigamme  slate.  It  is  tlierefore  treated  as  a  member  of  the  Michi- 
gamme slate  m  this  district.     The  immetliately  adjacent  wall  rocks  of  slate  are  as  a  rule  highly 


CRYSTAL  FALLS  IRON  DISTRICT.  299 

carbonaceous  and  pyritiferous.  The  iron-bt^aring  layers  range  in  thickness  from  a  few  inches 
to  300  feet,  and  are  even  thicker  where  repeated  by  buckling.  This  buckling  is  of  a  drag  type, 
giving  steep  pitches  and  not  materially  changing  the  dip  and  trend  of  the  member  as  a  whole. 
Folds  of  similar  types  are  characteristic  of  the  Iron  River  and  Menominee  districts.  (See 
pp.  324,  347.)  Explorations  are  not  yet  sufficient  to  correlate  the  individual  iron-bearing  layers 
in  different  parts  of  the  district  satisfactorily,  and  it  is  impossible  now  to  say  wdiether  there  are 
one,  two,  or  more  independent  layers  separated  by  slate,  though  the  probabiHty  is  in  favor  of 
there  being  at  least  two  principal  horizons  near  the  base  of  the  upper  Huronian,  as  in  the 
Menominee  district. 

The  map  of  the  Crystal  Falls  district  (PI.  XXII,  in  pocket)  shows  that  the  distribution  of 
the  Vulcan  iron-bearing  member  has  certain  linear  characteristics.  One  bolt  follows  the  baSe 
of  the  upper  Huronian.  Beginning  4  miles  north  of  Amasa,  it  has  been  followed  by  mag- 
netic lines  and  intermittent  mines  and  explorations  southeastward  past  Amasa  and  Balsam; 
thence  southeastward  to  the  vicinity  of  the  Holhster  and  Armenia  mines  near  the  east  side 
of  T.  43  N.,  R.  32  W. ;  thence  southwestward  and  westward  through  the  Lee  Peck,  Hope, 
West  Hope,  Morrow,  May,  Kimball,  and  Tobin  mines  south  of  Crystal  Falls;  and  thence 
southward  through  the  Dunn  and  Mastodon  mines.  The  real  continuity  of  this  belt  has  not 
been  estabhshed  at  every  point,  but  the  probabihty  of  continuity  is  so  great  that  exploration 
is  being  vigorously  conducted  at  many  points  along  the  belt.  Another  belt  of  iron-bearing 
rocks  extends  from  the  Crystal  Falls  mine  east  of  Crystal  Falls  westward  through  the  Great 
Western,  Pamt  River,  Lamont,  and  Bristol  mines.  This  belt  may  be  at  a  higher  horizon  in  the 
upper  Huronian.  It  has  not  yet  been  cormected  with  the  one  previously  noted,  though  it  is 
too  early  to  say  that  a  connection  may  not  exist.  A  possibihty  of  comrection  seems  to  be 
indicated  by  certain  explorations  between  the  two  belts  just  east  of  Paint  River  east  of  the 
town  of  Crystal  Falls.  Developments  in  the  Iron  River  district  have  shown  the  iron-bearing 
member  to  extend  eastward  toward  the  Crystal  Falls  district,  raising  the  question  of  connection 
with  one  of  the  iron  belts  in  the  vicinity  of  Crystal  Falls,  but  so  far  as  the  Crystal  Falls  district 
itseK  is  concerned  such  a  coimection  is  not  yet  shown  by  explorations. 

The  magnetic  belt  marking  the  extension  of  the  iron-bearing  member  of  the  Amasa  dis- 
trict to  the  north  and  south  is  caused  partly  by  the  iron-bearmg  member  and  partly  by  mag- 
netic surface  portions  of  the  ellipsoidal  basalts  of  the  Hemlock  formation  near  their  contact 
with  the  upper  Huronian.  At  certain  places,  as  in  the  vicinity  of  the  Hollister  mine,  there 
are  really  two  parallel  magnetic  belts  rather  than  a  single  belt.  One  of  these  belts  follows 
the  magnetic  phase  of  the  greenstone  and  the  other  the  iron-bearing  member  immediately 
adjacent.  It  is  apparent,  therefore,  that  not  much  reliance  may  be  placed  on  the  assumption 
that  the  iron-bearing  member  exists  every whei-e  beneath  the  magnetic  belt. 

It  is  an  imexplained  fact  that  parts  of  the  Vulcan  iron-bearing  member  away  from  the 
Hemlock  formation,  particulai'ly  near  Crystal  Falls  and  farther  south,  are  but  slightly  magnetic. 
This  is  also  true  of  the  Vulcan  iron-bearing  member  in  the  Iron  River  district  to  the  west. 

INTRUSIVE  AND  EXTRUSIVE  ROCKS  IN  UPPER  HURONL&N. 

The  upper  Huronian  is  penetrated  in  this  district  by  intrusive  rocks  of  acidic,  interme- 
diate, and  basic  composition.  wSome  of  these  have  been  intruded  before  the  folding  and  are 
very  schistose.     Most  of  them,  however,  are  later. 

Basaltic  extrusive  rocks,  identical  in  all  features  with  the  Hemlock  formation,  appear  in 
isolated  areas  in  the  upper  Huronian.  The  principal  areas  are  immediately  northwest  of 
Crystal  Falls  and  in  the  vicinity  of  the  Mastodon  mine.  There  is  as  yet  no  evidence  to  show 
whether  these  extrusive  rocks  are  the  correlatives  of  the  main  mass  of  Hemlock  formation 
and  owe  their  present  distribution  to  folding  or  whether  they  are  really  later  extrusives  inter- 
bedded  with  the  upper  Huronian  Michigamme  slate. 


300  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

RELATIONS  OF  THE  UPPER  HaRGITIAN  TO  UNDERLYING  ROCKS. 

In  genonil  tho  dip  and  the  disLribution  of  tlie  upper  Iluroiiian  about  the  cores  of  older 
rocks  show  it  to  bo  distinctly  younger  than  these  rocks.  The  next  underlying  rocks  for  the 
most  part  are  those  of  the  Hemlock  formation. 

The  upper  Ihiionian  is  doubtfully  unconformable  structurally  with  the  Hemlock  formation. 
In  the  earlier  Geological  Survey  rej)orts  on  this  district  the  two  were  described  as  unconfornuiljle, 
largely  because  of  their  marked  difference  in  lithology  and  because  of  the  fact  than  an  unconform- 
ity exists  at  the  base  of  the  upper  Huronian  of  the  Marquette  district,  with  which  the  upper  Huro- 
nian  of  the  C'r\'stal  Falls  district  is  satisfactorily  correlated.  No  direct  evidence  is  j-et  available 
to  show  that  there  is  not  some  time  break  between  the  Hemlock  formation  and  the  upper 
Huronian  slates,  but  the  apparent  structural  conformity,  together  with  the  conformable  rela- 
tions of  the  upper  Huronian  to  underlying  formations  in  the  Menominee  and  Felch  Mountain 
districts,  seems  to  point  to  the  possibility  of  nearly  if  not  quite  contiimous  deposition  of  Huron- 
ian sediments  beginning  with  the  Ranilville  and  Sturgeon  formations  and  continuing  through 
the  upper  Huronian.  On  the  other  hand,  the  existence  of  an  unconformity  is  strongly  sug- 
gested by  the  relations  of  the  two  magnetic  belts  taken  to  represent  respectively  the  iron-bearing 
Vulcan  and  Negaunee  formations  northward  from  tlio  Red  Rock  mine,  where  there  is  a  break 
in  the  magnetic  field  and  a  difl'erence  in  lithology  and  metamorphism,  and  where  a  conglomerate 
at  the  base  of  the  upper  Huronian  can'ies  pebbles  of  an  earlier  (supposedly  Negaunee)  iron- 
bearing  formation.     (See  p.  297.) 

CAMBRIAN    SANDSTONE. 

In  the  southern  and  eastern  portions  of  the  district  the  edges  of  the  tilted  older  rocks  are 
partly  covered  by  a  blanket  of  gently  dipping  sandstones  of  Cambrian  age,  very  soft  and  easily 
dismtegrating.  These  rocks  appear  near  Michigamme  River  as  detached  outhers.  To  the 
south  and  east  from  that  river  the  separated  patches  become  larger  and  more  abundant,  until 
finally  a  few  miles  bej^ond  the  eastern  limit  of  the  Felch  Mountain  range  they  unite  and  entirely 
cover  the  pre-Cambrian  formations. 

STURGEON   RF^ER   DISTRICT." 

LOCATION  AND  AREA. 

The  Sturgeon  River  area  of  Algonkian  sediments,  like  the  Felch  Mountain  area,  is  an  east- 
west  tongue,  very  narrow  at  its  eastern  extremity  and  wadening  out  toward  the  west  imtil  it 
finally  plimges  under  drift  deposits  that  separate  it  from  the  large  Huronian  area  of  the  Crystal 
Falls  district.  The  tongue  occupies  the  western  portions  of  T.  42  N.,  R.  27  W.,  and  the  central 
and  northern  portions  of  T.  42  N.,  Rs.  28,  29,  and  30  W.  The  best  exposures  of  the  roclvs 
constitutmg  the  tongue  are  foimd  in  sees.  7,  8,  17,  and  18,  T.  42  N.,  R.  28  W.,  and  sees.  1  and 
3,  T.  42  N.,  R.  29  W.,  on  or  near  the  northwest  branch  of  the  east  branch  of  Sturgeon  River; 
hence  the  name  Sturgeon  River  tongue. 

GENERAL,   SUCCESSION. 

The  succession  is  as  follows : 

Algonkian  system: 

Keweenawan  series  (?) - Sandstone. 

Huronian  series: 

,,.,',,    TT         .       ,„,  [Basic  igneous  rocks,  largely  intrusive. 

Middle  Huronian  (?) {  ^  [■      ,•       ^       ■     \ 

(Negaunee  (?)  formation  (iron  bearing). 

Unconformity  (?). 

,  .        „         .  IRandville  doli)niile. 

Lower  Huronian ^ , i  , 

(Murgeon  quartzite. 

Unconformity. 

Archcan  system: 

Laurentian  series Granites  and  gneisses. 

a  Bayley,  W.  S.,  Tho  Sturgeon  River  tongue :  Mon.  V.  S.  Ceol.  Survey,  vol.  36, 1899,  pp.  458-487. 


STURGEON  RIVER  DISTRICT.  301 

ARCHEAN   SYSTEM. 

LAUBENTIAN  SERIES. 

Laurentian  granites  and  gneisses  bound  the  Algonkian  sediments  on  the  north  and  south. 
Also  between  the  northern  and  the  southern  boundaries  of  the  sedimentary  area  as  defined, 
and  in  the  midst  of  the  sediments,  are  two  areas  of  granite,  the  rociv  of  one  of  which  is  unques- 
tionably and  that  of  the  other  presumably  older  than  the  conglomerates  v/ithin  the  tongue. 
The  better  dcfuied  of  these  two  areas  lies  in  the  northern  i)ortions  of  sees.  7  and  8,  T.  42  N., 
R.  28  W.,  and  sec.  12,  T.  42  N.,  R.  29  W. 

ALGONKIAN   SYSTEM. 

HTJRONIAN  SERIES. 
LOWER    HUKONIAN. 
STURGEON  QTJARTZITE. 

In  this  district  the  Sturgeon  quartzite  is  represented  by  schists,  conglomerates,  arkoses, 
and  quartzites  1,000  feet  or  more  thick.     Nowhere  is  there  any  marked  discortlance  between 
.  the  schistosity  of  the  Archean  and  Sturgeon  rocks,  but  the  conglomerate  indicates  a  marked 
unconformity. 

RANDVILLE  DOLOMITE. 

In  the  Sturgeon  River  trough  the  dolomites  have  relatively  more  fragmental  material 
with  them  than  in  the  Felch  Mountain  trough.  Exposures  are  few  and  occupy  here  the  central 
■portion  of  the  trough.  Tlie  dolomites  do  not  themselves  show  the  synclinal  structure  of  the 
Sturgeon  trough,  but  the  fact  that  they  are  bounded  by  the  quartzite  on  the  northeast  and 
southwest  and  this  in  turn  by  the  Archean  granite  suggests  trough  structure.  No  definite 
contacts  of  Archean  granites  and  the  dolomites  are  known. 

MIDDLE    HTJRONIAN  (1). 
NEGADNEE  (!)  FORMATIOIT. 

Bordering  the  north  side  of  the  dolomite  in  sees.  34  and  35,  T.  43  N.,  R.  29  W.,  is  non- 
magnetic red  and  black  hematitic  chert,  associated  with  red  slate,  shown  in  the  Deerhunt  mine 
explorations.  Neither  hanging  or  foot  wall  was  determined  in  the  exploratory  work  and 
the  relations  of  the  iron-bearing  formation  to  the  other  formations  are  therefore  imknown. 
The  iron-bearing  formation  is  doubtfully  correlated  with  the  Negaunee. 

IGNEOTTS  ROCKS. 

Associated  with  the  sedimentary  rocks  are  great  masses  of  basic  igneous  rocks.  Some  of 
these  are  unquestionably  intrusive  masses,  as  shown  by  their  relations  to  the  conglomerates; 
others  appear  to  be  interleaved  sheets.  A  very  few,  apparently  bedded  greenstones,  on  close 
examination  seem  to  be  composed  of  intermingled  sedimentary  and  igneous  material.  These 
may  be  altered  tuffs. 

KEWEENAWAN  SERIES  (?). 

In  the  SW.  i  sec.  34,  T.  43  N.,  R.  29  W.,  are  wliite  .calcareous  sandstones  associated  wdth 
purple  slates,  mth  dips  ranging  from  35°  to  40°.  These  are  similar  in  all  respects  to  tlie  upper 
series  in  the  east  end  of  the  Felch  Mountain  trough,  and  there  are  the  same  elements  of  doubt 
with  reference  to  their  correlation.  They  are  provisionally  assigned  to  the  Keweenawan,  but 
they  may  prove  to  be  of  Cambrian  age. 


302  GEOLOGY  OF  THE  LAKE  SLTERIOR  REGION. 

FELCH  MOUNTAIN  DISTRICT." 

LOCATION,   STRUCTURE,  AND   GENERAL.  SUCCESSION. 

The  Felch  Mountain  district  is  an  east-west  synclinorium  constituting  a  narrow  strip 
nowhere  more  than  1^  miles  and  usually  less  than  a  mile  ^^'ide,  which  as  a  whole  runs  almost 
exactly  east  and  west  for  a  distance  of  over  13  miles.  It  lies  in  the  southern  portion  of 
T.  42  N.,  Rs.  28,  29,  and  30  W.  On  tiie  north  and  south  it  is  Ijordcrcd  by  tlie  older  Archean. 
The  lowest  member  of  the  Algonkian  occupies  parallel  zones  next  to  the  Archean  on  both  the 
north  and  tlic  south  and  is  succeeded  toward  the  interior  of  the  strip  by  the  younger  members. 
Although  tlic  general  structure,  therefore,  is  synclinal,  a  single  fold  of  sunple  type  has  nowhere 
been  found  to  occupy  the  whole  cross  section  of  the  Algonkian  formations,  but  usually  two  or 
more  svnclines  occur,  separated  by  anticlines,  which  may  have  diiTerfiit  degrees  and  directions 
of  pitcli  and  dilTerent  strikes,  or  may  be  sunk  to  different  depths,  and  complicated  besides  both 
by  subordinate  folds  and  by  faults. 
The  succession  is  as  follows: 

Intrusive  rocks  (basic  and  acidic). 
Algonkian  system: 

Ke weenawan  series  (?) Mica  8chist.s  and  f erruginou.'i  and  micaceous  quartzites. 

Unconformity. 

Huronian  series: 

TT         TT         ■      / 1    •     1  •  \     I  Vulcan  formation  (iron  bearing). 

Upper  Huronian  (Anmiikie  group).. <  „     ...  ^  ^' 

*^'  ^  hi/     [Felch  schist. 

Unconformity  (?). 

,  „        •.  f Randville  dolomite. 

Lower  Uuronian <  „ 

l&turgeon  quartzite. 

Unconformity. 

Archean  system: 

Laurentian  series - Granites  and  gneisses, 

ARCHEAN   SYSTEM. 

LAURENTIAN  SERIES. 

The  Laurentian  series  is  the  same  as  that  of  the  other  areas  in  Michigan  here  described. 
The  contact  between  the  Laurentian  and  Hm-onian  is  not  exposed,  but  the  existence  of  con- 
glomerate elsewhere  along  the  contact  and  tlie  fact  that  the  contact  is  followed  uniformly 
by  quartzite  are  believed  to  indicate  unconformity. 

ALGONKIAN   SYSTEM. 

HURONIAN  SERIES. 
LOWER    IIUROXIAN. 
STURGEON  QTJAHTZITE. 

In  the  Felch  Moimtain  district  the  Sturgeon  quartzite,  less  than  500  feet  tliick,  forms 
two  liands  in  contact  with  the  Archean  granite  bordering  the  Felch  Moimtain  synchne.  It  is 
here  principally  tjuartzite  but  contains  conglomerate  near  the  contact.  It  is  in  most  places 
extremely  diflicult  to  deternune  the  attitude  of  the  quartzite  owing  to  its  massive  and  homo- 
geneous character.  Closely  associated  with  the  massive  quartzites  are  mashed  quartzites  or 
micaceous  quartz  schists. 

RANDVILLE  DOLOHITE. 

Owing  both  to  its  great  thickness  and  to  its  intermediate  position,  the  Randville  dolo- 
mite in  the  Felch  Mountain  range  covers  a  larger  share  of  the  surface  than  any  otlier  member 
of  the  Algonkian  succession.     No  actual  contacts  between  the  Sturgeon  and  Randville  for- 


aSmyth,  H.  L.,  The  Felch  Mountain  range :  Uon.  U.  S.  Oeol.  Survey,  vol.  36, 1S99,  pp.  374-t26. 


FELCH  MOUNTAIN  DISTRICT.  303 

mations  have  been  found,  but  from  their  close  association  and  continuity,  as  well  as  from  the 
structural  characters,  where  these  are  determinable,  they  seem  evcrywiicre  to  be  strictly  con- 
formable. The  best  sections  give  a  wide  range  of  thickness,  from  a  minimum  of  about  500 
feet  near  Felch  Mountain  to  a  maximum  of  nearly  1,000  feet  in  the  western  part  of  the  district. 
Though  the  discrepancies  may  be  partly  due  to  lack  of  precision  in  the  data,  it  is  probable 
that  the  tliickness  of  the  formation  is  not  uniform  but  really  increases  from  east  to  west. 

UPPER    HUEONIAN     (aNIMIKIE    GROUP). 
FELCH  SCHIST. 

In  the  Felch  Mountam  district  schists  not  more  tlian  200  feet  thick  lie  between  the  dolo- 
mite on  the  one  hand  and  the  Vulcan  formation  on  the  other.  They  do  not  outcrop  but  have 
been  piercetl  by  many  drill  holes.  The  greater  part  of  them  are  fuie-grained  mica  schists, 
containing  garnet  and  tourmaline.  Near  the  contact  with  the  overlying  Vulcan  formation  the 
schists  become  more  siliceous  and  more  ferruginous  and  there  is  a  passage  between  the  two 
formations.  These  schists  were  called  "Mansfield  schists"  by  Smyth"  and  correlated  with 
the  slates  at  Mansfield  and  Michigamme  Mountam.  The  slates  at  Mansfield,  however,  are 
regarded  in  this  report  as  older  than  the  schists  of  the  Felch  Mountain  district,  and  m  any 
event  not  certainly  to  be  correlated  with  tliem.  The  new  name  "Felch  schist"  is  therefore 
introduced  for  tliis  formation  from  its  typical  development  at  Felch  Momitain. 

VTTLCAN  FORMATION. 

In  the  Felch  Mountain  district  the  Vulcan  formation  is  magnetic  and  has  been  traced 
by  means  of  compass  and  dip  needle.  Excellent  natural  as  well  as  niunerous  artificial  expo- 
sures render  the  data  concernmg  the  distribution  of  the  formation  very  satisfactory. 

On  the  west  tlie  iron-bearing  formation  is  exposed  in  ledges  and  test  pits  in  sec.  5, 
T.  41  N.,  R.  30  W.,  from  which  a  line  of  attraction  extends  southwestward  through  sec.  6  into 
sec.  12,  T.  41  N.,  R.  31  W.,  where  it  is  lost.  The  presence  of  the  Vulcan  formation  through 
sees.  34,  35,  and  36,  T.  42  N.,  R.  30  W.,  is  shown  by  one  principal  and  other  minor  lines  of 
attraction,  as  well  as  by  test  pits  and  outcrops.  The  principal  line  of  attraction  begins  in 
sec.  34,  near  the  southwest  corner,  and  rmis  to  the  northeast,  in  conformity  with  the  strike 
of  the  northern  belt  of  dolomite,  finally  ending  m  the  northeastern  portion  of  sec.  36.  This 
line  of  attraction  is  very  vigorous  and  strongly  marked.  Two  other  lines,  parallel  with  the 
principal  line  but  more  feeble  and  much  shorter,  cross  the  boundary  between  sees.  35  and  36, 
and  on  the  northern  of  these  Imes  ferruginous  rocks  outcrop  in  the  western  part  of  sec.  36. 
Another  line,  marking  the  west  end  of  the  Groveland  syncline,  begins  near  the  center  of  sec.  36 
and  contmues  for  1^  miles  eastward  to  the  eastern  portion  of  sec.  31,  T.  42  N.,  R.  29  W. 
Along  the  western  portion  of  this  line  are  many  test  pits  and  in  sec.  31  occur  the  fine  exposures 
of  the  Groveland  liill. 

Another  line  of  attraction  begins  400  paces  north  of  the  center  of  sec.  32,  T.  42  N., 
R.  29  W.,  which  may  be  followed  eastward  without  interruption  nearly  to  the  east  line  of 
sec.  33  of  the  same  township.  Along  this  line,  which  is  comparatively  feeble  and  crosses  wet 
ground,  there  are  but  few  test  pits.  In  the  eastern  part  of  sec.  33,  beyond  the  pomt  at  wliich 
the  attraction  ceases,  many  pits  have  been  sunk  to  and  into  the  Felch  schist,  which  is  there 
somewhat  ferruginous.  From  this  point  eastward  for  4  miles  the  Vulcan  formation  has  not 
been  recognized. 

In  the  northern  part  of  sees.  32  and  33,  T.  42  N.,  R.  28  W.,  the  ferruginous  rocks  are  well 
exposed  on  Felch  Mountain  for  nearly  a  mile  along  the  strike,  and  may  be  identified  for  half  a 
mile  farther  by  the  vigorous  disturljances  produced  in  the  magnetic  neetUes.  In  the  SE.  | 
sec.  33  the  Vulcan  formation  is  again  encountered  in  a  small  and  much  disturbed  area,  in 
faulted  contact  with  the  Archean. 

"Van  Hise,  C.  R.,  Clements,  J.  M.,  and  Smyth,  H.  L.,  The  Crystal  Falls  iron-bearing  district  of  Michigan:  Mon.  U.  S.  Geol.  Survey,  vol.  36, 
1899,  pp.  411-415. 


304  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

The  most  prominent  hills  in  the  Algonkian  belt  owe  their  relief  to  the  fact  that  tlicy  are 
underlain  by  the  Vulcan  formation. 

Petrographically  two  main  kinds  of  rock  may  be  recognized  in  tliis  formation.  The  more 
common  kind  consists  of  quartz  and  the  anhydrous  oxides  of  iron;  the  other  and  mucii  rarer 
Icind  consists  essentially  of  an  inMi  amphibole  with  quartz  and  the  iron  oxides  as  associates. 
Both  of  these  kinds  are  clearly  of  detrital  origin.  The  conclusion  is  reached,  based  on  certain 
microscopic  structures,  .that  iron  and  silica  were  originally  present  largely  in  the  form  of  green- 
alite.  Between  the  ferruginous  quartzites  of  the  Vulcan  formation  and  the  ferruginous  cherts 
of  the  Mesabi  range  there  is  a  very  close  resemblance,  esjjeciall}-  in  structure.  Tiie  essential 
difference  is  that  the  former  contain  little  or  no  chalcedony,  the  silica  being  crystallized  quartz, 
whereas  the  latter  Imve  a  great  deal  of  chalcedonic  silica.  Also  the  former  contain  small 
amounts  of  detrital  material,  wliich  the  latter  generally  lack;  but  the  essential  difference 
between  them  is  one  of  degree  of  crystallization  only. 

In  Smyth's  report  on  tliis  area"  the  iron-bearing  formation  of  the  Felch  Mountain  (Ustrict 
was  called  the  "Groveland"  formation  from  its  occurrence  at  the  Groveland  mine.  The  e\ddence 
is  now  regartled  as  sufTicient  for  correlating  it  with  the  Vulcan  formation  of  the  Menominee  and 
Crystal  Falls  districts,  and  hence  the  name  "Groveland"  is  discarded. 

KEWEENAW  AN  SERIES  (?). 

In  the  east  end  of  the  Felch  Mountain  range  the  Iluronian  rocks  are  overlain  unconfomiably 
by  a  series  of  soft  iron-stained  mica  scliists,  with  tliin  interbanded  beds  of  ferruginous  and 
micaceous  quartzite.  From  their  structures  and  general  relations  they  are  believed  to  have  been 
derived  from  sedimentary  rocks  by  metamorpliism.  At  an  old  open  pit  just  west  of  Felch,  on 
the  east  side  of  sec.  33,  this  series  may  be  seen  to  rest  in  unconformable  contact  with  the  Rand- 
ville  dolonute,  the  basal  conglomerate  being  heavily  ferruginous  and  having  been  mined  as  iron 
ore.  These  rocks  are  tentatively  assigned  to  the  Keweenawan  series,  although  they  may  prove 
to  be  of  Cambrian  age. 

INTRUSIVE  ROCKS. 

Basic  and  acidic  intrusive  rocks  cut  the  Huronian  at  several  localities.  Some  of  the  basic 
intrusives  are  in  the  form  of  sheets,  some  of  them  highly  schistose  and  greatly  altered. 

PALEOZOIC  SANDSTONE  AND  LIMESTONE. 

The  Paleozoic  is  represented  by  the  Lake  Superior  sandstone,  supposedly  of  Upper  Cam- 
brian age,  and  the  overlying  calciferous  limestone.  These  formations  were  originally  laid  down 
over  the  upturned  edges  of  the  older  rocks  in  flat  sheets  or  with  low  initial  dips  and  have  not 
since  suffered  relative  displacement  to  any  notable  degree.  As  has  already  been  stated,  sub- 
sequent erosion  has  to  a  great  extent  removed  this  overlying  blanket  and  laid  bare  tlie  older 
rocks,  except  for  the  covering  of  recent  glacial  deposits.  The  Cambrian  sandstone  and  to  a 
less  extent  the  calciferous  limestone  still,  however,  occupy  considerable  outlying  areas,  detached 
from  one  another  throughout  most  of  the  district  but  gradually  coalescing  beyond  the  east  end, 
where  they  completely  cover  the  older  rocks  and  limit  all  further  geologic  study  of  those  rocks 
in  that  direction. 

CORRELATION. 

Laurentian  fienes. — The  correlation  of  the  main  mass  of  granite  gneiss  north  and  south  of 
the  Felch  Mountain  district  with  the  Laurentian  series  of  the  Arcliean  is  fairly  certain  in  view 
of  its  essential  unconformity  beneath  the  Sturgeon  quartzite,  but  granitic  dikes  also  penetrate 
the  Huronian  series,  suggesting  that  a  part  at  least  of  the  Ijaurentian  complex  may  be  intrusive. 
Such  part  has  not  been  discriminated. 

a  The  Crystal  Falls  iron-bearing  district  of  Michigan:  Mon.  U.  S.  Geol.  Survey,  toI.  36, 1899,  pp.  41&-423. 


FELCIi  MOUNTAIN  DISTRICT.  305 

Lower  Huronian. — The  correlation  of  the  Sturgeon  quartzite  and  the  Randville  dolomite 
respectively  with  the  Mesnard  quartzite  and  the  Kona  dolomite  of  the  lower  Huronian  of  the 
Marquette  district  seems  to  be  reasonable,  the  rocks  of  both  areas  being  highly  folded,  much 
metamorphosed,  and  near  the  base  of  the  lower  Huronian,  and  no  evidence  being  known  in  the 
Upper  Peninsula  of  the  existence  of  a  second  dolomite  series. 

Upper  Huronian  {AnimiJcic  group). — The  Felch  scliist  grades  up  into  the  iron-bearing  Vulcan 
formation  and  undoubtedly  constitutes  a  fragmental  base  of  the  iron-bearing  formation.  Con- 
tacts of  the  Felch  schist  with  the 'underl3dng  dolomite  are  lacking.  From  the  sheared  nature 
of  the  slate  it  seems  hkely  that  the  contact  is  scliistose  and  that  any  evidence  of  conglomerate 
at  the  base  may  have  been  obhterated,  but  the  structure  of  the  Felch  and  Vulcan  formations 
is  essentially  conformable,  so  far  as  can  be  determined,  with  that  of  the  Randville  dolomite  and 
Sturgeon  cjuartzite  below.  The  two  have  been  folded  together.  There  has  been  question  whether 
the  Felch  and  Vulcan  formations  should  be  assigned  to  the  middle  Huronian,  which  includes 
the  Negaunee  formation,  or  to  the  upper  Huronian,  which  includes  the  Vulcan  formation  of  the 
Menominee  district.  Both  the  mitldle  Huronian  and  the  upper  Huronian  have  an  unconformity 
at  the  base,  and  hence  the  lack  of  evidence  of  unconformity  at  the  base  of  the  iron-bearing 
formation  of  the  Felch  Mountain  district  does  not  aid  in  the  selection  of  one  of  these  alternatives. 
The  iron-bearing  formation  of  the  Felch  Mountain  district  is  geograpliically  separated  from  both 
the  Negaunee  formation  of  the  Marquette  district  and  the  Vulcan  formation  of  the  Menominee 
cHstrict.  At  the  west  end  of  the  Felch  Mountain  district  the  formation  may  be  followed  by 
magnetic  work  to  the  west  under  the  Uiain  area  of  the  upper  Huronian.  A  few  miles  south,  in 
the  Calumet  trough,  an  iron  formation  similar  to  the  Vulcan  and  underlain  by  slate  and  schist  of 
the  Felch  variety  may  be  traced  to  the  west  and  southwest  with  reasonable  continuity  and  ^vith 
uniform  lithology  into  the  broad  area  of  upper  Huronian  joining  areally  with  the  upper  Huronian 
of  the  Menominee  district.  To  the  southeast  also  there  is  probable  connection  with  the  Menomi- 
nee district.  The  lithology  of  the  iron  formation  in  the  Felch  Mountain  district  is  more  like 
that  of  the  Vulcan  than  that  of  the  Negaunee  formation.  These  facts  suggest  strongly  that  the 
iron-bearing  formation  of  the  Felch  Mountain  district  is  an  eastward  projection  of  the  mam 
upper  Huronian  of  Michigan,  other  eastward  extensions  of  this  area  being  fount!  in  the  Menominee, 
Calumet,  and  Marquette  districts.  If  not,  the  line  of  demarkation  between  the  upper  Huronian 
and  the  iron-bearing  formation  of  the  Felch  district  is  not  yet  known.  The  evidence  for  corre- 
lating the  iron-bearing  formation  of  tliis  district  with  the  Vulcan  formation  of  the  Menominee  and 
Crystal  Falls  districts  is  regarded  sufTicient,  and  the  old  name  "Groveland,"  as  heretofore  used 
in  this  district,  has  therefore  been  abandoned  for  Vulcan.  The  apparently  conformable  relations 
of  the  Felch  and  Vulcan  formations  with  the  underlying  Randville  and  Sturgeon  formations  do 
not  disprove  unconformity.  The  relations  may  really  be  the  same  as  in  the  Menominee  and 
other  adjacent  districts. 

Keweenawan  series{1). — The  purple  sandstones  overlying  the  Vulcan  formation  at  the  east 
end  of  the  trough  look  in  many  of  their  outcrops  like  Canabrian  rocks,  and  were  it  not  for  their 
dip  of  30°  or  thereabouts  they  would  probably  be  mapped  as  Cambrian,  for  farther  east  the 
Cambrian  is  flat-lying.  It  is  entirely  possible  that  the  Cambrian  has  been  tilted  up  in  tliis  place. 
However,  a  similar  series  of  sandstones  in  the  Sturgeon  trough  is  also  tilted  up,  and  tliis,  in 
connection  with  the  reddish-purple  color  of  the  beds,  suggests  the  possibihty  of  Keweenawan 
sediments  intervening  between  the  Huronian  on  the  one  hand  and  the  Paleozoic  on  the  other. 
Their  position  above  the  Vulcan  formation  and  their  friable  character,  however,  seem  to  preclude 
the  probability  of  their  being  Huronian. 

47517°— VOL  52—11 ^20 


306  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

CALUMET  DISTRICT. 

LOCATION  AND  GENERAL  SUCCESSION. 

Tlio  Calumet,  district  is  an  east-west  trough  soutli  of  the  Felch  Mountain  trough,  extending 
through  T.  41  N.,  lis.  27  to  30  W.  (PI.  XXIII). 
The  succession  is  as  foflows: 

Cambro-Ordovician Hermansville  limestone.  • 

Cambrian  system Sandstone  (Potsdam  sandstone). 

Algonkian  system: 
Huronian  series: 

{Michigamme  slate. 
Vulcan  formation  (iron  bearing). 
Felch  schist . 
Unconformity  (?). 

Lower  Huronian jRandville  dolomite. 

ISttu'geon  quartzite. 
Unconformity. 
Archean : 

Lauren tian  series Granites  and  gneisses. 

ARCHEAN   SYSTEM. 

LAUKENTIAN    SEKIES. 

The  Laurentian  series  consists  of  granites  similar  in  all  respects  to  those  of  other  districts, 
and  will  not  be  here  described.  It  borders  the  trough  on  both  the  north  and  tlie  south  and  forms 
a  nearly  isolated  area  in  the  vicinity  of  Granite  Bluff. 

ALGONKIAN   SYSTEM. 

HURONIAN  SERIES. 
LOWER    HURONIAN. 
STURGKON  QTTARTZITE. 

The  Sturgeon  quartzite  is  exposed  principally  in  the  western  part  of  the  district.  One 
belt,  contmuous  with  that  of  the  Menominee  district,  swings  northward  from  the  northwest 
side  of  the  Menominee  district  into  sees.  20  and  29,  T.  41  N.,  R.  29  W.  Another  belt  extends 
from  the  southwest  end  of  the  southern  Felch  Mountain-Sturgeon  quartzite  belt  and  swings 
southward  around  the  granite  mass  to  sec.  20,  T.  41  N.,  R.  30  W.  An  eastward  embayment 
carries  this  belt  into  sees.  9  and  16,  T.  41  N.,  R.  30  W.,  and  this  may  possibly  also  connect 
still  farther  east  with  quartzite  in  sees.  9  and  16,  T.  41  N.,  R.  29  W.  There  is  some  difiiculty 
in  all  these  places  in  discrinainating  this  quartzite  from  the  base  of  the  upper  Huronian.  On 
the  whole,  however,  the  Sturgeon  quartzite  is  much  more  massive  and  cherty  and  stands  at 
steeper  angles  than  the  upper  Huronian  quartzite,  which  is  more  or  less  thiidy  beddeil  with 
slate,  is  infolded  into  gentle  rolls,  and  has  been  rendered  micaceous  and  schistose  along  the 
slaty  layers. 

RANDVILLE  DOLOMITE. 

The  underground  workings  at  the  Calumet  mine  indicate  a  ih'sccnding  succession  of  upper 
Huronian  slate,  iron-bearing  formation,  schist,  cherty  or  cjuartzitic  rock,  and  dolomite.  The 
dolomite  occurs,  with  southward  diji,  only  a  short  distance  south  of  the  Archean  granite,  and 
may  with  reasonable  certauit}'  be  correlated  with  the  Randville  dolomite. 


U.  S.  GEOLOGICAL  SURVEY 
GEORGE   OTtS    SMITH,  DIRECTOR 


MONOGRAPH     Lll        PLATE     XXIH 


GEOLOGIC  MAP  OF  THE  CALUMET  DISTRICT,  MICHIGAN 

CoBmiledbv'(.'.KLeiHifYxjinaun\'p\'s  tn'W'S.Bavi(?v, 

H,C.AIleu.Edwar<l  Steidtmaim.and  olh  ers ' 

Scale    soodb 


ORDOVtCIAN  AND  CAMBRIAN 


UPPER  HURONIAN  (ANIMIKtE  GROUP) 


1909 

LEGEND 

ALQONKIAN(HtJRONIAN  SERIES) 


LOWER  HURONJAN 


HermaasviHe  hmestone  and 
Upper  Canibriaa  sandstone 


H 


Ayv 


Ba  s  alt  eactrusive  s 


MinhiganuQfi  slate  with 
some  basalt  estrusives 


Vul  c  ail  fopniau  o  n 
iron  hearing  J 


Kandville  doloimte 


Ats 


Stair;5^on  quart  zite 


Ejiposure.dip  and  strike 
notsnown 


G3q)osurea'wi^  obser\'^d 
dip  and  strike 


Shaft  or  pit 


AgCHEAN 
'TaURENTiAN  SERIES  ^ 


CALUMET  DISTRICT.  307 

UPPER    HUEONIAN   (aNIMIKIE  GROUP). 
FELCH  SCHIST. 

The  slate  and  c[uaitz-scliist  formation  forming  the  base  of  the  upper  Hurorian  is  exposed 
north  of  the  iron-bearing  Vulcan  formation  and  south  of  the  Laurentian  granite  at  Calumet. 
Its  characters  here  are  identical  with  those  of  the  Felch  schist  of  the  Felch  Mountain  trougli. 
To  the  south  it  appears  again  near  tlie  south  quarter  post  of  sec.  16  and  along  Sturgeon  River 
in  sees.  19,  20,  and  21,  where  it  contains  somewhat  more  quartzite  but  still  maintains  its  essential 
character  as  a  micaceous  slate.  The  formation  extends  from  this  locality  westward  into  the 
west  end  of  the  Calumet  trough,  where  it  open.s  out  into  the  great  area  of  upper  Huronian 
connecting  with  the  Michigamme  ("Hanbury")  slate  of  the  Menominee,  Crystal  Falls,  antl 
Florence  districts.  Quartz  schist  which  may  belong  to  tliis  formation  appears  in  the  minor 
trough  extending  eastward  from  sees.  9  and  10,  T.  41  N.,  R.  .30  W.,  but  as  already  indicated  in 
the  discussion  of  the  Sturgeon  quartzite,  it  has  not  been  satisfactorily  discriminated  from  the 
Sturgeon  quartzite  in  this  trough. 

VULCAN    FORMATION. 

Tlie  iron-bearing  formation  is  best  exposed  at  the  Calumet  mine,  where  it  has  been  crosscut 
for  700  feet.  It  is  here  interlayered  with  slate.  Its  dip  is  steep  to  tlie  south.  Magnetic  attrac- 
tions, drill  holes,  and  test  pits  show  iron-bearing  formation  and  ferruginous  quartzite  also  through 
the  northern  part  of  sees.  16  and  17,  a  mile  to  the  south,  where  the  dip  is  low  to  the  north, 
suggesting  that  tliis  is  the  same  belt  as  the  Calumet  brought  up  by  anticlinal  folding.  Magnetic 
work  shows  that  these  two  belts  extend  to  the  west  and  coalesce  against  the  soutli  margin  of 
the  granite  through  sees.  14  and  15.  West  of  this  locality  no  trace  of  it  has  been  found.  East 
from  the  Cahmiet  mine  the  belt  has  been  shown,  principally  l)y  magnetic  work,  to  extend  for 
several  miles. 

To  the  southeast,  in  the  southwest  corner  of  T.  41  N.,  R.  27  W.,  at  the  Hancock  mine,  is 
a  heavily  ferruginous  micaceous  slate  and  quartzite  which  may  be  correlated  with  the  iron- 
bearing  formation  at  the  Calumet  mine  or  may  belong  at  a  higher  horizon  in  the  upper  Huronian. 
Still  farther  southeast,  in  the  southeastern  part  of  the  same  township  and  tlie  north-central  part  of 
T.  40  N.,  R.  27  W.,  there  is  a  magnetic  field  with  trend  suggesting  its  correlation  with  the  iron- 
bearing  formation  of  the  Calumet  district.  A  drill  hole  on  the  magnetic  belt  in  sec.  3  of  this 
township  discloses  ferruginous  slates  and  quartzites  similar  to  those  at  the  Hancock  mine. 

MICHIGAMME  SLATE. 

Slates  and  micaceous  quartzites  overlie  the  iron-bearing  formation  at  the  Calumet  mine 
and  occupy  most  of  the  Algonkian  area  of  the  Calumet  trough.  Thence  they  presumably 
extend  southeastward  through  T.  40  N.,  R.  27  W.,  and  probably  connect  with  the  Michigamme 
("Hanbury")  slate  of  the  Menominee  district  in  this  direction.  West  and  southwest  from  the 
Calumet  mine  they  also  connect  with  the  upper  Huronian  slate  of  the  Menominee  district. 

PALEOZOIC   LIMESTONE  AND    SANDSTONE. 

Cambrian  sandstone  (Potsdam)  and  Cambro-Ordovician  limestone  (Hermansville)  rest 
in  flat  beds  over  much  of  the  Calumet  area  and  its  eastward  extensions.  They  cover  most 
of  the  upper  Huronian  of  T.  41  N.,  R.  28  W.,  and  T.  40  N.,  Rs.  27  and  28  W.,  and  thicken  rapidly 
to  the  east.     These  rocks  form  a  serious  obstacle  to  exploration  in  this  district. 

CORRELATION. 

The  correlation  of  the  rocks  of  tliis  district  is  discussed  in  connection  with  the  Felch  trough 
(p.  305)  as  well  as  in  the  general  correlation  chapter  (pp.  597  et  seq.). 


308  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

IRON  RIVER  DISTRICT. 

By  R.  C.  Allkn." 

LOCATION  AND   EXTENT. 

The  Iron  River  district  lies  west  of  the  Crystal  Fulls  district  and  extends  southward  a 
few  miles  beyond  Brule  River  into  Wisconsin,  to  the  <^ranitcs  and  fjrecn  schists  of  the  Archean. 
North,  east,  anil  west  of  this  boundary,  which  is  a  natural  one,  only  arbitrary  limits  may  be 
drawn.  Recent  detailed  studies  have  been  made  of  the  area  in  Michigan  extending  from 
Brule  River  north  to  latitude  45°  1.5',  east  to  longitude  S5°  .30',  and  west  to  longitude  85°  45', 
embracing  an  area  of  201.5  scjuare  niiles.     (See  PI.  XXIV,  in  pocket.) 

TOPOGRAPHY  AND   DRAINAGE. 

The  topography  is  of  glacial  origin,  slightly  affected  by  preglacial  forms  and  modified  little 
by  postglacial  erosion.  In  general  the  area  presents  a  series  of  hills  or  parallel  chains  of  hills 
elongated  in  a  direction  about  S.  20°  W.,  which  is  the  direction  of  ice  movement  as  recorded 
on  striated  and  grooved  rock  surfaces  in  the  southwestern  and  northern  parts  of  the  area. 
The  ridges  are  separated  by  corresponding  hollows,  which  hold  swamps  and  lakes  connected 
by  creeks  forming  the  minor  drainage  courses.  The  major  drainage  is  independent  of  the 
natural  northeast-southwest  "grain"  of  the  country,  for  the  larger  streams,  Bnde,  Iron,  and 
Paint  rivers,  cross  diagonally  the  general  southwest  trend  of  the  hills  and  valleys.  Paint 
River  in  the  northern  part  of  the  district  follows  in  general  the  strike  of  the  underlying  rocks, 
outcrops  being  comparatively  numerous  along  its  course.  The  same  may  be  said  of  the  Brule 
in  the  southern  part  of  the  district.  Both  of  these  streams  seem  to  follow  modified  preglacial 
courses.  However,  this  is  certainly  not  true  of  Iron  River,  for  this  stream  is  known  to  cross  at 
least  two  well-defined  drift-filled  preglacial  valleys  which  fall  toward  the  northeast  nearly 
at  right  angles  to  the  course  of  the  Iron.  These  valleys  are  separated  by  a  rock  ridge  which 
protrudes  tlirough  the  drift  in  Stambaugh  Hillj^  on  which  is  built  the  village  of  Stambaugh. 
(See  PI.  XXIV,  in  pocket.)  In  contrast  to  the  independence  of  the  major  streams,  the  minor 
drainage  is  controlled  absolutely  by  the  topography  of  the  drift  mantle,  which  may  be  readily 
inferred  from  a  study  of  the  map.  Many  of  the  lakes  occupying  depressions  between  the 
ridges  are  likewise  elongated  in  a  northeast-southwest  direction.  The  best  examples  are  Stanley 
and  Iron  lakes;  others  are  Minnie,  Chicagon,  and  Trout  lakes,  occupying  parts  of  the  same 
depression  in  the  eastern  part  of  the  area.  Most  of  the  lakes  are  drained  by  streams,  but  some, 
as  Bennan,  Snipe,  and  Scott  lakes,  have  no  outlets. 

The  combination  of  elongated  ridges  and  corresponding  depressions  above  described  forms 
a  distinctly  drumloid  type  of  topography.  However,  there  are  but  few  typical  drumlins. 
The  most  perfect  example  occurs  just  north  of  Iron  River  village,  crossing  the  south  line  of 
sec.  23,  T.  43  N.,  R.  35  W.  It  will  be  interesting  to  note  that  a  terminal  nioraine  formed  by 
the  Langlade  lobe  (Weidman)  of  the  Wisconsin  ice  sheet  occurs  not  far  to  the  south  in  Wis- 
consin, following  a  general  course  at  right  angles  to  the  trend  of  the  drumloid  hills  of  this  area. 
This  is  the  characteristic  relation  between  drumlins  and  terminal  moraine  found  elsewhere, 
notably  in  New  York  and  southern  Wisconsin. 

The  thickness  of  the  drift  ranges  from  a  knife-edge  up  to  more  than  300  feet.  It  is  of 
course  least  along  the  depressions  and  drainage  courses,  where  the  unilerlying  rocks  are  exposed 
at  many  places,  and  greatest  in  the  hills  between  them;  but  this  is  not  everywhere  true,  as  is 
abundantly  demonstrated  in  drill  borings.  Some  postglacial  and  preglacial  valleys  coincide 
in  general  trend  and  carry  greater  thicknesses  of  drift  than  the  bordering  hills.  This  is  true 
of  the  valley  extending  diagonally  northeastward  through  sec.  1,  T.  42  N.,  R.  35  W.,  and 
sees.  31  and  29,  T.  43  N.,  R.  34  W.     (See  map,  PI.  XXIV.)     Although  the  elevation  of  many  of 

aState  geologisi  of  Michigan.  Based  on  survey  by  \\.  S.  Bayley  for  the  United  States  Geological  Survey,  on  private  surveys  by  C.  K. 
Leith  and  R.  C.  Allen,  and  on  recent  survey  for  the  Michigan  Geological  Survey  by  R.  C.  Allen.  See  Allen,  R.  C,  The  Iron  River  iron-bearing 
district  of  Michigan:  Michigan  Oeol.  Survey,  Pub.  3  (Geol.  ser.  2),  1910. 


IRON  RIVER  DISTRICT.  :^0'J 

the  hills  is  accounted  for  hy  the  relatively  great  thicknesses  of  drift  under  them,  there  is  aljun- 
dant  evidence  that  the  preglacial  topography  of  this  region  was  more  rugged  and  presented 
greater  vertical  range  between  hills  and  valleys  than  does  the  present  surface.  The  higliest 
hills  are  in  the  southwestern  part  of  the  district  and  are  of  preglacial  origin.  Sheridan  Hill, 
in  sec.  20,  T.  42  N.,  R.  35  W.,  has  an  altitude  of  1,840  feet  and  rises  460  feet  above  the  lowest 
point  in  the  district,  the  valley  of  Paint  River,  where  it  leaves  the  area  in  sec.  36,  T.  44  N., 
R.  34  W.  The  elevation  of  the  rock  surface  near  the  center  of  sec.  29,  T.  43  N.,  R.  34  W.,  is 
1,280  feet.  Thus  the  maximum  difference  in  elevation  was  in  preglacial  time  at  least  100  feet 
greater  than  it  is  now. 

CHARACTER   OF   THE   GLACIAL  DRIFT. 

The  ridges  and  higher  lands  in  general  are  composed  of  liowlderv  till  intercalated  with 
lenses  of  sand  and  gravel.  The  till  is  in  many  places  composed  almost  entirely  of  clay  but  is 
more  commonly  somewhat  sandy  and  maintains  good  tilth  under  cultivation.  The  soil  of  the 
till  areas  is,  in  general,  excellent  and  supports  a  heavy  stand  of  hardwood.  Where  cultivated 
it  ])roduces  good  crops  of  small  grains,  hay,  and  vegetables  adapted  to  the  climate.  The  gen- 
eral abimdance  of  bowlders  is  the  main  obstacle  confronting  the  farmer  on  the  till  areas,  but 
this  is  not  insurmountable,  as  is  abundantly  shown  by  the  many  prosperous  farms  in  the  central 
and  eastern  parts  of  the  district. 

The  valleys  of  Brule,  Iron,  and  Paint  rivers  are  partly  filled  with  sandy  and  gravelly  out- 
wash  of  glaciofluvial  origin.  In  places  sand  and  gravel  plains  of  considerable  extent  have 
formed,  notably  at  the  junction  of  the  Iron  and  Brule,  in  the  valley  of  Net  River,  and  north, 
west,  and  south  of  the  junction  of  the  nortli  and  south  branches  of  Paint  River. 

GENERAL   SUCCESSION. 

The  general  succession  of  rocks  in  the  Iron  River  district  from  youngest  to  oldest,  is  as 
follows : 

(Quaternary  system: 

Pleistocene  deposits Bowlder  till,  sand,  and  gravel. 

Unconformity. 

Ordovician  system Limestone,  sandstone,  and  conglomerate. 

Unconformity. 
Algonkian  system: 

lluronian  series: 

(Intrusive  and  extrusive  greenstones. 
Michigamme  slate,  containing  Vulcan  iron-bearing 
member. 
Unconformity  (?). 

Lower  Huronian Saunders  formation  (interbedded  cherty  dolomite 

and  quartzite  and  slates,  believed  to  be  equiva- 
lent   of    Randville     dolomite    and    Sturgeon 
quartzite). 
Unconformity  (?). 
Archean  (?)  system: 

Keewatin  series  (?) Ellipsoidal  greenstone,  green  schists,  and  tuffs. 

ARCHEAN  (?)  SYSTEM. 

KEEWATIN  SERIES  (?). 

Basaltic  extrusive  rocks  with  surface  textures  similar  to  those  of  the  Quinnesec  schist  of 
the  Menominee  district  and  the  Hemlock  formation  of  the  Crystal  Falls  district  are  exposed 
in  isolated  outcrops  north  and  south  of  Brule  River  in  an  east-west  belt  across  the  southern 
part  of  the  district.  These  rocks  possess  no  lithologic  or  structural  characteristics  which  may 
safely  be  used  as  a  basis  for  their  correlation.  Most  of  the  outcrops  are  north  of  the  adjacent 
Saunders  formation,  but  a  few  are  south  of  it.  However,  detailed  mapping  has  not  been  done 
south  of  the  Brule,  and  consequently  the  extent  of  the  volcanic  rocks  in  this  direction  is  not 
yet  known.     They  are  nowhere  exposed  in  contact  with  the  Saunders  formation,  hence  their 


310  GEOLOGY  OF  THE  LAI^E  SUPERIOR  REGION. 

stratigrapliic  position  can  l)i'  determined  only  bj'  their  areal  relati(jn  to  tlie  Saunders  forma- 
tion in  reference  to  the  structural  attitude  of  the  latter.  The  available  data,  though  not  abso- 
lutely conclusive  for  all  parts  of  the  Saunders  formation,  indicate  a  general  northward  dip.  By 
appljang  tliis  criterion,  the  volcanic  rocks  north  of  the  Saunders  are  stratigraphically  above 
it  and  those  south  of  it  are  stratigraphically  below  it. 

The  volcanic  rocks  in  the  southern  part  of  sees.  19,  20,  21,  and  22,  T.  42  N.,  R.  .35  W.,  are 
the  only  greenstones  in  the  area  wliich  are  known  to  lie  south  of  the  Saunders  formation.  Being 
probably  below  the  Saunders  formation  they  are  tentatively  correlated  with  the  Keewatin 
scliist.  It  is  possible  that  they  may  be  interbedded  with  the  Saunders  formation  or  they  may 
be  of  later  age,  in  wliich  case  their  occurrence  here  maj'^  be  exj)lained  as  the  result  of  faulting. 

The  greenstones  tentatively  referred  to  the  Archean  are  of  basaltic  composition.  Ellip- 
soidal structure  is  well  developed.  The  elhpsoids  have  been  elongated  in  an  east-west  direction 
parallel  to  the  east- west  axes  of  major  folding  in  tliis  part  of  the  district.  Agglomeratic  struc- 
tures are  less  common  and  in  one  outcrop  the  rock  is  a  green  chloritic  shist. 

ALGONKIAN   SYSTEM. 

HTJRONIAN  SERIES. 

LOWER    HUEONIAN. 
SAUNDERS  FORMATION. 

Distribution. — The  Saunders  formation  occurs  in  a  l)elt  of  varj'ing  width  extending  in  a 
general  direction  a  little  north  of  west  across  the  southern  part  of  the  district  and  westward 
an  unknown  distance.  Outcrops  are  few,  on  the  whole,  and  are  absent  in  large  areas  supposed 
to  be  underlain  by  this  formation.  It  is  well  developed  in  Sheridan  Hill  in  sec.  20,  T.  42  X., 
R.  35  W.,  and  vicinity.  Tliis  Iiill  owes  its  altitude,  1,840  feet,  to  the  resistive  ciiaracter  of  the 
Saunders  formation.  East  of  Saunders  village  and  south  of  Brule  River  in  Wisconsin  this 
formation  again  assumes  topograpliic  prominence  in  an  east^west  ridge  about  2  miles  long. 
In  sees.  26  and  35,  T.  42  N.,  R.  35  W.,  slatj^  and  dolomitic  ])hases  are  exposed  in  a  number  of 
pits  and  an  outcrop  occurs  on  the  west  side  of  Brule  River  a  short  distance  southwest  of  the 
north  quarter  corner  of  sec.  34. 

Lithologic  characters. — The  Saunders  formation  embraces  a  wide  variety  of  facies.  Cherty 
dolomite  and  quartzite  are  the  most  prominently  developed.  Associated  with  them  are  mas- 
sive wliite  and  pink  dolomite,  impure  carbonate  slates,  and  talcose  slates. 

Cherty  dolomite  and  quartzite  are  best  developed  in  Sheridan  Hill  and  vicinity  and  in 
the  ridge  south  of  Brule  River  southeast  of  Saunders.  In  both  these  localities  the  rock  is  ex- 
ceedingly brecciated.  The  crushed  and  fractured  cherty  fragments  are  embedded  in  the  great- 
est confusion  in  secondarj^  infiltrated  silica  and  carbonate,  silica  being  dominant.  In  the 
Saunders  ridge  are  masses  of  almost  pure  quartz  associated  with  pure  massive  wliite  dolomite 
and  banded  chert  and  cherty  dolomite.  The  more  siliceous  bands  stand  out  prominently  on 
weathered  surfaces,  producing  a  ribbed  appearance. 

A  liighlj'  ferruginous  phase  of  the  Saunders  formation  is  exposed  in  a  cut  on  the  Connorsville 
branch  of  the  Chicago  and  Northwestern  Railway  about  2,100  feet  south  of  its  crossing  of  Brule 
River.  In  general  the  rock  is  intensely  sheared,  with  marked  slaty  structure  of  nearly  vertical 
dip  and  an  almost  easf^west  strike.  Here  there  are  gradations  to  more  massive  bluish  phases, 
which  are  seen  under  the  microscope  to  consist  chiefly  of  carbonate  with  coarse  interlocking  tex- 
ture. Inclosed  in  the  carbonate  arc  areas  of  fniely  granular  silica.  Sericite  occurs  as  a  secontlarv 
mineral,  ferric  oxide  is  abundant,  and  pyrite  occurs  commonlj'  in  aggregates  of  small  grains. 
The  ferruginous  character  of  the  carbonate  is  evident  from  the  abundance  of  ferric  oxide  devel- 
oped in  weathering. 

The  abundance  of  iron  oxide  in  weathered  portions  of  these  rocks  has  invited  explorations 
for  iron-ore  deposits,  particularly  in  the  SW.  J  sec.  26  and  the  NW.  \  sec.  35,  T.  42  X.,R.  35 
W.,  where  a  number  of  pits  have  been  dug.  The  deepest  of  these  which  have  penetrated  the 
weathered  mantle  are  bottomed  in  a  bluish  carbonate  rock  described  above.     Apparently  inter- 


IRON  EIVEE  DISTRICT.  311 

bedded  here  with  the  purer  carbonate  rocks  are  impure  shxty  phases  showing  cruin])lcil  bedding 
lamina;  whicii  are  cut  in  general  at  a  high  angle  by  the  plane  of  schistosity. 

Scliistose  slaty  phases  of  the  8aunders  formation  are  exposetl  on  the  west  banli  of  Brule 
River,  a  short  distance  southwest  of  the  north  quarter  corner  of  sec.  34,  T.  42  N.,  R.  35  W., 
and  on  the  north  bank  of  this  stream  in  the  NW.  ^  sec.  19  of  the  same  township. 

Talcose  ferruginous  slates  are  exposed  in  pits  400  paces  west  and  75  paces  south  of  the 
northeast  corner  of  sec.  20,  T.  42  N.,  R.  35  W. 

Stnicture. — Satisfactory  structural  observations  can  not  })e  made  on  known  exposures  of 
tins  formation.  In  the  cherty  and  quartzitic  phases  bedding  is  destroyed  by  excessive  breccia- 
tion,  in  the  slaty  phases  it  is  obscured  by  schistosity,  and  in  the  purer  massive  dolomitic  phases 
bedding  is  not  shown,  being  doubtless  destroyed  by  recrj^stallization  and  rearrangement  of  the 
minerals  in  the  rock.  In  the  north  face  of  the  ridge  southeast  of  Saunders  there  are  banded 
cherty  phases  showing  steep  northward  dip,  but  the  folding  and  brecciation  are  here  of  such 
character  as  to  indicate  that  these  dips  may  be  local.  Wliere  develoj)ed  the  schistosity  is  as  a 
rule  steeply  inclined  northward  and  is  parallel  to  the  trend  of  the  formation.  Distinct  bedding 
is  shown  in  slaty  fragments  on  the  dumps  of  pits  in  the  SW.  \  sec.  26,  T.  42  N.,  R.  35  W.,  but 
here  the  pits  are  filled  with  debris  and  the  rock  could  not  be  observed  in  place.  At  this  place 
tiie  schistosity  cuts  the  crumpled  laminas  nearly  at  right  angles.  As  the  scliistosity  is  elsewhere 
steeply  inclined  northward,  it  may  be  inferred  that  the  dip  of  the  bedding  is  here  northward  at 
a  lower  angle.  These  observations  are  unsatisfactory,  but  considered  with  the  position  of  the 
Saunders  formation  between  the  older  rocks  south  of  them  and  rocks  to  the  north,  which  are 
certainly  younger,  they  seem  to  indicate  a  general  northward  dip. 

East  of  sec.  21,  T.  42  N.,  R.  35  W.,  the  Saunders  formation  seems  to  widen  and  swing 
southeastward.  This  is  probably  due  to  flattening  of  dip  on  an  anticlinal  cross  fold.  If  the 
axis  of  this  fold  is  extended  northward  it  coincides  approximately  with  the  direction  of  the  axis 
of  a  broad  anticline  in  the  northern  part  of  the  district.  As  will  be  pointed  out  later,  it  is  prob- 
able that  the  entire  district  has  been  folded  on  this  axis  thus  extended. 

TMcl-ness. — A  close  estimate  of  the  thickness  of  the  Saunders  formation  can  not  be  made. 
If  the  width  of  the  formation  across  Sheridan  Hill  is  taken  at  4,000  feet  and  the  dip  assumed 
to  be  75°,  the  thickness  will  be  3,750  feet.  Doubtless  the  formation  is  very  thick,  but  the 
above  figures  may  be  a  thousand  feet  or  more  too  great. 

Relations  to  adjacent  formations. — Contacts  between  the  Saunders  formation  and  overlvmg 
and  underlying  rocks  are  not  exposed.  The  dip  of  the  Saunders  is,  on  the  whole,  steeply  north, 
from  which  it  is  inferred  that  it  is  probably  younger  than  the  Keewatin  (?)  rocks.  \\Tiether 
the  latter  are  unconformably  below  or  are  interbedded  with  the  Saunders  formation  is  not  here 
apparent.  In  the  southern  part  of  the  Florence  district  the  Quinnesec  schist  is  bounded  on 
the  north  by  quartzites  and  conglomerates,  which  are  clearly  unconformable  upon  the  Quinnesec 
schist  and  whose  bedding  and  contact  planes  trend  northwestward  toward  the  Brule  River  sec- 
tion of  the  southern  Iron  River  district.  The  quartzites  and  dolomites  of  the  Saunders  forma- 
tion may  be  the  extension  of  the  quartzite  and  conglomerate  belt  of  the  southern  Florence 
district,  and  if  so  they  would  be  unconformably  above  the  Keewatin  (?)  rocks. 

The  Saunders  formation  is  structurally  beneath  the  upper  Iluronian,  with  probable  con- 
formity. It  is  paralleled  on  the  north  by  a  belt  of  scattered  outcrops  of  volcanic  greenstones, 
b_v  which  it  is  overlain  and  with  which  it  may  be  to  some  extent  interbedded. 

UPPER   HURONIAN    (aNIMIKIE    GROUP). 
MICHIGAMME  SLATE. 

DISTRIBUTION    AND   GENERAL   CHARACTERS. 

The  Michigamme  slate  occupies  much  the  larger  part  of  the  district.  It  is  limited  on  the 
south  by  the  Saunders  formation  and  extends  north,  west,  and  east  beyond  the  hmits  of  the 
district,  on  the  east  connecting  with  the  upper  Huronian  (Michigamme)  sRite  of  the  Menominee, 
Crystal  Falls,  and  Florence  districts. 


312  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

The  rocks  of  this  formation  inckulc  a  wide  variety  of  facies.  Graywackes,  with  textures 
varying  from  oonclomcratic  to  fine  grained,  and  their  schistose  equivalents  arc  dominant  in 
tiie  northern  part  of  tlie  area.  Here  tliey  are  interliedded  with  lenses  of  black  pyritiferous  and 
carbonaceous  slates,  micaceous  and  chloritic  slates,  and  narrow  iron-bearing  lenses  which  occur 
in  the  vicinity  of  Atkinson,  in  sec.  24,  T.  44  N.,  R.  35  W.,  and  doubtless  in  other  areas  wliich 
are  drift  covered.  Toward  the  south  the  clastic  rocks  become  finer  grained  on  tlie  wliole  and 
perhaps  less  metamorphosed.  Slates  are  dominant  and  the  iron-bearing  member  is  more 
extensively  developed.  However,  graywackes  and  fine  conglomerates  are  not  lacking  and  are 
here  and  there  associated  with  the  Vulcan  iron-bearing  member.  Black  pyritiferous  and  car- 
bonaceous slates  are  common  associates  of  the  iron-bearing  member. 

Tlie  relations  between  the  various  facies  of  the  Michigammc  slate  are  those  of  gradation 
and  interbedding.  Any  single  type  of  the  rock  may  grade  by  muicralogical  and  textural  varia- 
tions into  any  other  type.  The  variations  take  place  both  in  the  direction  of  the  bedding  and 
across  it,  with  the  result  that  in  general  the  entire  formation  is  made  up  of  dovetailed  lenses  of 
various  dimensions  and  compositions,  with  indefinite  gradationul  borders  between  them. 
Although  gradation  is  the  rule,  abrupt  transitions  across  the  bedding  from  one  type  to  another 
are  not  uncommon,  especially  between  black  slates  and  iron-bearing  beds,  the  former  forming 
the  footwalls  of  many  of  the  ore  bodies. 

Elhpsoidal,  agglomeratic,  and  tuffaceous  extrusive  basaltic  greenstones  are  interbedded 
at  various  horizons  with  the  Michigamme  slate.  They  seem  to  be  especially  abundant  at  the 
base  of  the  formation  just  north  of  the  Saunders  formation  and  at  higher  horizons  in  the  northern 
part  of  the  district.  Of  less  common  occurrence  are  igneous  rocks  of  similar  composition  but 
with  well-developed  interlocking  crystalline  texture.     These  are  probably  intrusive. 

GENERAL    STRUCTURE. 

In  attempting  to  work  out  the  general  structure  there  is  the  same  difficulty  in  identifying 
horizons  in  the  slates  which  has  prevented  satisfactory  structural  work  in  the  Crystal  Falls 
district.  Rocks  of  identical  character  are  repeated  at  different  stratigraphic  horizons  and  the 
same  stratigraphic  Jiorizon  may  exhibit,  even  in  a  small  area,  facies  which  are  of  very  different 
composition  and  texture.  Inasmuch  as  this  fact  is  not  appreciated  by  many  who  explore  for 
iron  ore  in  this  district,  it  should  be  emphasized  here. 

(1)  The  rocks  at  any  particular  horizon  of  the  Michigamme  slate  can  not  be  depended  on 
to  maintain  the  same  character  over  an}-  considerable  area.  It  follows  that  (2)  cross  sections 
tlirough  the  same  stratigraphic  horizons  ma}^  differ  widely  in  a  given  small  area  and  conse- 
quently (3)  similar  sec[uence  of  formations  in  adjacent  areas  does  not  necessarily  imply  strati- 
graphic equivalence  unless  the  beds  are  known  to  be  continuous  from  the  one  area  into  the  other. 
Especially  is  this  true  if  the  two  areas  compared  are  wideh^  separated.  Observations  in  the 
field  and  in  mine  workhigs  and  microscopic  study  of  the  rocks  establish  beyond  doubt  the  truth 
of  the  above  statement. 

Guides  to  the  structure  in  the  southern  part  of  the  district  are  found  in  the  iron-bearing 
layers  and  in  the  structure  of  the  underlying  and  presuma])ly  conformable  Saunders  formation. 
In  the  northern  part  of  the  district  structures  are  well  brought  out  by  graywacke  phases,  abund- 
antly exposed,  exhibiting  bedding. 

The  general  east-west  trend  of  the  steeply  inclined  Saunders  formation  and  the  east-west 
strike  of  the  secondai-y  structures  in  it  and  the  adjacent  greenstones  indicate  the  main  struc- 
tural line  for .  this  part  of  the  district.  As  the  upper  and  lower  Huronian  are  probably  in 
structural  conformity  liere  as  well  as  fartlier  east  in  the  Crystal  Falls  and  ^lenominee  districts, 
the  Michiganune  slate,  with  its  interbedded  lenses  of  the  Vulcan  iron-bearing  member  wliich  are 
best  developed  in  the  southern  part  of  the  area,  may  be  expected  to  extend  beneath  the  drift 
west  of  Iron  River,  beyond  the  limits  of  the  district.  The  westernmost  exposure  of  tiie  Vulcan 
member  is  in  the  SW.'i  SW.  {  sec.  33,  T.  43  N.,  R.  35  W. 


IRON  RIVER  DISTRICT.  313 

The  folding  along  the  main  east-west  axis  is  considerably  nunlilied  in  tlio  central  and  northern 
parts  of  the  district  by  fokling  along  an  axis  trending  north  of  east  and  south  of  west.  Begin- 
ning on  the  east  side  of  T.  44  N.,  R.  34  W.,  along  Paint  River,  the  rocks  are  observed  to  strike 
slightly  west  of  north  and  to  cUp  vertically  or  steeply  to  the  northeast.  Upstream  along  Paint 
River  to  its  junction  with  the  Net  and  thence  westward  toward  Atkinson,  the  strike  swings 
sharply  westward  and  then  south  of  west,  the  dip  varying  from  north  to  northwest.  South- 
west of  Atkinson,  to  tlie  limits  of  the  district,  and  at  least  several  miles  beyond,  the  southwesterly 
trend  becomes  more  marked  ami  the  dips  are  to  the  northwest.  Brittle  layers  have  been  gashed 
by  tension  cracks,  in  general  normal  to  the  strike.  Cleavage  is  subordinate  to  bedding  in  the 
nortiieastern  part  of  the  district,  but  toward  the  west  tlie  rocks  become  more  and  more  scliis- 
tose  until  the  beddmg  is  mainly  obliterated.  This  is  due  chiefly  to  a  change  in  tlie  character 
of  the  setliments.  The  rocks  in  the  northeastern  part  of  the  area  are  commonly  coarse  gi'ained 
to  li:iely  conglomeratic,  becoming  fhier  grained  toward  the  west.  In  this  direction  the  dip  of 
schistosity  becomes  on  the  average  flatter  and  where  compared  with  the  bedding  the  two  struc- 
tures both  dip  northward,  the  schistosity  being  the  more  steeply  inclined. 

From  the  data  given  above  it  seems  that  the  structure  of  the  northern  part  of  the  district 
is  that  of  a  broad  northward-pitcliing  asymmetrical  anticline,  with  steeper  limb  on  the  east 
and  axis  trending  15°  or  20°  east  of  north.  If  this  axis  is  ])rojected  southwestward  across  the 
center  of  the  district  it  will  coincide,  with  slight  allowance  for  change  in  (hrection,  with  the 
axis  of  the  anticlinal  cross  fold  affecting  the  Saunders  formation  and  indicated  in  the  widening 
and  the  southeastward  swing  of  the  formation  in  the  big  bend  of  Brule  River. 

The  existence  of  this  north-south  cross  axis  of  fokling  is  further  indicated  by  the  trend  of 
the  iron-bearmg  member,  which  enters  the  district  from  the  southeast,  bends  to  the  west  in  the 
central  part  of  the  district  as  it  crosses  the  cross  fokl,  and  then  extends  southwestward. 


VULCAN   IRON-BEARING   MEMBER. 


Distribution  and  exposures. — There  are  few  exposures  of  the  Vidcan  iron-bearing  member. 
Knowledge  of  its  distribution  is  basetl  mainly  on  occurrences  in  underground  workings  and 
in  drill  holes  put  down  in  search  of  iron  ore  and  therefore  is  largely  limited  by  the  extent  to 
which  these  operations  have  been  conducted.  On  the  map  (PI.  XXIV,  in  pocket)  are  indi- 
cated those  areas  which  are  known  to  be  underlam  by  this  member  and  the  position  of  the 
drill  holes  in  which  the  member  has  been  penetrated.  Most  of  the  drill  cores  were  examined, 
but  some  are  unavailable;  in  the  latter  case  it  has  been  necessary  to  rely  on  the  superintend- 
ent's and  drUl  runner's  records.  An  attempt  has  been  made  to  discriminate  between  the  more 
unaltered  iron-bearing  rocks  on  the  one  hand  and  ferruginous  cherts  and  slates  and  iron  ores  on 
the  other.  There  are  all  gradations  between  the  various  phases  of  the  iron-bearmg  member, 
but  as  the  ores  and  highly  oxidized  phases  are  related  to  structural  conditions  that  largely 
influence  ore  concentration,  it  is  thought  that  the  discrimination  attempted  will  have  some 
practical  usefulness  in  suggesting  lines  for  further  exploration. 

The  knowm  main  occurrences  of  the  Vulcan  member  may  be  referred  to  three  different 
areas — (1)  the  Jumbo  belt,  just  south  of  Brule  River  in  Florence  County,  Wis.,  about  1^  miles 
east  of  Saunders;  (2)  the  central  area  of  unestablished  boundaries  extending  north,  east,  south, 
and  west  of  Iron  River;  and  (3)  the  northern  area,  including  the  ilorrison  Creek  belt,  in 
sec.  24,  T.  44  N.,  R.  35  W.,  and  the  Atkinson  belt,  southwest  of  Atkinson. 

Possible  extensions  of  these  belts  are  to  be  inferred  from  the  general  structure  of  the  dis- 
trict already  described.  These  are  specifically  discussed  under  later  headings.  Of  especial 
interest  in  this  stage  of  development  are  the  possibilities  of  connection  with  iron-bearing  belts 
in  the  Florence  and  Crystal  Falls  district,  toward  which  much  exploration  is  being  directed. 

Relations  to  Michigamme  slate. — All  the  iron-bearing  areas  include  more  or  less  slate,  and 
interbedded  slate  is  shown  in  many  of  the  drill  holes  which  are  indicated  as  cutting  the  Vulcan 
member.  It  will  be  seen  by  a  study  of  the  data  on  Plate  XXIV  that  in  the  central  part  of 
the  district  the  areal  relations  between  slate  and  iron  member  are  exceedingly  complex  and 


314  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

for  tlie  most  part  it  is  impossible  to  exclude  the  slate  from  any  considerable  area.  The  expla- 
nation lies  in  tlie  intcrbedding  of  the  slate  and  iron-l)earing  membex,  couphMl  with  complicated 
foldinfj. 

From  the  foregoing  statements  it  is  evident  that  the  Vulcan  member  is  not  confined  to  a 
single  horizon  in  tlic  Michigamme  slate.  From  analogy  with  the  Vulcan  bods  of  the  Menomi- 
nee and  Crystal  Falls  tlistricts  it  might  be  inferred  that  the  meniljer  occujiies  at  least  two  hori- 
zons near  the  base  of  the  Michigamme  slate,  but  it  is  reasonably  <'ertain  that  there  are  at  least 
four  horizons  of  iron-l)earing  rocks  in  the  Iron  River  district,  without  making  allowances  for 
the  possible  occurrence  of  two  or  more  horizons  in  the  producing  part  of  tlie  areas  near  Iron  River 
and  Stambaugh.  From  the  general  structure  of  the  district  it  is  probable  that  the  several  areas 
of  iron-bearing  rocks  occupy  as  many  different  general  horizons  of  the  Michigamme  slate,  the 
southernmost  belt  being  at  the  lowest  horizon,  the  central  area  being  somewiiat  higher,  and 
the  northern  area  being  liigher  still.  In  fact,  slate  and  iron-bearing  member  are  interbedded 
in  such  a  way  that  the  rocks  at  any  horizon  of  the  Michigamme  slate  may  somewhere  become 
iron  bearing.  There  are  areas  where  the  facts  are  more  nearly  expressed  by  the  phrase  "Vul- 
can formation  containing  lenses  of  Micliigamme  slate"  than  ' ' Micliigamme  slate  containing 
Vulcan  iron-bearing  member,"  and  this  is  especially  true  of  the  central  and  southern  parts  of 
the  district.  Any  attempt  to  unravel  tlie  structure  of  the  slate  and  the  iron-bearing  member 
wliich  does  not  take  into  account  these  relations  will  certainly  lead  to  erroneous  results. 

TJiicl-ness  and  structure .-^T\\c  iron-formation  bands  probably  do  not  exceed  .300  feet  in 
thickness  except  where  repeated  by  local  buckling.  They  are  closely  and  intricately  folded 
with  the  associated  slates  and  are  as  a  rule  steeply  dipping.  Erosion  has  cut  deeply  into  the 
series,  doubtless  removing  the  iron-bearing  member  over  considerable  areas  where  it  once 
existed.  Where  exj)osed,  it  occurs  at  the  surface  mainly  in  narrow  bands,  many  of  them  twist- 
ing and  contorted,  but  some  retaining  an  approximately  straight  course  for  distances  at  least 
greater  than  2  miles.  With  this  general  idea  in  mind,  it  will  be  readily  understood  that  any 
attempt  to  draw  boundaries  of  the  Vulcan  member  will  be  more  misleading  than  helpful.  The 
major  structui'e  of  the  Vulcan  member  is  discussed  under  the  general  structure  of  the  district. 

Lithologic  characters. — The  Vulcan  member  is  made  up  of  ferruginous  cherts  and  slates, 
cherty  iron  carbonate  rocks,  magnetitic  sideritic  slates,  and  iron  ores.  The  various  facies 
possess  no  characteristics  which  are  peculiar  to  this  district  and  therefore  will  not  be  described 
in  detail.  The  relations  between  the  different  types  are  those  of  gradation.  The  original 
iron-bearing  rock  was  niamly  a  cherty  iron  carbonate  similar  in  all  respects  to  those  which 
occur  in  neighboring  iron-bearing  districts. 

However,  there  are  two  characteristics  which  arc  worthy  of  notice  in  this  place.  Micro- 
scopic study  of  these  rocks  has  revealed  the  original  presence  of  small  quantities  of  greenalite. 
The  altered  forms  of  this  mineral  are  abundant  in  some  sections,  but  generally  they  are  not 
shown.  It  is  probable  that  greenalite  was  origmally  present  in  much  greater  abundance  than 
might  be  inferred  from  an  examination  of  the  rock  sections.  It  was  only  after  itlentification  of 
better-preserved  forms  in  a  few  sections  that  its  original  presence  in  others  was  determineil. 

In  the  more  highly  altered  ])hases  all  traces  of  original  greenalite  have  been  obliterated  bj' 
recrystallization  and  rearrangement  in  ilifferent  combinations  of  the  elements  forming  the  min- 
erals in  the  rock.  Various  later  stages  of  the  alteration  of  the  greenaUte  granules  are  observable 
in  tliin  sections,  but  nothing  approaching  unaltered  greenalite  has  yet  been  found. 

A  second  characteristic  of  the  Vulcan  member  which  should  be  noted  is  the  abundance 
of  associated  clastic  material  and  resultmg  alteration  products.  Fragmental  quartz  grains 
are  abuntlant  in  many  s])ecimens  and  are  clearly  distinguisliable  from  the  matrbc  of  crystalline 
silica  of  fine  interkx'king  texture  in  which  they  are  liK'ally  inclosed.  Less  conunonly  there  are 
grains  of  feldspar.  By  increase  in  the  relative  proportions  of  quartz  and  feklspar  grains  the 
rock  takes  on  the  characters  of  a  graywacke.  If  the  intermixed  clastic  nuitorial  is  of  veiy  fine 
grain,  impure  sideritc  and  ferruginous  slates  result  and  these  by  tiecroase  in  the  carbonate  and 
the  cherty  constituents  grade  into  ordinary  slate.  By  metamorphism  the  impurities  in  the 
iron-boaring  rocks  give  ri.se  to  secondarj'  products,  mainly  chlorite,  which  is  neaily  always 


IRON  RIVER  DISTRICT.  315 

associated  with  biotite  and  lesser  amounts  of  sericite.  Carbonaceous  imjjurities  are  espcciallv 
abundant  and  are  responsible  for  the  dark  color  of  much  of  the  cliiMt  of  tlie  iron-bearing  member. 
Pyrite  is  a  common  associate  of  the  carbonaceous  impurities  l)ut  may  occur  in  smaller  amount 
in  the  purer  phases  of  the  iron-bearinj^  rocks.  In  the  least-altered  rocks  the  iron  is  present 
mainly  as  carbonate,  being  changed  to  limonite  and  hematite  as  oxidation  progresses,  but  by 
anamorphism  occasionally  giving  rise  to  magnetitic  cldoritic  slates,  usually  carrying  more  or 
less  residual  iron  carl)onate.  Such  rocks  occur  on  the  to])  of  Stamljaugh  Hill  near  the  village  of 
Stambaugh  and  are  indicated  in  a  small  magnetic  field  in  tlie  SW.  i  sec.  33,  T.  43  N.,  R.  34  W. 
(See  PI.  XXIV,  in  pocket.) 

In  short,  tlie  typical  iron-bearing  rock  of  the  Vulcan  member — mainly  a  cherty  iron  carbon- 
ate— shows  all  possible  gradational  phases,  on  the  one  hand  to  slate,  which  is  nearly  always 
higlily  chloritic,  usually  biotitic  and  sericitic,  and  in  places  more  or  less  carbonaceous,  grading 
into  highly  graphitic  varieties,  and  on  the  other  to  graywacke;  and  further,  it  is  to  be  noted  that 
the  purer  forms  of  iron-bearing  rocks  are  subordinate  in  amount.  A  laboratory  study  of  these 
rocks  discloses  the  characters  that  they  may  be  inferred  to  possess  from  their  intimate  field 
relations  to  various  types  of  interliedded  slates  and  graywackes.  It  is  impossible  to  describe 
the  rocks  of  the  Vulcan  member  without  ref  -ence  to  the  clastic  rocks  with  which  they  are  so 
closely  associated. 

Distribution  and  local  structure. — (1)  The  only  natural  exjiosure  on  the  so-called  Jumbo  belt 
occurs  on  the  east  side  of  Brule  River  about  200  paces  east  of  the  southeast  corner  of  the  NE.  \ 
SE.  I  sec.  22,  T.  42  N.,  R.  34  W.  The  rock  is  mainly  a  finely  banded  cherty  iron  carl)onate, 
locally  altered  to  ferruginous  chert  and  interbedded  with  carbonaceous  and  pyritic  black  slate. 
The  strike  is  east  and  west  and  the  dip  is  about  vertical  on  the  average,  although  it  varies 
widely  on  the  limbs  of  the  minor  folds.  From  this  exi)osure  the  member  is  traced  eastward 
for  three-quarters  of  a  mile  by  numerous  test  pits  of  the  old  Jumbo  exploration.  The  pits  are 
now  filled  with  debris,  but  the  tlumps  disclose  slate  and  iron-bearing  member  of  the  characters 
shown  in  the  outcrop.  In  the  dump  of  the  old  Jumbo  shaft  at  tlie  east  end  of  the  belt  are  found 
an  abundance  of  much  altered  greenstone,  black  carbonaceous  and  pyritic  slate,  roughly  banded 
iron-bearing  rocks  carrying  plentiful  pyrite  and  secondary  cjuartz  and  a  little  lean  iron  ore. 
The  relations  between  the  Vulcan  member  and  the  greenstone  are  not  shown,  but  these  rocks 
are  probably  interbedded.  Interbedded  sihceous  chloritic  pyritiferous  slate  and  much-altered 
greenstone  are  well  exposed  in  an  outcrop  on  the  south  bank  of  Brule  River  just  north  of  the 
Vulcan  member  and  seem  to  lie  conformably  above  it.  The  Jumbo  belt  of  iron-bearing  member 
and  slate  is  overlam  on  the  north,  in  jjrobable  conformity,  by  extrusive  ellipsoidal  greenstone 
which  is  well  exposed  in  numerous  outcrops  north  and  south  of  the  Chicago  and  Northwestern 
Railway.  It  is  underlain  by  the  Saunders  formation,  which  occurs  about  one-quarter  of  a  mile 
farther  south.     The  Jumbo  belt  extends  east  and  west  beyond  known  limits. 

(2)  The  boundaries  of  the  central  area,  the  iron-ore  producing  area  of  the  Iron  River 
district,  are  not  yet  definitely  known  and  are  being  rapidly  widened  by  exploration.  If  Iron 
River  and  Stambaugh  are  taken  as  a  center,  the  iron-bearing  member  is  known  to  occur  north- 
ward to  the  southern  part  of  sec.  11,  T.  43  N.,  R.  35  W.;  eastward  to  the  Chicagon  mine,  in 
the  NE.  i  sec.  26,  T.  43  N.,  R.  34  W.;  southeastward  to  the  NW.  J  NW.  J  sec.  16,  T.  42  N., 
R.  34  W.;  and  westward  to  the  SW.  {  SW.  I  sec.  33,  T.  43  N.,  R.  35  W.  The  area  seems  to 
be  Hmited  on  the  south  by  greenstone.  By  connecting  the  scattered  outcrops  of  greenstone 
occurrmg  just  north  of  the  Saunders  formation  a  belt  of  varying  width  is  outlined  extendino^ 
across  the  entire  district.  Although  it  is  certam  that  this  belt  as  shown  on  the  map  (PL  XXIV, 
in  pocket)  contains  considerable  interbedded  slate  and  possibly  iron-bearing  member,  it  seems 
to  mark  in  a  general  way  the  south  limit  of  the  main  Michigamme  slate  and  Vulcan  member. 
Beginning  at  the  outcrops  in  sec.  23,  T.  42  N.,  R.  34  W.,  a  magnetic  line  probably  marking  the 
nqrtli  eilge  of  the  greenstone  extends  slightly  north  of  west  for  about  2  miles  and  dies  out. 
If  extended,  this  line  would  pass  just  north  of  the  greenstone  exposure  in  the  NW.  J  NW.  { 
sec.  21.  Thence  the  boundary  swings  more  to  the  north  and  jiasses  through  the  Wildcat  shaft 
near  the  center  of  the  S.  J  sec.  18,  and  thence  just  north  of  the  outcrops  of  greenstone  in  the 


316  GEOLOGY  OF  THE  LAKE  Sl'PERlOR  REGION. 

N.  A  N.  i  sec.   13,  T.  42  X.,  R.  .3.5  W.      F-'arther  westwunl  tlic  Ix.iiiKhiiy  can  not  1)C  followed, 
from  lack  of  exposures  and  ex])loration. 

Data  for  drawirijj  a  north  boundary  of  tliis  area  arc  entirely  iua<lc(juate.  Probably  it 
has  no  well-defmed  north  limit.  A  few  greenstone  outcrops  occur  in  a  broad  belt  of  country 
several  miles  \\ndc,  bcfxinning  about  the  middle  of  the  east  side  of  the  district,  where  they 
connect  with  the  greenstone  ai'ea  tiiat  extends  eastward  almost  to  Crystal  Falls,  and  extending 
thence  northwestward  to  the  middle  of  the  district  and  thence  southwestward.  In  this  belt 
there  are  a  greater  number  of  square  miles  of  territory  than  there  are  outcrops,  and  those 
tiiat  occur  are  confmed  to  the  eastern,  central,  and  western  parts.  However,  tlie  wide  distri- 
bution of  the  few  outcrops  that  are  known  indicates  a  belt  composed  dominantly  of  greenstone 
extending  across  the  district  in  a  curving  course  in  line  witii  the  structure  of  the  graywacke 
and  slate  area  north  of  it. 

Of  tbe  structure  and  distribution  of  the  Vulcan  member  within  this  area  the  available 
information  is  by  no  means  full.  Exploration  has  been  very  active  for  the  last  few  years,  but 
is  still  far  fi'oni  adequate.  Locally,  in  tlie  mine  workings,  the  structure  is  well  known,  but 
it  may  be  very  diflicult  to  comiect  the  structure  and  stratigraphy  shown  in  workings  on  a, 
single  40  acres  with  those  of  an  adjacent  40  acres.  The  explanation  for  this  complexity  ha^ 
already  been  discussed.  In  a  later  publication  details  of  structure  and  distribution  so  far  as 
known  will  be  given,  but  here  a  general  outline  will  suffice. 

To  begin  m  the  southeastern  part  of  the  district,  the  iron-bearing  member  is  foimd  in  the 
drill  iioles  m  the  NW.  i  NW.  i  sec.  16,  T.  42  N.,  R.  34  W.,  and  thence,  in  a  cui-ving  line  parallel 
to  the  north  bomidary  of  the  greenstone,  northwestward  to  the  Zimmerman  mine.  Eastward 
from  sec.  16  the  iron-bearing  member  extends  in  all  jirobability  through  sees.  1.5  and  14,  and 
])orhaps  still  farther  east,  but  in  this  direction  exploration  has  not  yet  been  cari'ied.  It  is  a 
favorable  line  for  exploration.  North  and  east  of  this  belt  borings  have  generally  penetratetl 
black  slate.  From  the  Zimmerman  and  Baltic  mines  the  general  course  of  the  member  is 
northwestward  up  the  valley  of  Iron  River.  In  detail  the  structure  is  exceedingly  complex, 
and  thorough  understanding  would  involve  a  description  of  the  structure  and  succession  in 
every  mine  on  the  belt.  The  Vulcan  member  is  here  very  generally  underlain  and  interbedded 
with  black  slate  and  is  usually  in  a  highly  inclined  position.  It  attains  its  greatest  known 
width  on  the  Caspian  mine  location,  where,  ^^dth  allowances  for  repetition  by  cross  folding, 
it  is  probably  over  300  feet  thick.  At  the  Hiawatha  mine  and  thence  westward  for  about  a 
mile  the  Vulcan  member  strikes  a  little  north  of  east  and  seems  to  dip  on  the  whole  steeply 
northward.  Farther  west  this  belt  has  not  been  traced.  From  the  Caspian  mine  northeast 
to  the  SW.  i  SW.  i  sec.  21,  T.  43  N.,  R.  34  W.,  drill  holes  have  penetrated  what  seems  to  be 
a  more  or  less  continuous  belt  of  the  Vulcan  member.  This  belt  is  about  at  riglit  angles  to  the 
belt  along  Iron  River,  Avith  which  it  and  the  extension  of  the  Hiawatha  belt  fonn  a  cross. 

North  of  Iron  River  the  strikes  are  prevailingly  aliout  east  and  west.  The  ^'ulcan  member 
occurs  in  one  main  belt  at  least,  more  than  2^-  miles  long,  extendmg  from  the  James  mine 
slightly  south  and  east  through  the  Spies  and  Hall  explorations  to  the  NE.  \  sec.  19,  T.  43  N., 
R.  34  W.,  and  slightly  north  of  west  to  tlie  SE.  i  SE.  i  sec.  1.5,  T.  43  N.,  R.  3.5  W.  The  thick- 
ness of  this  belt  in  the  James  muie  ajjpears  to  be  not  over  250  feet,  making  due  allowance 
for  thickenmg  by  minor  folding.  Black  slate  here  forms  both  foot  and  hanging  walls.  Tlie 
di|)  varies,  but  is  vertical  or  steeply  southward  or  northward.  Other  lenses  of  the  iron- 
bearing  member  occur  both  north  and  south  of  the  James  belt,  but  their  importance  and 
extent  have  yet  to  be  proved  by  exploration. 

(3)  In  the  northern  area  the  Morrison  Creek  belt  is  a  narrow  band  of  ferruginous  chert 
and  sideritic  slate  disclosed  in  the  dumj)s  of  numerous  test  pits  following  the  north  bountlary  of 
the  S.  ^  SW.  i  sec.  24,  T.  44  N.,  R.  3.5  W.  A  few  outcrops  of  sideritic  slates  occur  on  the  banks 
of  Morrison  Creek  in  an  east-west  line  with  the  pits.  The  dip  is  vertical  or  slightly  nortJiward. 
The  iron-l)eanng  member  is  ■here  underlain  by  and  pro])al)ly  interlxMlded  witii  black  carbona- 
ceous slate.  The  overlying  rock  is  a  scricitic  schist,  a  inctamor])liosc(l  e(|iiivalent  of  tiie  gray- 
wacke exposed  to  the  east  and  nortii  in  numerous  outcrops.     (Jn  the  soulii  the  slate  seems  to 


IRON  RIVER  DISTRICT.  317 

be  underlain  by  volcanic  greenstone,  which  outcrops  for  about  a  mile  to  the  soutli  along  the 
line  between  T.  44  N.,  R.  34  W.,  and  T.  44  N.,  R.  35  W. 

Southwest  of  Atkinson  the  Vulcan  member  occui's  in  a  double  belt,  separated  by  a  belt  of 
volcanic  greenstone  breccia.  The  dip  of  the  greenstone  and  associated  iron-bearing  member 
and  slate  here  seems  to  be  uniformly  northwest  at  an  angle  of  about  55°. 

It  will  be  interesting  to  consider  in  some  detail  the  Atkinson  section,  for  the  interbeddcd 
relations  of  the  various  rocks  in  the  Michigamme  slate  are  here  best  exhibited.  Tlie  southern- 
most rock  is  mainly  black  slate,  carrying  considerable  but  varying  amounts  of  carbonaceous 
matter  and  in  places  becoming  cherty  and  ferruginous,  especially  toward  the  top  of  tlie  forma- 
tion, where  it  gives  place  to  thin  iron-bearing  rock  about  80  feet  thick,  according  to  plats  of 
the  McColman  exploration  furnished  by  the  Verona  Mining  Company.  The  Vulcan  member 
at  this  horizon  has  not  been  followed  beyond  the  McColman  workings.  The  iron-bearing 
member,  as  shown  by  an  examination  of  the  rocks  on  the  dump  of  the  McColman  shaft,  includes 
hard  limonitic  iron  ore,  ferruginous  chert,  and  brownish  and  gray  banded  sideritic  slate.  The 
slaty  phases  are  sericitic,  chloritic,  and  biotitic,  and  m  one  place  abundant  titanite  was  found. 
The  ore  occurs  in  lenses  in  the  slaty  phases  of  the  member.  From  an  inspection  of  the  Verona 
Mining  Company's  plats  it  appears  that  the  highly  sericitic,  biotitic,  and  chloritic  slates  are 
abmidant  just  under  the  overlying  greenstone. 

'  The  greenstone  belt  extends  from  the  northeast  corner  of  sec.  18,  T.  44  N.,  R.  35  W.,  north- 
eastward into  the  SW.  5  NE.  {  sec.  9  of  the  same  township  and  doubtless  farther  in  both  directions 
where  exposures  are  lacking.  Its  thickness  ranges  fi-om  700  or  800  feet  up  to  possibly  1,400 
or  1,500  feet  at  the  northwest  end.  In  places  tliis  rock  is  very  schistose,  but  usually  its  original 
agglomeratic  structure  is  retained.  Brecciation  is  common,  but  the  resulting  structures  can 
usually  be  discrunmated  fi-om  its  original  agglomeratic  structure,  the  fractures  of  the  former 
cutting  indifferently  across  the  latter.  The  rock  is  extremely  altered.  Weathered  surfaces 
have  the  green  colors  of  chlorite  and  epidote  and  show  abundant  secondary  calcite  and  dolo- 
mite filling  fracture  planes  and  disseminated  through  the  rock. 

The  greenstone  is  overlam  by  a  belt  of  ferruginous  slates  and  cherts,  which  become  more 
siliceous  in  the  upper  horizons.  Near  the  underlying  greenstone,  black  carbonaceous  slates 
are  found,  but  these  seem  to  be  less  prominent  in  the  higher  beds,  which  are  composed  dominantly 
of  very  lean  ferruginous  granular  chert.  Only  one  natural  exposure  is  kno\sii,  but  numerous 
pits  and  a  few  drill  holes  disclose  the  character  of  the  rocks.  This  belt  is  less  tlian  a  cjuarter 
of  a  mile  wide.  North  of  it  are  sericitic  slates,  and  these  in  tmni  grade  northward  into  micace- 
ous schists  and  graywackes,  wliich  are  the  dommant  rocks  in  the  northern  part  of  the  Iron 
River  district. 

While  little  is  known  of  the  extent  of  the  Vulcan  member  in  the  Atkmson  district,  it  should 
be  noted  that  to  the  southwest,  on  the  strike  of  these  beds,  in  the  SE.  I  sec.  14,  T.  44  N., 
H.  36  W.,  lean  ferruginous  white  granular  cherty  beds  of  the  character  of  similar  beds  at  Atkinson 
are  associated  with  black  slate  and  overlain  by  micaceous  schistose  graywacke.  Similar 
white  granular  chert  occurs  on  the  strike  of  the  Atkinson  rocks  in  the  bed  of  Paint  River  in 
the  SW.  i  NE.  i  sec.  1,  T.  44  N.,  R.  35  W.  These  two  occurrences  seem  to  be  at  about  the 
horizon  of  the  beds  in  the  Atkinson  district,  but  it  should  not  be  inferred  that  the  iron-bearing 
member  is  continuous  from  one  locality  to  the  other  along  tliis  indicated  belt.  The  proba- 
bilities are  that  the  reverse  is  true. 

Local  magnetism  in  the  Vulcan  iron-hearing  member. — Although  in  general  the  Vulcan 
member  is  nonmagnetic,  there  are  a  few  local  areas  in  which  magnetism  is  well  developed. 
Other  magnetic  areas  woukl  probably  be  discovered  were  the  district  carefully  magneticaUy 
surveyed.  Reference  has  already  been  made  to  the  magnetic  line  apparently  following  the 
northern  edge  of  the  greenstone  m  sees.  21,  22,  and  23,  T.  42  N.,  R.  34  W.  Wliether  this  line 
is  caused  b}-  magnetism  in  the  greenstone  or  in  one  of  the  lower  members  of  the  Jilichigamme 
slate  is  not  known. 

A  magnetic  field  of  irregular  and  widely  varying  strength  in  diff'erent  parts  covers  about  60 
acres  on  the  crest  of  Stambaugh  Hill,  in  the  W.  i  sec.  36,  T.  43  N.,  R.  35  W.     (See  Pi.  XXIV, 


318 


GEOT>OGY  OF  THE  T.AKE  SirPERIOR  REGION. 


in  pocket.)  Here  tlie  rocks  are  well  exposed  in  numerous  outcrops.  The  dip  is  about  vertical 
iind  the  strike  sH<;iitly  west  of  nortli,  wliicii  is  the  direction  of  cloiij^ation  of  the  field.  Under 
the  microscope  tlie  rocks  are  seen  to  contain  innumcraljie  small  f^rains  of  maf^nctite  associated 
with  iilmndaiit  chlorite  and  finely  crystalline  quartz  and  considerable  siderite. 

A  ina<;netic  field  of  about  the  same  size  and  shape  occurs  in  the  SW.  \  sec.  .33,  T.  43  N., 
R.  34  W.  (see  PI.  XXIV),  but  here  the  field  is  elongated  in  a  northwest-southeast  direction, 
which  is  likewise  believed  to  indicate  the  strike  of  the  rocks  at  this  place,  although  no  exposures 
occur. 

Local  magnetism  occurs  also  in  separated  patches  in  sees.  35  and  30,  T.  43  X.,  R.  34  W. 
Here  the  magnetic  rock  is  mainly  a  graywacke  carrying  abundant  magnetite  associated  with 
chlorite,  biotitc,  and  siderite. 

To  the  west  of  the  Iron  River  district  projjcr  a  belt  of  magnetic  attraction  has  been 
traced  in  an  area  of  heavy  drift  from  a  point  near  the  center  of 
T.  43  N.,  R.  37  W.,  westward  to  the  Michigan  boundary  and  thence 
probably  into  Wisconsin. 

Slate  and  i  ■' 

graywacke  INTRUSIVE  AND  EXTRUSIVE  ROCKS  IN  THE  UPPER  HURONIAN  (ANIMIKIE  GROUP,. 

Igneous  rocks  of  basaltic  type  are  abundant  in  the  upper  Hu- 
ronian.  The  distribution  of  those  now  known  is  indicated  on  the 
accompanying  map  of  the  Iron  River  district.  (See  PI.  XXIV,  in 
pocket.)  There  is  much  difficulty  in  determining  the  general  distribu- 
tion of  tliese  rocks,  because  the  relations  to  the  slates  are  so  intricate 
that  it  is  never  safe  to  conclude  that  adjacent  exposures  are  or  are  not 
separated  by  slate. 

The  rocks  are  principally  of  extrusive  type  and  have  surface 
textures,  especially  the  ellipsoidal  and  agglomj-ratic  textures,  that 
are  characteristic  of  the  Hemlock  formation  and  of  the  volcanic 
rocks  associated  with  the  upper  Iluronian  of  tlie  C'lystal  Falls  dis- 
trict. Some  of  these  extrusive  rocks  arc  distinctly  contemjioraneous 
with  the  slates.  Southwest  of  Atkinson  agglomeratic  and  tuffaceous 
phases  of  the  greenstone  are  interbedded  with  upper  Iluronian  slate 
SE  and  iron-bearing  member  (fig.  44).     In  the  southern   part  of  the  dis- 

FiGURE44.-sectionshomng  roughly    ^j.^^^^  j,^  gg^.   93   T.  42  X.,  R.  34  W.,  elHpsoidal  and  tuffaceous  green- 

the  succession  of  beds  in  the  \  i:!-  1         c      i  tt  •  1  •  11 

can  iron-bearing  member  near  Ai-    stoue  occurs  north   of   the  Upper  Huroniau  slates   m  a   uorthward- 
kinson,  in  the  Iron  River  district,    clipping  series.     From   the  lack  of  contact  metamorphism  and   the 

abundance  of  tuffaceous  phases  and  effusive  rocks  they  were  prob- 
ably nearly  all  deposited  contemporaneously  with  the  sediments.  The  deposition  was  prob- 
ably submarine.  (See  pp.  510-.512.)  Definite  evitlence  of  relations  is  lacking  for  many  of  the 
greenstones,  especially  those  not  adjacent  to  slates  or  some  of  those  which  have  been  developed 
by  mining  operations  and  explorations. 


Iron  formation 

Slate 

Iron  formation 

Tuff 

Iron  formation 

Black  slate 


RELATIONS  OF  UPPER  HURONIAN  (ANIMIKIE  GROUPS  TO  UNDERLYING  ROCKS. 

No  direct  evidence  of  the  relations  of  the  upper  Iluronian  with  the  undcrlnng  Saunders 
formation  is  yet  available.  Certain  slates  conformable  with  the  Saunders  formation  in  Sheriilan 
Hill  may  be  upper  Huronian  slates  and  may  therefore  indicate  the  conformable  relations  between 
the  upper  Huronian  slates  and  the  Saunders  formation.  The  fact  that  rocks  of  the  Saunders 
type  form  a  continuous  belt  between  the  upper  Huronian  slates  and  the  supposed  Archean 
shore  to  the  south  is  evidence  of  nearly  conformable  relations.  It  is  noteil  in  the  sections  on 
the  Crystal  Falls,  Menominee,  Felch  Mountain,  and  Calumet  districts  tluit  the  succession  from 
underlying  quartzite  and  dolomite  to  the  upj)er  Huronian  shows  similar  relations.  (For  dis- 
cussion of  correlation  and  nomenclature,  see  pp.  597  et  seq.) 


IRON  RIVER  DISTRICT.  319 

ORDOVICIAN   ROCKS. 

Remnants  of  flat-lying  Paleozoic  rocks  occur  in  the  southern  part  of  the  district,  on  Sheri- 
dan Hill  and  vicinity  and  farther  southwest  in  the  SW.  i  sec.  27,  T.  42  N.,  R.  35  W.,  also  in 
the  SE.  }  sec.  24,  T."  44  N.,  R.  35  W. 

The  base  of  these  rocks  on  Sheridan  Hill  is  a  conglomerate  made  up  almost  entirely  of 
material  from  the  underlying  Saunders  formation.  Angular  fragments  of  chert  and  vitreous 
quartzite  up  to  2  inches  in  diameter  lie  in  a  matrix  of  materials  of  the  same  general  composi- 
tion, but  finer  grained.  The  rock  is  cemented  mainly  with  iron  oxide  and  calcium  carbonate. 
The  tliickness  of  the  conglomerate  is  unknown  but  is  not  great.  The  rock  has  not  been  found 
in  natural  exposure,  but  is  abundant  on  the  dumps  of  pits  wliich  have  been  sunk  through  it 
into  the  Saunders  formation. 

The  conglomerate  is  overlain  by  a  coarse  quartz  santlstone  of  buff  and  red  color  and  gen- 
erally very  friable  texture.  The  cement  is  mainly  iron  oxide.  Under  a  slight  tap  of  the  hammer 
the  rock  falls  apart  into  its  constituent  sand  grains.  The  thickness  of  this  sandstone  is  not 
known,  but  it  probably  ranges  from  a  knife-edge  up  to  perhaps  .S5  or  40  feet. 

In  the  southeast  corner  of  sec.  24,  T.  44  N.,  R.  35  W.,  a  film  of  red  sandstone  is  found 
mantling  black  slate.  Here  the  rock  carries  considerable  iron  oxide,  doubtless  derived  from 
the  Vulcan  member  occurring  about  a  quarter  of  a  mile  north  of  it. 

The  conglomerate  and  sandstone  of  tliese  areas  have  the  lithologic  characters  of  the  lower- 
most Cambrian  beds  in  the  Menominee  district  and  were  formerly  correlated  w^th  the  Cambrian. 
Also  Seaman  has  suggested  that  they  perhaps  represent  the  base  of  the  upper  Iluronian. 
Recent  fossil  discoveries,  however,  in  flaggy  limestone  beds  in  the  S.  5  SW.  |  sec.  27,  T.  42  N., 
R.  85  W.,  have  fixed  witliin  narrow  limits  the  age  of  these  rocks.  In  this  area  there  is  one 
natural  exposure  on  the  east  side  of  Brule  River  and  several  pits,  all  showing  nonmagnesian 
dove-colored  to  buff  flaggy  hmestone  of  the  same  general  characters.  The  rock  seems  to  be 
flat-l3ang,  although  the  beds  iii  the  outcrop  on  the  Brule,  where  observations  were  made  and 
where  most  of  the  fossils  were  found,  have  been  disturbed  by  slump,  following  undercutting 
by  the  river.  From  the  position  of  this  outcrop  in  reference  to  an  exposure  of  the  Saunders 
formation  on  the  west  side  of  the  river  about  500  paces  south,  it  woul^l  seem  that  these  rocks 
are  not  far  above  the  eroded  surface  of  the  Saunders  formation.  'VMiether  they  are  underlain 
by  the  conglomerate  and  sandstone  of  Sheridan  Hill  is  not  known.  The  beds  are  practically 
undisturbed  in  both  areas,  but  the  lowermost  kno\vn  occurrence  of  the  conglomerate  on  Sheri- 
dan Hill  is  about  150  feet  higher  and  the  uppermost  known  beds  of  sandstone  are  about  300 
feet  liigher  than  tjie  hmestone  outcrops  on  Brule  River  in  sec.  27.  It  would  seem  from  this 
that  the  conglomerate  and  sandstone  on  Sheridan  Hill  are  stratigraphically  higher  than  the 
limestone  of  sec.  27.  Doubtless  the  conglomerate  originally  formed  a  continuous  mantle  at 
the  base  of  the  Paleozoic  rocks,  but  owing  to  the  rugged  character  of  the  surface  over  wliich 
the  sea  advanced  there  was  probably  a  considerable  time  interval  between  the  submergence  of 
the  lower  areas  and  that  of  the  tops  of  the  liills.  Consequently  the  relative  age  of  the  basal 
mendaer  formed  at  any  point  is  a  function  of  its  altitude  at  that  place.  The  occurrence  of 
sandstone  on  Sheridan  Hill  at  an  altitude  of  about  1 ,760  feet  makes  it  certain  that  the  entire 
district  was  almost  if  not  entirely  covered  by  a  Paleozoic  sea. 

The  lowest  exposure  of  the  Paleozoic  beds  is  the  limestone  member  in  sec.  27,  T.  42  N., 
R.  35  W.  Tliis  hmestone  is  correlated  by  E.  O.  Ulrich  on  paleontologic  grounds  with  the 
Lowville  of  New  York  and  the  Platte\'ille  hmestone  of  Wisconsin — that  is,  with  the  Middle 
Ordovician.     The  following  is  Mr.  Ulrich's  report  to  T.  W.  Stanton: 

I  beg  leave  to  report  as  follows  on  the  fossils  collected  in  the  Iron  River  district,  Michigan,  by  R.  C.  Allen  and 
forwarded  to  the  Survey  for  examination  and  report  by  C.  K.  Leith  November  18,  1909: 

This  discovery  of  fossils  in  northern  Michigan  is  of  great  interest,  as  it  adds  an  important  link  in  proving  the 
former  connection  of  the  early  Mohawkian  limestone  of  Minnesota  and  western  Ontario  across  northern  Wisconsin. 
In  discussing  the  Lowville  limestone  in  my  paper  on  revision  of  Paleozoic  systems  I  state  my  conviction  that  this 
ind  perhaps  other  Mohawkian  formations  must  have  originally  extended  from  New  York  through  Ontario,  northern 


320 


GEOLOGY  OF  THE  LAIvE  SUPERIOR  REGION. 


Michigan,  and  northern  Wisconsin  to  Minnesota  and  Iowa.  Tliis  direct  westerly  connection  wa.s  indicated  by  the 
great  similarity  in  fauna  and  lithology  noted  in  comparing  the  Lowville  limestone  in  New  York  and  the  more  typical 
part  of  tlie  Platteville  limestone  of  southern  Minnesota,  Iowa,  southern  Wisconsin,  and  northwestern  Illinois.  I 
objerted  to  comraiitiication  via  southeastern  Wisconsin  because  there  the  beds  supposed  to  correspond  in  age  to  the 
Lowville  are  dolomites  instead  of  pure  limestone,  with  no  indication  of  transition  in  lithic  characters  northward. 
Hitherto  the  northern  connection  could  not  be  established  farther  west  from  New  York  than  Escanaba,  Mich.  This 
Iron  River  occiurence,  which  is  of  the  .same  fine-grained  noinnagnesian  dove-colored  limestone  everywhere  charac- 
terizing the  Lowville  and  lying  well  up  on  the  old  "Wisconsin  Peninsula,"  may  therefore  ju.stly  be  regarded  as 
tending  to  establish  a  \iew  hitherto  based  only  on  inference. 

The  following  20  species  are  more  or  less  confidently  identified.     All  are  older  than  the  Trenton  limestone  and 
younger  than  the  latest  Stones  River. 


?Coreniatocladus  densus. 

Tetr.uiium  cellulosum  (fragment  of  tube  only). 

Rhinidictya  cf.  nicholsoni  and  mutabilis-minor  (fragment). 

R.  cf.  major  (fragment). 

Escharopora  angularis. 

?Homotrypa  arbuscula. 

Raftne?quina  minnesotensis. 

Strophomena  incurvata  (Lowville  var.). 

Zygospira  recurvirostris  (Lowville  var.). 

Ctenodonta  sp.  undet.  (near  C.  levata). 


Leperditia  fabnlites. 

Lcperditella  tumida. 

L.  germana. 

Bythocj'pris  granti  var. 

Eurychilinia  reticulata. 

E.  subradiata. 

E.  n.  sp. 

Isotelus  cf .  obtusus. 

Thak'ops  ct.  ovatus. 

Pterj'gometopus  sp.  undet.  (pygidium). 


The  fossils  of  the  above  list  indicate  a  horizon  at  the  extreme  top  of  the  Platteville  limestone  in  the  Lead  district. 
Compared  with  the  New  York  section  the  bed  corresponds  in  age  to  the  uppermost  beds  of  the  Lowville,  asdescribed 
by  Gushing,  or  to  the  cherty  bed  at  the  base  of  the  Black  Ri^'er  limestone,  as  defined  by  the  same  author. 

FLORENCE    (COMJklONWEALTII)   IRON   DISTRICT   OF   WISCONSIN. 
LOCATION  AND   GENERAL   SUCCESSION. 

The  Florence  district  is  the  westward  geographic  extension  mto  Wisconsin  of  the  Menomi- 
nee district  bej'ond  Menomuree  River.  It  is  essential]}"  included  between  the  two  tributaries 
of  the  Menominee,  the  Brule  on  the  north  and  the  Pine  on  the  south  (PI.  XXV,  in  pocket). 
On  the  east  it  is  separated  from  the  Menominee  district,  as  this  is  limited  on  the  geologic  map, 
by  Menominee  River,  ^he  area  is  one  of  low  relief,  hke  the  Iron  River  district  to  the  north- 
west.    Exposures  are  relatively  few  except  along  the  rivers  and  lakes. 

Part  of  the  Florence  district  has  been  studied  by  members  of  the  United  States  Geological 
Survey,  and  a  complete  outcrop  map  of  the  district  has  been  prepared  by  ^Mi-.  W.  N.  Merriam 
and  assistants  for  the  OUver  Iron  Mmmg  Company.  As  yet,  however,  the  district  has  not  been 
studied  with  sufficient  exhaust iveness  to  definite^  estabhsh  the  succession  and  structure. 
Such  a  study  is  now  being  conducted  by  W.  O.  Hotchkiss,  State  geologist  of  Wisconsm.  So 
far  as  the  facts  are  now  known,  including  those  developed  in  recent  work  of  Hotchkiss,  the 
succession  in  the  Florence  district  seems  t(5  be  as  follows : 

Quaternary  system: 

Pleistocene  deposits. 

Paleozoic  rocks Patches  of  sandstone,  probably  Cambrian. 

Algonkian  system: 

Keweenawan(?)  series Granite  and  gneiss. 

(Juinncsec   schist,   intrusive    and    extrusive   green- 
Huronian  series:  stones  and  green  schists. 

Upper  Huronian  (Animikie  group) . . .  P>  "•'■gamme  slate,  includmg  the  \  u  can  u-on-beanng 

member  (inlerbedded  with  base  of  the  slates),  and 
also  quartzites  and  conglomerates  of  doubtful  age 
but  believed  to  be  phases  of  the  slate. 


FLORENCE  IRON  DISTRICT.  321 

ALGONKIAN   SYSTEM. 

HtTBONIAN  SERIES. 

UPPER    HUEONIAN    (aNIMIKIE    GROUp). 
MICHIGAMME  SLATE. 

General  character  and  distribution. — The  Animikie  group  seems  to  occupy  nearly  all  the 
area  of  the  Florence  district  north  of  the  Qumnesec  schist  belt,  except  where  small  patches  of 
intrusive  or  extrusive  greenstone  appear  at  the  surface.  Tlie  rocks  are  cliiefly  slate.  In  less 
quantity  occur  conglomerate,  quartzite,  tuffs,  and  iron-bearing  rocks.  It  has  not  been  proved 
that  all  these  rocks  belong  to  one  group,  but  as  yet  they  have  not  been  certainly  separated. 

The  Michigamme  slate  is  poorly  exposed  in  the  district  as  a  whole,  except  along  Brule 
River,  in  the  vicinity  of  Keyes  Lake,  and  northwest  and  southeast  of  Florence.  It  is  ahnost 
identical  in  petrographic  characters  with  the  u]iper  Iluronian  slates  of  the  Menominee  and 
Crystal  Falls  districts,  and  has  been  regarded  as  belonging  to  the  same  formation. 

Quartzites,  associated  with  more  or  less  conglomerate,  appear  m  three  main  areas — (1)  at 
Island  Rapids,  on  Menominee  River,  m  sees.  1.3  and  14,  T.  40  N.,  R.  18  E.;  (2)  in  a  belt 
running  north  of  Keyes  Lake;  and  (3)  in  a  belt  running  through  sec.  28,  T.  39  N.,  R.  18  E., 
north  of  Pine  River.  The  quartzite  at  Island  Rapids  stands  vertical  or  dips  steeply  to  the 
south,  and  the  top  is  to  the  south.  In  tlie  Keyes  Lake  belt  the  rock  is  vertical  or  dipping 
steeply  to  the  southwest.  The  relations  of  these  two  belts  witli  the  slates  are  not  known  defi- 
nitely, but  are  probably  conformable.  The  southern  belt  of  quartzite  just  north  of  Pine  River 
dips  southwestward  at  a  lower  angle.  It  is  thought  by  Hotchkiss  to  rest  unconformably  upon 
the  slates  to  the  north  of  it.  If  this  is  true  the  so-called  upper  Iluronian  of  this  district  con- 
sists really  of  two  groups,  the  correlation  of  which  is  doubtful.  The  southern  quartzite  is 
overlain  conformably  by  slates  which  upward  become  uiterbedded  with  tuffs  and  eruptives 
belonging  to  the  Quinnesec  schist. 

Vulcan  iron-hearing  member. — The  Vulcan  iron-bearing  formation  is  somewhat  widely  dis- 
tributed through  the  upper  Huronian  area,  but  here  it  is  so  interbedded  with  the  slates  that  it 
is  difficult  to  map  independently.  In  this  district,  therefore,  as  in  some  other  districts,  it  is 
treated  as  a  member  of  the  Michigamme  slate.  In  the  Florence  district  tliere  are  only  five 
areas  in  which  the  ferruginous  phases  of  the  upper  Huronian  are  now  known  sufficiently  well 
to  warrant  a  separate  color  on  the  map — one  is  immediately  northwest  of  Florence  in  sees.  20 
and  21,  T.  40  N.,  R.  18  E.,  and  in  a  belt  extending  northwestward  to  Brule  River;  two  south- 
east of  Commonwealth,  in  sees.  33  and  34,  T.  40  N.,  R.  18  E. ;  one  extending  east  and  west 
south  of  the  greenstone  belt  in  sees.  8  and  9,  T.  39  N.,  R.  19  E.  These  three  exposed  areas  are 
connected  by  a  belt  of  magnetic  attraction,  indicating  that  the  ii'on  formation  is  probably 
continuous  from  Brule  River  on  the  northwest  nearly  to  Menominee  River  on  the  southeast. 
Another  area  is  in  the  vicinity  of  the  Buckeye  mine,  just  to  the  southwest  of  Commonwealth. 
This  connects  with  a  magnetic  belt  running  southeastward  to  Menominee  River,  in  sec.  22, 
T.  39  N.,  R.  19  E.  To  the  east,  across  the  river,  this  magnetic  line  connects  with  the  principal 
iron-formation  l)elt  of  the  Menominee  district.  Another  belt  of  iron-bearing  formation  out- 
crops west  of  Keyes  Lake,  whence  it  is  followed  by  magnetic  Imes  to  the  southeast  to  about 
the  east  side  of  T.  39  N.,  R.  18  E.,  and  northwestward  toward  the  northwest  corner  of  T.  40  N., 
R.  17  E.  Belts  of  attraction  not  connected  with  any  well-exposed  areas  of  iron  formation  are 
known  elsewhere  m  the  tlistrict.  Particularly  to  be  mentioned  are  the  belts  extendmg  north- 
westward from  Pine  River  from  sec.  28,  T.  39  N.,  R.  18  E. 

The  iron-bearmg  member  is  magnetic  in  places,  especially  along  the  contacts  with  the 
intrusive  greenstones.  The  map  shows  a  number  of  disconnected  magnetic  lines  which  have 
been  traced  in  this  area.     Some  of  these  may  represent  altered  iron-bearing  rock. 

The  Vulcan  iron-bearing  member  consists  of  (1)  ferruginous  chert,  siderite,  and  hydrated 
hematite;  (2)  various  phases  intermediate  between  these  and  the  slates,  called  sideritic  slates 
47517°- VOL  52— 11 2] 


322  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

and  ferruginous  slates;  and  (3)  griineritic  and  magnetic  slates.  They  are  similar,  except  for 
type  3,  to  the  rocks  of  the  Vulcan  iron-beuring  member  in  tlio  Iron  River  and  Crystal  Falls 
districts.  Iron  ores  are  exploited  at  the  Florence  mine,  immeiliately  northwest  of  the  town  of 
Florence;  at  the  Commonwealth  and  Badger  mines,  southeast  of  the  town  of  Commonwealth; 
and  at  the  Buckeye  mme,  south  of  Commonwealth.  (.See  p.  323.)  The  ores  seem  to  be  in 
minor  drag  folds,  pitching  steeply  northwestward  m  the  Florence  and  Commonwealth  mines. 
The  major  trend  of  the  iron-bearing  exposures  of  magnetic  belts  and  of  exposures  of  other 
rocks  is  north  of  west  in  this  district,  a  trend  which  would  tend  to  connect  the  iron-bearing 
belts  with  those  of  the  Menominee  district  on  the  southeast  and  with  those  of  the  Mastodon 
area  in  the  southern  part  of  the  Crystal  Falls  district  on  the  northwest.  (See  p.  292.) 
Ex])loration  has  been  very  slight,  as  there  has  been  little  to  guide  it.  However,  there  is  a  large 
territory  along  the  trend  here  noted  which  inust  soon  receive  attention. 

The  horizon  in  the  upper  Huronian  slates  at  which  the  iron-bearing  member  of  this  dis- 
trict occurs  has  not  been  determined.  The  proximity  to  the  upper  Huronian  iron  formation 
of  the  Menominee  district  suggests  its  occurrence  near  the  base  of  the  upjier  Huronian. 

INTRUSIVE  AND  EXTRTTSIVE  GREENSTONES  AND  GREEN  SCHISTS. 

Quinnesec  schist. — The  Quinnesec  schist  outcrops  in  an  east-west  belt  1  to  3  miles  wide 
along  the  south  side  of  the  district,  probably  constituting  the  northwestern  extension  of  the 
southern  Quinnesec  schist  belt  of  the  Menominee  district.  The  best  exposures  are  along  Pine 
River,  especially  in  sees.  29  and  30,  T.  39  N.,  R.  18  E.  The  schists  are  cliiefly  hornblendic 
gneiss,  locally  micaceous.  They  are  cut  by  basic  and  acidic  intrusive  rocks,  the  former  being 
the  more  abundant.  The  detailed  petrographic  description  of  these  schists  given  in  the  Menom- 
inee chapter  will  suffice  for  this  district. 

The  continuation  of  these  schists  along  the  south  side  of  the  Menominee  district  has  been 
assigned  to  the  Keewatin  series  of  the  Archean  in  previous  reports  of  the  LTnited  States  Geo- 
logical Survey. °  Later  work  showed  this  assignment  to  be  a  very  doubtful  one,  and  the  question 
of  the  correlation  of  the  schists  has  been  largely  left  open  for  the  Menominee  district.  The 
work  of  Hotchkiss  along  the  south  side  of  the  Florence  district  shows  clearly  an  interbedding 
of  upper  Huronian  slate  with  tuffs  and  cruptives  of  the  Quinnesec  schist  in  a  manner  showing  the 
main  body  of  schist  to  be  later  in  origin  than  the  upper  Huronian  to  the  north  of  it. 

Intrusive  and  extrusive  greenstones  and  green  schists  other  than  Quinnesec. — Massive  and 
schistose  intrusive  and  extrusive  greenstones  appear  in  several  small  areas  in  the  upper  Huronian. 
Two  of  them  cross  Menominee  River  on  the  east,  where  they  join  the  northern  Quinnesec  schist 
area  of  the  Menominee  district.  Another  group  is  exposed  along  Brule  River  and  others 
between  the  Brule  and  Florence.  Isolated  outcrops  of  green  schistose  and  tuffaceous  rocks 
of  doubtful  structural  relations  are  somewhat  widely  distributed  through  the  district.  They 
are  in  places  associated  with  amphibole-magnetite  schists,  some  of  which  represent  phase  s  of  the 
intrusive  rocks,  but  some  of  which  doubtless  also  are  metamorj)hosed  phases  of  the  Huronian 
ferruginous  slates. 

Petrographically  these  rocks  are  very  similar  both  to  the  Hendock  formation  and  to  the 
Quinnesec  schist,  and  the  description  of  the  northern  Quinnesec  schist  area  of  the  Menominee 
district  will  apply  to  them. 

The  areas  of  intrusive  rocks  are  longer  from  east  to  west  than  from  north  to  south.  Evi- 
dence of  the  intrusive  character  of  the  greenstones  is  found  along  Brule  and  Menonunee  rivers 
in  T.  40  N.,  R.  18  E.  Especially  good  evidence  is  the  area  just  west  of  Keyes  Lake.  In  sec.  9, 
at  several  points  along  the  Brule,  are  to  be  found  outcrops  of  the  massive  greenstones  in  contact 
with  the  slates.  Invariably  the  slates  are  more  micaceous  near  the  contact  than  elsewhere. 
In  fact,  they  become  mica  schists,  and  here  and  there  is  seen  a  slight  development  of  some 
secondary  niineral,  probably  garnet.  In  every  outcrop  along  the  Brule  the  contacts  of  the 
greenstones  and  sediments  are  not  sharply  defuied,  the  greenstones  being  schistose  and  chloritic 
at  the  contacts.     In  sec.  13,  T.  40  N.,  R.  18  E.,  greenstone  is  found  in  contact  with  a  micaceous 

oMon.  U.  S.  Geol.  Survey,  vol.  !f..  190^;  Monominoo  sppclM  folio  iNo.  02),  Geol.  Atlas  V.  S.,  C.  S.  r.eol.  Survey,  1900. 


IRON  ORES  OF  CRYST.-VL  FALLS,  IRON  RIVER,  AND  FLORENCE  DISTRICTS.     32S 

quartzitc.  The  actual  well-defined  contact  may  be  seen  here,  and  the  intrusive  character  of  the 
greenstone  is  clearly  shown.  A  wedge  of  the  greenstone  cuts  the  quartzite  at  1 ,650  paces  north 
antl  200  paces  west  of  the  southeast  corner  of  sec.  13,  T.  40  N.,  R.  19  E.  The  quartzite  at  this 
place  is  much  fissured  and  shattered. 

Brule  River,  where  it  crosses  the  E.  i  sec.  9,  T.  40  N.,  R.  18  E.,  is  a  favorable  place 
to  see  the  way  in  which  the  intrusive  greenstones  stand  out  prominently  as  hills  in  the  slate 
area.  The  river  here  cuts  through  the  slates  and  greenstones,  giving  a  well-exposed  cross 
section.  The  conclusion  is  here  forced  on  the  observer  that  the  outcrops  of  the  greenstones  of 
this  area  represent  with  a  very  fair  degree  of  accuracy  the  actual  distribution  of  the  greenstones. 
The  greenstone  outcrops  are  many  times  longer  east  and  west  than  north  and  south,  as  has 
been  noted.  This,  however,  does  not  justify  the  correlation  of  greenstone  knobs  because  they 
happen  to  align  in  the  direction  of  their  long  dimensions.  The  areas  mapped  as  intrusive  and 
extrusive  greenstones  and  green  schists  on  the  Florence  map  (PI.  XXV,  in  pocket)  may  there- 
fore be  regarded  as  containing  much  slate  in  lower,  covered  ground. 

GRANITE  AND  GNEISS  INTB.USIVES. 

Bordering  the  Quinnesec  schist  on  the  south  is  an  area  supposed  to  be  underlain  by  granites 
and  gneisses.  Exposures  are  few,  but  to  the  east,  south  of  the  Menominee  district,  they  are  more 
abundant.  The  relations  are  those  of  intrusion  into  the  Quinnesec  schist,  and  the  rocks  are 
doubtfully  correlated  with  the  Keweenawan. 

PALEOZOIC    SANDSTONE. 

A  few  patches  of  Paleozoic  sandstone  he  unconformably  upon  the  pre-Cambrian  rocks. 
These  are  well  shown  just  west  of  the  Buckeye  mine  and  north  of  Keyes  Lake. 

QUATERNARY  DEPOSITS. 

This  district  is  covered  by  Pleistocene  glacial  drift.     (See  Chapter  XVI,  pp.  427-459.) 

THE  IRON  ORES  OF  THE  CRYSTAL  FALLS.  IRON   RIVER,  AND  FLORENCE, 

DISTRICTS. 

By  the  authors  and  W.  J.  Mead. 

DISTRIBUTION,  STRUCTURE,  AND   RELATIONS. 

The  principal  ores  of  this  region  are  found  in  iron-bearing  layers  infolded  \vith  upper 
Huronian  slate  in  the  vicinity  of  Florence,  Commonwealth,  Crj^stal  Falls,  Amasa,  and  Iron 
River,  and  in  the  middle  Huronian  slate  near  Mansfield.  These  districts  are  usually  considered 
as  a  part  of  the  Menominee  district  in  returns  of  ore  sliipments,  and  their  ores  are  similar, 
geologically  and  structurally,  to  those  of  the  Menominee  district.  Though  not  chrectly  continu- 
ous with  the  iron  formation  of  the  Menominee  district,  so  far  as  explorations  yet  show,  they 
mainly  belong  in  a  formation  which  is  closely  correlated  with  that  iron-bearing  formation  (the 
Vulcan),  and  is  given  the  same  name.  Also  the  upper  Huronian  slate  with  wliich  this  iron- 
bearing  formation  is  associated  is  similar  to  and  continuous  with  the  Michigamme  ("Hanbury") 
slate  of  the  jNIenominee  district,  and  is  therefore  called  by  the  same  name. 

The  Micliigamme  slate  over  this  great  area  is  remarkably  uniform  in  character,  anil  it  is 
difficult  to  tell  at  what  horizon  in  the  slate  formation  the  ores  occur  in  any  particular  locality. 
In  tlie  vicinity  of  Crj'stal  Falls  and  Amasa  the  upi)er  Huronian  slate  rests  upon  greenstones  of 
the  Hemlock  formation,  so  that  in  tliis  part  of  the  district  it  is  easy  to  determine  the  base  of 
the  upper  Huronian,  and  the  occurrence  of  the  ore  at  a  short  though  varying  distance  from  the 
volcanic  Hemlock  formation  shows  that  for  this  locality  at  least  the  iron-bearing  rocks  occur  at^ 
a  fairly  persistent  horizon  near  the  base  of  the  upper  Huronian  slate. 

Most  of  the  ore  deposits  of  these  districts  are  accompanied  by  black  and  pyritiferous  slate 
walls,  in  places  associated  with  greenstone,  or  they  maj^  be  separated  from  such  walls,  especially 
the  hanging  wall,  by  a  small  amount  of  lean  cherty  iron-bearing  rock.     Along  the  trend  of  the 


324 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


iron-bearing  member  and  in  (l("i)tli  the  iron-ore  layers  jiass  info  lean  clierty  layers.  The  ore 
bodies  throughout  show  a  strong  tendency  to  follow  the  steeply  inclined  and  uniformly  trending 
bechiing  of  the  iron-bearing  member,  liaving  tlius  distinct  linear  shape  and  distribution  at  the 
surface  and  tabular  or  lens  shape  in  three  dimensions.  In  certain  of  the  Crj'stal  Falls  deposits 
these  characteristics  are  much  more  apparent  than  in  others.  For  instance,  the  ores  at  the 
Hemlock  mine  at  Amasa  constitute  a  lens  in  a  narrow  band  of  iron-bearing  rock,  with  consid- 
erable extent  vertically  and  horizontally,  parallel  to  the  strike  of  the  upper  Iluronian.  The 
same  is  true  of  the  ore  deposits  in  the  so-called  "Mansfield  slate."  Though  minor  folds  are 
present  in  both  of  these  deposits,  they  are  subordinate  to  the  general  tabular  sha{)e  of  the 
deposits. 

Other  ore  bodies  follow  the  axial  Hues  of  drag  folds,  thus  jiitching  at  various  angles  beneath 
the  surface.  Their  shape,  considered  in  three  dimensions,  tends  to  be  linear  rather  than  tabular. 
As  few  of  these  axial  lines  are  uniform  for  long  distances,  offsets  of  the  ore  body  are  common. 
The  ores  of  the  Florence  district  seem  to  be  in  drag  folds,  with  ])itches  to  the  northwest.  Their 
distribution  suggests  sharp  offsets  by  drag  folding. 

The  iron-bearing  rocks,  and  therefore  the  ore  bodies,  are  usually  not  more  than  300  feet 
tliick,  though  locally  the  thickness  may  be  much  increased  l)y  buckling.  It  will  be  noted  by 
figure  12  (p.  123)  that  folding  of  that  type  multiplies  the  thickness  by  3.  The  depth  to  which 
mining  has  thus  far  extended  is  1 ,000  feet,  but  exploration  has  shown  ore  to  a  greater  depth. 
It  can  not  yet  be  said  what  the  maximum  depth  of  the  ores  may  be.  At  the  Florence  mine  the 
formation  becomes  pyritiferous  below  this  depth,  although  it  is  not  demonstrated  that  the 
pyritiferous  portion  continues  indefinitely. 

The  iron  formations  near  the  main  area  of  the  Hemlock  formation  in  the  Crystal  Falls 
district  and  part  of  those  in  the  Florence  district  are  distinctly  magnetic.  Elsewhere  in  the 
Crystal  Falls  district  and  in  the  Iron  River  district  the  formations  are  weakly  or  not  at  all 
magnetic. 

The  structural  relations  of  the  ores  of  this  group  are  less  satisfactorily  known  than  those 
of  almost  any  other  district  in  the  Lake  Superior  region,  partly  because  of  the  lack  of  sufficient 
development  and  partly  because  of  the  uniformity  of  the  slate,  making  it  difficult  to  find  recog- 
nizable horizons  as  a  basis  for  working  out  the  structure.  Because  of  the  lack  of  continuity  of 
the  iron  formation  in  tliis  great  slate  area  and  the  covering  of  a  large  part  of  the  area  by  glacial 
drift,  it  seems  altogether  likely  that  there  are  still  many  deposits  to  be  found  through  the  slate. 
Magnetic  work  sometimes  indicates  places  to  begin  exploration,  but  much  of  the  exploration 
must  begin  blindly. 

CHEMICAL   COMPOSITION. 

The  ores  of  these  cfistricts,  with  the  exception  of  the  Mansfield  deposit  and  the  Amasa- 
Porter,  south  of  Amasa,  are  non-Bessemer  hydrated  hematites  of  medium  to  low  grade.  The 
average  composition  and  range  for  each  constituent  of  the  ores  minetl  in  these  districts  in  1907 
and  1909  are  as  follows: 

Arcniye  rhcmicd}  composition  of  ores  from  carrjo  anahjscsfor  1907  arid  1909. 


Crystal  Falls  dLs- 
trict. 

Iron  Kiver  dis- 
Iriut. 

Florence  district. 

1907.           1909. 

1907. 

1909. 

1907. 

1909. 

8.46 

8.42 

8.23 

8.34 

10.86 

9.76 

-Analysis  of  ore  dried  at  21'2°  F.: 

54.10 
.437 
0.  27 
1.27 
2.94 
2. 62 
2.15 
.050 
5. 89 

54.79 
.495 
7.71 
.799 
2.50 
2.63 
2.16 
.071 
4.11 

55.70 
.390 

8.62 
.20 

2.54 
.92 
.76 
.057 

5.25 

54.35 

.404 

8.77 

..30 

3.07 

1.34 

1.49 

.056 

5.74 

54.50 

.32 

0.72 

.26 

3.35 

1.51 

2.40 

.1.12 

5.20 

54.70 

.319 

6.89 

.08 

4.17 

I.ime                                                                            

1.80 

2.86 

.173 

S.20 

IRON  ORES  OF  CRYSTAL  FALLS,  IRON  RIVER,  AND  FLORENCE  DISTRICTS.     325 


Range  in  pcrrentage  of  each  constituent  in  ores  mined  in  1909. 


Crystal  Falls 
districl. 


Iron  River 
district. 


Florence  dis- 
trict. 


Moisture  (loss  on  drying  at  212°) 

Analysis  of  ore  dried  at  212°  F.: 

Iron 

Phosphorus 

Silica 

Man^nnese . . . .- 

Alumina 

Lime 

Macnesia 

Sulphur 

Loss  on  ignition 


2.  S3    to  13. 75 


35.74  to  57. 20 
.04010  1.28 

5..';i  to  30.  .13 
.15  to    2.93 

1.20  to    3.41 

1.20  to  4.96 
.71  to  2.  NO 
.007  10      .100 

l..-)S   to    7.  CO 


49.87  to  50.  07 

.70910    3.13 

5.35   to  14.  IB 

.18   to    2,10 


.99  to 
.40  to 
.20  to 
.  009  to 
2.45   to 


4.  23 
2.74 
2.40 


8.46   to    9.1 


53.  .30   to.M.OO 
.  297  to      .  410 

0.  .50   lo 
.00   to 

2.S2   to 

1.01    to 

2.74   to 
.11    to 

5.05   to 


S.05 
.20 
4.47 
2.03 
2.88 
1.87 
.5.80 


MINERAL   COMPOSITION. 

The  ore  of  these  districts  is  cliiefly  soft  red  hematite,  though  in  places  it  is  hydrated  and 
graded  as  brown  hematite  (limonite).  Goethite  has  been  identified  at  Iron  River.  In  achlition, 
there  are  quartz  and  some  kaoUn,  with  small  amounts  of  magnetite,  calcium,  and  magnesium 
carbonates,  and  minute  amounts  of  sulphides. 

The  average  mineral  composition  of  the  ores  of  these  districts,  calculated  from  average 
analyses  for  1909  given  in  the  above  table,  is  as  follows: 

Approximate  mineralogical  composition  of  ores,  calculated  from  the  average  analyses  for  1909. 


Crystal  Falls 
district. 

Iron  River 
district. 

Florence 
district. 

71.90 
7.  50 
4.  311 
4.70 
3.50 
4.00 
2.  00 
1.44 

54.00 
27.80 
4.82 
5.80 
3.80 
.45 
2  12 
\.2\ 

62.42 

18.10 

1.26 

7.70 

6.20 

2  85 

Apatite  (all  phosphorus  calculated  as  apatite) 

1.65 

100.  00 

100.00 

100.18 

The  above  mineral  compositions  are  necessarily  only  approximate,  as  ferrous  and  ferric 
iron  are  not  separated,  and  the  combined  water,  COj,  ami  a  possible  small  amount  of  organic 
material  are  included  together  under  loss  on  ignition.  All  the  phosphorus  with  proper  amounts 
of  limestone  was  calculated  as  apatite;  the  remaining  lime  with  proper  amounts  of  magnesia 
and  water  was  calculated  as  dolomite.  The  remaining  magnesia  with  alumina,  silica,  and 
water  was  calculated  as  chlorite.  The  alumina  not  used  in  the  chlorite,  together  with  sufficient 
silica  and  combined  water,  was  taken  as  kaolin.  Sufficient  iron  was  combined  with  the  remain- 
ing water  to  form  limonite  and  the  remaining  iron  figured  as  hematite.  Hematite  and  limonite 
probably  do  not  exist  in  the  ores,  but  as  a  means  of  comparison  and  to  show  the  degree  of 
hydration  the  hydrated  iron  oxide  is  calculated  in  terms  of  these  two  minerals. 

PHYSICAL  CHARACTERISTICS. 

The  ore  is  very  porous  and  shows  many  crystal-lined  cavities.  At  places  a  hard  steel 
hematite  ore  is  fomid,  which  rims  high  in  metallic  iron.  It  breaks  into  a  mixture  of  small 
blocks  and  soft  ore  similar  to  the  ores  of  the  Menominee  district. 

The  average  mineral  density  of  the  ores,  calculated  from  the  above  analyses,  is  4.38  for 
the  Crystal  Falls  ores  and  4.30  for  the  Iron  River  ores. 

The  porosity  of  the  ores  ranges  from  less  than  5  per  cent  to  over  40  per  cent  of  their  volume. 

The  cubic  contents  of  the  ores  vary  from  8.5  to  15  cubic  feet  to  the  ton,  with  an  average  of 
about  1 1  cubic  feet.  The  volume  composition  of  these  ores,  in  comparison  with  those  of  the 
Menominee  district,  is  represented  in  figure  50  (p.  352). 


326  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

SECONDARY  CONCENTRATION  OF  THE  ORES  OF  THE  CRYSTAL  FALLS,  IRON 

RIVER,  AND   FLORENCE  DISTRICTS. 

Structural  conditions. — The  ores  of  the  Crystal  Falls,  Iron  River,  and  Florence  districts  are 
enrichments  of  narrow  bods  and  lenses  of  iron-bearing  roc'ks,  as  a  rule  not  more  than  300  feet 
wide,  usually  between  steeply  inclined  walls  of  slate,  generally  graphitic  and  pjTitiferous  near 
the  contact,  and  commonly  associated  with  greenstone.  The  iron-lx'aring  member  Uiay  trend 
in  the  same  direction  for  considerable  distances  and  yet  be  closely  corrugated  by  minor  folds  of 
the  drag  type  illustrated  in  figure  12  (p.  123).  These  steeply  pitching  drag  folds  furnish  an 
impervious  basement  of  slate  along  which  the  waters  have  followed  the  o])enings  in  the  iron- 
bearing  member  in  especial  abundance  and  have  effected  the  concentration  of  the  ore.  The 
iron-bearing  rock  is  brittle,  but  the  slate  is  not,  the  result  being  that  breccias  are  common  in 
such  troughs,  greatly  favoring  the  flow  of  water.  The  folds  are  of  various  magnitudes  and  the 
concentration  may  follow  either  the  minor  or  the  major  folds. 

The  circulation  has  been  controlled  by  the  fracture  openings  in  the  iron-bearing  member 
and  the  bedding  in  it,  and  the  confining  strata  hav(>  been  foot-wall  slates,  hanging-wall  slates, 
and  iron-bearing  member.  The  essential  parallelism  of  the  ores  to  the  trend  of  the  iron-bearing 
member  shows  the  obvious  tendency  of  the  waters  to  follow  that  trend  but  to  be  deflected  by 
the  minor  bends  in  it.  This  is  especially  well  seen  along  the  main  belt  of  iron-bearing  rocks 
along  Iron  River. 

The  depths  to  which  the  waters  have  acted  is  yet  largely  unknown.  The  deepest  mines 
0])erate  to  a  depth  of  1,000  feet  in  the  Gystal  Falls  district,  500  feet  in  the  Iron  River  district, 
and  950  feet  in  the  Florence  district.  In  certain  deposits  the  ore  has  apparently  given  out  with 
depth.  It  is  possible  that  in  some  mines  it  has  been  lost  because  of  considerable  offset  by  the 
folding.     Deeper  exploration  is  warranted. 

The  topographic  relief  of  the  region  is  so  great  that  different  ])arts  of  the  iron-bearing 
member  may  differ  as  much  as  300  feet  in  elevation.  The  ores  are  as  a  rule  closely  associated 
with  the  hills  but  seem  to  follow,  indifferently,  crests,  slopes,  and  adjacent  valleys.  In  the  Iron 
River  district  the  ores  favor  especially  the  valleys.  These  are  discernible  with  difficulty  tlirough 
the  thick  drift,  but  are  being  found  by  drilling.  The  depth  to  which  a  head  given  by  the  observed 
topography  would  carry  a  vigorous  circulation  through  the  iron-bearing  member  can  not  be 
woi'ked  out  theoretically  because  of  the  imcertamty  of  the  factors  mvolved.  Certainly  nothing 
is  now  known  which  would  prevent  exploration  as  deep  as  in  other  districts  of  the  Lake  Superior 
region,  although  here,  as  in  other  districts,  many  of  the  deposits  have  certainly  been  found  to 
be  only  a  few  hundred  feet  deep. 

Ohemical  and  mineralogical  changes. — The  iron-bearing  member  was  originally  pyritiferous 
iron  carbonate  interbedded  with  more  or  less  slate.  The  alteration  to  ore  has  occurred  in  two 
phases — first,  the  oxidation  of  the  iron  without  removal  of  silica,  producing  ferruginous  cherts; 
second,  partly  simultaneous  and  more  local,  the  leaching  of  the  silica,  leaving  the  iron  oxide 
concentrated  as  ore.  The  phj^sical  and  chemical  features  of  these  alterations  have  not  been 
worked  out  tjuantitatively  as  they  have  for  other  districts,  but  qualitativel}'  they  are  knowTi 
to  be  similar  to  those  of  other  districts  in  all  respects. 

Time  of  concentration. — The  ores  were  concentrated  after  the  upper  ITuronian  folding  and 
before  the  Cambrian  deposition,  and  since  their  concentration  they  have  been  little  affected  by 
further  folding. 

THE  IRON  ORES  OF  THE  FELCH  MOUNTAIN  AND  CAI.LTMET  DISTRICTS. 

By  the  authors  and  \V.  J.  Mead. 

The  Felch  Mountam  and  Cahmiet  districts  are  eastward  branches  of  the  Crystal  Falls 
district.  Except  for  low  grade  and  low  ])hosphorus,  their  ores  are  the  same  in  horizon,  relations, 
and  mineralogical  and  j)liysical  character  as  the  ores  of  the  CVystal  Falls  and  Menominee  districts. 

The  shipment  from  these  districts  has  been  small. 


IRON  ORES  OF  FELCH  MOUNTAIN  AND  CALUMET  DISTRICTS.  327 

FELCH  MOUNTAIN   DISTRICT. 

Iron  ores  have  been  mined  at  two  localities  in  the  Felch  Mountain  district  near  Groveland 
and  near  Felch.  In  both  these  localities  the  iron-bearing  Vulcan  formation  lies  in  a  closely 
compressed  s\'ncline  with  basement  of  impervious  slate  or  schist,  called  "Mansfield"  schist  by 
Smyth,  but  called  Felch  schist  in  this  report.  The  lenses  at  the  east  end  of  the  Felch  Moimtain 
trough  are  now  largely  worked  out.     At  the  Groveland  mine  dikes  of  granite  cut  the  ore  body. 

The  average  composition  of  the  ores  mined  in  the  Felch  Mountain  district  in  1907  is  as 
follows : 

Average  analysis  of  ore  mined  in  the  Felch  Mountain  district  in  190/. 

Mointure  (loss  on  drying  at  212°) 4.  05 

Analysisof  ore  dried  at  212°  F.:  == 

Iron 52.  50 

Phosphorus 040 

Silica 11.  22 

Manganese 1. 10 

Alumina. . . .- 2.  49 

Lime 3.  51 

Magnesia 4.  62 

Sulphur 008 

Loss  by  ignition 5.  29 

The  volume  composition  of  these  ores,  in  comparison  with  the  Crystal  Falls,  Menominee, 
Iron  River,  and  Florence  ores,  is  given  in  figure  50  (p.  352). 

CALUMET  DISTRICT. 

Ore  is  mined  in  the  Calumet  district  only  at  the  Calumet  mine,  a  comparatively  recent 
development,  where  there  is  a  steeply  southward-dipping  succession  beginning  with  Archean 
granite  on  the  north,  followed  successively  by  Sturgeon  quartzite,  Randville  dolomite,  Felch 
schist,  Vulcan  formation  (iron  bearing),  and  Michigamme  slate.  The  strike  of  the  ore  body  is 
parallel  to  the  bedding.  The  bedding  trends  east  and  west,  but  has  minor  folds  with  steep 
pitches  parallel  to  the  strike.  The  ore  body  with  its  associated  iron-bearing  formation  is 
divided  longitudinally  into  three  parts  by  layers  of  slate,  from  north  to  south  60,  15,  and  60 
feet  thick.  The  foot  wall  is  slate,  quartzite,  and  dolomite.  The  hanging  wall  is  slate  or  iron- 
bearing  formation.  Along  the  strike  the  ore  abuts  irregularly  against  unaltered  iron  formation. 
The  depth  of  mining  operations  to  the  date  of  writing  is  200  feet.  The  possibilities  of  the 
extension  of  the  deposits  are  discussed  on  page  324.  The  iron  ore  is  banded  cherty  hematite 
and  limonite  and  some  magnetite.  It  is  nonmagnetic  in  individual  pieces,  but  collectively  it 
exerts  a  powerful  magnetic  pull.  The  ore  runs  from  40  to  45  per  cent  of  iron  and  is  sold  on  the 
basis  of  0.028  per  cent  of  phosphorus.  Its  density  is  about  4  and  its  porosity  IS  per  cent;  it 
averages  about  10.5  cubic  feet  to  the  ton. 

The  average  composition  of  the  ores  mined  in  the  Calumet  district  in  1907  is  as  follows: 

Averaije  analysis  of  ore  mined  in  the  Calumet  district  in  1907. 

Moisture  (loss  on  drying  at  212°) 5.  00 

Analysis  of  ore  dried  at  212°  F.:  =^ 

Iron 42.  82 

Phosphorus 028 

Silica 32.  27 

Manganese 20 

Alumina 2.  53 

Lime .- .74 

Magnesia 1.  06 

Sulphur Oil 

Loss  by  ignition 1.  86 

The  volume  composition  of  these  ores,  in  comparison  with  Crystal  Falls,  Iron  River,  Florence, 
and  Menominee  ores,  is  given  in  figure  50  (p.  352). 


328  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

SECONDARY    CONCENTRATION    OF    THE    FELCH    MOUNTAIN    AND    CALUMET 

ORES. 

Structural  conditions. — Tlie  iron-bearing  Vulcan  formation  of  the  Fclch  Mountain  district 
is  in  closely  compressed  synclinal  folds  in  the  upper  lluronian  Felch  schist.  It  stands  out  as 
erosion  remnants  forming  the  crests  of  the  hills.  The  concentration  has  evidently  been  con- 
trolled bj'  the  impervious  basements  of  slate,  and  also  to  some  extent  by  the  o[)enings  along 
fracture  planes,  especially  north-south  fracture  planes  crossing  the  axis  of  the  trough.  The 
granite  dikes  at  the  Groveland  mine  may  also  have  been  influential  in  controlling  circulation. 

In  the  Calumet  district  there  is  no  essential  difference  in  the  structural  relations  governing 
the  How  from  those  in  the  Crystal  Falls  and  Iron  River  districts.  The  dip  is  steep  and  the  forma- 
tion has  the  usual  drag  type  of  corrugation. 

Che^nical  and  mincralogical  changes. — The  iron-bearing  member  was  originally  iron  car- 
bonate mterbedded  with  more  or  less  slate.  The  alteration  to  ore  has  occurred  m  two  phases — 
first,  the  oxidation  of  the  iron  without  removal  of  silica,  producing  ferruginous  cherts;  second, 
partly  simultaneous  and  more  local,  the  leachmg  of  the  silica,  leaving  the  iron  oxide  concentrated 
as  ore.  The  physical  and  chemical  features  of  these  alterations  have  not  been  worketl  out 
quantitatively  as  they  have  for  other  districts,  but  quahtatively  they  are  known  to  be  similar 
to  those  of  other  districts  in  all  respects. 


CHAPTER  XIII.     THE  MENOMINEE  IRON   DISTRICT  OF  MICHIGAN." 

LOCATION   AND   EXTENT. 

The  portion  of  the  Menominee  district  covered  by  tiie  accompanying  map  (PI.  XXVI,  in 
pocket)  is  bounded  on  tlie  west  by  Menominee  River,  on  the  soutli  by  tlie  same  river  and  the 
south  hne  of  T.  39  N.,  on  the  north  by  the  north  Una  of  T.  40  N.,  and  on  the  east  by  the  east 
hue  of  sees.  10,  15,  22,  27,  and  34,  T.  39  N.,  R.  28  W.  The  area  thus  outlined  constitutes  a 
tongue  of  sedimentary  deposits  lying  between  a  granite  area  to  the  north  and  a  greenstone  schist 
area  to  the  south. 

At  about  tlie  line  between  Rs.  27  and  28  W.  the  characteristic  rocks  of  the  Menominee 
trough  become  so  deeply  buried  under  later  sediments  that  they  can  be  traced  no  farther  by 
outcrop.  Lines  of  magnetic  attraction,  however,  have  been  obtained  still  farther  east,  and  these 
are  taken  to  mean  that  the  Huronian  deposits  continue  for  a  considerable  distance  beyond 
the  places  where  they  are  last  seen  on  the  surface. 

The  area  of  sedimentary  rocks  belonging  in  the  Menominee  trough  is  about  125  square  miles, 
entirely  within  the  State  of  Michigan.  This  area  is  narrowest  in  the  vicinity  of  Vulcan,  where 
it  measures  about  4  miles  in  width  from  the  contact  witli  the  granite  on  the  north  to  the  contact 
with  the  greenstone  schist  on  Menominee  River  to  the  south.  To  the  east  the  area  widens 
gradually,  until  m  the  eastern  portion  of  R.  28  W.  its  width  measures  about  7  miles.  To  the 
west  it  also  widens  gradually  and  finally  loses  its  identity  as  a  distinct  trough  at  al)out  the 
center  line  of  R.  30  W.,  where  it  merges,  with  the  Calumet  trough,  into  the  wide  area  of  Huronian 
sediments  on  the  west. 

TOPOGRAPHY. 

There  are  thi'ee  important  ridges  in  the  district  with  axes  parallel  to  its  length,  a  northern 
one  and  two  others,  nearly  parallel,  near  the  central  part  of  the  district.  The  northern  ridge 
is  composed  of  Archean  granite  and  the  Sturgeon  cpiartzite.  The  central  ridges  are  composed 
of  the  Randville  dolomite  and  the  ii-on-bearing  Vulcan  formation,  capped  in  much  of  the  dis- 
trict by  Cambrian  sandstone.  The  higher  points  of  these  ridges  range  in  altitude  from  about 
1,000  feet  to  nearly  1,600  feet.  The  valleys  between  the  ridges,  as  well  as  the  valley  to  the 
south  of  the  main  central  ridge  sloping  to  Menoininee  River,  are  composed  mainly  of  the  Michi- 
gamme  ("Hanbury")  slate.  The  southern  lowland  area  of  the  Michigamme  slate  continues  into 
the  area  of  the  Quinnesec  schist.  The  lower  areas  have  altitudes  varying  for  the  most  part  from 
800  to  1,000  feet. 

The  minor  streams  follow  to  a  considerable  extent  the  valleys  of  the  Michigamme  slate, 
and  the  same  is  true  of  the  chief  stream  of  the  district,  the  Menominee,  for  a  considerable  part 
of  the  area,  but  this  and  a  number  of  the  other  more  important  streams,  such  as  Sturgeon 
River  and  Pine  Creek  and  some  of  its  branches,  flow  transverse  to  the  ridges.  Several  of  even 
the  smaller  branches  break  through  either  one  or  both  of  the  iron  ranges  and  the  cpiartzite  and 
granite  range  to  the  north.     Sturgeon  River  crosses  all  the  formations  of  the  district. 

SUCCESSION   OF  FORMATIONS. 

The  rocks  of  the  Menominee  district  belong  to  the  Archean,  Algonkian,  Cambrian,  and 
Ordovician  systems.  The  oldest  rocks  bordering  the  Menominee  tongue  are  greenstone  schists 
and  granite.     These  are  regarded  as  Archean.     Resting  unconformably  upon  the  Archean  rocks 

o  For  further  detailed  description  of  the  geology  of  this  district  see  Men.  U.  S.  Geol.  Survey,  vol.  46, 1904,  and  references  there  given. 

329 


Huronian  scries: 

Upper  Huronian  (Animikie  group) 


330  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

are  Algonkian  sediments,  which  belong  to  the  Huronian  series.  These  are  (hvisible  into  lower 
Huronian,  middle  Huronian,  and  upper  Huronian,  and  an^  separated  by  unconfornjities.  The 
Paleozoic  rocks  comprise  horizontal  Cambrian  sandstones  and  Cambro-Ordovician  Umestones. 
These  occur  in  patches  on  the  tops  of  the  hills,  capping  the  closely  folded  and  truncated  Huronian 
rocks.  The  Huronian  series  is  divisible  into  a  number  of  formations,  each  representing  a  time 
during  which  the  conditions  of  deposition  were  apjjroximately  uniform.  The  following  table 
gives  the  list  of  the  formations  arranged  in  descending  order  according  to  age.  The  members 
of  the  Vulcan  formation  are  distinguished  in  the  descriplion  but  not  on  the  map. 

Cambro-Orclo\dcian Uermansvillc  limestone. 

Cambrian  system Lake  Superior  sandstone. 

Unconformity. 
Algonkian  system: 

Keweenawan  series Granite  (?). 

Quinnesec   schist  and   other  green  schists 

representing  surface  eruptions  overlying 

and  interbedded  with  Michi<;amme  slate. 

Michigamme  ("Hanbury")  slate,  including 

iron-bearing  beds. 
Vulcan   formation,   subdivided   into  Curry 
iron-bearing  member,  Brier  slate  member, 
and  Traders  iron-bearing  member. 
Unconformity. 

Middle  Huronian Quartzite,  not  separated  from  Rand\'ille  dolo- 
mite in  mapping  for  most  of  the  district. 
Unconformity. 

T  „        ■  [Randville  dolomite. 

Lower  Huronian <„^  ^  .^ 

ISturgeon  quartzite. 

Unconformity. 
Archean  system : 

Laurentian  series Granites  and  gneisses,  cut  by  granite  and 

diabase  dikes. 
Keewatin  series  (not  separated  in  mapping  from 
Laurentian) Green  schists. 

The  Quinnesec  schist  is  so  named  because  the  formation  is  typically  developed  at  the 
Quinnesec  Falls,  on  Menominee  River.  The  Sturgeon  cjuartzite  is  so  called  because  this  forma- 
tion in  the  Menominee  district  has  been  traced  almost  continuously  to  a  like  formation  in 
the  Crystal  Falls  district  which  has  been  called  the  Sturgeon  quartzite.  The  dolomite  in  the 
Menominee  district  is  called  the  Randville  dolomite  because  it  has  been  practically  connected 
with  the  Randville  dolomite  of  the  Crj^stal  Falls  district. 

In  the  upper  Huronian  the  Vulcan  formation  is  so  named  because  it  occurs  in  typical 
development  with  full  succession  and  fine  exposures  near  the  town  of  Vulcan.  The  "Hanbury" 
slate  was  thus  named  because  in  the  vicinity  of  Lake  Hanbury  this  formation  is  better  exposed 
than  anywhere  else  in  the  district.  This  slate,  however,  has  been  proved  to  be  equivalent  to 
and  continuous  with  the  Michigamme  slate  of  the  Marquette  district,  and  the  older  name, 
Michigamme,  is  therefore  used  in  tliis  report. 

ARCHEAN  SYSTEM. 

LAURENTIAN  SERIES  AND  UNSEPARATED  KEEWATIN. 

The  complex  north  of  the  Menominee  district  is  composed  largely  of  Laurentian  rocks. 
They  are  principally  gneissoid  granites  and  finer-grained  banded  gneisses.  In  addition  to  these 
there  are  also  present  in  subordinate  qiuintity  hornblende  schists  and  certain  feklspathic  green- 
stone schists  identical  lithologically  with  some  of  the  mashed  eruptive  rocks  among  the  Quin- 
nesec schist.     These  are  intruded  by  Laurentian  granites  and  are  believed  to  represent  the 


MENOMINEE  IRON  DISTRICT.  331 

Keewatin  series.  They  have  not  been  sejjarated  in  map])ing.  Mica  scMsts  arc  founfl  only  in 
a  few  exposures  in  the  interior  of  the  Ai'chean  area  north  of  the  region  shown  on  the  map 
(PI.  XXVI,  in  pocket).  The  granites,  gneisses,  and  schists  are  cut  by  small  dikes  and  veins  of 
granite,  pegmatite,  and  aplite,  by  numerous  quartz  veins,  and  by  coarse  granite,  massive  basalt, 
diabase,  and  gabbro. 

Some  of  the  hornblende  schists  (Keewatin)  and  some  of  the  gneisses  appear  to  be  older 
than  most  of  the  granites.  Others  of  the  scliists  are  unquestionably  mashed  intrusive  rocks 
that  are  younger  than  some  of  the  granites.  The  aplites,  pegmatites,  and  some  of  the  basic 
intrusives  are  the  youngest  rocks  belonging  exclusively  in  the  complex,  but  even  these,  as  they 
are  not  known  to  cut  through  the  Huronian  deposits,  are  thought  to  have  taken  their  present 
position  before  the  sediments  were  deposited.  The  latest  of  all  the  intrusive  rocks  are  certain 
coarse-grained  massive  diabases  and  gabbros.  These  rocks  not  only  occur  as  members  of  the 
complex  but  are  found  also  in  the  lower  division  of  the  Huronian  series,  overlying  the  Archean 
complex.  There  is  no  reason  to  believe  that  any  of  these  rocks  are  metamorphosed  sediments. 
Most  of  them  are  clearly  of  igneous  origin. 

The  massive  granites  and  the  gneissoid  granites  differ  from  each  other  in  no  essential 
respect.  The  latter  are  merely  schistose  phases  of  the  former.  They  both  embrace  medium- 
grained  to  fine-grained  gray  and  pink  rocks  with  a  granitic  texture  that  locally  approaches  in 
appearance  the  texture  of  some  quartzites. 

The  banded  gneisses  consist  of  alternate  bands  of  pink  and  gray  material,  each  band  having 
the  look  of  granite.  These  bands,  though  appearing  to  be  approximately  parallel  in  the  ledges, 
are  found  on  close  inspection  to  run  parallel  to  one  another  for  short  distances  only  and  then  to 
anastomose  or  interlace.  The  red  layers  cut  across  the  gray  gneiss  as  if  they  were  veins  of 
granitic  material.  The  only  difference  that  can  be  discerned  between  the  banded  gneisses 
and  the  fine-grained  gray  gneisses  cut  by  red  granite  veins  is  that  the  latter  are  irregularly 
injected  by  the  granitic  material,  while  in  the  former  the  injections  are  largely  parallel. 

The  hornblende  scliists  (Keewatin)  are  usually  lustrous  greenish-black  schists  with  the 
normal  characteristics  of  such  rocks.  They  are  cut  by  the  granites  in  some  places.  In  other 
places  large  blocks  are  found  included  in  granite.  Plainly  they  are  older  than  the  granites, 
and  probably  they  are  the  oldest  rocks  in  the  northern  complex.  A  second  kind  of  hornblende 
sclust  exists  in  which  the  rocks  are  so  related  to  the  granites  and  gneisses  that  they  must  be 
regarded  as  dikes.  In  some  places  they  a]^pear  as  bands  cutting  across  the  banding  of  the 
gneisses,  and  in  others  as  bands  conforming  in  strike  and  dip  with  the  lighter-colored  bauds  of 
these  rocks.     These  schists  are  therefore  looked  upon  as  mashed  intrusive  rocks. 

ALGONKIAN   SYSTEM. 

GENERAL  CHARACTER  AND  LIMITS. 

The  Algonlvian  rocks  constituting  the  Menominee  trough,  though  strongly  metamorphosed, 
are  recognized  as  mainly  sediments.  The  greater  mass  of  these  sediments  is  mechanical,  clastic 
textures  being  still  plainly  apparent.  The  iron-bearing  formation  is  largely  mechanical,  but 
with  the  mechanical  material  an  important  amount  of  chemical  and  organic  material  was 
deposited,  and  some  of  the  jaspers  of  the  formation  may  be  wholly  chemical  or  organic.  The 
limestones  are  chemical  or  organic  sediments.  The  sedimentary  rocks  have  been  intruded  by 
a  few  coarse-grained  and  some  fine-grained  igneous  rocks.  The  latter  are  now  usually  scliistose. 
The  lowest  formation  of  the  Algonkian  system  has  at  its  bottom  basal  conglomerates,  which 
rest  unconformably  upon  the  Ai'chean  rocks  of  the  northern  complex.  These  conglomerates 
may  be  seen  at  a  number  of  places  along  the  border  of  the  trough,  and  notably  at  the  falls  of 
Sturgeon  River. 


332  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

The  formations  of  the  Algonkian  system  are  likewise  separated  from  the  overlying 
Cambrian  sandstone  by  a  profound  unconformity.  The  Algonkian  rocks  are  folded ;  the  Cam- 
brian sandstone  is  horizontal  and  thus  lies  across  the  truncated  ends  of  the  eroded  folds.  Its 
lowt-r  layer  is  formed  largely  of  the  debris  of  the  more  ancient  rocks.  Hence  the  Algonkian 
rocks  formed  a  land  surface  for  a  vast  period  of  time  before  the  deposition  of  the  Cambrian 
sandstone. 

HTJKONIAN  SEKIES. 

LOWER   HURONIAN. 

SUCCESSION  AND  DISTRIBUTION. 

The  lower  Huronian  is  divided  into  two  formations — the  Sturgeon  quart  zite  and  the 
Randville  dolomite,  the  former  bemg  the  older.  These  formations  are  observed  only  in  the 
center  and  on  the  north  side  of  the  Menominee  trough.  On  the  south  side  of  the  trough  no 
evidence  of  their  existence  is  obtainable.  This  may  possibly  be  due  to  the  thick  covering  of 
drift  that  blankets  the  rocks  north  of  the  southern  area  of  Quinnesec  schist ;  but  it  is  thought 
to  be  more  probable  that  these  formations  are  not  present  at  the  rock  surface  in  this  portion 
of  the  district. 

STURGEON  QUARTZITE. 

Distribution. — The  Sturgeon  quartzite  forms  a  continuous  border  of  bare  hiUs  on  the  south 
side  of  the  northern  complex.  The  formation  lies  between  the  Archean  complex  and  the 
northern  belt  of  dolomite.  Prominent  bluffs  of  the  typical  quartzite  may  be  conveniently 
studied  northeast  of  the  Loretto  mine. 

Lithology. — At  many  places  at  the  base  of  the  Sturgeon  quartzite  there  is  a  conglomerate 
made  up  of  bowlders  and  fragments  of  granites,  gneisses,  and  hornblende  schists  identical 
with  the  corresponding  rocks  in  the  adjacent  Archean  complex  to  the  north.  The  matrix  in 
which  these  are  embedded  is  in  some  places  a  quartzite,  in  others  an  arkose  composed  of  the 
fine-grained  debris  of  granitic  rocks.  In  many  places  this  matrix  is  schistose  and  a  large  quan- 
tity of  a  micaceous  mineral  has  been  produced  by  alteration  of  the  feldspar  of  the  original 
sediment,  so  that  the  matrix  is  now  lithologicaUy  a  sericite  schist. 

The  major  portion  of  the  formation  consists  of  massive  beds  of  a  very  compact,  vitreous 
quartzite,  usually  white,  but  here  and  there  tinted  with  some  shade  of  pink  or  green.  In  its 
upper  portion  the  cement  between  the  quartz  grains  is  locally  calcareous.  This  calcareous  con- 
stituent increases  in  quantity  as  the  overlying  dolomite  is  approached,  until  the  rock  becomes 
a  calcareous  quartzite  and  finally  a  quartzose  dolomite.  The  change  from  the  cjuartzite  to  the 
dolomite  is  thus  a  transition.  This  indicates  a  gradual  deepening  of  the  waters  during  the 
later  part  of  the  Sturgeon  epoch. 

Deformation. — The  main  belt  of  the  Sturgeon  quartzite  is  a  nearly  vertical  southward- 
dipping  monochne.  The  outcrop  of  this  monocline  varies  in  strike,  thus  indicating  that  cross 
folding  has  taken  place  to  some  extent.  At  the  west  end  of  the  district  the  quartzite  turns 
northward,  ^Tapping  around  the  Archean  complex  and  then  passing  eastward  into  the  area  of 
the  Calumet  trough.  On  the  turn  to  the  north  several  small  folds  are  developed,  the  synclines 
of  which  are  now  represented  by  embayments  extending  eastward  into  the  Archean.  The  dips 
of  the  quartzite  beds  may  vary  a  few  degrees — 25°  in  one  place — from  perpendicularity.  There 
arc  almost  as  many  northern  dips  toward  the  granite  and  gneiss  complex  as  there  are  southern 
dips  toward  the  center  of  the  trough. 

Relations  to  adjacent  formations. — The  Sturgeon  quartzite  rests  unconformably  upon  the 
Archean  rocks  of  the  northern  complex.  This  is  shown  by  the  character  of  the  lower  bed  of 
the  quartzite,  which,  as  already  said,  is  a  basal  conglomerate.  This  basal  conglomerate  con- 
tains almost  every  variety  of  fragment  derivable  from  the  rocks  of  the  northern  complex. 
Some  of  this  material  in  its  original  position  must  have  been  formed  at  great  dei)th  in  the  earth. 
Therefore  there  was  deep-seated   denudation  of  the  Archean  before  the   deposition  of  the 


MENOMINEE  IRON  DISTRICT.  333 

quartzite.     Upward  the  Sturgeon  quartzite  grades  into  the  Randville  dolomite.     The  nature 
of  ^,he  gradation  is  discussed  in  tlie  section  on  tliat  formation. 

Thickness. — Two  difficulties  stand  in  the  way  of  determining  the  thickness  of  the  Sturgeon 
quartzite.  The  first  is  the  inqiossibihty  of  deciding  how  much  of  the  apparent  thickness  of  the 
many  rock  layers  in  a  closely  folded  district,  like  the  Menominee,  is  due  to  the  duphcation  of 
beds  in  consequence  of  close  folds.  The  other  difficulty  is  the  impossibihty  of  fixing  the  upper 
limit  of  the  formation.  There  is  everywhere  between  the  c[uartzites  and  the  nearest  ledges  of 
the  overlying  dolomite  a  belt  of  country  without  exposures  of  any  kind.  If  we  assume  that 
the  southward-facing  cliffs,  which  in  so  many  places  mark  the  southern  limit  of  the  quartzites, 
are  cliffs  of  differential  degradation,  that  the  low  ground  at  the  base  of  the  cliffs  is  underlain 
by  the  dolomite  formation,  and  that  the  exposures  are  monochnal,  the  maximum  thickness  of 
the  formation  is  between  1,000  and  1,250  feet. 

RANDVILLE  DOLOMITE. 

Distribution. — The  Randville  dolomite  occupies  three  separate  belts,  whose  positions  and 
shapes  are  determined  by  the  folding  to  which  the  formation  has  been  subjected.  These  will 
be  referred  to  as  the  northern,  central,  and  southern  belts  of  dolomite. 

The  northern  belt  is  south  of  the  belt  of  Sturgeon  quartzite.  Only  a  few  exposures  are 
found  in  this  area,  but  they  are  so  uniformly  distributed  that  on  the  map  (PI.  XXVI,  in  pocket) 
the  whole  belt  has  been  colored  for  the  formation.  It  is  quite  possible,  however,  that  in  some 
places  erosion  has  carried  away  the  dolomite  and  that  the  upper  Huronian  rests  immediately 
upon  the  quartzite. 

The  central  belt  of  dolomite  borders  the  north  side  of  Lake  Antoine  for  a  portion  of  its 
length,  passes  eastward  between  the  Cuff  and  Indiana  mines,  and  ends  at  the  bluff  known  as 
Iron  Hill  in  the  E.  i  sec.  32,  T.  40  N.,  R.  29  W.  This  belt  is  well  marked  by  numerous  and 
large  exposures. 

The  southern  belt  of  dolomite  extends  all  the  way  from  the  west  side  of  the  sandstone 
bluff  west  of  Iron  Mountain  to  the  village  of  Waucedah,  at  the  east  end  of  the  district.  Where 
not  exposed  the  rock  has  been  found  in  mines,  test  shafts,  and  pits,  so  that  there  is  a  reason- 
able certainty  that  it  exists  throughout  this  distance  of  16  miles.  Where  there  is  any  doubt  of 
its  existence  at  the  surface  this  is  due  to  a  considerable  thickness  of  overlying  Cambrian  sand- 
stone. From  Iron  Mountain  as  far  east  as  Sturgeon  River  tlie  country  underlain  by  the  dolo- 
mite is  a  range  of  high  hills,  broken  only  at  a  few  points  by  north-south  gaps.  On  the  southern 
slope  of  this  ridge  are  the  principal  producing  iron  mines  of  the  district. 

LitTiolo'jy. — The  Randville  dolomite  is  composed  of  a  heterogeneous  set  of  beds  Lq  which 
dolomite  is  dominant.  With  the  pure  dolomites  are  siliceous  dolomites,  calcareous  quartzites, 
argillaceous  rocks,  and  cherty  quartz  rocks.  The  Randville  dolomite,  lying  upon  the  Sturgeon 
quartzite,  grades  dowmward  into  it.     The  intermediate  rock  is  a  calcareous  quartzite. 

The  predominant  rock  of  the  Randville  dolomite  is  an  almost  massive,  apparently  homo- 
geneous, fine-grained  white,  pink,  blue,  or  bufl'  dolomite,  occurring  m  beds  from  a  few  inches 
to  many  feet  in  thickness.  This  is  interstrattfied  with  beds  of  siliceous  dolomite  m  which  are 
observable  numerous  grams  of  quartz.  In  many  places  on  the  weathered  surfaces  of  the  dolo- 
mites are  thin  projecting  Ijands  of  vein  cpiartz  parallel  to  the  bedding,  which  the  microscope 
shows  to  be  calcareous  quartzite.  In  other  places  projecting  bands  anastomose  or  run  irregu- 
larly over  the  weathered  surfaces,  here  and  there  intersecting  the  bedding  planes  of  the  rock 
at  acute  angles.  Their  abundance  proves  clearly  that  the  dolomites,  in  spite  of  their  homo- 
geneous appearance,  have  been  extensively  fractured  and  crushed.  In  many  places  the  crush- 
ing has  produced  a  breccia  of  dolomite  fragments  in  a  siliceous  matrix.  In  a  few  localities  the 
fragments  are  rounded,  so  that  the  rock  is  a  pseudoconglomerate. 

The  greater  part  of  the  argillaceous  rocks  interstratified  with  the  dolomite  is  soft  light- 
gray  or  dark-gray  slate.  Another  part  is  typical  black  slate,  still  plainlj^  marked  by  bedding 
lines.     Still  other  parts  are  purplish-pink  schistose  argillaceous  dolomite.     Many  of  the  thin 


334  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

layers  of  the  pui'plisli-piiik  shitelike  material  between  massive  dolomite  beds  appear  to  be- 
largely  the  selvajje  of  the  softer  lajers  of  dolomite,  rendered  schistose  by  the  movement  of 
accommodation  between  the  stronger  beds. 

Dcfommiion. — Structurally  the  northern  belt  of  dolomite  is  a  southward-dipping  mono- 
cline. The  central  and  southern  belts  are  anticlines.  The  three  belts  are  separated  by  two 
sjTiclines. 

In  the  anticlinal  belts  the  beds  at  first  sight  appear  to  bo  isoclinal,  but  a  close  examination 
of  the  southern  belt  reveals  the  existence  of  a  number  of  minor  folds  having  almost  vertical 
pitches.  In  the  western  part  of  the  district  the  folds  are  overturned  to  the  south,  the  axial 
planes  dipping  northward  at  high  angles.  In  the  central  and  eastern  parts  of  the  district,  east 
of  Quiimescc,  the  minor  and  major  folds  have  their  axial  planes  steeply  inclined  to  the  south. 
Although  the  minor  folds  are  rather  easilj'  recognizable,  it  is  only  on  the  south  side  of  the 
southern  belt  that  they  become  prominent.  Here  the  synclines  open  out,  forming  basins  in 
which  the  ore  bodies  lie.  The  small  folds,  with  a  few  exceptions,  pitch  west  in  the  western 
portion  of  the  range  and  east  in  the  eastern  portion. 

The  attitude  of  minor  folds  is,  as  is  well  known,  an  indication  of  the  attitude  of  the  major 
folds  on  which  they  are  superimposed.  By  using  this  principle,  it  is  concluded  that  the  major 
anticlines  in  this  district  disappear  to  the  east  and  to  the  west  by  plunging  beneath  the  upper 
Huronian  sediments. 

From  the  above  statements  it  is  clear  that,  in  addition  to  the  major  east-west  anticlines  and 
synclines  that  are  so  prominent  in  the  district,  the  dolomite  formation  is  also  affected  by  a^ 
gentle  but  large  cross  anticlinorium  whose  axis  runs  approximately  north  and  south.  It  is- 
remarkable  that  eiosion  has  nowhere  exposed  the  Sturgeon  quartzite  in  association  with  the 
central  belts  of  dolomite. 

Relatione  to  adjacent  formations. — The  dolomite  formation  is  nowhere  seen  in  actual  con- 
tact witli  the  Sturgeon  quartzite,  nor  are  ledges  of  the  two  formations  seen  in  close  proximity. 
It  is  knowii,  however,  that  the  upper  layers  of  the  quartzite  are  calcareous  and  that  the  lower 
beds  of  the  dolomite  are  quartzose.  The  inference  seems  to  be  safe  that  the  two  formations  are 
conformable,  and  that  they  grade  into  each  other  through  calcareous  quartzites.  The  rela- 
tions of  the  dolomite  to  the  overlying  formations  are  discussed  in  connection  with  the  upper 
Huronian. 

Thicl'ness. — At  no  place  i^dthin  the  area  mapped  is  the  dolomite  known  to  be  exposed 
from  the  bottom  to  the  top  of  the  formation.  On  the  north  side  of  the  trough  the  formation 
is  bordered  by  the  Sturgeon  quartzite  on  the  north  and  the  Vulcan  formation  on  the  south, 
but  exposures  between  these  limits  are  so  few  that  it  is  not  ceitain  that  the  dolomite  occupies 
the  entire  breadth,  and  on  this  account  and  because  of  the  minor  folds  it  is  impossible  to  give 
anything  like  an  exact  estimate  of  the  thickness  of  the  formation.  By  making  calculations  so 
as  to  obtain  a  minimum  figure,  1,000  feet  or  less  could  be  obtained.  If,  on  the  other  hand, 
calculations  were  made  on  the  supposition  that  all  of  the  isoclinal  beds  are  different  layers,  the 
estimate  might  be  as  great  as  5,000  feet.  Probably  the  truth  is  much  nearer  the  lower  figure 
than  the  higher.  The  original  thickness  of  the  dolomite  is  probably  somewhere  between  1,000 
and  1,500  feet. 

MIDDLE    HURONIAN. 

The  identified  middle  Huronian  of  tJic  Menominee  district  consists  entirely  of  chert}' 
quartzite  resting  in  a  thin  film,  from  a  few  feet  to  70  feet  thick,  on  the  Randville  dolomite 
near  its  contact  with  the  upper  Huronian  (Animikie  group),  and  it  is  included  with  tlie  Rand- 
ville dolomite  on  the  general  map  of  the  district  (PI.  XXYI,  in  pocket).  These  rocks  were 
formerly  regarded  as  a  part  of  the  dolomite  formation,  but  recent  work  has  shown  them  to  be 
sci>aral)lc  from  the  dolomite.  The  quartzite  has  been  separated  from  the  dolomite  in  the 
mapping  for  several  small  areas  near  Norway  and  the  east  end  of  Iron  Hill.     (See  fig.  4.').) 

The  chcrty  quartz  rocks  are  fine  grained,  drusy  in  places,  and  white,  light  red,  or  dark 
purple.     The  darker  colorcil  kinds  look  very  much  like  some  varieties  of  jaspilite.     Under th& 


MENOMINEE  IRON  DISTRICT. 


335 


LEGEND 


Cambrian 
sandstone 


microscope  the  cherty  quartz  rocks  seem  to  be  composed  almost  exclusively  of  a  fine-grained 
crystalline  aggregate  of  quartz  which  incloses  a  few  grains  of  hematite,  magnetite,  and  other 
iron  compounds.  Here  and  there  a  fragmental  quartz  grain  may  be  seen,  but  usually  no  trace 
of  fragmental  constituents  can  be  discerned. 

Pebbles  in  the  conglomerate  at  the  base  of  the  upper  Huronian  are  partly  jasper  and  iron 
ore,  obviously  derived  from  some  preexisting  formation  not  now  appearing.  A  reasonable 
inference  is  that  these  pebbles  represent  fragments  of  the  Negaunee  formation,  which  would 
normally  lie  above  the  middle  Huronian  quartzite.  In  the  previous  report  on  this  district" 
several  masses  of  iron-bearing  rocks  were  doubtfully  referred  to  the  Negaunee.  Subsequent 
work  has  demonstrated  these  to  be  upper  Huronian. 

The  middle  Hiu-onian  quartzite  rests  unconformabl}-  on  the  Randville  dolomite,  with 
discordance  in  bedding.  The  quartzite  may  be  observed  to  fill  fissures  and  depressions  in  the 
dolomite.  At  Norway  Hill  erosion 
cut  ofl^  100  feet  or  more  of  the  dolo- 
mite before  the  quartzite  was  de- 
posited. On  the  south  side  of 
Iron  HiU  there  is  a  thin  film  of 
conglomerate,  taken  to  represent 
the  base  of  the  micklle  Huron- 
ian quartzite,  plastered  against 
the  dolomite  escarpment.  The 
quartzite  is  not  shown  directly 
above  the  conglomerate  but  ap- 
pears a  few  hundred  feet  to  the 
east,  resting  against  the  dolomite 
escarpment.  (See  PI.  XVII,  ^4,  of 
Monograph  XL VI.)  In  fact,  much 
of  the  mitldle  Huronian  quartzite 
itself  on  Iron  Hill  is  conglomeratic 
and  brecciated,  and  a  considerable 
part  of  it  may  possibly  represent  a 
coarsely  fragmental  basal  phase  of 
the  middle  Huronian.  The  intri- 
cacy of  the  relations  of  the  middle 
Huronian  quartzite  with  the  Rand- 
ville dolomite  on  Iron  HiU  is  rep- 
resented in  figure  45.  The  hill  is  a  normal  anticlinorium,  of  the  type  to  be  expected  in  com- 
petent formations  of  this  type.  It  contrasts  in  every  essential  feature  with  the  abnormal 
anticlinorium  in  the  weak,  incompetent  beds  of  the  Michigamme  slate  on  Han  bury  Hill. 


MichigammeC'Hanbury"^  slata 

with  iron  formation 

(upper  Huronian) 


Quart-zite 
(^middle  Huronian) 


Randville  dolomite 
(lower  Huronian) 


Direction  and  pitch  of 
'  axial  lines  of  minor  folds 


a5| 
Strike  and  dip 


Outcrop 


Outcrop  with  strike 


^S5S3ffiS3.ff 


Figure  45.- 


Axis  of  folds 
Cross  section  A-B,  looking  east 

Geologic  map  and  cross  section  of  Iron  HiU,   Menominee  district,   showing 
relations  of  lower  and  middle  Huronian. 


UPPER    HURONIAN    (aNIMIKIE    GROUP). 

All  the  formations  between  the  Randville  dolomite  and  the  unconformity  at  the  base  of 
the  Cambrian  sandstone  are  placed  in  the  upper  Huronian.  For  the  purpose  of  the  present 
monograph  the  group  may  be  divided  into  two  formations;  the  lower,  the  Vidcan  formation, 
includes  all  the  known  iron-bearing  rocks  of  the  district  except  the  conglomerate  beds  at  the 
base  of  the  Cambrian;  the  upper,  the  Micliigamme  ("Hanbxiry")  slate,  comprises  the  great 
upper  slate  formation  of  the  upper  Hiu-onian. 

VULCAN  FORMATION. 

Subdivision  into  members. — The  u'on-bearing  Vulcan  formation  embraces  tlu-ee  members; 
these  are,  from  the  base  up,  the  Traders  iron-bearing  member,  the  Brier  slate  member,  and  the 
Curiy  iron-bearing  member.  In  this  monograph  the  three  members  are  mapped  as  a  single 
formation  because  they  are  not  so  well  exposed  that  they  can  everywhere  be  separately  out- 


u  Mon.  U.  S.  Qeol.  Survey,  vol.  40, 1904,  pp.  273-279. 


336  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

lined.  However,  at  several  places  the  three  members  are  known  to  exist,  and  ran  be  separately 
mapped.  The  Traders  iron-bearin<z;  member  is  so  named  because  of  its  typical  occurrence  at 
the  Traders  mine,  iiortli  of  Iron  Mountain.  The  Brier  slate  is  so  named  because  it  is  well 
exliibited  at  Brier  Hill.  The  Curry  member  is  so  called  because  the  Curry  mine  is  located  at 
its  horizon. 

Distribution. — From  the  position  of  the  ^  ulcan  formation  inmiediately  upon  the  lower 
and  miildle  Huronian  it  would  be  natural  to  expect  its  distribution  to  be  dete'rmined  by  the 
distribution  of  those  rocks,  and  as  a  matter  of  fact,  wlien'ver  the  Vulcan  formation  occurs  it 
lies  immediately  above  the  Randville  dolomite  or  middle  Huronian  quartzite  and  below  the 
Michigamme  slate.  But  at  some  places  within  the  district  the  dolomite  or  quartzite  is  in 
immediate  contact  ^\^th  the  Micliigamme  slate  or  is  separated  from  exposures  of  it  by  intervals 
so  narrow  as  to  show  that  tlie  ^'uk•an  beds  are  lacking. 

The  principal  area  of  the  Vulcan  formation  extends  as  a  belt  from  900  to  1,300  feet  wide 
along  the  south  side  of  the  soutliern  belt  of  dolomite  for  nearly  its  entire  extent.  The  belt 
follows  the  sinuosities  of  the  southern  border  of  the  dolomite  area  rather  closelj-,  but  it  is  much 
wider  in  the  reentrants  caused  by  the  pitching  synclines  of  the  dolomite  than  elsewhere.  The 
widening  of  the  formation  at  these  places  is  of  course  due  to  the  repetition  of  beds  in  consecjuencc 
of  close  foldmg.  Along  onl}'  one  stretch,  about  a  mile  in  length,  is  the  iron-bearing  formation 
known  to  be  absent.  This  is  in  the  W.  \  sec.  1  and  the  E.  ^  sec.  2,  T.  39  N.,  R.  30  W.,  where 
the  Michigamme  slate  lies  upon  ledges  of  the  typical  dolomite. 

On  the  north  side  of  the  southern  dolomite  belt,  in  the  central  or  western  part  of  the  dis- 
trict, the  iron-bearing  formation  has  nowhere  been  found  nor  has  any  indication  of  its  presence 
been  detected.  In  the  eastern  part  of  the  district  the  Vulcan  formation  appears  at  the  Loretto 
mine  in  an  eastward-pitching  syncUne.  From  this  place  it  extends  eastward  along  the  north 
side  of  the  dolomite,  as  shown  by  a  line  of  magnetic  attractions,  to  a  point  within  a  short  dis- 
tance of  the  east  end  of  the  area  mapped,  where  the  thick  deposits  of  Paleozoic  beds  prevent 
ftu-ther  tracing. 

The  second  important  area  of  the  Vulcan  formation  is  that  in  which  the  Traders  and 
Forest  mines  are  situated.  It  stretches  for  about  5  miles  along  the  south  side  of  the  central 
dolomite  belt,  running  north  of  Lakes  Antoine  and  Fumec  and  ending,  so  far  as  present  informa- 
tion indicates,  somewhere  about  the  east  line  of  R.  30  W.  On  the  north  side  of  this  same  dolo- 
mite belt  the  iron-bearing  formation  is  known  to  extend  for  only  a  short  distance  on  both  sides 
of  the  Cuff  mine,  in  the  southern  portion  of  sec.  22,  T.  40  N.,  R.  30  W. 

The  tliird  strip  of  country  in  which  the  iron-bearing  beds  are  to  be  expected  is  that  which 
borders  the  northern  dolomite  belt.  This  area,  however,  is  in  the  valley  of  Pine  Creek.  The 
siu-face  is  tliickly  covered  \\'ith  sand.  There  is  no  indication  of  the  character  of  the  under- 
lying rock  anywhere  west  of  the  Loretto  mine  except  that  afforded  by  a  group  of  pits  near  the 
center  of  sec.  14,  T.  40  N.,  R.  30  W.,  at  the  western  extremity  of  the  belt.  These  pits  liave 
shown  the  presence  of  lean  ore  associated  with  cherts,  jaspilites,  and  black  slates.  Tjie  cherts 
are  filled  with  the  "shots  and  bands"  of  ore  characteristic  of  the  cherts  in  the  Michigamme 
slate  and  present  to  some  extent  in  the  jaspilites  of  the  Curry  iron-bearing  member.  The 
rocks  in  this  locaUty  are  believed  to  belong  to  the  Curry  horizon. 

From  the  foregoing  account  of  the  distribution  of  the  Vulcan  formation  it  will  be  noticed 
that  the  belts  of  iron-bearing  rocks  are  not  continuous.  From  the  stratigra|)hic  relations  of 
the  non-bearmg  formation  it  would  be  expected  to  occur  as  continuous  belts  surrounding  the 
dolomite  anticlines,  bordering  the  south  side  of  the  northern  dolomite  monocline.  In  several 
places,  however,  these  relations  do  not  exist.  It  is  known  that  in  jiarts  of  the  district  the 
iron-bearing  formation  is  absent  from  the  position  it  would  naturallj-  be  expected  to  occupy, 
and  that  the  Michigamme  slate,  which  stratigraphically  overlies  the  ore-bearing  strata,  is  in 
immediate  contact  with  the  dolomite  that  underlies  tlie  Vulcan  formation.  It  is  probable  that 
tlie  larger  parts  of  the  belts  niaj)ped  as  doubtful — the  areas  in  wliich  tlie  underlying  rock  is 
unknowTi — are  underlain  by  the  Michigamme  slate  rather  than  the  Vulcan  formation,  but  it 
is  possible  tluit  the  Vulcan  formation  underlies  a  portion  of  these  areas. 


MENOMINEE  IRON  DISTRICT.  337 

Traders  iron-hearing  member. — The  Traders  iron-bearing  member  of  the  Vulcan  formation 
consists  of  a  comformable  set  of  betls  composed  of  ferruginous  conglomerates,  ferruginous  quartz- 
ites,  heavily  ferruginous  quartzose  slates,  and  iron-ore  deposits.  The  conglomerates  and 
quartzites  are  usually  at  the  base  of  the  member,  resting  upon  the  Randville  dolomite.  These 
rocks  vary  in  thickness  from  a  few  inches  to  20  feet  or  more.  They  contain  fragments,  usually 
small  but  here  and  there  large,  of  quartzite,  jaspilite,  white  cjuartz,  and  rocks  that  make  up 
the  Archean  complex.  In  many  places,  however,  the  conglomerate  contains  so  much  ore  and 
jasper  that  it  is  an  ore  and  jasper  conglomerate  or  quartzite,  of  which  some  is  so  rich  that  it  is 
mined.  In  these  rocks  the  matrix  is  a  mass  of  small  grains  of  hematite,  embedded  in  which 
are  bowlders  and  pebbles  of  ore  and  of  jaspilite.  The  conglomerates  and  quartzites  of  this 
kind  are  usually  schistose.  The  ore  and  jaspilite  fragments  are  mashed  into  lenticular  bodies, 
and  the  matrix  into  a  mass  of  thin  scales  like  those  characterizing  the  specular  ores  of  the 
Marquette  district.  Typical  occurrences  of  the  ore-bearing  quartzites  and  conglomerates  may 
be  seen  at  the  open  pits  of  the  Traders  mme  and  at  the  bottom  and  along  the  west  side  of  pit 
No.  3  of  the  Penn  Iron  Company,  in  the  SW.  \  NE.  i  sec.  9,  T.  39  N.,  R.  29  W.  In  the  vicmity 
of  the  Forest  mine  are  heavy  quartzites,  some  of  them  white  and  vitreous,  interbedded  with 
what  is  taken  to  be  the  Traders  member.  This  is  the  largest  mass  of  quartzite  developed  at 
this  horizon  in  the  district. 

The  conglomerates  and  quartzites  pass  upward  mto  the  ferruginous  quartzose  slates. 
These  consist  of  alternating  layers  of  heavily  ferrugmous  quartzites,  iron  oxides,  and  in  some 
cases  jaspilites.  The  cjuartzose  layers  are  dark  red  or  purple  jasper-like  beds,  from  a  fraction 
of  an  inch  to  18  inches  or  2  feet  m  thickness.  Some  of  them  on  fresh  fractures  exliibit  the 
quartzitic  texture  very  plainlj'.  The  coarser  of  them  approach  ferruginous  quartzites.  Others, 
however,  resemble  A^ery  closely  a  typical  jaspilite,  which  a  number  of  them  are  believed  to  be. 
These  varieties  are  m  places  mottled  by  red  and  purple  blotches  that  appear  to  be  due  to  the 
presence  of  red  jaspilite  grains  in  a  ferruginous  quartzose  matrix.  Some  of  these  mottlings 
are  secondary  concretions  and  some  are  alterations  of  greenalite  granules,  more  fully  described 
in  connection  with  the  Curry  iron-bearing  member  of  the  Vulcan  formation.  As  a  rule  the 
motthng  is  in  small  elongated  areas  and  the  rock  possesses  an  mcipient  schistosity  in  the  direc- 
tion of  the  longer  axes  of  the  areas.  This  phenomenon  is  the  result  of  mashing,  which  flattened 
the  jaspilite  grauas  and  the  smaller  components  of  the  quartzose  matrix,  producmg  a  parallel 
arrangement  of  the  particles.  It  is  difficult  to  determine  with  certainty  the  relative  amounts 
of  the  detrital  material  and  true  jaspilite,  which  is  nonfragmental,  but  apparently  the  former 
is  more  abundant  and  it  may  be  dominant. 

Brier  slate  member. — The  Brier  slate  member  lies  immediately  above  the  Traders  iron- 
bearing  member.  The  slates  are  heavj^  black,  ferruginous,  and  quartzose,  presenting  in  many 
places  a  very  even  and  fine  banding,  due  either  to  the  presence  of  layers  richer  than  the  average 
in  iron  oxides  or  merely  to  the  presence  of  small  quantities  of  pigments.  On  exposed  surfaces 
the  banding  is  emphasized  by  slight  weathering.  Wliere  the  weathering  has  progressed  very 
far  the  slates  are  stained  red.  They  open  along  the  bedding  planes  and  become  very  shaly. 
In  this  form  they  yield  an  abundant  talus  at  the  base  of  all  cliff  faces  in  which  they  are  exposed. 

Curry  iron-bearing  member. — The  Curry  iron-bearing  member  consists  of  interbedded 
jaspiUtes  and  ferruginous  quartzose  slates,  with  various  mLxtures  of  the  two,  and  ore  deposits. 
Many  of  the  jaspilite  bands  are  in  the  center  of  the  quartzose  slate  layers,  but  a  few  are  along 
one  side;  all  are  parallel  to  the  bedding  planes.  Both  the  jaspilites  and  the  ferrugmous  quartz- 
ose slates  are  dark  red  or  purple.  The  two  can  usually,  however,  be  distinguished.  The 
jaspilites  are  homogeneous  rocks,  with  a  flinty  fracture  and  luster.  They  consist  as  a  rule  of 
very  finely  crystalline  quartz  and  hematite,  with  abundant  concretionary  and  granular  struc- 
tures marked  by  varying  combinations  of  iron  oxide  and  chert.  Some  of  the  concretionarj' 
structures  are  similar  to  those  figured  by  Van  Hise  and  Irving  for  the  Gogebic  district."  Much 
more  numerous  granules  have  the  same  shape  as  the  concretions  but  differ  from  them  in  lackmg 


a  Mod.  U.  S.  Geol.  Survey,  vol.  19, 1892. 
47517°— VOL  52—11 22 


338  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

nulial  or  concentric  structures.  Such  granules  are  identical  in  their  characteristics  witli  altered 
greenalite  granules  of  the  Mesabi  district  of  Minnesota  described  by  Leith  "  and  of  tlie  Felch 
Mountain  district  of  Michigan  described  by  Smytli.'' 

It  is  concluded  tluit  the  iron-bearing  formation  is  essentially  the  result  of  alteration  of 
greenalite  rocks  like  those  in  the  Mesabi  district  and  of  iron  carbonates  like  those  in  the 
Gogebic  ihstrict.  None  of  the  original  greenahte  or.  iron  carbonate  is  now  present,  but 
pseudomorjjlis  of  both  of  tliem  are  abundant. 

The  ferruginous  quartzose  slates  consist  largely  of  plainly  fragmcntal  rjuartz.  The  coarser 
varieties  approach  quartzites.  Between  tlie  grains  of  fragmental  quartz  tliere  is  finelj'  crys- 
talline quartz  and  iron  oxide.  What  part  of  the  matrLx  is  truly  detrital  and  what  part,  like 
the  jaspilite,  is  nonfragmental  in  origin  it  is  difficult  to  say.  Between  the  bands  wliicli  are 
plainly  true  jaspilites  and  nondetrital  and  those  which  are  ]>lainly  detrital  there  are  all  grada- 
tions. It  is  difficult  to  ascertain  whether  the  fragmental  or  the  nonfragmental  material  is  the 
more  abundant  in  the  Curry  iron-bearing  member  as  a  whole,  for  it  is  poorly  exposed.  The 
ferruginous  quartzose  slates  are  beheved  to  have  been  derived  largely  from  the  erosion  of  the 
lower  Iluronian.  But  mingled  with  tliis  detrital  material  m  many  places  was  apparently-  a 
considerable  amount  of  nonfragmental  material.  There  are,  therefore,  in  the  Curry  iron- 
bearing  member  all  gradations  between  clastic  and  nonclastic  sediment. 

Deformation. — The  Vulcan  formation  occupies  a  position  on  the  upper  sides  of  the  dolomite 
anticlines.  Its  major  folds,  or  folds  of  the  first  order,  correspond  exactly  to  the  major  folds 
of  the  RandviUe  dolomite.  The  folds  of  the  second  order  correspond  also  with  those  of  the 
dolomite.  The  troughs  on  the  south  side  of  the  southern  dolomite  area  are  occupied  by  the 
members  of  the  iron-bearing  formation.  Moreover,  within  the  Vulcan  formation  are  numerous 
still  smaller  folds  of  the  thirtl  order,  which,  because  of  the  hardness  of  the  rocks  and  the  perfec- 
tion of  the  bandmg,  are  well  exliibited.  These  small  folds  may  be  observed  at  nearh'  every 
place  where  minmg  has  progressed  to  any  considerable  extent  and  at  many  other  places  wliere 
only  lean  ores  have  been  developed.  The  folds  of  the  third  order  pitch  in  the  same  direction 
as  those  of  the  second  order,  upon  which  they  are  superimposed,  but  the  strikes  of  their  axes 
may  diverge  slightly. 

Still  smaller  folds  are  superimposed  on  the  folds  of  the  third  order  m  the  same  \vay  in 
which  the  latter  are  superimposed  on  the  folds  of  the  second  order.  On  exposed  surfaces  the 
folds  of  the  higher  orders  appear  as  a  series  of  crinkhngs  or  flutings,  with  heights  ranging  from 
one-quarter  inch  to  5  or  6  inches  from  trough  to  crest.  Even  in  the  troughs  of  these  minute 
folds,  under  favorable  circumstances,  iron  ore  was  deposited,  especially  where  crusMng  and 
brecciating  took  place  in  connection  \\\i\\  the  folding. 

Wherever  folding  is  observed  within  the  iron-bearing  formation  it  is  noticeable  that  the 
bedding  is  best  preserved  in  the  siliceous  bands.  The  iron-ore  layere  between  the  siliceous 
laj^ers,  while  yielding  to  the  stresses  that  produced  the  folding,  were  mashed  and  sheared  and 
became  schistose.  Where  the  compressing  forces  were  very  powerful  a  slat}'  cleavage  developed 
in  both  the  iron-ore  and  the  siliceous  layers. 

In  the  western  part  of  the  south  belt  of  the  iron-bearing  formation  cariying  the  piincipal 
ore  bodies  the  minor  folds  show  considerable  regularity  of  pitch  to  the  west  at  angles  ap])roaching 
30°.  The  ore  bodies  follow  these  axial  lines.  Not  uncommonly  these  nainor  folds  pass  into 
overthrust  faults.  The  distribution  of  the  formation  suggests  that  more  overthnist  faidts  are 
really  present  than  have  been  found.  In  this  area  the  rocks  to  the  south  have  moved  westward 
and  upward  with  reference  to  the  rocks  to  the  north,  developing  drag  or  buckle  folds  and 
thrust  faults  in  the  relatively  incompetent  upper  Huronian  beds  near  the  contact  with  the 
relatively  competent  lower  Huronian.  The  eastern  part  of  the  south  belt  shows  some  eastward 
pitches. 

Relations  hetween  the  members  of  the  Vulcan  formation  and  the  Michigamme  slatt. — Where 
the  relations  between  the  Traders  iron-bearing  member  and  the  Brier  slate  member  are  normal 


o  Mon.  U.  S.  Geol.  Survey,  vol.  '.3, 1903.  '  Idem,  vol.  3C.  1899. 


MENOMINEE  IRON  DISTRICT.  339 

the  Traders  grades  into  the  Brier  by  diminution  of  tlie  amount  of  ferruginous  material  and  by 
increase  in  the  number  and  tiiickness  of  the  quartzose  beds.  At  the  same  time  there  is  an 
increase  in  tlie  proportion  of  shity  material.  Where  the  ferruginous  material  is  much  reduced 
in  quantity  the  Traders  iron-bearing  bed  becomes  the  Brier  slate  member.  This  gradation 
occupies  only  a  veiy  short  vertical  range,  so  that  the  line  between  the  iron-bearing  member 
and  the  slate  member  is  usually  determinable  within  a  few  feet. 

Where  marked  disturbances  have  occurred,  as  in  the  vicinity  of  Norway  and  for  several 
miles  to  the  east,  the  relations  between  the  two  members  are  vevy  different.  Wherever  it  can 
be  seen  the  contact  between  the  Traders  and  Brier  members  is  shaqj.  In  many  places  the 
contact  seems  to  be  slickensided  and  locally  to  be  a  plane  of  differential  movement.  At  the 
o])en  pits  of  the  Norway  mine  and  those  north  of  the  Curry  mine  and  between  this  mine  and 
the  West  Vulcan  the  Traders  rocks  are  in  ])laces  ])seudoconglomeratic.  The  Brier  slate  member 
also  may  be  brecciated.  Moreover,  the  brecciation  is  not  confined  to  these  two  members,  but 
the  underlying  dolomite  is  at  some  places  likewise  brecciated  for  a  short  distance  beneath  its 
upper  surface.  The  phenomena  wherever  studied  appear  to  indicate  that  at  the  time  of  folding 
fault  slipping  occurred  along  the  contact  between  the  upper  Huronian  and  the  lower  Huronian 
and  between  the  Traders  and  Brier  members.  The  dolomite  was  brecciated  to  some  extent, 
the  Traders  detrital  ores  were  crushed  and  brecciated,  and  in  several  places  the  lower  portions 
of  the  Brier  slate  member  were  likewise  included  wiihin  the  zone  of  movement  and  were  frac- 
tured and  brecciated.  Later  the  breccias  were  enriched  by  the  deposition  of  hematite  and 
other  iron  compounds,  and  both  the  Traders  member  and  the  lower  part  of  the  brecciated 
Brier  slate  member  became  sufficiently  ferruginous  to  warrant  mining. 

The  Brier  slate  member  passes  upward  into  the  Curry  iron-bearing  member  by  the  diminu- 
tion of  argillaceous  material  and  the  introduction  of  ferruginous  material,  especially  bands  of 
jaspilite,  the  somewhat  ferruginous  quartzose  Brier  slate  meml)er  thus  becoming  heavily  ferru- 
ginous. At  one  place  this  transition  is  seen  to  occur  laterally  as  well  as  vertically.  No  strati- 
graphic  break  has  been  discovered  anywhere  within  the  Vulcan  formation. 

The  relations  between  the  Vulcan  formation  and  the  overlying  Michigamme  slate  are 
those  of  conformity.  The  contact  is  usually  very  sharp.  No  difficuUy^  is  experienced  in 
defining  the  upper  limit  of  the  iron-bearing  formation.  The  slates,  however,  are  in  places  so 
very  schistose  on  the  upper  side  of  the  contact  that  their  bedding  planes  can  not  be  recognized, 
suggesting  fault  slipping.  The  bedding  of  tlie  iron-bearing  formation,  on  the  other  hand,  is 
still  almost  perfectly  preserved  and  is  parallel  to  the  contact. 

The  relations  of  the  Vulcan  formation  with  the  lower  and  middle  Huronian  are  discussed 
on  pages  342-343. 

ThicTcness. — A  number  of  sections  offer  opportunities  for  determining  the  thickness  of  the 
separate  members  of  the  Vulcan  formation,  but  in  only  a  few  can  its  total  thickness  be  deter- 
mined. All  along  the  south  side  of  the  southern  dolomite  belt,  from  the  Aragon  mine  eastward 
to  Sturgeon  River,  the  iron-bearing  formation  stretches  as  a  narrow  belt,  which  for  much  of 
the  distance  appears  to  be  without  important  folds.  At  several  places  mining  operations  have 
afforded  excellent  sections  from  the  base  of  the  productive  portion  of  the  Traders  iron-bearing 
member  to  the  top  of  the  Curry  iron-bearing  member,  and  at  a  few  places  the  sections  extend 
downward  to  the  top  of  the  Randville  dolomite.  At  Brier  Hill,  where  practically  the  whole 
formation  can  be  seen  on  the  surface,  its  thickness  is  about  600  feet.  At  the  Curry  shaft  No.  2 
it  is  700  feet  thick  and  at  the  Aragon  mine  about  675  feet. 

At  a  number  of  sections  the  thickness  of  the  individual  members  comprising  the  formation 
is  easily  measured.  The  Brier  slate  member  has  been  measured  at  seven  places,  yielding 
results  between  100  and  360  feet.  Five  of  these  measurements  fall  between  320  and  360  feet. 
Eight  measurements  of  the  Curry  member  have  given  results  vaiying  between  100  and  225 
feet.  Six  of  these  fall  between  160  and  225  feet.  Measurements  of  the  Traders  iron-bearing 
member  have  yielded  no  such  concordant  results.  In  the  first  place,  its  thiclvness  probably 
varies  widely,  as  should  be  expected  of  a  formation  composed  largely  of  detrital  deposits. 


340  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Moreover,  only  a  few  sections  reach  as  low  as  the  dolomite;  hence  the  exact  position  of  the 
contact  between  tliis  rock  mikI  the  iron-bearing  formation  must  be  guessed  at.  Only  three 
ineasurement.s  have  been  made  from  the  known  top  of  the  dolomite  to  the  known  top  of  the 
Traders  member.     These  give  170  feet,  85  feet,  and  155  feet. 

An  interesting  feature  of  these  figures  apjiears  when  the  estimated  thickness  of  the  Brier 
and  Currj'  members  is  compared  with  the  total  thickness  of  the  two.  In  almost  every  section 
where  the  estimated  tliickness  of  either  of  these  members  falls  below  tlie  average  of  all  the 
measurements  for  that  member  the  tliickness  of  the  other  member  exceeds  the  average,  and 
the  total  of  the  two  is  fairly  constant.  Thus,  whereas  seven  estimates  of  the  tluckcess  of 
the  Brier  slate  member  vary  between  240  and  360  feet,  and  eight  estimates  for  the  Curry  iron- 
bearing  member  vary  between  112  and  225  feet,  measurements  of  the  total  thickness  of  the 
two  vary  only  between  400  and  5.30  feet.  The  apparent  greater  variation  in  tliickness  of  the 
two  members  separately  than  in  that  of  the  two  combined  may  be  partly  explained  as  due 
to  the  gradation  between  the  two  and  the  consequent  difficulty  of  fixing  the  exact  place  at 
which  one  ends  and  the  other  begins. 

From  a  careful  consideration  of  the  figures  given  above  and  a  few  others  that  are  not 
here  recorded,  it  is  estimated  that  the  average  thickness  of  the  Vulcan  formation  is  approxi- 
mately 650  feet,  divided  as  follows:  Traders  u-on-bearing  member,  150  feet;  Brier  slate  mem- 
ber, 330  feet;  Curry  iron-bearing  member,  170  feet — that  is,  the  two  ore-bearing  members 
combined  about  equal  in  thickness  the  intervening  slates.  It  is  conceded,  however,  that  the 
Traders  member  departs  considerably  from  this  average  and  that  the  total  thickness  of  the 
ormation  varies  accordingly. 

MICHIGAMME    ("HANBURT")  SLATE. 

Di-ttrihution. — The  Micliigamme  slate  occurs  mainly  in  three  large  belts  constituting  valleys 
which  correspond  wdth  synchnes  between  the  older  rocks.  It  occupies  nearl}-  all  the  low  ground 
in  the  Menominee  trough,  forming  a  plain  broken  only  by  heaps  of  glacial  material  deposited 
upon  it,  by  the  protrusions  of  a  few  liillocks  composed  of  the  harder  slates,  or  by  equally 
resistant  greenstones.  The  slate  areas  are  narrowest  at  the  east  and  gradually'  widen  toward 
the  west.  The  northern  belt  is  divided  into  two  portions  by  the  western  area  of  Quinnesec 
scliist.  The  northern  part  turns  northwest  and  leaves  the  Menominee  district  at  the  northern 
limit  of  the  mapped  area;  the  southern  portion  coalesces  with  the  middle  belt  and  crosses 
Menominee  River  into  Wisconsin.  East  of  Iron  Hill  the  two  northern  belts  again  coalesce 
and  extend  as  a  single  belt  to  Sturgeon  River.  Near  the  longitude  of  Waucedah  all  the  slates 
disappear  to  the  east  beneath  the  Paleozoic  beds. 

Name. — In  previous  reports  on  this  district  this  slate  has  been  called  the  Hanbury  slate, 
but  the  formation  has  been  proved  to  be  equivalent  to  and  continuous  witli  tlie  Micliigamme 
slate  of  the  Marquette  district,  and  the  older  name,  Micliigamme,  is  therefore  used  m  this  report. 

Lithology. — The  formation  is  dominantly  a  pelite.  It  comprises  black  and  gray  clay  slates, 
gray  calcareous  slates,  graphite  slates,  graywackes,  tldn  beds  of  cjuartzite,  local  beds  of  ferru- 
ginous dolomite  and  siderite,  and  rarer  bodies  of  ferruginous  chert  and  iron  oxide.  Tilie 
formation  is  cut  by  dikes  of  schistose  greenstones,  and  in  one  or  two  places  sheets  of  the  same 
rock  have  been  intruded  between  the  sedimentary  beds.  The  predominant  rocks  of  the  forma- 
tion are  gray  clay  slates  and  calcareous  slates.  The  latter  are  more  abumlant  m  the  lower 
portions  of  the  formation  and  the  former  in  the  upper  portions.  The  exact  vertical  relations 
of  the  two  rocks  have  not  been  made  out,  because  of  the  scarcity  of  exposures  and  the  very 
intricate  folding  to  which  they  have  been  subjected.  The  clay  slates  are  normal  argillaceous 
slates,  in  wliich  there  is  always  more  or  less  ferruginous  matter.  Those  exposed  to  the  weather 
are  light  in  color  and  have  a  slialy  character,  iluscovite  then  becomes  prominent.  Their 
iron  components  are  decomposed  to  red  ocherous  compounds.  \Yliere  most  altered  the  rocks 
are  light-red  sericite  slates  or  shales.  The  weathering  of  the  slates  that  contain  small  quantities 
of  calcareous  components  is  somewhat  different.  They  tend  to  bleach  to  a  very  ])ale-green 
or  white  color  and  to  become  porous  tlirough  the  loss  of  their  calcareous  cement.     The  ferru- 


MENOMINEE  IRON  DISTRICT.  341 

ginous  components  oxidize,  forming  red  ocher,  and  this  lies  in  an  irregular  pattern  on  the  light- 
colored  background.  The  result  of  tliese  changes  is  a  red  and  wliitc  or  pale-green  mottled 
friable  slate,  known  locally  as  "calico  slate." 

By  the  Sedition  of  carbonates  the  argillaceous  slates  pass  into  the  carbonate  slates. 
These  in  places  contain  as  much  as  .50  per  cent  of  carbonate  as  a  cement.  With  an  increase 
in  the  carbona;te  the  slates  lose  their  slaty  character,  become  more  massive,  and  finally  pass 
into  beds  of  f#rodolomite  and  siderite  measuring  from  a  few  inches  to  20  feet  in  thickness. 
On  many  of  the  weathered  surfaces  both  the  dolomite  and  the  calcareous  slates  are  coated 
with  a  skin  of  brown  ocherous  limonite,  which  on  some  of  the  massive  dolomites  reaches  a 
tliickness  of  an  inch  or  more.  Much  of  the  limonite  is  pseudomorphous  after  the  carbonate 
siderite. 

The  ferruginous  cherts  and  u"on  oxides  are  not  known  to  be  present  in  the  Michigamme 
slate  in  large  quantity.  Indeed,  they  are  as  a  rule  only  locally  developed  in  association  with 
the  sideritic  dolomites  and  calcareous  slates  where  these  have  been  severely  crushed  or  folded. 
The  source  of  the  iron  oxides  is  clearly  iron-bearmg  carbonate  in  the  calcareous  slates  and 
the  dolomites.  The  cherts  are  wliite  or  yellow  massive  rocks  with  finely  granular  texture. 
They  occur  as  tliin  seams  and  veuis  traversing  the  slates  and  dolomites,  and  as  tliin  beds  inter- 
laminated  with  the  tlucker  beds  of  the  last-named  rocks. 

Wlierever  the  cherts  occur  there  is  usually  found  also  a  greater  or  less  quantity  of  some 
iron  oxide.  Tliis  occurs  as  small  veins  of  pure  hematite  cutting  through  the  cherts,  as  coatings 
of  hematite  on  the  walls  of  cracks  traversmg  the  slates,  as  small  vugs  inclosed  in  shattered 
cherts,  as  druses  covering  the  walls  of  the  cavities  in  an  extremely  porous  chert,  m  distinct  bands 
interlaminated  with  bands  of  graywacke  or  cpiartzite,  and  in  the  form  of  a  mixture  of  oxides 
anil  hydroxides  impregnating  slaty  material.  In  short,  the  iron  oxitles  occur  in  all  forms 
characteristic  of  deposits  precipitated  from  percolating  waters.  The  slates  impregnated  with 
ferruguious  matter  are  naturally  dark  red.  Those  that  are  but  slightly  ferruginous  still  plamly 
exliibit  their  true  character.  In  those  containing  a  large  proportion  of  the  iron  oxides,  how- 
ever, but  few  traces  of  the  original  slate  remain  and  the  rock  resembles  a  slaty  ocher. 

The  grapliite  slates  are  black,  very  fissile,  tliinly  laminated  rocks.  They  appear  to  be 
limited  to  the  lower  portions  of  the  Micliigamme  slate.  At  any  rate,  they  have  been  seen 
only  in  association  with  the  underlying  Curry  iron-bearing  member  and  at  horizons  a  few 
hundred  feet  above  the  base  of  the  slate  formation,  but  they  do  not  everywhere  occur  at  the 
base  of  the  formation.  Their  association  with  iron-beaiing  beds  at  many  places  in  tliis  and 
other  districts  probably  has  some  significance  as  to  the  origin  of  the  ore.  (See  p.  .502.) 
The  graphite  slates  appear  to  grade  laterally  mto  the  normal  gray  slates,  of  wliich  they  seem 
to  be  local  modifications.  The  graywackes  and  quartzites  of  the  Micliigamme  slate  are  normal 
rocks  of  their  kind,  requiring  no  special  description.  They  both  occur  in  comparatively  tliin 
beds,  more  commonly  in  the  lower  part  of  the  formation  than  in  the  upper  part.  The  quartzites 
are  more  abundant  than  the  graywackes,  but  neither  are  common. 

Deformation. — The  major  folding  of  the  Michigamme  slate  seems  to  correspond  with  that 
of  the  underlying  formations,  and  the  slate  therefore  lies  in  three  major  synchnes.  This 
structure  is  inferred  from  areal  relations  to  older  rocks  rather  than  from  structures  seen  in 
the  slates  themselves,  which  are  poorly  exposed,  lack  easily  identifiable  horizons,  and  have 
their  bedding  much  obscured  by  cleavage. 

Many  of  the  folds  are  of  the  abnormal  type  characteristic  of  incompetent  strata.  The 
hmbs  are  thinned  and  the  crests  thickened,  as  would  be  expected  in  folds  of  this  type,  con- 
trasting in  every  essential  detail  with  folds  in  the  competent  quartzites  and  dolomites,  as,  for 
instance,  in  Iron  Hill.     (See  fig.  45,  p.  33.5.) 

The  strong  north-south  compression  of  the  slate  beds,  producing  the  close  east-west  folds, 
also  produced  in  all  the  weaker  members  of  the  formation  a  perfect  slaty  cleavage  with  a 
nearly  east-west  strike  and  a  dip  that  varies  but  a  few  degrees  on  either  side  of  the  vertical. 
There  is  also  a  set  of  fracture  planes  or  joints  at  right  angles  to  the  cleavage.  These  joints 
intersect  the  rocks  at  approximately  equal  intervals  of  several  inches.     In  some  places  they 


342  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGIOX. 

are  bordered  by  narrow  shear  zones  in  which  the  total  displacement  of  tlio  slate  beds  is  an 
inch  or  more.  On  some  flat  horizontal  surfaces  two  sets  of  these  joints  are  seen  cutting  each 
other  at  acute  angles,  and  about  each  slight  faulting  has  occurred.  Extensive  thrust  faults 
are  suggested  by  the  close  folding  of  cleavage,  but  these  have  not  been  identified. 

Tliickness. — No  even  approximately  correct  estimate  of  the  thickness  of  the  Michigamme 
slate  can  at  present  be  made.  The  similarity  of  the  beds  and  their  redujjhcation  in  consecpience 
of  the  close  folding  render  it  impossible  to  determine  what  proportion  of  the  apparent  thickness 
of  the  formation  is  due  to  folding  and  what  proportion  is  due  to  successive  deposits.  There 
is  no  (l(iul)t  that  the  Michigamme  slate  is  little  thicker  than  any  of  the  other  formations  in 
the  district. 

RELATIONS  OF  TIPPER  HrRONIAN  TO  TINDERLYING  ROCKS. 

Relations  between  Vulcan  formation  and  the  lovxr  Ibtronian. — The  ir<)n-l)eariIlg^'ul(■an  forma- 
tion, except  in  very  small  areas,  rests  upon  the  Randville  dolomite  or  middle  Huronian  (juartzite. 
If  the  Vulcan  formation  exists  in  the  do-.btful  areas  adjacent  to  the  Quinnesec  schist,  it  there 
rests  against  that  schist.  Where  the  Vuican  formation  rests  upon  the  middle  Huronian  quartzite 
or  Randville  dolomite  the  lower  layers  of  the  younger  formation  appear  to  he  conformably 
upon  the  older  one,  with  an  extremely  sharp  hne  of  definition  between  them.  In  j)laces  the 
contact  rock  is  a  talc  schist  derived  from  the  dolomite  or  cherty  cpiartzite.  The  basal  member 
of  the  Vulcan  formation  is  either  a  quartzite  which  in  i)laces  contains  ore  and  jaspilite  fragments, 
or  an  ore  and  jasper  conglomerate  containing  large  and  small  pebbles  of  ore,  or  a  breccia  con- 
taining fragments  of  all  the  adjacent  rocks.  The  relative  abundance  of  autoclastic  rocks 
and  true  water-deposited  conglomerates  is  uncertain.  The  Traders  iron-bearing  member 
appears  to  be  nearly  conformable  in  attitude  with  the  underlying  dolomite,  but  detailed  work 
discloses  distinct  though  slight  discordance. 

Contacts  between  the  Randville  dolomite  or  middle  Huronian  quartzite  and  the  overlying 
formation  are  found  in  many  of  the  mines,  but  they  are  nowhere  discoverable  on  the  surface. 
In  the  little  ravine  just  east  of  the  old  Brier  Hill  mine  the  dolomite  and  the  lower  members 
of  the  iron-bearing  formation  are  very  close  together,  but  their  actual  contact  is  covered.  The 
space  between  the  ledges  of  the  two  forrnations  is  filled  with  loose  fragments,  and  among  these 
fragments  are  large  pieces  of  quartzite  holding  pebbles  of  jaspilite,  quartzite,  granite,  and 
other  members  of  the  Archean. 

In  many  of  the  mines  and  the  open  pits  a  similar  conglomerate  or  a  coarse  quartzite  is 
found  lying  upon  the  dolomite  or  quartzite. 

The  dolomite  near  the  contact  is  usually  schistose,  so  much  so  that  in  most  places  it  is  a 
pure  talc  schist.  The  calcium  of  the  dolomite  has  been  removed  and  much  of  it  has  been 
deposited  in  the  ore  bodies  as  calcite,  while  the  magnesium  has  remained  in  the  talc.  A 
surprisingl}^  similar  schist  has  been  formed  from  the  middle  Huronian  quartzite,  though  on 
the  whole  it  is  more  siliceous  and  less  talcose.  This  talc  schist  serves  as  an  impervious  lining 
to  many  of  the  folds  in  which  the  ore  deposits  lie,  and  afforded  better  conditions  for  the  concen- 
tration of  the  ore  material  than  were  afforded  by  the  massive  and  shattered  dolomite  under- 
lying the  ore  formation  at  many  places.  The  schist  was  probably  formed  in  connection  with 
movement  along  the  contact  plane  after  the  upper  Huronian  deposits  were  laid  down,  contem- 
poraneously with  the  folding  and  mctamorphism  that  affected  both  the  lower  Huronian  and 
upper  Huronian.  The  contact  between  the  schist  and  the  superjacent  quartzite  is  extremel}' 
sharp,  and  in  many  places  the  plane  of  contact  is  slickensided. 

In  those  places  where  the  basal  member  of  the  iron-bearing  formation  is  not  a  coarse 
quartzite,  it  is  usually  a  bedded  red  slate,  or  more  nearly  a  schist  composed  of  small  grains 
of  quartz,  considerable  dolomite,  and  locally  talc.  Alternate  bands  are  composed  of  layers  in 
which  dolomite  and  talc  are  predominant  and  those  in  wliich  siliceous  material  predominates. 
Tlie  contacts  between  the  schist  and  the  rocks  on  both  sides  of  it  are  usually  covered.  There 
is  in  some  localities  an  apparent  gradation  between  these  underh'ing  rocks  and  the  rocks  Ij'ing 
above  them,  but  in  others  the  line  of  division  between  them  is  well  defined. 


MENOMINEE  IRON  DISTRICT.  343 

In  earlier  reports  certain  dense  jaspilites  were  diseriniinated  from  the  fragmental  and 
micaceous  jaspilites  of  the  Vulcan  formation  above  them  and  were  regarded  as  belonging  to 
the  middle  Huronian,  unconformably  below  the  Vulcan  formation.  The  principal  evidence 
of  the  existence  of  such  a  formation  is  the  presence  of  fragments  of  jaspilite  in  the  conglomerate 
at  the  base  of  the  Vulcan  formation. 

In  general,  then,  there  is  a  slight  structural  discordance  between  the  beds  of  the  Vulcan 
formation  and  the  middle  and  lower  Huronian,  and  schistosity  and  autoclastic  rocks  seem  to 
inilicate  that  this  has  been  a  plane  of  considerable  faulting.  Also  the  fragmental  phases  of  the 
Vulcan  formation  point  to  a  preceding  erosion  interval,  though  evidence  of  great  differential 
erosion  is  lacking,  and  so  far  as  these  fragmental  j)hases  are  autoclastic  this  evidence  is  weakened. 
The  general  significance  of  the  unconformity  will  be  discussed  in  the  chapter  on  general 
correlation  (pp.  597  et  seq.). 

Relations  between  AficMgamme  {"Ilanbury")  slate  and  the  middle  or  lower  Ilurnnian. — The 
Michigamme  slate  rests  upon  the  Vulcan  formation  conformably.  Where  the  Vulcan  formation 
is  absent  the  slat.e  rests  directly  upon  the  Randville  dolomite  or  the  middle  Huronian  quartzite. 
This  relation  is  seen  for  a  short  distance  in  the  central  part  of  the  southern  belt  of  the  slate, 
and  it  is  the  relation  which  prevails  generally  in  the  two  northern  belts,  for  in  this  part  of  the 
district  the  Vulcan  formation  occurs  only  locally.     Contacts  are  not  exposed. 

At  Iron  Hill,  in  sec.  32,  T.  40  N.,  R.  29  W.,  there  is  at  the  top  of  the  middle  Huronian 
quartzite  a  conglomerate  the  debris  of  which  is  derived  largely  from  that  formation  and  which 
may  be  a  basal  conglomerate  of  the  Michigamme  slate.  At  other  locahties  also  there  is  a 
breccia  which  appears  to  be  a  brecciated  conglomerate. 

The  absence  of  the  Vulcan  formation  east  of  Quinnesec  could  be  explamed  by  the  hypothesis 
that  the  Michigamme  slate  had  been  thrust  over  the  lower  formation  of  the  upper  Huronian 
so  as  to  rest  upon  the  Randville  dolomite.  The  absence  of  the  Vulcan  formation  between  the 
Michigamme  slate  and  the  middle  Huronian  quartzite  at  Iron  Hill  might  be  similarly  explained 
only  here  it  would  be  necessary  to  believe  that  folding  accompanied  the  faulting,  else  the  manner 
in  which  the  slate  wraps  around  the  east  end  of  the  central  belt  of  dolomite  would  l)e  inexplicable. 
There  are  undoubted  mmor  faults  in  the  Menominee  district,  but  most  of  them  are  extremelv 
small,  that  in  the  Pewabic  mine  being  the  only  one  of  sufficient  magnitude  to  be  mapped  on  the 
mine  plats.  It  is  clear  that  certain  crushed  zones  of  the  Traders  and  Brier  members  near 
Vulcan  are  due  to  faulting.  Further,  there  have  been  marked  movements  of  accommodation 
between  the  different  formations  at  their  contacts,  which  might  be  called  faidting.  AJl  these 
faults  are  local,  and  in  none  of  them  is  the  displacement  of  the  faidted  beds  known  to  be  great. 

On  the  other  hand  the  existence  of  overthrust  folds  grading  into  faults,  so  clearly  indicated 
in  the  distribution  of  the  southern  belt  of  the  iron-bearing  formation,  is  the  best  of  evidence 
of  the  extensive  relative  displacement  of  the  upper  and  knver  Huronian  beds,  a  displacement 
brought  about  largely  by  the  close  deformation  of  the  lower  beds  of  the  upj)er  Huronian  as  they 
are  crowded  against  the  competent  beds  of  the  lower  Huronian.  It  is  entirely  likel}'  that  more 
faults  are  present  than  ha^-e  been  found,  and  there  is  little  difficulty  in  believing  that  overthrust 
faulting  may  have  been  a  large  factor  in  this  deformation  and  may  have  thrust  the  slate  locally 
over  the  iron-bearing  formation  against  the  dolomite,  or,  on  the  farther  side  of  the  fold,  may 
have  carried  the  dolomite  up  and  over  the  slate. 

Although  faultmg  is  doubtless  a  factor  m  determming  the  distribution  of  the  Vulcan 
formation,  from  present  evidence  faulting  is  inadequate  to  explain  the  uniform  absence  of  the 
formation  through  such  long  belts  of  country  where  it  might  be  supposed  to  exist. 

The  presence  of  doubtfid  conglomerates  at  the  base  of  the  Michigamme  slate  where  it  rests 
upon  the  middle  or  lower  Huronian  suggests  unconformable  overlap  of  the  Michigamme.  It  is 
possible  also  that  the  iron-bearing  formation  was  originally  deposited  in  discontinuous  lenses, 
with  intervening  slate,  resting  directly  upon  the  lower  or  middle  Huronian. 


344  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

IGNEOUS  BOCKS  IN  THE  ALGONKIAN. 

QtrlNNESEC  SCHIST. 

The  Quinnesec  schist  lies  along  and  adjacent  to  Menominee  River,  from  the  sharp  north- 
ward bend  in  the  river  due  west  of  Iron  ih)untiiin  to  the  eastern  limit  of  the  area  mapped. 
The  river  is  bordered  by  scliistose  greenstones  and  various  rocks  that  cut  them,  except  at  a  few 
places  where  rock  ledges  are  absent.  The  Quinnesec  Falls  and  Sturgeon  Falls  are  on  some  of  the 
harder  ledges  of  these  rocks.  South  of  the  river,  in  Wisconsin,  at  a  distance  ranging  from  half 
a  mile  to  -  miles,  is  the  north  side  of  a  large  area  of  granite.  This  granite  sends  apoi)hyses 
into  the  greenstone  schists,  and  consequently  is  of  later  age.  For  the  most  part  the  schists  are 
arranged  in  belts  striking  a  little  north  of  west  at  Sturgeon  Falls,  but  trending  more  toward 
the  north  as  they  pass  up  the  river,  until  at  the  Upper  Quimiesec  Falls  they  strike  about  north- 
west.    Their  schistosity  is,  as  a  rule,  nearly  vertical. 

The  Quinnesec  schist  is  composed  of  schists  of  two  classes,  basic  and  acidic.  The  ba.sic 
scliists  comprise  greenstone  schists,  chlorite  schists,  and  amphibolites.  Elhpsoidal  and  other 
extrusive  structures  are  common.  The  acidic  schists  comprise  gneissoid  granites,  porphyritic 
cneisses,  felsite  schists,  and  sericite  schists.  Associated  with  the  schists  are  both  basic  and 
acidic  massive  rocks.  The  basic  rocks  include  gabbro,  diorites,  diabases,  and  basalts.  The 
acidic  rocks  include  granite  and  granite  porphyry.  The  greenstones  and  the  basic  scliists  are 
closely  allied,  as  are  also  the  granites  and  granite  porphyries  and  the  acidic  schists. 

A  microscopic  study  "  of  the  basic  scliists  shows  that  they  comprise  schistose  gabbros, 
diorites,  diabases,  basalts,  and  basalt  tuffs.  Where  the  schistosity  is  not  strongly  developed 
the  original  structures  of  the  massive  eruptive  rocks  may  be  recognized,  so  that  there  is  no 
doubt  that  the  greenstone  schists,  chlorite  schists,  and  amphibohtes  are  merely  altered  phases 
of  the  greenstones.  The  amphibolites  are  limited  in  their  distribution  to  the  neighborhood  of 
the  sreat  granite  mass  of  Wisconsin,  and  nearlv  all  of  them  occur  directlv  in  contact  with  this 
granite.  It  is  clear  that  the  schistosity  in  these  rocks  has  developed  in  connection  with  the 
folding  of  the  district  and  that  the  extreme  phase  of  metamorphism  represented  by  the  amphib- 
olites has  taken  place  in  connection  with  the  intrusion  of  the  great  batholithic  granite  of 
Wisconsin. 

The  acidic  schists  are  limited  principally  to  the  neighborhood  of  Horserace  Rapids  and  Big 
Quinnesec.  The  sericite  schists  in  many  places  grade  into  the  felsite  schists.  They  occur  mainly 
in  bands  parallel  to  the  trend  of  the  bands  of  basic  schists.  The  coarser-grained  gneissoid 
granites  and  porphyritic  rocks  clearly  represent  metamorphosed  phases  of  the  great  granite 
mass  to  the  south  in  Wisconsin,  but  some  of  the  felsite  schists  and  the  sericite  schists  may 
represent  acidic  lavas  contemporaneous  with  the  basic  igneous  rocks. 

From  the  field  relations  and  microscopic  study  of  the  Quinnesec  schist  and  associated 
rocks  it  must  be  concluded  that  all  are  of  igneous  origin.  Many  of  tlieni  were  lava  flows;  some 
were  beds  of  volcanic  ashes,  or  tuffs;  others  were  dikes  cutting  through  the  bedded  deposits. 

A  few  small  dikes  cutting  the  scliists  are  normal  diabases  and  basalts,  identical  in  com- 
position with  some  of  the  rocks  cutting  through  the  iron-bearing  beds. 

Within  the  Menominee  district  itself  there  are  no  contacts  between  the  Quinnesec  schist 
and  the  Huronian  sediments.  A  sand  plane  covers  the  area  of  contact.  Exposures  and  explora- 
tions indicate  that  upper  Huronian  slates  are  the  rocks  nearest  to  the  Quinnesec  scliist,  and 
these  have  not  been  found  nearer  than  200  3'ards. 

In  earlier  reports  on  the  Menominee  district  *  the  Quimiesec  schist  was  provisionally  cor- 
related witli  the  Koowatin  scries  of  the  Archean  because  of  its  relatively  high  degree  of  meta- 
niori)hisni  and  similarity  to  certain  schists  in  the  kiio\\Ti  ^Vrchean  on  the  north  siile  of  the  district. 
The  apparent  absence  of  the  Vulcan  formation  at  the  base  of  the  upper  Huronian  was  exjilained 
by  overlap,  and  later  it  was  suggested  that  faulting  might  play  a  part.     During  the  simimer 

o  Williams,  G.  U..  The  treenstone  scliist  areas  of  the  Menominee  and  Marquette  regions  of  Michigan ;  Bull.  V.  S.  Geol.  Survey  No.  02. 1S90. 
i>  Mon.  r.  S.  Geol.  Survey,  vol.  -Id,  1904;  Menominee  special  folio  (No.  02),  Geol.  .Vtlas  U.  S.,  U.  S.  Geol.  Survey,  1900. 


MENOMINEE  IKON  DISTRICT.  345 

of  1910  the  Wisconsin  Geological  and  Natural  History  Survey,  under  direction  of  W.  O.  Hotch- 
kiss,  mapped  what  is  prol^al^ly  the  continuation  of  the  Quinnesec  schist  to  the  northwest  along 
the  south  side  of  the  Florence  district  of  Wisconsin,  and  determined  the  green  schists  there 
clearly  to  overlie  the  upper  Huronian  sediments  to  the  north  of  them  and  to  be  locally  inter- 
l)cdded  with  upper  Huronian  sediments.  However,  it  is  yet  possible  that  the  Quinnesec  schist 
in  the  Menommee  district  may  be  really  pre-Huronian,  for  continuous  exposures  do  not  connect 
the  two  areas,  and  green  schists  of  this  type  are  known  in  at  least  tlu'ee  different  horizons  in  the 
pre-Cambrian  of  Michigan. 

GREEN  SCHISTS  AT  FOTJRFOOT  FALLS. 

Another  area  of  igneous  rocks  of  Algonkian  age  occupies  about  .5  square  miles,  extending 
from  about  the  center  of  sec.  15,  T.  40  N.,  R.  30  W.,  to  Menominee  River.  The  Fourfoot 
Falls  are  on  the  south  side  of  the  area,  and  the  old  village  of  Badwater  at  its  northern  edge. 
The  rocks  of  this  area  are  mainly  schists,  but  they  are  cut  by  altered  diabase  dikes. 

The  schists  are  gi-ayish-grecn  fine-grained  greenstones,  in  which  schistosity  is  nearly  every- 
where noticeable.  In  some  places  the  rocks  are  well-defined  schists,  with  a  cleavage  almost 
as  perfect  as  that  in  slates;  in  other  places  they  are  nearly  massive.  On  many  of  the  exposures 
a  typical  ellipsoidal  structure  is  discernible.  The  ellipsoids  vary  in  diameter  from  a  few  inches 
to  3  or  4  feet.  There  is  no  striking  contrast  between  the  material  of  the  ellipsoids  and  that 
of  the  matrix  between  them.  In  both  the  rock  is  a  dense  grayish  greenstone  without  any 
distinct  textural  features.  The  matrix  is  usually  slightly  more  schistose  than  the  ellipsoids, 
but  otherwise  it  is  like  them.  At  the  Fourfoot  Falls  the  exposures  consist  of  alternating  beds 
of  massive,  schistose,  and  slaty  rocks,  striking  about  N.  80°  W.,  almost  at  right  angles  to  the 
course  of  the  river,  and  yet  these  rocks  are  mostly  schistose  on  the  Wisconsin  side  of  the  river 
and  mostly  massive  on  the  Michigan  side. 

The  microscopic  examination  of  tliin  sections  shows  that  some  of  the  rocks  in  the  western 
area  are  altered  dolerites  still  preserving  their  characteristic  textures.  Others  are  so  much 
changed  that  their  original  nature  can  only  be  inferred  fi-om  the  character  of  their  alteration 
products.  Some  of  these  appear  to  have  been  fine-grained  dolerites  and  others  perhaps  glassy 
basalts.  A  few  others  were  originally  basic  tuffs.  All  are  now  aggregates  of  actinolite,  uralite, 
zoisite,  epidote,  quartz,  and  other  well-known  decomposition  products  of  basic  igneous  rocks. 

TMs  area  of  schists,  at  the  time  the  Menominee  monograph  was  written,  was  supposed  to 
be  equivalent  in  age  with  the  Quinnesec  schist  of  Menominee  River,  then  regarded  as 
Archean.  Later  work  by  G.  W.  Corey  and  C.  F.  Bowen  "  has  shown  that  they  are  really  intru- 
sive and  extrusive  in  the  upper  Huronian  or  in  part  contemporaneous  flows.  That  these  igneous 
rocks  antedate  the  chief  folding  of  the  district  is  shown  by  the  fact  that  they  are  so  extensively 
transformed  to  schists. 

The  only  other  large  masses  of  igneous  rocks  which  have  been  found  m  the  Huronian 
series  are  in  the  Micliigamme  slate  and  the  Sturgeon  quartzite.  In  each  of  these  formations 
in  a  number  of  places  are  found  greenstones,  locally  in  the  form  of  dikes,  in  other  places  as 
siUs,  and  in  others  as  interbedded  eruptives.  In  the  Michigamme  slate  the  form  of  the  igneous 
bodies  is  known  in  but  few  places.  In  then-  present  condition  they  are  much-altered  diabases 
or  basalts  comj^osed  of  uralitized  augite  or  hornblende,  decomposed  plagioclase,  and  a  consider- 
able quantity  of  quartz  that  is  probably  entirely  secondary. 

PALEOZOIC  ROCKS. 

Small  areas  of  Paleozoic  sediments  in  horizontal  sheets  lie  on  the  eroded  edges  of  the  Huro- 
nian and  Archean  rocks.  The  Paleozoic  rocks  are  represented  by  two  formations,  one  of 
Cambrian  age  and  the  other  of  Cambro-Ordovician  age.  The  lower  formation  consists  mainly 
of  red  sandstone,  and  is  known  as  the  Lake  Superior  sandstone.  The  upper  formation  is  a 
porous  arenaceous  limestone,  identified  by  Rominger  as  corresponding  to  the  Chazy  and  "Cal- 
ciferous"  of  the  Eastern  States,  and  designated  the  Hermansville  limestone.     The  sandstones 

a  Unpublished  field  notes,  1905. 


346  GEOLOGY  OF  THE  LAKE  SLTPERIOR  REGION. 

and  limestones  were  at  on(>  time  spread  continuously  over  the  entire  Menominee  district.  To 
the  east  of  the  district  tiicy  still  cover  all  the  older  rocks.  West  of  Waucedah,  however,  they 
have  been  generally  eroded  from  the  valleys,  leaving  remnants  as  isolated  patches  on  the  tops 
of  the  higher  hills. 

CAMBRIAN  SYSTEM. 

LAKE  SUPERIOR  SANDSTOITE. 

Lithohr/i/. — The  Lake  Sii])erior  sandstone  consists  of  a  lower  portion  partly  cemented 
by  an  iron  oxide  and  conscciucntly  red  in  color  and  an  upper  portion  in  wliich  the  cement  is 
partly  calcareous  and  the  color  white.  The  total  thickness  is  estimated  by  Rominger"  at  300 
feet.  Several  ])ieces  of  the  sandstone  have  been  obtained,  wliich  according  to  reliable  authority 
came  from  the  ledge  tlu-ougli  wliich  one  of  the  Pewabic  mine  shafts,  near  Iron  Mountain,  was 
driven.  These  contain  numerous  fragments  of  fossils,  some  of  which  were  determined  by  Wal- 
cott  as  "  the  heads  of  small  trilobites,  probably  Dicelhcephalus  misa;  also  fragments  of  a  large 
species  of  DiceUocephalus."  According  to  Walcott,  "These  indicate  tlie  Upper  Cambrian 
horizon  of  the  Mississijipi  Valley  section." 

Relations  to  adjacent  formations. — The  relations  of  the  sandstone  to  the  underlying  forma- 
tions are  everywhere  practically  tiie  same.  Whetlier  on  the  tops  of  hills  or  in  tlie  depressions 
between  the  hills,  the  horizontal  beds  of  the  younger  rock  rest  unconformably  upon  tlie  up- 
turned and  truncated  layers  of  the  older  series.  Moreover,  the  basal  layers  of  the  sandstone 
contain  a  great  deal  of  material  derived  from  the  immediately  subjacent  formations.  Where  the 
underlying  rocks  belong  to  the  Vulcan  formation  the  basal  member  of  the  sandstone  is  an  ore 
and  jasper  conglomerate,  composed  of  huge  rounded  bowlders  of  ore  and  large  sharp-edged 
fragments  of  ferrughious  quartzose  slate  and  jasper  in  a  matrix  consisting  of  quartzose  sand, 
numerous  small  pebbles  and  fragments  of  ore-formation  materials,  quartzite,  and  a  few  pebbles 
of  white  ((uartz,  of  granite,  or  of  other  Archean  rocks.  In  a  few  places  their  proportion  of 
ferruginous  material  is  so  great  that  they  have  been  utilized  as  sources  of  iron  ore. 

CAMBRO-ORDOVICIAN. 

HERMANSVILLE  LIMESTONE. 

The  general  character  of  the  Hermansville  limestone  "is  that  of  a  coarse-;  rained  sandstone, 
with  abundant  calcareous  cement,  in  alternation  with  pure  dolomite  or  sometimes  oolitic  beds." 
The  limestone  may  be  seen  near  the  top  of  the  hill  east  of  Iron  Mountain,  on  the  bluff  northeast 
of  Norway,  and  at  several  places  on  the  hills  north  of  Waucedah.  Its  maximum  thickness, 
according  to  Rominger,'' is  about  100  feet,  but  this  maximum  is  rarely  reached  in  the  Menominee 
district.  Only  a  few  fossils  have  been  reported  from  it.  Romin^-er  states  tliat  it  has  yielded 
a  few  fragments  of  molluscan  shells.  To  these  may  now  be  adiled  a  broken  OrtJioceras.  a  frag- 
ment resembling  a  piece  of  a  fh/rtoceras,  a  gastropod,  am.!  several  otlier  fra.Lrmeiitary  forms 
found  in  the  top  layer  on  the  ])\iiiY  northeast  of  Norway. 

THE  IRON  ORES  OF  THE  MENOMINEE  DISTRICT. 

By  the  authors  and  W.  J.  Mead. 
DISTBIBUTION,   STRUCTURE,  AMU  RELATIONS. 

The  ore  deposits  of  the  Menominee  district  occur  in  the  two  iron-bearing  members  of  the 
Vidcan  formation  known  as  the  Traders  iron-bearing  member  and  the  Curry  iron-bearing 
member.  These  are  separated  by  the  Brier  slate  member.  Much  the  larger  tonnage  of  ore 
mined  lias  come  from  the  Traders  member,  lying  south  of  the  southern  dolomite  belt.  The 
ores  may  occur  at  any  horizon  within  these  members,  but  otlier  comlitions  being  equal  they 
are  more  likely  to  occur  at  low  and  high  horizons  than  at  middle  horizons.     A  number  of  the 


o  liomlDger,  Carl,  Paleozoic  rocks:  Geol.  Survey  Michigan,  vol.  1,  pt.  3, 187»,  p.  '<1.  » Idem,  p.  71. 


U.  S.  GEOLOGICAL  SURVEY 


MONOQRAPH  Lll      PL.    XXVIl 


6 -5 


i&im 


No   3  SHIFT 


Si 


0     Ih  leittl 


/  Un 


i  V, 


s  / 


\*^ 


'Ta/c-schist     ,  ^ 


VERTICAL    NORTH-SOUTH    CROSS    SECTIONS    THROUGH    THE    NORWAY-ARAGON    AREA,    MENOMINEE    DISTRICT,    MICH,,    ILLUSTRATING    GEOLOGIC    STRUCTURE. 

After  Bayley.     See  page  347 


MENOMINEE  IRON  DISTRICT. 


347 


large  ore  bodies  extend  entirely  across  the  members  in  which  they  occur.  The  deposits  of  large 
size  rest  upon  relatively  impervious  formations,  which  are  in  such  positions  as  to  constitute 
pitching  troughs.  A  pitching  trough  may  be  made  (a)  by  the  Randville  dolomite  or  middle 
Huronian,  underlying  the  Traders  iron-bearing  member  of  the  Vulcan  formation;  (b)  by  a  slate 
constituting  the  lower  part  of  the  Traders  member;  or  (c)  by  the  Brier  slate  member,  between 
the  Traders  and  Curry  iron-bearing  members.  (See  PI.  XXVII;  figs.  40,  47,  48.)  The  dolomite 
or  quartzite  formation  is  especially  likely  to  furnish  an  impervious  basement  where  its  upper 
portion  has  been  transformed  into  a  talc  schist,  as  a  consequence  of  folding  and  shearing  between 
the  formations. 

These  pitching  troughs  are  minor  folds  of  the  drag  type.    In  tJie  western  and  central 
parts  of  the  south  belt  of  Vulcan  formation  carrying  the  principal  ore  bodies  there  is  consider- 


►  No3   SHAFT 


■  No  2  SHAFT 


Figure  40.— Horizontal  spotion  of  the  Aragon  mine  at  the  first  level,  Menominee  tlistrict,  Michifran.    Scale,  1  inch  =  250  feet.    After  Bayley. 

able  uniformity  of  pitch  to  the  west,  resulting  from  the  westward  and  upward  shearing  of  the 
southerly  beds  with  reference  to  the  northerly  beds.  At  the  east  end  of  tiie  district  some  of  the 
pitches  are  eastward.  Any  portion  of  the  iron-bearing  member  may  have  yielded  to  the  shear- 
ing by  a  series  of  tliese  drag  folds.  The  ore  bodies  following  the  axial  lines  may  thus  be  in  a 
series  of  parallel  shoots,  one  pitcliing  below  the  other  along  the  strike.  This  is  well  illustrated 
in  the  Chapin  and  Millie  mines. 

In  these  folds  the  strike  of  the  shoots  at  the  surface  is  at  a  slight  angle  from  the  strike  of 
the  bedding,  as  shown  in 'figure  49  (p.  3.50). 

The  wall  rocks  of  the  ore  l)0tlies  may  be  unaltered  phases  of  the  iron-bearing  member, 
especially  the  ferruginous  cherts,  or  any  of  the  rocks  forming  the  impervious  basement.  The 
be<ls  in  the  ore  botly,  when  followed  along  the  strike  and  dip,  usually  pass  into  ferruginous 
cherts  or  iron  carbonates. 


348 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


At  first  sight  tlic  forms  of  the  ore  deposits  might  be  thought  to  be  exceedingly  irregular,  but 
when  the  above  relations  are  understood  they  appear  to  have  orderly  forms.  A  main  mass  of 
ore  is  likely  to  be  at  the  bottom  of  a  trough,  but  from  this  main  mass  a  considerable  belt  of  ore 
may  extend  along  tlio  limbs  of  the  trough  to  a  much  higher  altitude  than  in  the  center  of  the 
trough.  Many  of  the  ore  bodies  in  cross  sections  thus  constitute  a  U,  which  is  very  tliick  at  the 
bottom,  the  center  of  the  U  bcmg  occupied  by  the  iron-bearing  rocks  wliich  have  not  been  trans- 
formed to  ore.  If  the  fold  is  very  much  compressed  the  limbs  of  the  U  may  unite  at  the  center 
and  produce  a  pitching  Ions,  with  its  lower  extremity  rounded  to  conform  with  the  shape  of  the 
trough  of  the  fold  and  its  upper  end,  where  not  at  the  surface,  more  or  less  irregular  in  shape 
in  consecjuence  of  the  gradual  passage  of  the  ore  into  jaspihte.  The  deposits  at  Chajjui  are  good 
illustrations  of  such  lenses. 


N3^. 


:--^5«? 


400  feet 


Randville  formation) 

trail'"  •"<!  I"arhg  j 


Figure  47.— Horizontal  section  of  ttie  Aragon  mine  at  the  eighth  level,  Menominee  district,  Michigan.    After  Bayley. 

Though  all  the  largest  iron-ore  bodies  are  confined  to  the  pitchmg  troughs  with  impervious 
basements  of  dolomite,  quartzite,  or  talc  scliist,  smaller  ore  deposits  occur  at  contacts  between 
the  different  members  and  at  places  within  the  iron-bearing  members  where  severe  brecciation 
has  occurred.  The  deposits  formed  at  contacts  are  usually  much  more  irregular  than  those 
formed  in  troughs.  In  general,  they  are  broad  and  thua  sheetlike  masses  with  irregular  bomnla- 
ries  on  all  sides.  Their  lower  surfaces  are  the  more  even  and  the  better  defined,  but  even  these ' 
are  umlulatory.  For  the  most  part  they  remain  near  the  contact  of  the  iron-bearing  formation 
with  the  imderlying  rocks,  but  at  many  places  they  leave  tliis  contact,  rise  into  the  iron-bearing 
beds,  and  thus  become  separateil  from  the  base  of  the  formation  by  considerable  thicknesses  of 
jasjiilitos.  The  upper  surfaces  are  much  more  uneven  than  tlie  lower  ones.  Not  only  are  they 
umlulatory  to  a  greater  degree,  but  ore  projections  extend  upward  uito  the  overlying  jaspilites 
and,  ramifying  through  these  in  an  extremely  irregular  manner,  in  places  coalesce  and  inch)se 
lenses  of  jaspihte  and  then  continue  their  separate  courses  until  the  contact  with  the  overlying 


MENOMINEE  IRON  DISTRICT. 


349 


/ 

■55/ 


JiGUEE  48.— Vertical  north-south  cross  section  through  Burnt  shaft,  West  Vulcan  mine,  Menominee  district,  Michigan.    After  Bayley. 


350 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


slates  is  reached,  where  they  again  coalesce,  spread  out,  and  form  a  second  sheetHke  body, 
which,  however,  is  usually  much  thinner  and  much  less  extensive  than  the  deposit  at  the  lower 
contact.  Deposits  of  this  kind  occur  principally  in  tiie  straight  portions  of  tiic  member,  where 
folding  is  absent  and  where  the  dip  is  not  overturned.  A  portion  of  tlie  deposits  of  the  West 
Vulcan  and  Verona  mines  ar(^  of  this  class. 

Tiie  Menominee  ores  rest  f(>r  the  most  part  on  the  middle  slopes  of  tlie  ridges  formed  by  the 
Kandville  dolomite  and  middle  Huronian  quartzite,  but  they  also  go  beneath  the  lower  ground. 


....                                                    ....                              V    Ol 

ii 

"% 

WMM&MmifimMif^:-iM'Mi0!yMi 

"-%. 

:'■;.' •.W-.'.';.;.;.V;.'-:.;'.V' 

'y\yy:'\:'-'-'^]{-yrr\:\'-:y^-yy^\y\^y'-y'.\-'^-^ 

fk 

fm^i^!Miy^^M$M0-^M^^^ 

,-—~—~ — ^^ 

. -___^ — ^^— ^_— ^_ 

Figure  49. — Sketch  to  show  pitch  of  a  drag  fold  in  a  monodinal  succession.  The  ores  in  some  places  follow  the  axis  of  the  fold.  It  will  be  noted  that: 
the  strike  of  the  ore  body,  measured  at  th?  surface,  is  at  a  slight  angle  to  the  strike  of  the  bedding,  notwithstanding  the  fact  that  the  ore  body 
follows  throughout  a  single  bed  or  set  of  beds. 


CHEMICAL  COMPOSITION  OF  THE  ORES. 


The  averages  of  the  cargo  analyses  of  ore  shipped  from  the  district  in  1907  and  1909,  with 
the  range  for  each  constituent,  are  as  foUows: 

Average  chemical  composition  of  ores  from  cargo  analyses  for  1907  and  1909,  with  range  for  each  constituent. 


Average. 

Range. 

1907. 

1909. 

1907. 

1909. 

Moisture  (loss  on  drying  at  212°  F.) 

5.92 

6.67 

2.16    to   7.92 

1.07    to   8.77 

Analysis  of  ore  dried  at  212°  F: 

50.70 
.0.38 
1.54 
21.15 
.17 
.28 
2.12 
.013 
1.92 

52.23 

.074 

L41 

Hi.  77 

.19 

1.31 

2.70 

.012 

2.52 

39.00    to  59. 90 
.010  to      .084 
.80    to    2.73 
5.70    to41.M 
.03    to      .47 
.35    to    1.70 
.14    to    3.71 
.005  to      .022 
.60    to    3.50 

3S.  46    to  01. 20 

Phosphorus 

008  to        620 

.86    to    2.28 

Silica 

4  ''t    to  "iQ  14 

.07    to    1.27 

Lime 

63    to    2  90 

.70    to    3.9S 

Sulphur 

.  006  to      .  041 

Loss  on  ignition , 

90   to   4  30 

Comparison  of  analyses  of  all  the  ores  of  the  district  shows  tlie  rich  ores  to  consist  principally 
of  slightly  hydrated  hematite,  witli  additional  varymg  amoimts  of  magnetite,  silica,  alumina, 
lime,  magnesia,  carbon  dioxide,  phosphorus  pentoxide,  and  water.  Most  of  the  ores  contairt 
also  manganese,  potash,  and  soda,  and  a  few  of  them  titanium  and  carbon. 


MENOMINEE  IRON  DISTRICT. 

Following  are  three  complete  analyses  of  high-grade  Menominee  ore: 

Complete  analyses  of  Menominee  ores." 


351 


1. 

■     2. 

3. 

Fe ' 

60.64 

65.63 

o7  03 

FP.O3 ■ 

85.44 
.47 

L.'iS 
.76 

1.26 

3.02 
.004 
.060 

4.M 
.15 
.002 

91.51 
1.97 
1.53 

80  15 

FeO 

1.10 

AI^Gi 

3  88 

MiijOj 

CaO 

..■i6 
.21 
.57 
.03 
3.03 
.021 
.099 
.38 
.27 

.17 

MgO 

.48 

K.O 

2  29 

Na.O 

.30 

SiC). 

10.72 
.074 

P^Oi 

s 

146 

CO, 

.08 

H-OCabove  100°) 

2.75 

56 

99. 842 

99.980 

99.950 

a  Mon.  U.  S.  Geol.  Survey,  vol.  40,  1904,  p.  383. 


1.  Chapin  ore:  analysis  furnished  by  E.  E.  Brewster. 

2.  "Soft  specular"  Quinnesec  ore. 

3.  "Soft  specular  blue  ore"  from  Cornell  ir.ine. 


AVERAGE  IRON  CONTENT  OF  THE  IRON-BEARING  FORMATION. 

An  average  of  1,681  analyses,  representing  5,287  feet  of  drilling  from  the  district  away 
from  the  available  ores,  gives  37.93  per  cent  of  iron.  Ores  of  this  class  are  so  much  more 
abundant  than  the  "available"  ores  that  the  average  of  the  entire  formation,  including  ores, 
is  not  much  higher  than  this  figure.  The  composition  of  the  lean,  unaltered  jaspers  where 
not  altered  to  ore' has  not  been  averaged,  but  presumably  the  iron  content  is  about  25  per  cent, 
as  in  other  districts. 

MINERAL  COMPOSITION  OF  THE  ORES. 

The  approximate  mineral  composition  of  the  average  ores  of  the  Menominee  district, 
calculated  from  the  preceding  average  chemical  analysis,  follows: 

Approximate  mineral  composition  of  average  Menominee  ore,  calculattrl  from  average  cnemical  analysis. 


Hematite  (including  a  small  amount  of  magnetite) . 

Limonite 

Quartz 

Kaolin 

Serpentine  and  talc 

Dolomite 

Miscellaneous 


100.00 


The  richer  ores  are  usually  bluish  black,  porous,  fine-grained  aggregates  of  crystallized 
hematite.  These  rich  ores  grade  into  leaner  phases  containing  more  or  less  hydrated  hematite, 
with  varying  amounts  of  quartz,  soijientine,  talc,  clay,  and  carbonates  of  calcium  and  magne- 
sium, rangmg  in  color  from  the  bluish  black  of  the  richer  ores  through  various  shades  of  red 
and  brown  to  yellow. 

All  the  minerals  occurring  as  constituents  of  the  ores  are  found  also  as  visible  masses  either 
hi  veins  cuttmg  the  ore  bodies  or  in  ^'ngs  or  pores  within  them.  Dolomite,  calcite,  and  pyi-ite 
occur  locally  in  excellent  crystals,  and  serpentine  as  large,  white,  almost  pure  masses.  Talc 
also  occurs  in  thick  scams  of  almost  ideal  jiurity,  and  chalcopyrite  in  small  crystals  associated 
with  pyrite.  The  carbonates  and  sulphiiles  are  found  near  watercourses  and  the  silicates 
mainly  in  the  lower  portions  of  the  ore  bodies. 


352  GEOLOGY  OF  TPIE  LAKE  SUPERIOR  REGION. 

Tlie  ores  when  exposed  to  the  action  of  the  atmosphere  become  coated  with  a  white 
elllorescence,  consistiiifi  of  a  mixture  of  the  sulphates  of  sodium,  magnesium,  and  calcium,  in 
wliicli  tire  sodium  sulpluito  is  greatly  in  excess. 

PHYSICAL  CHARACTERISTICS  OF  THE  ORES. 

The  lean  ores  differ  vcmt  little  in  appearance  from  the  jaspilites,  of  which  they  are  essentially 
a  part.  They  are  banded,  brecciated,  and  m  places  specular.  The  brecciated  ores  may  consist 
of  jas})er  fragments  in  a  mass  of  hematite,  or  of  hematite  fragments  in  a  mass  of  dolomite,  or 
fragments  of  ore,  jasper,  and  slate  m  a  mass  consisting  largely  of  slate  debris  that  has  been 
strongly  ferruginized. 

IRON    MINERALS 


SILICA  PORE   SPACE 

Figure  50.— Triangular  diagram  representing  the  volume  composition  of  the  various  grades  ot  ore  mined  in  the  Menominee.  Crystal  Falls,  and 
neighboring  districts  in  1907.  M,  Menominee;  CF,  Crj'stal  Falls;  IR,  Iron  River;  F,  Florence;  FM,  Felch  Mountain:  C.  Calumet.  The  pore 
space  fur  each  grade  was  calculated  from  the  average  moisture  content,  and  hence  represents  the  true  pore  space  only  when  the  moisture  in  a 
particular  grade  was  at  a  maximum.  The  true  porosity  of  the  various  grades  of  ore  would  therefore  be  slightly  greater  than  is  shown.  For 
description  of  the  method  of  platting  on  triangular  diagram,  see  page  189. 

The  average  texture  of  the  Menominee  ores  is  shown  by  the  following  table  of  screening 
tests,  made  by  the  Oliver  Iron  Mming  Companj'  on  six  typical  grades  of  ore  representing  a 
total  of  1,033,491  tons.  Each  test  was  made  on  a  sample  of  100  pounds,  representative  of  the 
entire  3-ear's  output  of  that  grade.  A  comparison  of  the  textures  of  the  ores  of  the  several  ranges 
is  shown  in  figure  72  (p.  481). 


MENOMINEE  IRON  DISTRICT.  353 

Textures  of  Menominee  ores  as  shown  by  screening  tests. 

Per  cent. 

Held  on  ^-inch  sieve 39. 44 

^-inch  sieve 30.  63 

No.  20  sieve 11.56 

No.  40  sieve 4.73 

No.  60  sieve -  1-31 

No.  80  sieve !■  I'J 

No.  100  sieve 1-35 

Passed  through  No.  100  sieve 9-67 

The  mineral  density  of  the  ores  varies  with  the  iron  content.  The  average  mineral  density 
of  the  ores  calculated  from  the  average  of  the  cargo  analyses  for  1907,  by  computing  the  mineral 
composition  and  properly  combining  the  densities  of  the  component  minerals,  is  4.28. 

To  test  the  accuracy  of  this  method  of  computing  mineral  density,  pycnometer  determina- 
tions were  made  on  the  average  pulp  samples  '^  of  Ajax  antl  Cluipin  grade  for  1907,  with  the 

following  results: 

Mineral  density  of  Menominee  ores. 


Determined  by  means  of  pycnometer. 
Calculated  from  chemical  analysis .... 


Ajax  grade. 


4.21 
4.34 


Chapin  grade. 


4.601 
4.607 


The  porosity  of  the  ores  ranges  from  1  per  cent  or  less  in  some  of  the  lean  jaspilite  ores  to 
as  much  as  45  per  cent  in  some  of  the  richer  hematites  and  especially  in  the  limonitic  ores. 

The  cubic  contents  of  the  ores  vary  greatly.  The  bulk  of  the  ores,  however,  lies  between 
9  and  14  cubic  feet  to  the  ton. 

Volume  comparisons  of  the  Menominee  ores  with  each  other  and  with  ores  of  the  Crystal 
Falls,  Iron  River,  Florence,  Felch  Mountain,  and  Calumet  districts  are  made  in  figure  50. 

IRON  ORE  AT  BASE  OF  CAMBRIAN  SANDSTONE. 

The  basal  conglomerate  of  the  Cambrian  sandstone  where  it  rests  upon  tlie  iron-bearing  . 
formation  contains  abundant  fragments  of  that  formation.  In  a  few  places  the  proportion  of 
ferruginous  material  is  so  great  that  the  conglomerates  have  been  utilized  as  sources  of  iron  ore. 
A  deposit  of  this  kind  was  formerly  worked  by  the  operators  of  the  Quinnesec  inine,  and  another 
has  recently  been  worked  by  the  Pewabic  company.  The  latter  was  reached  by  the  open  pit 
in  the  SE.  J  sec.  32,  T.  40  N.,  R.  30  W.,  known  as  the  Pewabic  pit.  Although  at  this  place 
the  rock  immediately  underlying  is  dolomite,  the  amount  of  iron  ore  in  the  conglomerate  is  so 
great  that  the  company  operating  the  pit  felt  warranted  in  erecting  concentrating  works  on  the 
property  for  the  separation  of  the  ore  from  the  sandstone. 

SECONDARY  CONCENTRATION  OF  THE  MENOMINEE  ORES. 

Structural  conditions. — The  ore  deposits  in  the  Menominee  district  rest  upon  steeply  dipping 
impervious  basements  of  sheared  dolomite  or  slate.  The  hanging  wall  may  be  of  slate  or  iron- 
bearing  formation.  The  greater  dimensions  of  the  deposits  are  parallel  to  the  bedding.  Fold- 
ing, of  the  type  illustrated  in  figure  12  (p.  123),  develops  minor  corrugations  in  the  foot  wall  and 
other  rocks,  with  pitches  parallel  to  the  main  strike  of  the  formation.  In  tliese  pitching  folds 
the  ore  deposits  are  hkely'to  be  larger  and  better  concentrated  than  elsewhere.  It  is  obvious  that 
the  flow  of  water  concentrating  the  ore  has  been  principally  parallel  to  the  bedding,  that  it  has 
been  especially  strong  where  the  bedding  has  been  folded  into  pitching  troughs,  and  that  the 
fracturing  of  the  brittle  iron-bearing  rocks  during  tliis  folding  has  aided  greatly  in  the  circu- 
lation of  waters  in  pitcliing  troughs  and  elsewhere  in  the  formation.  The  ores  are  associated 
with  marked  topographic  relief,  affording  abundant  head  for  the  waters.     The  larger  number 

o  Kindly  furnished  by  Mr.  J.  H.  Hitchens,  chief  chemist  for  the  Oliver  Iron  Mining  Company  at  Iron  Mountain. 
47517°— VOL  52—11 ^23 


354  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

of  them  are  on  the  upper  or  middle  slopes  of  the  rock  elevations,  though  some  of  them  extend 
})eneath  tiio  depressions. 

Mincralofjical  and  chemical  changes.— The  iron-bearing  formation  was  originallj'  iron  car- 
bonate and  greenalite  interbedded  with  more  or  less  slate  and  containing  much  detrital  ferric 
oxide  at  the  base  of  the  formation.  The  alteration  of  the  chcrtj'  iron  carbonate  and  greenalite 
to  ore  has  been  acconij)lisiic(l  in  the  general  manner  already  described  as  typical  for  the  region — 
(1)  oxidation  and  hydration  of  the  iron  minerals  in  place,  (2)  leadiing  of  silica,  and  (3)  intro- 
duction of  secondary  iron  oxide  and  iron  carbonate  from  other  parts  of  the  formation.  These 
changes  may  start  simultaneously,  but  1  is  usually  far  advanced  or  complete  before  2  and  3 
are  conspicuous.  The  early  products  of  alteration  therefore  are  ferruginous  cherts — that  is, 
rocks  in  which  the  iron  is  oxidized  and  hydrated  and  the  silica  not  removed.  The  later  removal 
of  silica  is  necessary  to  produce  the  ore. 

SEaUENCE  OF  ORE  CONCENTKATION  IN  THE  MENOMINEE  DISTRICT. 

The  first  considerable  concentration  of  ore  in  the  district  which  is  now  minetl  did  not  take 
place  until  the  erosion  period  following  upper  Huronian  time.  As  indicated  in  the  general 
discussion,  the  process  was  well  advanced  before  Cambrian  time  and  has  practically  continued 
to  the  present. 


CHAPTER  XIV.     NORTH-CENTRAL  WISCONSIN  AND  OUTLYING  PRE- 
CAMBRIAN  AREAS  OF  CENTRAL  WISCONSIN. 

NORTHERN  WISCONSIN  IN  GENERAL. 

The  only  work  done  by  the  United  States  Geological  Survey  in  northern  Wisconsin  is  in 
the  Florence  district;  the  southern  extension  of  the  Menominee  district,  in  the  northeastern 
part  of  the  State ;  the  Penokee  range,  in  the  northern  part  of  the  State ;  and  the  Keweenawan 
belt  crossing  the  northwest  corner  of  the  State.  These  districts  are  described  on  other  pages. 
Other  areas  in  northern  Wisconsin  have  been  examined  in  reconnaissance  work  by  members 
of  the  Survey,  but  no  detailed  mapping  has  been  done.  Outside  of  the  areas  named,  the  chs- 
tribution  of  the  rocks  of  northern  Wisconsin  shown  on  the  general  map  (PI.  I)  is  taken  from 
the  Wisconsin  Geological  Survey  reports,  particularly  that  of  Weidman  '^  for  north-central  Wis- 
consin. The  recent  map  of  Douglas  County  made  by  Grant ''  for  the  Wisconsin  Geological 
Survey  is  used  in  place  of  the  earlier  map  by  Irving. 

Granites  and  gneisses,  with  subordinate  amounts  of  sedimentary  rocks  and  basic  igneous 
rocks,  constitute  a  highland  in  the  northern  part  of  the  State,  roughly  oval  in  its  outline,  extend- 
ing from  the  vicinity  of  Grand  Rapids  and  Stevens  Point,  on  the  south,  to  the  State  boundary, 
on  the  north,  and  from  Barron  County  eastward  to  the  Micliigan  boundary.  The  area  is  bounded 
on  the  northwest  by  the  Keweenawan  rocks  described  in  Chapter  XV,  and  on  the  north  and 
northeast  by  the  Huronian  formations  of  Michigan;  on  the  southeast,  south,  and  southwest  it 
is  overlapped  on  the  lower  ground  by  Paleozoic  sediments  which  outcrop  in  wide  belts  sur- 
rounding the  pre-Cambrian  core.  The  predominating  granites  and  gneisses  were  called  Lauren- 
tian  and  the  sedimentary  rocks  Huronian  by  the  geologists  of  the  first  Wisconsin  Geological 
Survey  (1882).  The  highlands  as  a  whole  have  been  often  referred  to  as  a  "Laurentian  liigh- 
land."  The  drift  cover  is  heavy,  exposures  are  few,  except  in  certain  localities,  antl  much  of  it 
has  been  difficult  of  access  even  to  the  present  time.  The  only  published  detailed  work  is  that 
of  Weidman, °  which  is  summarized  below. 

WAUSAU  DISTRICT. 
LOCATION,  AREA,  AND    GENERAL   GEOLOGIC   SUCCESSION. 

The  pre-Cambrian  area  in  north-central  Wisconsin  mapped  and  described  by  Weidman 
includes  the  counties  of  Marathon,  Portage,  Wood,  Clark,  Tajdor,  Lincoln,  and  adjacent  parts 
of  Ru.sk,  Price,  and  Langlade,  containing  in  all  about  7,200  square  miles.  From  90  to  95  per 
cent  of  the  pre-Cambrian  rocks  of  this  area  are  of  igneous  origin. 

The  following  table  is  compiled  from  the  succession  worked  out  by  Weidman.  The  rocks 
he  classes  doubtfully  as  lower  and  middle  Huronian  we  classify  doubtfully  as  middle  and  upper 
Huronian,  respectively.     The  names  of  the  formations  are  those  used  by  Weidman. 

Quaternary  system: 

"Wisconsin  drift. 


Pleistocene  series. 


Third  drift. 

Second  drift. 

First  drift. 

Alluvial  deposits  (contemporaneous  with  drift). 


a  Weidman,  Samuel,  The  geology  of  north-central  Wisconsin;  Bull.  Wisconsin  Geol.  and  Nat.  Hist.  Survey  No.  16,  1907. 
t  Grant,  U.  S.,  Preliminary  report  on  the  copper-bearing  rocks  of  Douglas  County,  Wisconsin;  Bull.  Wisconsin  Geol. and  Nat.  Hist.  Survey 
No.  6,  2d  ed.,  1901. 

355 


356  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Unconformity. 

■  Cambrian  system Upper  Cambrian  or  Potsdam  sandstone. 

Unconformity. 
Algonkian  system : 
Huronian  series: 

fNorth  Mound  oonglomerate  and  quartzite. 


Upper  Huronian?  ("Middle  Huronian?"  or 
"Upper  sedimentary  group,"  of  Weid- 
man).  (Stratit;raphic  relations  unknown; 
formations  presumably  contemporaneous.) 


Arpiu  conslomorate  and  quartzite. 
Mosince  confjlomerate. 
Marshall  Hill  conglomerate. 
Marathon  conglomerate. 


Unconformity. 

,  .     f3.  Granite  and  nepheline  syenite  series. 

Intrusive  igneous  rocks.     (In  order  of  in-L,  ^  ,  ,  i   ,•     •, 

^   "  ^  ^2.  Gabbro  and  diorite  senes. 

trusion) 


Middle  Huronian?  ("Lower  Huronian?"  or 
"Lower  sedimentary  group,"  of  Weid- 
manV    (Stratigrapliic  relation.^  unknown.) 


1.  Rhyolite  series. 
Rib  Hill  quartzite. 
Powers  Bluff  quartzite. 
Hamburg  .slate. 
Wausau  graywacke. 


Unconformity. 

Archean  system  (?) Gneiss  and  schists. 

ARCHEAN  (?)  SYSTEM. 

The  basal  rocks,  believed  to  be  the  oldest  aud  to  belong  to  the  Archean  system,  consist 
of  a  complex  mixture  of  rocks,  such  as  contorted  and  crumpled  granite  gneiss,  diorite  gneiss, 
granite  scliist,  syenite  scliist,  and  diorite  schist.  The  gneisses  and  scliists  form  a  belt  which 
can  be  fairly  well  outlined,  extending  from  the  vicinity  of  Stevens  Point  and  Grand  Rapids 
in  a  northwesterly  chrection  through  Neillsville.  The  rocks  are  closely  intermingled  with  one 
another,  and  have  been  subjected  to  extensive  folding  and  metamorphism.  The  zone  in  which 
they  are  largely  comprised  lies  between  areas  of  later  igneous  and  sedimentary  rock  to  the  north 
and  to  the  south,  and  hence  appears  to  have  the  position  of  the  arch  of  an  anticline.  These 
basal  rocks  are  intruded  by  later  formations  of  rhyolite,  diorite,  and  granite.  Sedimentary 
rocks  have  not  been  found  in  contact  with  the  basal  rocks. 

ALGONKIAN   SYSTEM. 

HURONIAN  SERIES. 
MIDDLE   HUKONL\N  (?). 

The  rocks  next  succeeding  are  of  sedimentary  origin,  and  consist  of  quartzite,  slate,  and 
graywacke.  They  include  the  quartzite  of  Rib  Hill  and  ^^cinity,  the  quartzite  of  Powers 
Bluff  and  in  the  vicinity  of  Junction  and  Rudolph,  a  wide  belt  of  slate  in  northwestern  Mara- 
thon County,  and  graywackes  in  the  vicinity  of  Wausau.  These  rocks  are  almost  entirely  of 
fragmental  origin,  and  only  rarely  contain  phases  of  carbonaceous,  calcareous,  and  ferruginous 
deposits.  The  basement  upon  which  these  sediments  were  deposited  can  not  be  defuiitely 
determined,  for  all  the  observed  contacts  with  associated  rocks  are  those  either  of  later  intru- 
sive igneous  rocks  or  of  later  overlving  conglomerate.  The  quartzites  are  throughout  extremely 
metamorphosed  and  to  all  appearances  completely  recrystallized.  The  slates  and  gra3'wackes 
do  not  reveal  as  much  metamorpliism  as  the  quartzite,  although  in  places  rocks  presumably 
belonging  with  the  slate  have  been  changed  to  schists  bearing  staurolite,  cordierite,  and  garnet. 
These  sedimentary  rocks  appear  to  bear  the  relation  of  great  fragmentary  masses  intersected 
and  surrounded  by  later  igneous  intrusive  rocks.  They  constitute  the  lowest  and  oldest  sedi- 
mentary rocks  of  this  area. 


NORTH-CENTRAL  WISCONSIN  AND  OUTLYING  PRE-CAMBRIAN  AREAS.     357 

ROCKS    INTRUSIVE    IN    MIDDLE    IIURONIAN   (?)    AND    ARCIIEAN   (  ?). 

The  next  younger  rocks  are  of  igneous  origin.  They  form  about  75  per  cent  of  the  rocks 
of  the  area,  and  in  the  order  of  their  intrusion  arc  (1)  rhyoHte;  (2)  a  basic  scries  of  diorite, 
gabbro,  and  peridotite;  (3)  a  series  consisting  of  granite,  quartz  syenite,  nephehne  syenite, 
and  related  rocks.  Of  these  the  last-named  series  is  the  most  abundant,  the  granite  alone 
forming  about  50  per  cent  of  the  surface  rocks  of  the  area.  The  three  series  are  intrusive  in 
the  Archean(?)  of  the  area  and  also  in  the  middle  Huronian  (?).  They  are  in  turn  overlain 
by  later  Algonkian  sediments.  The  period  involved  in  the  intrusion  of  the  igneous  formations 
must  have  been  a  very  long  one,  and  evidently  constituted  an  important  portion  of  the  pre- 
Cambrian  era,  for  the  granite  and  syenite  series  itself  represents  a  complex  magma  of  varymg 
though  related  rocks,  intruded  at  different  dates.  In  the  stratigraphy  of  this  area,  therefore, 
these  igneous  intrusives  play  an  important  part  and  occupy  a  well-defined  position  between 
the  upper  Huronian  ( ?)  and  the  middle  Huronian  ( ?)  sediments. 

UPPER    HURONIAN   (?). 

The  latest  Algonkian  rocks  of  the  area  consist  mainly  of  conglomerate  and  quartzite  over- 
l3dng  all  the  other  rocks  above  referred  to.  North  of  Wausau,  at  Arpin,  and  at  North  Mound 
they  are  represented  by  conglomerate  and  quartzite,  and  at  Marathon  City  and  Mosinee  by 
conglomerate. 

CAMBRIAN  SYSTEM. 

In  the  north-central  area  the  pre-Cambrian  was  worn  down  to  base-level  by  subaerial 
erosion  before  the  much  later  Upper  Cambrian  or  Potsdam  sandstone  '^  was  dejjosited  upon  it.* 

BARRON,  RUSK,  AND  SAWYER  COUNTIES. 

In  Barron,  Rusk,  and  Sawyer  counties  the  pre-Cambrian  rocks  are  largely  of  igneous 
origin.  The  most  prominent  sedimentary  areas  are  the  prominent  ridge  of  quartzite  at  the 
junction  of  Flambeau  and  Chippewa  rivers  and  the  numerous  quartzite  I'idges  along  the  divide 
of  Cliippewa  and  Red  Cedar  rivers.  In  general,  these  quartzites  dip  westward,  away  from  the 
crystalline  and  schistose  area,  with  strongly  marked  eastward  escarpments  overlooking  the 
nearly  flat  plain  of  older  rocks.  Although  no  final  conclusion  has  been  reached  concerning  the 
relative  age  of  these  quartzites,  Weidman  is  of  the  opinion  that  there  are  here  represented  at 
least  two  and  probably  three  series.  The  quartzite  in  the  small  outcrops  along  the  railroad 
about  .3  miles  east  of  Weyerhauser  is  greatly  metamorphosed  and  is  correlated  with  the  Rib 
Hill  quartzite  at  Wausau.  The  prominent  ridge  of  quartzite  at  the  junction  of  the  Flambeau 
and  the  Chippewa  is  correlated  with  the  upper  sedimentary  series  in  north-central  Wisconsin 
and  the  Baraboo  quartzite.  The  prominent  ridges  of  quartzite  in  eastern  Barron  County  and 
in  the  adjacent  parts  of  Rusk  and  Sawyer  counties  are  but  slightly  metamorphosed,  the  bedding 
is  in  general  nearly  flat-lying,  and  the  formation  has  a  much  younger  aspect  than  the  other  two 
quartzite  formations  in  the  region  and  may  be  Keweenawan. 

o  The  term  Potsdam  sandstone  is  here  used  in  a  quotational  sense  from  the  Wisconsin  Geological  Survey. 

<>  Weidman,  Samuel,  The  pre-Potsdam  peneplain  of  the  pre-Cambrian  of  northKjentral  Wisconsin:  Jour.  Geologj',  vol.  11,  1903,  pp.  289-313. 


358 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


VICINITY  OF  LAKE  WOOD. 

Quartzites  arc  known  in  the,  vicinity  of  Lakewood,  indicating  the  presence  of  Huronian 
rocks  in  tliis  district.  Practically  all  that  is  known  concerning  the  distrii)iition  and  structure 
of  these  quartzites  is  shown  on  the  accompanying  sketch  (fig.  51).     They  stand  up  as  monad- 


R.  16  E. 


R.I7  E. 


20 

21 

22 

?3 

24 

19 

20 
P  P 

21 

22 

23 

24 

19 

20 

21 

22 

29 

28" 

27 

26 

,     25 

30 

29           26 

27 

26 

25 

29 

b\°*'S'?b 

?27 

32   Gr 

33 

34 

3S 

36 

31 

32 

% 

^V 

35 

* 

Or 
32 

33 

34 

/^ 

Gr*Gr 
4 

.8 

^B 

2 

1 

6 

5 

4 

3 

Z 

1 

6 

5 

4 

3 

8P* 

9 

10 

M 

12 

7 

8 

9 

10 

II 

12 

7 

6 

9 

10 

17 

16 

15 

14 

13 

18 

17 

16 

15 

14 

13 

IS 

17 

16 

15 

FlGtTRE  51.— Sketch  map  showing  occurrence  of  quartzites  of  Huronian  age  in  Tps.  33  and  34  N.,  Rs.  15,  16,  and  17  E.,  Wisconsin.    B.  Quartzite 
and  quartzite  breccia;  C,  conglomerate;  D,  diabase;  Gr,  granite;  P,  porphyry. 

nocks  above  the  surrounding  drift-covered  surface.     Associated  with  them  are  granite,  por- 
phyry, and  diabase  in  isolated  exposures. 

NECEDAH,   NORTH    BLUFF,   AND  BLACK    RIVER    AREAS. 


At  Necedah,  in  Juneau  County  (see  figs.  52  and  53),  and  at  North  Bluflf,  in  Wood  Count}'-, 
are  quartzite  exposures  projecting  tlirough  the  Cambrian. 


R.  3    E. 


R.   4-    E. 


DRILL  HOLE 
O 
Quartz    dionte  at  229' 
14 


23 


D/f/LL  HOLE 

o 

Granite  and  diorite^ 
at  202' 13 

Necedah, 

NIV        ft 


24      " 

DR/LL  HOLE 
O 
Quartz  dionte 
at  192' 


1/2 


zMiles 


FiatniE  52.— Sketch  map  shownng  occurrence  of  Huronian  quartzite  near  Necedah,  Wis. 

Drilling  at  Necedah  has  cUsclosed  the  presence  of  granite,  probably  intrusive  into  quartzite. 
The  quartzite  is  highly  metamorphosed  and  is  lithologically  similar  to  the  Huronian  rocks. 


BAEABOO    IRON   DISTRICT. 


359 


111  the  Black.  River  vallej',  north  of  Black  River  Falls,  are  exposures  of  gneiss,  granite, 
hornblende  schist,  magnesian  schist,  and  ferruginous  quartz  schist,  mapjied  l)_y  Irving  °  in  1873 
and  by  Hancock  *  in  1901.  The  relation  of  these  rocks  to  one  another  is  not  defmitely  Ivnown. 
All  are  pre-Cambrian. 

BARABOO  IRON  DISTRICT.'^ 

LOCATION  AND  GENERAL  GEOLOGIC  SUCCESSION. 

The  Baraboo  district  constitutes  an  outher  in  the  Paleozoic  rocks  and  centers  in  tlie  town 
of  Baraboo,  in  the  south-central  part  of  Wisconsin.     (See  fig.  53.)     It  is  south  of  the  area 


R.4-E;.         R.SE. 
PALEOZOIC 


R.6E. 


R.7E.        R,8E.       R9E..        RIOE. 
HURONIAN   SERIES  fMIDDLE  ?) 


RUE,        R.IZ.E.     R.I3E.       R.I4-E. 
LAURENTIAN?  SERIES 


(This  also  covers 
part   of  areas 
mapped  as  Huronian) 


<%' 


Seeiey  slate,  Freedom 
dolomite,  including 
iron-bearing  member 


Granite  and 
metarhyolite 


ZO  MILES 


FiGUEE  63.— Sketch  map  showing  Baraljoo,  Fox  lliver  valley,  Necedah,  Waushara,  and  Waterloo  pre-Cambrian  areas  o(  south-central  Wisconsin. 

shown  on  the  general  Lake  Superior  map  (PI.  I),  but  a  brief  description  is  here  given  because 
the  district  is  producing  iron  ore  and  is  similar  lithologically  and  structurally  to  the  iron- 
producing  area  of  the  Lake  Superior  region. 

a  Irving,  R.  D.,  The  Necedah  quartzite:  Geology  ol  Wisconsin,  vol.  2, 1873-1877,  pp.  523-524. 

6  Hancoclv,  E.  T.,  The  geology  of  the  area  at  Blacli  River  Falls,  Wisconsin:  Unpul)lished  thesis,  Geol.  Dept.  Univ.  Wisconsin,  1901. 

cSee  Weidman,  Samuel,  The  Baraboo  iron-bearing  district  of  Wisconsin:  Bull.  Wisconsin  Geol.  and  Nat.  Hist.  Survey  No.  13,  1904. 


360 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


The  area  is  elliptical  in  outline,  extending  20  miles  east  and  west  and  ranging  in  width 
from  2  to  12  miles.     It  is  essentially  a  quartzite  syncline. 
The  succession  is  as  follows: 

Quaternary  system Pleistocene  deposits. 

{Trenton  limestone. <" 
St.  Peter  sandstone. 
"  Lower  Magnesian  "  limestone. 

Cambrian  system Potsdam  sandstone." 

Unconformity. 
Algonkian  system: 
Hurouian  series: 

Upper  Huronian  (?)..  .Quartzite. 
Unconformity. 

fGranite,  intrusive  into  lower  formations. 
Freedom   dolomite,    mainly   dolomite,    including   an   iron-bearing 

member  in  its  lower  part. 
Seeley  slate,  500  to  800  feet. 
Baraboo  quartzite,  3,000  to  5,000  feet. 


Middle  Huronian  (?). 


Unconformity. 
Archean  system : 

I.aurentian  (?)  series Granites,  rhyolites,  tuffs,  etc. 

The  principal  structural  feature  is  an  east-west  synclinorium  of  middle  Huronian  ( ?)  rocks 
resting  on  the  -Archean  basement,  carrying  in  the  trough  unconformable  upper  Huronian  ( ?) 
c[uartzite  and  Paleozoic  sediments,  and  surrounded  by  Paleozoic  sediments.  The  edges  of  the 
basin,  composed  of  hard,  resisting  middle  Huronian  (?)  c[uartzite,  form  ridges  known  as  the 
north  and  south  Baraboo  ranges,  standing  700  to  800  feet  above  the  surrounding  country  and 
the  intervening  valley.     (See  fig.  54.) 


10000 


500O 

4.00O 
-3000 
-2000 

4-1000 
0  SealeveZ 

1000 

HOOO 
-3000 
..+000 

Lsooo 


4  Miles 


Figure  54. — Generalized  cross  section  extending  north  and  south    at-ross    the    Baraboo    district.    1,    Potsdam  sandstone;  2-4,    Htironian 
(2,  Freedom  dolomite;  3,  Seeley  slate;  4,  Baraboo  quartzite);  5,  rhyolite  and  granite  (Laurentian?).    Alter  Weidman. 

ARCHEAN   SYSTEM. 

LATJBENTIAN  SEKIES. 

The  Laurentian  roclis  outcrop  in  isolated  areas  bordering  the  outside  of  the  Baraboo 
syncline.  The  surface  volcanic  phases  are  best  exposed  west  of  the  Lower  Narrows  of  Baraboo 
River  on  tlie  northeast  .side  and  near  the  town  of  Alloa  on  the  southeast  side.  Thov  are 
similar  to  tlie  surface  volcanic  rocks  of  the  Fox  River  valley.  Granitic  rocks  appear  in  isolated 
areas  on  the  south  side  of  the  belt.  Some  of  these  rocks,  previously  considered  as  Archean, 
have  recently  been  found  to  be  intrusive  into  the  rhiddle  Huronian  ( ?) . 


o  Used  la  a  quotational  sense  from  the  Wisconsin  Geological  Survey. 


BAEABOO   IRON   DISTRICT.  361 

ALGONKIAN   SYSTEM. 

HTJKONIAN  SERIES. 

MIDDLE    HURONIAN   (?). 

BAEABOO  QUARTZITE. 

The  Baraboo  quartzite  is  a  massive  though  well-bedded  formation,  considerably  jointed, 
faulted,  and  brecciated,  but  showing  no  cleavage  as  e\ddence  of  rock  flowage  except  along  certain 
thin  slate  beds  in  which  readjustment  has  been  concentrated  during  folding.  Cross-bedding, 
ripple  marks,  and  thin  conglomeratic  layers  are  numerous.  In  the  north  range  the  beds  stand 
nearly  vertical;  in  the  south  range  they  dip  gently  toward  the  south.  Isolated  exposures  in 
the  north-central  side  of  the  trough  are  thought  to  be  brought  up  by  minor  folds.  There  is, 
however,  a  possibihty  that  faulting  has  been  a  factor. 

SEELEY  SLATE. 

The  Baraboo  c{uartzite  passes  up  into  the  Seeley  slate,  a  soft,  gray,  finely  banded  chlorite 
slate,  known  principally  by  drilling  along  the  south  hmb  of  the  syncline.  The  cleavage  is  some- 
what steeper  than  the  bedding,  corresponding  to  the  normal  development  of  cleavage  in  such 
relation  to  a  syncline. 

FREEDOM  DOLOUITE. 

The  Freedom  formation  consists  principally  of  dolomite  but  contains  near  its  base  slate, 
chert,  and  iron  ore  and  all  gradational  phases  between  these  kinds  of  rocks.  The  lowest  mem- 
ber is  a  ferruginous  kaolinitic  slate,  well  exposed  in  the  Illinois  mine,  representing  a  fernigi- 
nated  gradation  phase  of  the  Seeley  slate  into  the  Freedom  dolomite.  The  next  overlymg 
member  of  the  Freedom  dolomite  is  banded  ferruginous  chert  and  iron  ore,  known  principally 
along  the  south  hmb  of  the  syncline,  but  occurring  also  in  the  east  end  of  the  basin  and  in 
several  explored  areas  on  the  north  side.  Interbedded  mth  tlie  chert,  especially  near  its  upper 
parts,  are  calcite,  siderite,  kaolin,  and  dolomitic  slates.  Minor  folding  and  brecciation  are 
conspicuous  in  this  member,  part  of  it  at  least  resulting  from  secondary  chemical  changes, 
causing  slump  in  the  formation. 

The  cherty  dolomite  making  up  the  upper  member  and  by  far  the  greatest  part  of  the 
Freedom  formation  is  a  fine-grained  banded  rock  similar  in  some  of  its  phases  to  the  ferruginous 
cherts  but  usually  softer.     It  grades  locally  into  ferrodolomite. 

UPPER    HURONIAN     (?). 

Upper  Huronian  (?)  cjuartzite  has  been  found  only  by  drilhng  in  the  deeper  parts  of  the 
east  end  of  the  trough.  Only  recently  has  it  been  discriminated  from  the  Cambrian  sandstone 
above  it  or  the  middle  Huronian  ( ?)  quartzite  below.  When  the  drill  penetrated  tlie  Cambrian 
sandstone  and  conglomerate  and  reached  quartzite  below  it  was  usually  assumed  that  this  was 
-the  middle  Huronian  ( ?)  quartzite  and  the  drilling  was  stopped.  When  tliis  quartzite  was  pen- 
etrated by  the  drill,  however,  it  was  found  to  overlap  the  edges  of  all  the  middle  Huronian  (?) 
rocks  and  to  have  conglomerate  at  its  base.  The  thickness  of  tliis  quartzite,  as  shown  by 
the  drilling,  is  not  more  than  50  feet.  Its  attitude  is  not  definitely  known,  but  from  the  way 
it  lies  over  all  the  earlier  formations  it  is  beUeved  to  be  not  much  tilted.  No  exposures  of  the 
formation  are  recognized  as  sucli.  It  seems  to  remain  simply  as  a  residual  patch  in  the  deeper 
part  of  the  trough  where  protected  from  erosion.  However,  some  of  the  quartzite  on  the 
so-called  Baraboo  ridges  may  be  upper  Huronian  ( ?)  rather  than  middle  Huronian  ( ?) .  Still 
more  recently  red  slate  has  been  found  above  tliis  upper  Huronian  (?)  quartzite.  . 

PALEOZOIC  SEDIMENTS. 

The  Paleozoic  rocks  consist,  from  the  base  upward,  of  the  Potsdam  sandstone,  the  "Lower 
Magnesian"  Umestone,  the  St.  Peter  sandstone,  and  the  Trenton  hmestone.  The  Potsdam 
sandstone  occurs  on  the  lower  flanks  of  the  quartzite  ranges  and  in  the  valley  bottom;  the 


362  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

succeeding  formations  lie  at  higher  elevations.  The  Paleozoic  beds  rest  horizontally  uj)on  the 
more  or  less  folded  Huronian  beds,  a  conspicuous  basal  conglomerate  marking  the  great  uncon- 
formit}'. 

QUATERNARY  DEPOSITS. 

Pleistocene  deposits  cover  about  the  northeast  half  uf  tlie  district.  (See  Chapter  ^Yl, 
pp.  427-459.) 

THE  IRON   ORES   OF  THE  BARABOO   DISTRICT. 

l»y  the  authors  and  W.  J.  Mead. 

OCCURRENCE. 

The  iron-bearing  beds,  which  are  a  part  of  tiic  Freedom  dolomite,  have  been  productive 
thus  far  on  the  south  limb  of  the  basin.  They  dip  northward  at  angles  ranging  from  .50°  to 
70°.  The  foot  wall  is  Seeley  slate;  the  hanging  wall  is  cherty  dolomite,  with  small  amounts 
of  slate  and  iron  carbonate.  The  iron-bearing  member  itself  consists  of  ferruginous  chert, 
iron  carbonate,  ferruginous  slate,  and  iron  ore.  There  is  a  gradation  from  this  member  into 
both  hanging  and  foot  walls.  It  is  thin,  for  the  most  part  not  more  than  200  feet  thick,  and 
the  productive  ore  bodies  are  still  thinner,  20  to  30  feet.  The  ores  stand  as  lenses  arranged 
end  for  end  or  overlapping  paraUel  to  the  layers  of  chert.  These  have  been  found  by  drilling 
at  a  maximum  depth  of  1,500  feet,  but  mining  operations  do  not  yet  go  beyond  500  feet.  Their 
lateral  extent  has  been  found  to  be  at  least  2,000  feet.  Deep  drilhng  down  the  dip  discloses 
minor  folds.  Also  to  the  south  of  the  main  outcrop  the  ore  may  be  repeated  by  an  additional 
minor  fold. 

The  iron-bearing  member  has  been  found  also  on  the  north  side  of  the  basin,  where  it 
stands  almost  vertical  or  dips  south,  but  so  far  it  is  nonproductive  here. 

Only  one  mine  has  operated  to  the  present  time,  the  Illinois  mine  (see  fig.  55),  although 
three  other  shafts  are  now  being  sunk. 

CHEMICAL  COMPOSITION. 
The  following  is  a  complete  analysis  of  the  Baraboo  ore : " 

Chemical  composition  of  the  Baraboo  ore. 

Ferric  oxide  (FejOs) 88.  62 

Ferrous  oxide  (FeO) 92 

Alumina  (AUOj) 68 

Manganese  monoxide  (MnO) 26.5 

Silica  (SiO,) K06 

Lime  (CaO) 12 

Magnesia  (MgO) None. 

Titanium  oxide  (TiOo) None. 

Sulphur  (S) Trace. 

Chromium  oxide  (CrjOj) None. 

Water  at  110° 21 

Water  at  red  heat 55 

Carbon  in  carbonaceous  matter 04 

Carbon  dioxide  (CO,) 51 

Phosphoric  oxide  (P2O5) 004 

99.  979 
Total  iron 62.  75 


o  Weldman,  Samuel,  The  Baraboo  iron-bearing  district  of  Wisconsin:  Bull.  Wisconsin  Geol.  and  Nat.  Hist.  Survey  No.  13,  1904.  p.  12S. 


BARABOO   IRON   DISTRICT.  363 

A  commercial  analysis  showing  the  average  grade  shijDped  for  1007  is  as  follows: 

Partial  analysis  showing  average  grade  of  ore  shipped  for  1907. 

[Sample  dried  at  212°  F.] 

Fe 53.47 

P 043 

StO, .' 18.  51 

Mn 22 

Moisture U.  36 

An  average  of  1,517  analyses,  representing  4,814  feet  of  driUing  in  the  iron-bearing  member 
away  from  the  available  ores,  gives  36.40  per  cent  of  iron.  This  includes  both  the  lean  jaspers 
and  the  partly  altered  jaspers  but  not  the  ores.  Because  of  their  great  mass  compared  with 
the  ores,  they  represent  nearly  the  general  average  composition  of  the  entire  iron-bearing  member. 

MINERALOGICAL  CHARACTEB.o 

The  Baraboo  iron  ore  is  mainly  red  hematite  with  a  small  amount  of  hydrated  hematite. 
There  are  also  small  amounts  of  iron  carbonate  in  isomoiphous  combination  with  varying 
amounts  of  manganese,  calcium,  and  magnesium  carbonate.  Next  to  hematite  in  abundance 
is  quartz  or  chert,  which  occurs  either  in  bands  in  the  ore  or  somewhat  uniformly  distributed 
throughout  the  ore.  Chlorite,  mica,  and  kaolin  also  occur  in  the  ore  in  vaiying  but  usually 
small  quantities. 

The  vein  material  in  the  ore  is  to  a  very  large  extent  quartz,  to  a  small  extent  calcite  or 
ferrodolomite,  and  to  a  very  small  extent  iron  sulphide  and  iron  oxide.  The  quartz  of  the 
veins  has  the  usual  interlocking  granitic  texture  of  vein  quartz  and  is  generally  very  much 
coarser  than  the  finelj^  granular  cherty  quartz  in  the  ore  and  in  the  banded  ferruginous  chert. 
The  carbonate  of  the  veins  is  also  much  coarser  than  the  carbonate  of  the  beds. 

PHYSICAL  CHARACTER. 6 

The  common  phases  of  the  Baraboo  ore  are  soft  granular  ore,  hard  banded  ore,  and  hard 
Uue  ore.  The  soft  granular  phases  generally  carry  the  highest  percentage  of  iron,  the  banded 
and  hard  blue  ore  containing  usually  a  larger  amount  of  silica.  The  ore  in  its  prevailing  aspects 
is  more  like  the  hard  varieties  of  ore  of  the  old  ranges  of  the  Lake  Superior  district  than  the 
soft,  hydrated  hematite  ore  of  the  Mesabi  district. 

SECONDARY  CONCENTRATION. 

Structural  conditions. — The  circulation  of  waters  in  this  district  is  controlled  by  the  imper- 
vious foot-wall  slate  on  the  one  hand  and  the  impervious  dolomite  on  the  other.  The  zone 
between  is  a  narrow  one.  The  shaft  of  the  Illinois  mine  (see  fig.  55)  goes  down  in  the  foot-wall 
slate.  In  walking  from  the  shaft  in  the  drift  toward  the  ore  body  one  notes  the  conspicuous 
dryness  of  the  slate  as  contrasted  with  the  extreme  wetness  of  the  drift  where  it  passes  through 
the  iron-bearing  member.  Water  is  circulating  at  the  present  time  through  the  iron-bearing 
member  with  great  rapidity.  The  point  of  escape  of  the  waters  is  not  known;  neither  is  it 
possible  to  tell  what  the  depth  of  circulation  has  been.  Ores  have  been  found  to  a  depth  of 
1,500  feet,  but  the  deep  ores  were  not  so  rich  as  those  at  the  surface.  The  Baraboo  quartzite 
ridges  control  the  major  circulation.  The  ores,  however,  are  a  considerable  distance  from  the 
foot  wall  of  these  ridges  on  a  comparatively  flat  area,  although  the  hanging  walls  are  usually 
in  still  lower  ground. 

Original  character  of  the  iron-hearing  member. — The  iron-bearing  member  was  at -least  in 
larger  part  iron  carbonate,  as  shown  by  the  residual  iron  carbonate  into  which  the  member 
grades  below,  but  it  may  have  consisted  also  in  part  of  banded  ferric  hydrate  and  chert. 

o  Weidman,  Samuel,  op.  cit.,  pp.  127-128.  l>  Idem,  p.  127. 


364 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


Mineralogical  and  chemical  changes. — The  alterations  of  tlie  iron  carbonate  have  been 
accomplished  through  the  usual  processes  as  described  on  earlier  pages.  All  stages  of  altera- 
tion arc  to  be  observed  and  all  criteria  for  determining  these  alterations  are  known  to  be 
present.  Weidman  believes  that  the  iron  ore  of  the  Baraboo  district  was  originally  a  deposit 
of  ferric  hydrate,  or  limonite,  formed  in  comparatively  stagnant  shallow  water  under  condi- 
tions similar  to  those  existing  where  bog  or  lake  ores  are  being  formed  to-day,  and  that  J^hrough 
subsequent  changes  long  after  the  iron  was  deposited  as  limonite,  while  tiic  member  was 
deeply  buried  below  the  surface  and  subjected  to  heat  and  pressure,  the  original  limonite 
became  to  a  large  extent  dehydrated  and  changed  to  hematite,  and  that  therefore  its  structural 

relations  are  not  j)rimarily  con- 
trolled by  the  necessity  of  later 
water  circulation. 

Though  this  district  is  widely 
separated  from  the  principal  Lake 
Superior  ranges  and  may  have 
the  different  origiji  outlined  by 
Weidman,  its  close  similarity  in 
lithology  and  stnicture  to  the 
Lake  Superior  ranges  is  believed 
to  be  a  priori  evidence  of  simi- 
larity in  origin.  The  theorj*  of 
origin  of  the  Lake  Sujierior  ores 
adequately  explains  the  origin  of 
the  Baraboo  ores  and  is  combated 
by  no  facts  yet  shown  in  the 
Baraboo  district.  Moreover,  recent  deep  drilling  has  shown  an  abundance  of  original  iron 
carbonate.  Certainly  development  work  has  not  been  nearly  sufficient  in  the  Baraboo  dis- 
trict to  warrant  any  conclusions  at  variance  with  those  for  the  older  Lake  Superior  ranges 
at  the  present  time. 


Shale  bed  -V^ 


^V//////////Ai 


-— ^  -— •—  /'//Second  /eve/ 


FlOUBE  55.— Vertical  section  of  liilnoismine.    (After  Weidman,  Bull.  Wisconsin  Geol.  and 
Nat.  Hist.  Survey  No.  13, 1904,  fig.  1,  pi.  15.) 


WATERLOO   QUARTZITE   AREA. 

The  mapping  of  the  Waterloo  quartzite  at  Portland,  Hubbleton,  Mudlake,  and  Lake 
Mills  (see  fig.  53)  by  Buell  °  and  subsequently  by  J.  H.  Warner  *  shows  that  the  outcrops  of 
this  quartzite  have  a  distribution  and  stiiicture  such  as  to  suggest  that  they  represent  part 
of  a  great  eastward-pitching  syncline  of  quartzite.  The  quartzite  is  lithologically  almost 
identical  with  the  Baraboo  quartzite  and  its  synclinal  axis  has  the  same  direction  as  the  axis 
of  the  Baraboo  syncUne.  There  is  little  reason  to  doubt  that  the  Baraboo  and  Waterloo 
quartzites  are  of  the  same  age.  If  this  is  the  case,  one  would  expect  to  find  slate  and  ferru- 
ginous dolomite  formations  within  the  Waterloo  quartzite  syncline,  as  in  the  Baraboo  syn- 
cline,  but  drilling  has  thus  far  failed  to  locate  them.  Like  the  Baraboo  quartzite,  the  Waterloo 
quartzite  is  referred  to  the  Huronian,  and  its  similarity  with  the  middle  Huronian  is  emphasized. 
Well  drilling  outside  of  the  Waterloo  syncline  shows  the  presence  of  a  granite  basement. 


a  Buell,  I.  M.,  Geology  of  tlie  Waterloo  quartzite  area:  Trans.  Wisconsin  Acad.  Sci.,  vol.  9,  1893.  pp.  255-274. 

i  Warner,  J.  n.,  The  Waterloo  quartzite  area  of  Wisconsin:  Unpublished  bachelor's  thesis,  Dept.  Geology  Univ.  Wisconsin,  1904. 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION.  365 

FOX   RIVER   VALLEY." 

Several  small  isolated  outcrops  of  pre-Cambrian  crystalline  rocks  project  through  the 
PAleozoic  sediments  in  the  Fox  River  valley  at  Berhn,  Utley,  Waushara,  Marquette,  Montello, 
Observatory  Hill,  Marcellon,  and  Endeavor.  (See  fig.  53,  p.  359.)  The  rocks  are  mainly  acidic 
extrusives;  metarhyolites,  showing  gradation  mto  rocks  of  more  deep-seated  origin;  rhyolite 
gneiss;  quartz  rhyolite;  and  granite,  all  of  them  cut  by  basic  dikes.  The  characteristic  fea- 
ture in  the  metarhyolites  is  the  presence  of  abundant  and  well-jircserved  surface  volcanic 
textures,  such  as  fluxion,  perlitic,  spherulitic,  and  brecciated  textures.  The  hthologic  simi- 
larities of  the  rocks,  the  presence  of  the  surface  textures,  and  their  composition,  as  shown 
by  analysis,  indicate  clearly  their  consangumity  with  one  another  and  with  certain  of  the 
igneous  rocks  on  the  north  and  south  sides  of  the  Baraboo  range.  In  the  Baraboo  district 
these  rocks  have  been  found  by  Weidman  ''  to  lie  unconformably  below  the  sedimentary  rocks, 
and  hence  the  volcanic  rocks  of  Fox  River  may  be  supposed  to  be  pre-Huronian. 

a  Hobbs,  W.  H.,  and  Leith,  C.  K..  The  pre-Cambrian  volcanic  rocks  of  the  Fox  Eiver  valley,  Wisconsin:  Bull.  Univ.  Wisconsin  No.  158  (Sci. 
ser.,  vol.  3,  No.  G),  1907.  pp-  247-27S. 

b  Weidman,  Samuel,  The  Baraboo  iron-bearing  district  of  Wisconsin:  Bull.  Wisconsin  Geol.  and  Nat.  Hist.  Survey  No.  13,  1904,  p.  21. 


CHAPTER  XV.     THE  KEWEENAW  AN  SERIES." 
GENERAL  CHARACTERISTICS. 

The  Keweenawan  is  tlie  upper  series  of  the  Aljjonkian  sj^'stem  in  the  Lake  Superior  region. 
Its  most  cliaraeteristic  feature  is  that  its  abunchint  effusive  rocks  are  as  widespread  as  the  series 
itself.  Indeed,  they  probal)Iy  compose  from  a  third  to  a  lialf  of  the  series.  The  Keweenawan 
contrasts  with  the  Huronlan  in  that  in  tlie  latter  scries  tiie  efTusive  rocks  are  largely  concen- 
trated m  a  number  of  localities,  although  in  these  areas  they  may  he  of  veiy  great  thickness. 
In  short,  the  Keweenawan  was  a  period  of  regional  volcanic  activity  and  the  Huronian  was  a 
period  of  local  volcanism.  It  results  from  these  facts  that  in  the  earliest  studies  of  the  Kewee- 
nawan the  igneous  rocks  were  noted  and  described.  In  the  Huronian,  on  the  other  hand,  the 
sediments  were  more  conspicuous  and  were  especially  studied  in  the  early  years,  and  it  is  only 
recently  that  the  extent  and  magnitude  of  the  igneous  rocks  of  that  period  have  been  appreciatctl. 

In  the  following  discussion  of  tlie  Keweenawan  no  attempt  will  ho  made  to  give  detailed 
petrographic  descriptions.  The  most  salient  petrographic  features  will  be  mentioned,  and  a 
review  of  the  petrography  and  chemistry,  with  reference  to  nomenclature,  \\'ill  be  presented  by 
A.  N.  Wmchell.  In  order  to  give  a  somewliat  more  definite  impression  of  the  series,  the  more 
important  districts  will  be  briefly  described. 

DISTRIBUTION. 

The  Keweenawan  rocks  border  the  major  part  of  the  shore  of  the  western  half  of  Lake 
Superior,  occupy  islands  in  the  eastern  half,  and  are  found  on  the  mainland  at  the  extreme  east 
end  of  tlie  lake.  They  extend  to  a  maximum  distance  of  120  miles  northwest  of  Lake  Superior. 
To  the  southwest  Keweenawan  rocks  have  been  penetrated  by  drills  at  Stillwater,  and  still 
farther  southwest,  at  St.  Paul  and  vicinity,  certain  red  sandstones  have  been  drilled  which 
may  be  Keweenawan.  On  the  south  side  of  the  lake  they  occur  mainly  witliin  12  miles  of  the 
shore.  Sandstones,  Keweenawan  or  Cambrian,  are  known  also  at  the  east  end  of  the  Felch 
Mountain  trough.  This  distribution  shows  that  tliis  series  once  occupied  the  greater  portion 
of  the  Lake  Superior  basin  and  from  it  extended  for  varying  distances.  In  much  of  the  basin 
at  present  the  Keweenawan  rocks  are  overlain  by  Cambrian  sandstone. 

The  total  present  exposed  area  of  the  Keweenawan  rocks  is  approximateh"  15,000  square 
miles.  To  obtain  tlie  original,  area  there  must  be  added  a  very  large  but  unknown  portion  of 
the  Lake  Superior  basin.  Further,  there  must  be  added  the  numerous  masses,  large  and  small, 
of  the  rocks  of  Keweenawan  age  intrusive  into  the  Huronian  and  Archean  of  the  Lake  Superior 
region.  Irving ''  estimated  the  area  of  the  Keweenawan,  aside  from  the  rocks  intrusive  in 
older  series,  at  41,000  square  miles. 

It  is  thus  evident  that  Lake  Superior  in  Keweenawan  time  was  an  aiea  of  regional  activity 
extending  east  and  west  for  more  than  400  miles  and  north  and  south  for  scarcely  a  less  distance. 

SUCCESSION. 

A  broad  study  of  the  several  Keweenawan  districts  leads  to  the  conclusion  that  a  threefold 
division  of  the  seiies  as  a  whole  may  be  made,  beginning  at  tlie  bottom,  as  ft)lIows:  (1)  Lower 
Keweenawan,  comprising  conglomerates,  sandstones,  dolomitic  sandstones,  shales,  and  marls; 

a  For  further  detailed  description  of  the  Keweenawan  rocks  of  the  Lake  Superior  region  see  Mon.  C  S.  Geol.  Survey,  vol.  5,  and  references 
there  ^iven.    In  the  descriptions  of  the.  several  districts  accounts  of  local  features  of  the  Keweenawan  are  given. 
^  Irving,  K.  D.,  The  copper-bearing  rocks  of  Lake  Superior:  Mon.  U.  S.  Geol.  Survey,  voL  5,  1SS3,  p.  27. 

366 


THE  KEWEENAWAN  SERIES.  367 

(2)  middle  Keweenawan,  comprising  extrusive  and  intrusive  igneous  rocks  with  Important 
amounts  of  interstratified  sandstones  and  conglomerates  and  subordinate  amounts  of  shale; 
and  (3)  upper  Keweenawan,  comprising  conglomerates,  sandstones,  and  shales,  represented 
in  northern  Wisconsin  and  Michigan.  In  only  one  district,  nortliern  Wisconsin  and  Michigan, 
is  the  full  succession  found.  In  the  area  of  Black  and  Nipigon  bays  and  Lake  Nipigon,  in 
Minnesota,  and  at  the  east  end  of  Lake  Superior  the  lower  and  middle  Keweenawan  appear. 
On  Isle  Koyal  the  upper  and  middle  Keweenawan  occur,  and  on  Michipicoten  Island  only  the 
middle  Keweenawan  is  found. 

BLACK  AND  NIPIGON  BAYS  AND  LAKE  NIPIGON. 
LOWER  KEWEENAWAN. 

The  rocks  belonging  to  the  lower  Keweenawan  occupy  the  peninsida  between  Thunder 
and  Black  bays  and  the  neck  between  Nipigon  and  Black  bays  from  the  northwest  corner  of 
Nipigon  Bay  to  a  point  20  miles  west  of  Black  Sturgeon  River.  They  consist  of  quartzose 
sandstones,  dolomitic  sandstones,  and  red  marls.  According  to  Logan **  their  thickness  is  from 
800  to  900  feet.     Bell,  however,  estimated  it  from  1,.300  to  1 ,400  feet.     Bell's  section'  is  as  follows : 

Section  of  lower  Keweenawan  rocks  near  Black  and  Nipigon  bays. 

Feet. 

Alternating  red  and  white  dolomitic  sandstone,  with  a  red  conglomerate  layer  at  the  bottom, 
occurring  on  Wood's  location,  Thunder  Cape"^ 40 

Light-gray  dolomitic  sandstone,  with  occasional  red  layers  and  spots  and  patches  of  the  same 
color.  These  sandstones  occur  along  the  southwest  side  of  Thunder  Bay  and  on  Wood's  loca- 
tion d 200 

Red  sandstones  and  shales,  interstratified  with  white  or  light-gray  sandstone  beds,  frequently 
exhibiting  ripple-marked  surfaces,  and  also  with  conglomerate  layers  composed  of  pebbles  and 
'  *  bowlders  of  coarse  red  jasper  in  a  matrix  of  white,  red,  or  greenish  sand 500 

Compact  light-reddish  limestones  (some  of  them  fit  for  burning  into  quicklime),  interstratified 
with  shales  and  sandstones  of  the  same  color , 80 

Indvu'ated  red  and  yellowish-gray  marl,  usually  containing  a  large  proportion  of  the  carbonates  of 
lime  and  magnesia. «  This  di\dsion  runs  through  the  center  of  the  peninsula  between  Thunder 
Bay  and  Black  Bay,  and  may,  in  this  region,  have  a  thickness  of  350  feet  or  more 350 

Red  and  white  sandstones,  with  conglomerate  layers,  the  red  sandstones  being  often  very  argil- 
laceous and  variegated  with  green  spots  and  streaks,  and  having  many  of  their  surfaces  ripple- 
marked.  These  rocks  are  found  all  along  the  northwest  side  of  Black  Bay  as  far  up  as  the 
township  of  McTa\-ish 200 

There  are  no  lavas  interstratified  with  the  Black  Bay  and  Nipigon  Bay  rocks,  but  at 
numerous  places  they  are  cut  by  diabase  dikes  similar  to  those  which  cut  the  upper  Huronian 
(Animikie  group) . 

The  lower  Keweenawan  occurs  on  the  shore  of  the  southwestern  part  of  Lake  Nipigon  in 
relatively  small  areas  and  irdand  from  Lake  Nipigon  in  a  large  area  of  which  Black  Sturgeon 
Lake  is  the  center.  This  division  is  called  the  Nipigon  formation  by  Wilson./^  It  comprises 
basal  conglomerates  which  rest  unconformably  upon  the  Archean,  sandstones,  shales,  and 
dolomites — green,  ferruginous,  and  white.  For  this  area  Wilson  gives  the  succession,  in 
descending  order,  as  follows: 

Section  of  lower  Keweenaioan  rods  in  Nipigon  basin. 

Feet. 

Dolomites  and  dolomitic  shales 400 

Grits  and  sandstones 150 

Basal  conglomerate 4-6 

o  Logan,  W.  E.,  Report  of  progress  to  lS(i3,  Geol.  Survey  Canada,  18fj3,  p.  70. 
t  Bell,  Robert.  Report  of  progress  from  1800  to  18C9,  Geol.  Survey  Canada,  1870,  p.  319. 

c  Macfarlane  finds  the  red  sandstone  to  contain  12.5  per  cent  of  carbonate  of  lime  and  11  percent  of  carbonate  of  magnesia. 
d  Macfarlane  found  them  to  contain  13  percent  ofcarl)onate  of  lime  and  12  percent  of  carbonate  of  magnesia. 

c  The  amount  varying,  in  the  specimens  analyzed  by  Macfarlane,  from  21  to  34.5  per  cent  of  the  carbonate  of  lime,  and  from  7.5  to  13.5  per  cent 
of  the  carbonate  of  magnesia. 

/Wilson,  A.  W.  G.,  Geology  of  the  Nipigon  basin,  Ontario:  Canada  Dcpt.  Mines,  Geol.  Survey  Branch,  Memoir  No.  1,  1910,  pp.  69-70. 


368  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Nothing  is  said  by  Wilson  as  to  the  dips  of  the  lower  Keweenawan  rocks,  but  it  is  apparent 
from  liis  descriptions  that  they  are  relatively  flat. 

The  district  al)ovo  described  is  of  interest  as  being  the  only  district  in  wliich  the  accu- 
mulation of  detrital  material  before  the  outbreak  of  the  Keweenawan  lavas  covers  any 
considerable  area.  It  is  believed  that  these  rocks  really  represent  the  first  deposits  of  the 
transcressin"'  Keweenawan  sea  and  antedate  the  igneous  epoch  of  the  Keweenawan  altogether. 
The  absence  of  material  derived  from  the  Keweenawan  lavas  led  some  of  the  earlier  geologists — 
for  instance,  Macfarlane"  and  Hunt'' — to  question  whether  these  rocks  really  belong  with 'the 
Keweenawan.  These  lower  Keweenawan  rocks  pass  under  the  middle  Keweenawan  diabases 
and  amygdaloids,  which  form  the  southern  half  of  the  peninsula  southwest  of  Black  Bay.  On 
the  north  they  are  overlain,  according  to  Bell,'^  by  columnar  trap. 

MIDDLE  KEWEENAWAN. 

Aside  from  the  area  occupied  by  the  lower  Keweenawan  sediments  the  remainder  of  the 
Black  and  Nipigon  bays  and  Lake  Nipigon  district  is  occupied  by  the  middle  Keweenawan, 
consistmg  of  basic  igneous  rocks  with  subordinate  amounts  of  interstratified  clastic  material. 
These  igneous  rocks  are  partly  flows  and  partty  intrusions. 

BLACK  AND  NIPIGON  BAYS  AND  ADJACENT  ISLANDS. 

Black  and  Nipigon  bays  are  noted  for  their  conspicuous  and  interesting  topography,  wliicli 
has  originated  in  essentially  the  same  way  as  the  topography  of  Thunder  Bay.  In  both  locahties 
the  sediments  are  interleaved  with  great  sills  of  diabase,  sedimentary  and  igneous  rocks  ahke 
being  in  nearly  horizontal  attitude. 

The  rocks  of  the  middle  Keweenawan  constitute  the  shores  of  the  outer  parts  of  Black  and 
Nipigon  bays  and  of  the  adjacent  islands,  including  those  from  the  size  of  St.  Ignace  to  small 
rocks,  and  from  the  shore  they  extend  considerable  but  varying  distances  inland.  Over  large 
areas  these  rocks  present  f  acies  which  are  similar  to  those  of  the  Beaver  Bay  area  of  the  Mimiesota 
coast,  described  on  pages  371-374.  Locally  they  show  spheroidal  weathering,  as  at  Fluor  Island. 
They  are  cut  by  red  rock,  which  metamorphoses  the  diabase  to  an  orthoclase  gabbro,  just  as  on 
the  Minnesota  coast.  The  sediments  are  subordinate.  In  places  the  diabase  clearly  intrudes 
the  sediments  and  locally  the  latter  are  somewhat  modified  at  the  contact,  the  color  changmg 
toward  the  intrusive  rock  from  red  to  gray  or  white. 

For  the  most  part  the  dip  of  the  rocks  of  the  areas  of  Black  and  Nipigon  bays  is  very  gentle, 
here  m  one  direction  and  there  in  another,  but  near  the  shore  of  Lake  Superior  there  is  the  usual 
gentle  and  persistent  lakeward  slant  of  8°  to  10°.  Locally,  however,  the  dips  go  up  to  20°  or  30°, 
to  60°  or  70°,  or  even  to  the  vertical.  These  steep  dips  occur  at  places  where  the  diabases 
intrude  the  sediments  or  the  amygdaloids,  and  thus  disturb  their  normal  attitudes. 

LAKE  NIPIGON. 

The  middle  Keweenawan  igneous  rocks  extend  tliroughout  the  Lake  Nipigon  district,  except 
in  the  areas  of  the  lower  Keweenawan  already  mentioned.  They  occupy  about  half  of  the 
shore  line  on  the  east  and  north  sides  of  Lake  Nipigon,  where  they  mainly  constitute  the  pen- 
insulas and  headlands.  North  of  the  lake  they  extend  40  miles  or  more  to  the  Hudson  Bay 
divide.  They  occupy  all  the  hundreds  of  islands  of  the  lake,  varying  in  size  from  those  which 
are  several  miles  long  and  wide  to  those  which  are  mere  rocks. 

The  midtlle  Keweenawan  of  Lake  Nipigon  consists  mainly  of  great  masses  of  diabase, 
which  Wilson  says  are  in  sheets  and  dikes,  and  with  these  are  later  acidic  dikes.  English  Bay 
is  an  area  of  granite  porphyry,  which  Wilson  places  with  the  Archean,  but  which,  it  may  be 
suggested  from  the  association,  may.  belong  with  the  Keweenawan. 

0  Macfarlane,  Thomas,  Canadian  Naturalist,  new  ser.,  vol.  3, 180S,  p.  2S2;  vol.  4, 1809,  p.  38. 

b  Hunt,  T.  S.,  Special  report  on  the  trap  dikes  anil  Azaie  rocks  of  southern  Pennsylvania,  pt.  1:  Kept.  E,  Second  Geol.  Survey  rennsyl\-ania, 
1878,  p.  241. 

c  Bell,  Robert,  Report  of  progress  from  180C  to  1809,  Geol.  Survey  Canada,  1870,  p.  338. 


THE  KEWEENAW  AN  SERIES.  369 

There  has  been  much  discussion  as  to  whether  the  great  diabase  sheets  are  intrusive  or 
extrusive  rocks.     Wilson"  summarizes  the  evidence  in  favor  of  extrusiqn  as  follows: 

1.  The  very  widespread  occurrence  of  unconformities  between  diabase  sheets  and  underlying  formations. 

2.  The  occurrence  of  bowlders  of  granite  and  gneiss  and  schist  in  diabase,  the  latter  resting  on  similar  rocks  in  situ 
in  localities  where  there  is  direct  evidence  that  before  the  advent  of  the  trap  the  underlying  rocks  were  buried  beneath 
the  sediments  similar  to  those  now  present,  near  by,  under  the  same  diabase  sheet. 

3.  The  occurrence  of  old  soils  in  situ  at  the  bases  and  on  the  sides  of  sedimentary  ridges,  the  whole  being  covered 
in  places  with  a  diabase  cap. 

4.  The  nicety  of  the  adjustment  by  which  the  diabase  sheets  have  fitted  themselves  to  the  underlying  topography. 
MTiile  the  upper  surfaces  of  the  residuals  of  the  capping  sheets  are  everywhere  fairly  uniform  in  height,  the  base  of  the 
sheet  has  adju.sted  itself  to  a  topography  where  the  relief  was  at  times  as  much  as  300  feet. 

5.  The  mechanical  problem  which  arises  in  explaining  the  numerous  unconformities,  especially  those  on  the 
embossed  Archean  surface,  by  the  theory  of  intrusion  vanishes  completely  on  the  theory  of  surface  erosion  prior  to 
surface  extrusion. 

6.  The  features  characteristic  of  the  upper  surface  of  sills — the  occurrence  of  overlying  beds  or  fragments  thereof, 
aphanitic  structures,  included  fragments  of  an  old  cover  in  the  upper  parts  of  sheets — are  not  found. 

7.  The  medium  to  coarse  texture,  which  characterizes  the  sheets,  would  be  found  at  the  base  of  thick  surface 
flows  as  well  as  in  sills,  being  dependent  not  on  the  nature  and  thickness  of  the  cover  so  much  as  on  the  rate  of  cooling. 

8.  A  glassy  matrix,  amygdaloidal  or  porous  structure,  basaltic  texture,  flow  structure,  and  associated  volcanics 
would  not  be  characteristic  features  of  the  under  parts  of  surface  flows,  and  the  ujiper  parts  of  these  sheets  are  unques- 
tionably removed,  without  a  single  exception. 

In  favor  of  intrusive  sills  are: 

1.  Entire  absence  of  any  of  those  features  that  are  usually  associated  with  the  upper  parts  of  a  surface  flow — glassy 
matrix;  amygdaloidal,  porous,  or  basaltic  texture;  flow  structure;  associated  volcanic  rocks,  either  lava  breccias  or 
pyroclastic  rocks. 

2.  A  medium  to  coarse  crystalline  texture,  usually  indicative  of  a  slow  rate  of  cooling,  such  as  would  normally 
take  place  only  at  some  considerable  distance  below  the  surface. 

From  the  evidence  presented  Wilson  draws  the  following  conclusions :  * 

It  seems  that  we  have  no  data  relative  to  the  actual  character  of  the  upper  surface  of  the  trap  "caps;"  such  negative 
evidence  as  is  available  is  equally  applicable  to  both  theories.  With  regard  to  the  texture  of  the  residual  basal  por- 
tions of  the  sheets  there  are  no  recorded  differences  which  would  indicate  that  it  belonged  to  a  flow  and  not  to  a  sheet. 
On  the  other  hand,  numerous  unconformities  exist,  and  the  diabases  are  known  to  rest  successively  upon  Laurentian, 
Keewatin,  Huronian  (possibly  middle,  certainly  lower,  and  Animikie),  and  Keweenawan  (lower,  middle,  and  upper 
beds),  and  these  unconformities  are  very  widely  distributed.  Owing  to  the  mechanical  difficulties  involved  by  any 
other  interpretation  it  seems  to  the  writer  that  the  balance  of  evidence  available  is  distinctly  in  favor  of  considering 
these  capping  sheets  as  the  basal  residuals  of  a  once  very  extensive  flow  or  series  of  flows  of  a  very  fluid  diabase  over 
the  well-dissected  topography  of  a  previous  cycle. 

It  may  be  suggested  in  this  case,  as  in  so  many  others,  that  the  diabases  of  the  Keweenawan 
sheets  are  not  exclusively  intrusive  or  extrusive. 

It  has  heretofore  been  the  prevailmg  view  that  the  cappmg  diabases,  so  characteristic  of 
the  step  topography  of  the  Animikie  area  and  of  the  Keweenawan  area  on  the  northwest  side 
of  Lake  Superior,  are  sills  down  to  which  erosion  has  worked.  Wilson  has  held  that  some  are 
not  sills  but  are  flows  upon  an  old  erosion  surface.  His  conclusion  Ihat  the  flows  are  as  late  as 
Cretaceous  rests  on  very  slender  evidence — that  is,  on  the  identification  of  the  plane  on  which 
the  flows  rest  as  of  post-Cretaceous  age.  He  presents  no  evidence  to  show  that  the  flows  are 
not  Keweenawan  or  some  of  them  even  Animilde.  The  view  that  they  are  Keweenawan  is 
favored  by  their  petrologic,  areal,  and  structural  relations  with  known  Keweenawan  rocks  of 
the  northwest  and  south  sides  of  the  Lake  Superior  basm. 

RELATIONS  OF  THE  KEWEENAWAN  OF  BLACK  AND  NIPIGON  BAYS  TO  OTHER 

ROCKS. 

As  the  sediments  of  Black  and  Nipigon  bays  are  at  the  bottom  of  the  Keweenawan  series 
their  relations  to  the  underl_yTJig  rocks  are  important.  At  the  very  base  of  the  series  occur 
conglomerates  the  debris  of  wMch  is  derived  from  the  underlying  Huronian  series,  including  the 

a  Wilson,  A.  W.  G.,  Geology  of  the  Nipigon  basin,  Ontario:  Canada  Dept.  Mines,  Geol.  Survey  Branch,  Memoir  No.  1,  1910,  pp.  94-95. 
'  Idem,  pp.  95-96. 

47517°— VOL  52—11 24 


370  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Animikie  group,  showing  that  there  is  an  unconformity  between  the  normal  sediments  making 
uf)  the  earhpst  Kcweenavvan  and  the  latest  Iluronian.  One  of  the  best  exposures  of  this  uncon- 
formity is  at  a  cliff  adjacent  to  Surprise  Lake,  a  short  distance  from  Silver  Islet  village.  Here 
in  actual  contact  with  the  slates  of  the  Animikie  group  is  a  conglomerate  about  G  feet  in  thickness, 
which  is  largely  composed  of  angular  fragments  of  slates  from  the  Animikie  with,  however, 
detritus  from  granites,  mica  schists,  vein  quartz,  etc.,  but  no  fragments  of  any  of  the  Keweena- 
wan  lavas.  The  contact  between  the  conglomerate  and  slate  is  knifelike  in  shar[)ness.  Locally 
the  matrix  of  the  conglomerate  is  limestone.  The  conglomerate  grades  upward  into  wliite 
qiiartzite  interstratified  with  slaty  layers,  over  wliich  are  bands  of  red  and  white  dolomite. 
Here,  as  is  common  between  the  Keweenawan  and  Animikie,  the  discordance  is  shown  mainly 
by  the  conglomerate  and  not  by  an  important  difference  in  dip,  but  in  a  number  of  places  the 
conglomerate  cuts  across  the  slate  bands  in  a  minor  way. 

Other  very  satisfactory  contacts  between  the  Keweenawan  and  Animikie  are  those  in  a  cut 
of  the  Canadian  Pacific  Railway  about  a  mile  west  of  Loon  Lake  and  at  the  south  shore  of 
Deception  Lake.  Here  the  conglomerate  of  the  Keweenawan  resting  upon  the  Animikie  con- 
tains bowlders  as  much  as  2  feet  in  diameter.  At  the  railway  cut  the  phenomena  are  very 
similar  to  those  at  Surprise  Lake,  but  at  Deception  Lake  the  Animikie  rocks  have  been  some- 
what sharply  folded,  and  the  conglomerate  rests  horizontally  upon  the  truncated  beds  of  the 
Animikie. 

The  debris  of  the  Keweenawan  conglomerate  at  these  localities  includes  the  slates  from  the 
underlying  Animikie,  material  from  the  iron-bearing  formation  of  the  Animikie,  and  granites 
and  scliists  from  the  lower  Huronian  or  Archean.  At  all  these  localities  the  completely  indu- 
rated pebbles  of  the  Animikie  as  compared  with  the  much  less  cemented  Keweenawan  are 
notable.  Tliis,  combined  \vith  actual  discordance,  would  indicate  an  important  time  break 
between  the  two  series,  an  inference  wMch  is  confirmed  by  the  relations  of  the  two  in  the 
Penokee-Gogebic  district. 

According  to  Wilson,  in  the  Nipigon  basin  diabases  rest  unconformably  on  the  Keweena- 
wan, Animikie,  and  Archean  rocks. 

NORTHERN  MINNESOTA. 
THE  KEWEENAWAN  AREA. 

The  Keweenawan  rocks  of  northern  Minnesota  he  in  a  great  crescent-shaped  area,  opening 
lakeward,  extending  from  Fond  du  Lac,  on  St.  Louis  River,  at  the  southwest  to  Grand  Portage 
Bay  at  the  northeast.     Both  the  lower  and  the  middle  Keweenawan  are  represented. 

This  area  of  Keweenawan  rocks  is  undoubtedly  the  largest  continuous  area  of  the  series. 
It  covers  approximately  4,500  square  miles. "^  As  yet  tliis  great  region  has  been  too  insufficiently 
studied  to  jiermit  a  satisfactory  account  of  it,  and  many  points  remain  doubtful.  Granites  and 
diabases  intrusive  into  the  Animikie  of  the  Cuyuna  and  St.  Louis  River  areas  are  probably  of 
Keweenawan  age. 

LOWER  KEWEENAWAN. 

The  lower  Keweenawan  is  represented  by  the  Puckwunge  conglomerate.  AccorcUng  to 
Winchell,''  tliis  conglomerate  is  seen  in  various  locahties  at  tlic  top  of  the  .Vnimikie  group  from 
Grand  Portage  Island,  in  Grand  Portage  Bay,  as  far  west  as  the  middle  of  R.  3  E.,  a  distance  of 
about  20  miles.  He  states  that  the  basal  rock  of  the  Keweenawan  is  a  conglomerate  which 
grades  up  into  sandstone.  The  thiclcness  of  the  conglomerate  is  not  determined,  but  tliis  forma- 
tion is  just  what  one  would  expect  between  the  Animikie  group  and  the  Keweenawan  series 
from  the  character  of  the  lower  division  of  the  Keweenawan  about  Black  and  Nipigon  bays. 
Winchell  "^  also  states  that  a  (|uartzite  conglomerate  which  he  regards  as  Puckwunge  occurs  in 

a  Elftman,  A.  H.,  The  geology  of  the  Keweenawan  area  in  northeastern  Minnesota:  Am.  Geologist,  vol.  21, 1898,  p.  175. 
6  Winchell,  N.  H.,  The  geology  of  Minnesota,  vol.  4, 1899,  pp.  307, 327,  517-519;  vol.  5, 1900,  pp.  50-52. 
c  Idem,  vol.  4,  p.  13. 


THE  KEWEENAWAN  SERIES. 


371 


sec.  1,  T.  48  N.,  R.  16  W.,  on  St.  Louis  River,  and  that  its  total  tliickness  is  nearly  100  feet. 
There  are,  however,  rare  pebhles  of  Keweenawan  rocks  in  this  formation.  It  is  conformable 
below  the  younger  beds.  The  pebbles  of  this  conglomerate  are  largely  derived  from  the  quartz 
veins  of  the  slates  of  the  underlying  Animikie,  and  the  conglomerate  therefore  lies  unconform- 
ably  on  the  Animikie.  The  formation  grades  into  a  white  sandstone  and  then  into  a  shale. 
Thus  the  sechmentary  formation  is  seen  at  the  base  of  the  Minnesota  Keweenawan  at  both  the 
northeast  and  the  southwest  ends.  In  the  intervening  stretch  of  more  than  100  miles  the  exact 
base  of  the  sedimentary  or  volcanic  Keweenawan  has  not  been  traced  because  of  lack  of  expo- 
sures and  because  of  the  intrusion  of  the  great  Duluth  gabbro  to  be  mentioned  later. 

MIDDLE   KEWEENAWAN. 

The  middle  Keweenawan  rocks  comprise  all  of  the  Keweenawan  in  Minnesota  except  the 
relatively  insignificant  Puckwunge  conglomerate.  They  represent  the  volcanic  epoch  of  the 
Keweenawan.  Broadly  the  middle  Keweenawan  of  northeastern  Minnesota  may  be  divided 
into  two  great  divisions — (1)  the  effusive  rocks  and  the  associated  sediments  and  (2)  the  intru- 
sive rocks. 

EFFUSIVE  BOCKS. 

The  effusive  rocks  occupy  the  larger  part  of  the  Minnesota  coast  and  extend  for  varying 
distances  inland.  The  Minnesota  coast  line,  looked  at  as  a  whole,  presents  a  flat  crescentic 
shape,  with  the  concavity 
toward  the  lake.  The  same  is 
true  of  the  courses  of  the  effu- 
sive rocks,  but  the  crescents 
formed  by  them  have  a  smaller 
radius  and  hence  intersect  that 
formed  by  the  coast  line,  trend- 
ing more  to  the  north  at  the 
Duluth  end  and  more  to  the 
east  at  the  Grand  Portage  end. 
In  following  the  coast,  then, 
from  Duluth  to  Grand  Portage, 

we  ascend  in  geologic  horizon  to  a  point  near  Two  Islands  River  and  descend  from  a  point 
just  east  of  Temperance  River  to  Grand  Portage. 

These  rocks  consist  dominantly  of  a  well-stratified  series  of  volcanic  flows  having  a 
gentle  lakeward  dip,  winch  commonly  is  from  8°  to  10°  but  locally  is  as  low  as  5°  or  6°  and 
as  high  as  25°  or  30°,  or  rarely  even  45°  or  60°.  Numerous  minor  bowings  and  corrugations 
may  be  seen  in  the  incUvidual  layers  and  sets  of  layers,  which  may  be  followed  for  some  miles. 
These  may  be  seen  rising  into  arches,  locally  of  short  span,  and  sinking  into  synclines  to  reappear 
as  anticlines  a  short  distance  away. 

The  lavas  are  diabases  which  are  commonly  amygdaloidal.  Many  of  these  amygdaloids 
are  very  scoriaceous.  These  rocks  are  softer  than  the  intrusive  rocks  and  are  especially  Ukely 
to  constitute  the"  bays.  There  are  subordinate  masses  of  intermediate  rocks,  wliich  usually 
have  not  been  separated  on  the  maps  from  the  basic  flows.  At  one  place,  east  of  Kadonces 
Bay,  tins  intermediate  rock  has  a  peculiar  spheroidal  weathering  similar  to  that  of  the  Ely  green- 
stone, a  structure  which  has  been  regarded  as  evidence  of  subaqueous  extrusion. 

Associated  with  the  basic  lavas  are  masses  of  acidic  lavas  represented  by  quartz  jaor- 
phyrites  and  felsites.  One  of  the  more  notable  locahties  for  these  rocks  is  the  great  Palisades 
(fig.  56). 

The  conglomerates  and  sandstones  interstratified  with  the  lavas  are  subordinate  in  amount. 
In  the  lower  part  of  the  series  they  are  either  absent  altogether  or  are  represented  by  very  thin 
beds.     In  the  upper  part  of  the  series,  especially  the  portion  to  which  Irving  "■  has  given  the 


Water  line 


Figure  56. — Section  on  south  cliff  of  Great  Palisades,  Minnesota  coast.  (After  Irving.)  a, 
Amygdaloid;  6,  columnar  diabase-porphyrite;  c,  mingled  amygdaloid  and  detrital  matter; 
d,  quartz  porphyry. 


"  Irving,  B.  D.,  The  copper-bearing  rocks  of  Lake  Superior:  Mon.  U.  S.  Geol.  Survey,  vol.  5, 1883,  pp.  323-329. 


372  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

name  Temperance  River  group,  the  sandstone  and  conglomerate  beds  are  numerous.  Most  of 
these  beds  are  only  a  few  inches  to  a  few  feet  in  thickness,  but  there  are  some  beds  which  arc  100 
feet  tliick,  and  according  to  Elftman"  one  wliich  is  250  feet  thick.  Lawson**  estimates  that 
the  sandstones  and  conglomerates  oceupy  less  than  0.5  per  cent  of  the  coast  line. 

INTKUSIVE  BOCKS. 

The  intrusive  rocks  comprise  both  basic  and  acidic  types. 

BASIC    KOCKS. 

The  basic  rocks  include  the  Duluth  laccolith,  the  Beaver  Ba)^  and  similar  laccolitlis  and 
sills,  the  anorthosites,  and  the  dike  rocks. 

DULUTH  LACCOLITH. 

Area  and  character. — The  Dulutli  laccolith  is  a  gabbro.  It  extends  from  St.  Louis  River 
to  the  northeast,  grtuiually  widening  until  in  the  center  of  the  belt  it  is  30  miles  wide.  From 
this  maximum  breadth  it  narrows  toward  the  east  until  it  makes  a  point  at  the  Minnesota  coast. 

It  is  not  our  purpose  here  to  give  anything  more  than  a  most  general  petrographic  account 
of  the  Didutli  gabbro.  It  is,  for  the  most  part,  normal  gabljro,  but  it  has  many  facies.  Min- 
eralogically  it  ranges  from  a  very  magnetitic  gabbro  through  olivine  gabbro  hi  wliich  the  feldspar 
is  subordinate  and  ordinary  ohvine  gabbro  to  olivine-free  gabbro,  or  ordinary  gabbro,  and 
finally  to  a  rock  m  which  feldspar  is  the  dominant  mineral,  the  rock  beuig  a  labradorite  or  an 
anorthosite.  The  anorthosite  masses  vary  from  those  a  few  feet  across  to  those  liundieds 
of  feet  in  diameter.  The  anorthosite  appears  to  be  but  a  diH'erentiation  phase  of  the  gabbro, 
there  being  every  gradation  between  it  and  both  coarse  and  fine  grained  phases  of  the  main 
mass  of  the  rock.  These  relations  are  particular]}-  well  seen  at  Little  Saganaga  Lake, 
where,  accordmg  to  Clements,"  the  anorthosite  unquestionably  shows  gradations  into  the 
surroundmg  basic  masses.  Nowhere  is  there  a  sharp  line  of  contact  between  the  two  rocks. 
In  these  respects  the  occurrences  are  in  sharp  contrast  with  the  anorthosite  and  the  diabase  of 
the  Minnesota  coast,  to  be  later  described. 

Structurally  the  gabbro  is  ordinarily  massive.  However,  at  manj^  places,  especially  near  its 
borders,  it  has  a  sheeted  structure.  Some  of  the  sheets  are  verj'  tliui  and  strongly  resemble 
bedded  rocks.  This  variety  may  be  very  well  seen  in  the  north  bay  of  Basliitanequeb  Lake. 
In  addition  to  this  sheeted  structure  there  is  a  banded  structure,  due  to  the  parallel  arrangement 
of  the  mineral  constituents. 

TexturaUy  the  gabbro  varies  from  a  rock  of  very  coarse  grain  to  one  that  is  almost 
aphanitic.     All  varieties,  coarse  and  fine,  are  granulitic. 

Relations  to  other  formations. — The  structural  relations  of  the  Duluth  gabbro  are  veiy 
interesting.  On  the  north,  in  passing  from  St.  Louis  River  to  Grand  Portage,  the  gabbro  is  in 
contact  for  a  long  way  with  the  upper  Huronian,  then  for  many  miles  with  the  several  members 
of  the  lower  Huronian  and  the  Archean,  and  finally  for  many  miles  again  with  the  upper  Huronian. 
It  thus  cuts  diagonally  across  the  upper  Huronian  in  its  northern  and  southern  parts  and  in 
passing  toward  the  center  of  the  area  goes  through  the  lower  Huronian  and  deep  into  the  Archean. 

Evidence  of  its  intrusive  character  is  afforded  bj'  its  coarse  crystallization;  by  the  presence 
of  numerous  subordinate  bosses  and  dikes,  offshoots  of  the  gabbro  mass,  in  the  Huronian  series; 
by  the  inclusion  of  isolated  masses  of  upper  Hiu'onian  near  its  margin  and  the  profound  meta- 
morphic  effects  of  the  gabbro,  the  rocks  being  changed  to  schists  or  gneisses  or  even  to  com- 
pletely granular  crystalline  rocks  for  distances  up  to  half  a  mile  or  a  mile  from  the  main  gabbro 
mass,  an  effect  not  to  be  expected  from  a  rapidly  cooling  extrusive;  and  finally  by  the  higher 
density  of  the  gabbro  than  of  the  hitruded  rocks. 

a  Elftman,  A.  H.,  The  geolqgy  of  the  Keweenawan  area  in  northeastern  Minnesota:  Am.  Geologist,  vol.  21, 1898,  p.  185. 
!>  Lawson,  A.  C,  Sketch  of  the  coastal  topography  of  the  north  side  of  Lake  Superior:  Twentieth  Aon.  Kept.  Geol.  and  Nat.  Hist.  Survey 
Minnesota,  1893,  p.  190. 

c  Clements,  J.  M.,  The  Vermilion  Iron-bearing  district  of  Minnesota:  Mon.  U.  S.  Geol.  Survey,  vol.  45, 1903,  pp.  402-103. 


THE  KEWEENAWAN  SERIES.  378 

The  relations  of  the  gabbro  to  the  hivas  of  the  coast  have  not  been  satisfactorily  deter- 
mined. Their  contact  is  mainly  in  the  plateau  of  the  interior,  is  very  poorly  exposed,  and  has 
not  been  sufficiently  studied.  However,  it  is  believed  that  when  these  relations  are  worked  out 
it  will  be  found  that  the  galibro  is  intrusive  and  has  produced  profound  metamorphic  effects. 

If  this  inferred  intrusive  relation  is  confirmed,  the  Duluth  gabbro  is  a  great  laccolith,  which 
has  as  a  basement  the  Huronian  and  Archean  and  as  a  roof  the  Keweenawan  lava  flows.  The 
relations  of  the  Duluth  gabbro  to  the  Puckwunge  conglomerate  at  the  base  of  the  Keweenawan 
and  to  the  earlier  Keweenawan  lavas  have  not  been  established.  Until  this  is  done  it  is  impos- 
sible to  gain  any  definite  conception  as  to  how  far  Keweenawan  time  had  advanced  before  the 
appearance  of  the  gabbro.  If  the  Duluth  gabbro  is  interpreted  as  a  laccolith  it  surpasses  in 
magnitude  any  other  yet  described.  With  a  maximum  diameter  of  100  miles,  if  its  thickness 
has  approximately  the  ratio  shown  in  the  typical  laccoliths  of  the  Henry  Mountains,"  the  thick- 
ness would  be  75,000  feet.  If  an  average  dip  of  10°  for  50  miles  on  the  north  shore  is  assumed 
the  thickness  would  figure  45,000  feet. 

The  intrusion  of  so  vast  a  mass  of  material  must  have  required  a  long  time.  The  parts 
earlier  intruded  were  doubtless  solidified  long  before  magma  ceased  to  enter.  Thus,  offshoots 
of  these  later  parts  would  be  found  as  dikes  in  the  earlier  solidified  parts.  There  would  be  great 
variation  in  its  coarseness  of  crystallization.  Ample  time  would  be  afforded  for  differentiation 
by  fractional  crystallization,  separation  by  gravity,  and  other  processes,  and  thus  is  explained 
the  structural  complexit}^  of  the  gabbro  and  its  great  variation  in  mmeral  and  chemica 
character. 

THE  BEAVER  BAY  AND  OTHER  LACCOLITHS  AND  SILLS. 

Intruded  in  the  lavas  of  the  Minnesota  coast  are  a  great  many  laccoliths  or  sills  of  diabase. 
These  intrusive  rocks  are  especially  ])revalent  in  the  lower  pait  of  the  lavas,  and  particularly  in 
the  part  below  the  Temperance  River  group.  In  textiue  these  rocks  vary  from  diabases  to 
gabbros  and  include  the  so-called  black  gabbros  of  Irving.*  The  diabases  in  many  places  show 
a  remarkable  luster  mottling  due  to  the  inclusion  of  numerous  individuals  of  plagioclase  in  large 
individuals  of  augite.  Not  uncommonly  the  augites  are  several  inches  in  diameter  and  include 
hundreds  of  lath-shaped  feldspars. 

Many  of  these  laccoliths  and  sills  were  supposed  by  the  earlier  geologists' to  be  lava  flows, 
but  when  exammed  closely  they  are  found  to  cut  the  lava  beds  by  passing  gradually  across  their 
edges  and  by  sending  out  dike  offshoots.  In  not  a  few  places  they  show  a  distmct  columnar 
structure  at  right  angles  to  their  borders. 

The  local  steep  dips  of  the  lava  beds  mentioned  in  the  previous  section  are  apparently  all 
due  to  the  influence  of  the  intrusive  masses  and  thus  their  exceptional  character  is  explained. 

A  typical  illustration  of  these  laccoliths  is  seen  at  Beaver  Bay.  The  center  of  this  laccolith 
extends  from  a  point  near  Beaver  Bay  to  a  point  near  Two  Harbors  Bay.  In  this  distance  it 
occupies  the  entire  coast.  Neither  its  top  nor  its  bottom  is  seen.  In  this  part  it  is  not  luster 
mottled  but  is  the  coarse  black  gabbro  of  Irving.''  Its  central  part  is  sheeted  and  in  general  has 
a  coarse  or  imperfect  columnar  structure  at  right  angles  to  the  horizon  or  nearly  so.  Wliere  it 
is  foiuid  in  association  with  the  lavas  farther  east  and  west,  as  at  Split  Rock  and  Beaver  Bay, 
its  structure  corresponds  with  the  bedding  of  the  amygdaloids,  so  that  it  was  natural  for  Irving 
to  regard  it  as  a  bedded  flow,  although  even  he  recognized  that  at  some  places  it  cut  the  amygda- 
loids in  a  curious  way.  Indeed,  locally  it  cuts  the  amygdaloids  in  a  most  intricate  fashion, 
following  the  joints,  wmding  around  the  blocks,  intruding  itself  as  films  between  the  plating  of 
the  amygdaloid,  but  always  with  sharp  contacts.  It  is  a  significant  fact  that  near  the  lavas 
the  laccolith  is  luster  mottled.  Very  close  to  the  amygdaloid  it  is  locally  fine  grained.  In 
places  it  retains  its  coarse  texture,  even  in  narrow  strmgers.  The  laccoliths  and  sills,  being 
resistant  rocks,  usually  make  the  major  headlands  of  the  coast,  just  as  the  lavas  usually 
constitute  the  bays. 

a  Gilbert,  G.  K.,  The  geology  of  the  Henry  Mountains,  2d  ed.:  U.  S.  Geog.  and  Geol.  Survey  RockT,-  Mtn.  Region,  1880,  p.  55. 
4  Irving,  R.  D.,  The  copper-bearing  rocks  of  Lake  Superior:  Mon.  U.  S.  Geol.  Survey,  vol.  6, 1883,  pp.  267-268. 


374  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

The  Logan  sills  jiiul  capping  rocks  in  the  Animikie  and  Keweenawan  of  northeastern 
JVlinnosota  should  be  ])articularly  mentioned.  These  aic  doubtless  to  be  correlated  with  the 
great  gabbro  mass.  In  fact,  in  the  Gunllint  Lake  district  tliey  seem  to  l)e  directly  connected. 
To  these  sills  are  due  the  step  topography  of  tiiis  region.  Wilson  "  has  concluded  that  farther 
to  the  east  in  the  Nipigon  basin  some  of  tlie  capping  steps  instead  of  ])eing  sills  are  really 
flows.  It  is  possi})le  that  tliis  conclusion  may  be  applied  to  j)art  of  the  caj)j)ing  rocks  in  north- 
eastern Minnesota. 

.     ANORTHOSITES. 

The  anorthosites  of  the  Minnesota  coast  early  attracted  attention  because  of  their  brilliant 
light  color.  They  may  be  well  seen  at  Split  Rock,  BeaVer  Bay,  and  Carlton  Peak.  At  these 
places  a  large  portion  of  them  are  inclusions  in  the  basic  laccolitlis  and  sills,  such  as  the  BeaA'er 
Bay  laccolith.  Indeed,  at  many  jilaces  they  form  a  stucco  in  this  diabase  laccolith.  In  size 
the  inclusions  range  from  those  which  are  minute,  being  no  more  than  indiviihial  ciystals  of 
fclds])ars,  to  great  masses  .50  or  60  feet  in  diameter.  In  adtlition  to  these  masses,  wliicli  are 
plamlj'  inclusions,  there  are  other  masses  which  are  so  large  that  they  can  not  be  a.sserted  to 
be  inclusions.  These  are  mantled  by  the  Beaver  Bay  laccolitli,  as  described  by  Lawson,'  the 
relations  at  the  bottom,  however,  not  being  exposed.  Some  of  these  masses  on  the  Minnesota 
coast  are  as  large  as  a  cathedral,  and  the  largest  masses  are  found  at  Carlton  Peak,  the  different 
points  of  which  are  composed  entirely  of  anorthosite.  The  anorthosite  inclusions  are  not  con- 
tained in  tlie  central  part  of  the  Beaver  Bay  laccolith,  but  in  its  upper  part,  where  it  is  in 
contact  with  or  near  the  anwgdaloids. 

The  relations  above  described  conclusively  show  that  the  anorthosite,  as  a  rock,  antedated 
the  including  rocks.  Lawson  '^  has  interpreted  this  to  mean  that  the  anorthosite  marked  a 
pre-Keweenawan  terrane,  but  from  our  point  of  view  the  anorthosite  is  but  a  facies  of  the 
great  Duluth  gabbro  mass  which  had  been  segregated  before  the  diabase  intrusions  (seep.  372), 
and  therefore  has  been  included  in  the  diabase,  as  above  described. 

It  is  conjectured  that  the  very  abvmdant  diabase  laccoliths  and  sills  at  Beaver  Bay  and 
other  localities  are  but  later  offshoots  of  the  original  reservoir  of  magma  from  which  the  Diduth 
gal)bro  was  also  derived.  The  alliance  between  the  diabase  intrusives  of  the  coast  and  the 
Duluth  gabbro  is  shown  by  their  chemical  and  mmeralogical  likeness. 

BASIC  DIKES. 

Diabase  dikes  cut  the  lavas  and  sills  at  numerous  places.  As  a  rule  they  are  nearly  vertical. 
Many  of  them  lie  approximately  at  right  angles  to  the  coast,  and  are  likely  to  make  projections 
into  the  water.  Others  run  approximately  parallel  to  the  coast.  These  dikes  conform  to  the 
sets  of  strike  and  dip  fractures  which  were  produced  by  the  deformation.  Commonly  these 
diabase  dikes  are  less  than  50  or  60  feet  across.  At  some  places  they  have  a  columnar  stracture 
at  right  angles  to  the  walls,  parallel  to  the  bedding  of  the  lavas,  and  consequently  at  right 
angles  also  to  the  coluimiar  structure  of  the  laccoliths. 

ACIDIC    KOCKS. 

Along  the  northwest  shore  of  Lake  Superior  and  back  from  the  coast  are  many  areas  of 
acidic  rocks,  collectively  mapped  as  red  rock,  because  of  their  jirevaUing  red  color.''  The  red 
rock  consists  of  intrusives,  mainly  granite  and  augite  sA'enite,  and  their  equivalent  elTusives, 
quartz  porphyry.  These  are  later  than  the  associated  basic  extrusive  and  intrusive  rocks, 
succeeding  the  Duluth  gabbro  and  the  diabase  of  the  Beaver  Bay  laccolith.  The  red  rocks 
range  in  size  fi-om  considerable  masses  to  minute  stringers.  In  many  places  the  intrusives 
intricately  cut  tiie  basic  rocks.  This  is  well  illustrated  at  Beaver  Bay,  where  both  the  amygda- 
loidal  lavas  and  the  diabase  are  intruded.     Dikes  of  the  red  rock,  great  and  small,  cut  the  diabase 


a  Wilson,  A.  W.  G.,  Geology  of  the  Nipigon  basin:  Canada  Dept.  Mines,  Geol.  Survey  Branch.  Memoir  No.  1.  1910,  pp.  95-%. 

i>  I.awson,  .\.  C,  The  anorthosites  of  the  Minnesota  coast  of  Lake  Superior:  Bull.  Geol.  and  Nat.  Hist.  Survey  Minnesota  No.  8,  1S93.  p.  1& 

(■Idem,  p.  19. 

d  Klftmau,  \.  11.,  The  geology  of  the  Keweena*an  area  in  northeastern  Minnesota:  Am.  Geologist,  vol.  22,  1898,  pi.  7. 


THE  KEWEENAWAN  SERIES.  375 

through  and  through,  and  have  produced  an  important  exomorphic  effect.  Where  thus  altered 
the  diabase  grades  into  a  rock  of  a  somewliat  more  acidic  aspect  and  becomes  the  ortlioclase 
gabbro  of  Irving. °  Wherever  we  have  seen  tliis  rock  it  is  but  a  facies  of  the  diabase,  produced 
through  the  minute  penetration  of  the  acidic  magma  of  the  red  rock.  It  is  clear  that  the 
chemical  com])osition  of  the  diabase  has  been  affected  by  minute  penetration  of  the  acidic 
magma  and  its  emanations. 

KEWEENAWAN     ROCKS     IN     THE     CUYUNA     DISTRICT     OF     NORTH-CENTRAL 

MINNESOTA. 

Granite,  diabase,  and  gabbro  cut  the  slates  of  the  Animikie  in  the  great  north-central  area 
of  Minnesota,  including  the  Carlton,  Cloquet,  Cuyima,  and  Little  Falls  areas.  Being  later  than 
the  Animikie,  they  are  probably  to  be  correlated  with  the  Keweenawan  intiiisive  rocks  of  north- 
eastern Mimiesota.  They  are  probably  to  be  regarded  as  the  plutonic  equivalents  of  the  Kewee- 
nawan flows.  In  the  Cuyima  district  there  is  also  a  thin  layer  of  amygdaloidal  acidic  rock,  15 
feet  thick,  resting  upon  the  eroded  edges  of  the  slates  and  iron-bearing  formation  of  the  Animikie 
group.  Drilling  in  this  district  discloses  many  masses  of  basic  and  acidic  rock  intricately  asso- 
ciated with  the  slates  of  the  Animikie,  but  the  relations  are  not  yet  determined. 

THICKNESS   OF  THE  KEWEENAWAN   OF  MINNESOTA. 

Irving,*  in  his  monograph  on  the  copper-bearing  rocks  of  Lake  Superior,  makes  a  formal 
division  of  the  Keweenawan  of  the  Minnesota  coast  into  six  groups,  for  which  he  estimates 
thicknesses  as  follows,  from  the  top  down: 

Feet. 

Temperance  River  group 2, 500-3, 000 

Beaver  Bay  group 4, 000-6, 000 

Agate  Bay  group 1, 500 

Lester  River  group 2,  600 

Duluth  group 5,  000 

St.  Louis  gabbro  [now  called  Duluth  gabbro] Thickness  uncertain. 

Excluding  the  gabbro,  Irving'^  estimates  the  total  thickness  to  be  between  17,000  and 
IS, 000  feet.  It  is  to  be  remembered  that  these  estimates  of  thickness  include  large  masses  of 
intrusive  rocks,  as,  for  instance,  the  Duluth  gabbro  and  the  diabase  of  Beaver  Bay.  Also 
it  is  far  from  certam  that  the  lavas  on  the  Minnesota  coast  have  the  regidarity  of  superposition 
supposed  bj'  Irving.     Finally,  it  is  imcertain  what  part  of  the  present  dip  of  the  lavas  is  initial. 

Elftman,  the  one  other  geologist  who  has  made  an  extensive  study  of  the  Keweenawan  of 
the  Minnesota  coast,  gives  the  following  order:'' 

1.  Later  diabase  member. 

2.  Temperance  River  member. 

3.  Red  Rock  member. 

4.  Beaver  Bay  diabase  member. 

5.  Gabbro  member. 

This  is  the  structural  order.  It  is  clear  that  the  order  is  only  partly  one' of  age,  for  before 
the  gabbro  and  other  laccoliths  and  sills  could  be  intruded  in  the  Keweenawan  a  certain  amount 
of  sediments  and  lavas  must  have  been  buUt  up.  This  succession,  as  well  as  that  of  Irving,* 
ignores  the  Puckwimge  conglomerate. 

Elftman  supposed  that  between  the  "Temperance  River, member"  and  the  "Red  Rock" 
member  there  is  a  considerable  unconformity,  because  at  the  bottom  of  the  "Temperance 
River  member"  is  a  conglomerate  100  feet  thick.  This  conglomerate  contains  fragments  of 
diabase  similar  to  the  diabase  of  Beaver  Bay,  and  also  many  fragments  of  red  rock,  indicating 

a  Irving,  R.  D.,  The  copper-bearing  rocks  of  Lake  Superior:  Men.  U.  S.  Geol.  Survey,  vol.  5, 1883,  pp.  50  et  seq. 

(■  Idem,  pp.  200-268. 

<■  Idem,  p.  260. 

<i  Elftman,  A.  H.,  The  geology  of  the  Keweenawan  area  in  northeastern  Minnesota:  Am.  Geologist,  vol.  21, 1S9S,  pp.  1S3-185. 


376  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

tluit  tliese  lavas  were  formed  before  the  deposition  of  the  "Temperance  River  member."  As 
the  "Temperance  River  member"  is  cut  by  otlicr  diabase  dikes  and  ])y  red  rocks,  liowever, 
there  is  no  reason  to  behove  tliat  the  sui)posed  unconformity  is  diileicnt  from  tliat  marked 
elsewhere  in  the  Keneenawan  by  the  appearance  of  considerable  beds  of  sediments.  The  vol- 
canic epoch  had  not  ceased. 

In  view  of  the  great  uncertamty  as  to  the  exact  succession,  relations,  initial  dips,  and 
faulting  of  the  Minnesota  rocks,  it  is  almost  impossible  to  give  any  estimate  of  their  thickness. 
Probably  if  the  lavas  and  sediments  only  were  considered,  tlie  tliickness  woidd  be  very  much 
less  than  tlie  amount  that  Irving  "  mentioned,  but  if  the  thickness  of  tlie  intrusive  rocks, 
includin"  the  Duluth  gabbro,  were  computed,  and  this  added  to  the  thickness  of  the  extrusives, 
an  amount  vastly  in  excess  of  20,000  feet  would  be  ol)tained. 

NORTHERN  WISCONSIN  AND  EXTENSION  INTO  mNNESOTA. 

DISTRIBUTION. 

The  Keweenawan  rocks  of  northwestern  Wisconsin  and  their  extension  into  Minnesota 
include  an  area  estimated  at  over  5,000  square  miles.  The  greater  extent  of  the  area  is  m  a 
northeast-southwest  direction.  At  the  southwest  end  the  Paleozoic  strata  make  a  deep  embay- 
ment,  thus  partly  dividing  the  area  mto  two  belts  crossing  St.  Croix  River.  Farther  to  the 
southwest  the  Keweenawan  has  been  found  by  deep  drilhng  at  Stillwater,  and  it  is  not  impossible 
that  the  red  sandstone  fomid  at  St.  Paul  and  to  the  southwest  may  belong  to  the  same  (hvision. 
Granites  of  probable  Keweenawan  age  occupy  a  considerable  area  in  the  Florence  district  of 
northeastern  Wisconsin. 

STRUCTURE. 

Tlie  Keweenawan  area  of  northeastern  Wisconsm  is  a  synchnorium,  the  axis  of  which 
extends  southwest  from  Chequamegon  Bay  and  at  its  southwest  end  bends  more  to  the  south. 
On  the  northeast,  in  the  vicinity  of  Ashland  and  Clinton  Point,  the  work  of  Thwaites*  m  1910 
has  disclosed  minor  folding  and  possibly  faultmg  m  tins  synchnorium,  the  steeper  dips  of  the 
minor  folds  being  to  the  north.  On  the  southwest  end  of  the  district  in  Minnesota,  along  St. 
Croix  River,  the  work  of  Grout <^  has  disclosed  similar  complexity  of  structure. 

The  synclinorium  is  bordered  for  its  entire  length  on  the  north  by  a  fault  agamst  wliich 
the  Cambrian  is  faulted  down.  The  fault  plane  dips  38°  to  45°  S.  It  dies  out  in  Ba3-ficld  County. 
The  Lake  Superior  sandstone  beds  are  buckled  along  the  contact.  It  is  not  known  to  what 
extent  the  movement  has  been  vertical  or  horizontal,  although  striations  m  at  least  one  place 
point  to  a  vertical  movement.  The  net  result  in  any  case  has  been  to  bring  the  traps  up  over 
the  Cambrian  or  Lake  Superior  sandstone.  Muior  dip  faults  have  been  noted  m  northern 
Iron  Coimty  similar  to  those  on  Black  River  m  Micliigan. 

The  dip  of  the  upper  as  well  as  the  lower  divisions  of  the  Keweenawan  is  as  high  as  90°,  but 
averages  about  70°  to  80°  at  the  east  end  of  the  southern  part  of  this  area.  In  the  bottom  of  the 
synchnorium,  as  has  been  noted,  a  series  of  muior  rolls  show  dips  up  to  90°  on  the  north  hmbs 
and  as  low  as  25°  on  the  south  sides,  while  in  the  Apostle  Islands  to  the  north  the  overlying  quartz 
sandstone  (Lake  Superior)  dips  about  1°  to  5°  SE.  To  the  west,  along  the  sjmchne,  the  dips 
become  less  until  inclmations  of  only  15°  occur,  but  in  Minnesota  much  liigher  ones  are  recorded. 
On  the  north  hmb  in  Douglas  County  are  foimd  dips  of  30°  to  70°  S. 

LOWER  KEWEENAWAN. 

At  only  one  place  in  northern  Wisconsm  is  the  lower  Keweenawan  known  to  be  e.xposed. 
This  is  in  the  southeastern  portion  of  sees.  11  and  12,  T.  45  N.,  R.  1  W.,west  of  a  small  lake. 
At  this  point  there  is  a  considerable  mass  of  coarse  conglomerate,  the  pebbles  of  which  are  mostly 

"  Irvin.  n.  D.,  The  copper-bearing  rocks  of  Lake  Superior:  Mon.  V  S.  Gcol.  Siurey,  vol.  5.  1SS3,  p.  266. 

ft  Tliwaites.  l'\  T..  nnpul)lislied  fiel<i  noles  for  Wisi-oiisiii  <  !eol.  and  Nat.  Hist.  Survey.  1910. 

c  Grout.  !•'.  F.,  Com  ril>ut  ion  to  the  petrography  of  the  Keweenawan:  Jour.  Geology,  vol.  IS,  1910,  pp.  63S-657. 


THE  KEWEENAWAN  SERIES.  377 

white  quartz,  some  of  them  being  8  or  10  inches  in  diameter.  FUnt  and  black  hornstone  pebbles 
are  also  plentiful.  This  conglomerate  gradesup  into  a  coarse  quartzite,  and  this  mto  a  fine-grained 
compact  quartzite.  Immediately  to  the  north  of  the  latter  formation  are  the  basic  flows  of  the 
middle  Keweenawan,  and  400  or  500  feet  south  of  the  conglomerate  are  upper  Huronian  mica- 
ceous gravAvackes.  The  thickness  of  the  conglomerate  and  quartzite  exposed  is  probably  from 
300  to  400  feet. 

The  quartzites  adjacent  to  the  Keweenawan  in  Barron.  County,  Wis.,  may  be  in  part  Kewee- 
nawan. There  are  here  at  least  two  series  of  pre-Cambrian  quartzites,  the  upper  of  which  is 
reddish,  feldspathic,  and  not  strongly  consohdated,  and  has  comparatively  low  dips.  These 
facts,  together  with  the  position  of  the  quartzites  on  the  southeast  side  of  the  Keweenawan 
syncline,  have  suggested  to  Weidman'^  the  possibility  that  they  represent  lower  Keweenawan 
sediments,  but  this  has  not  been  proved. 

MIDDLE   KEWEENAWAN. 

The  general  characters  of  the  middle  Keweenawan  in  this  region  are  substantially  the  same 
as  those  of  northeastern  Muuiesota.  The  igneous  rocks  comprise  both  plutonic  and  volcanic 
masses.  The  volcanic  series  covers  a  much  greater  area  than  the  plutonic  rocks.  At  the  sec- 
tions which  have  been  studied,  Potato  River,  Tylers  Fork,  and  Bad  River,  the  igneous  rocks, 
accorduig  to  Irving,'  consist  dominantly  of  beds  of  diabases,  diabase  amygdaloids,  and  mela- 
phyres.  With  the  basic  igneous  rocks  are  subordinate  masses  of  felsite  and  quartz  porphyry. 
Interstratified  with  the  lavas  are  subordinate  beds  of  conglomerate  and  sandstone.  Along  the 
north  side  of  the  Keweenawan  of  Wisconsiji,  in  Douglas  County,  the  lower  part  of  the  series  is 
coniposed  wholly  of  igneous  rocks,  but  at  higher  horizons  in  the  southeastern  part  of  the  district 
conglomerates  are  interstratified  with  lava  flows.  On  the  whole  the  interbedded  detrital  rocks 
of  tliis  area  are  apparently  less  abundant  than  on  Keweenaw  Pomt  but  more  abimtUxnt  than 
in  Minnesota.  The  hthology  of  the  interstratified  conglomerates  and  sandstones  is  in  no  respect 
pecuhar. 

So  far  as  we  know,  there  has  been  no  approximately  accurate  determination  of  the  entire 
thickness  of  the  lava  flows  and  interstratified  sediments  of  the  middle  Keweenawan  in  Wisconsin. 
Berkey  has  estimated  the  thickness  of  the  Keweenawan  emptive  rocks  exposed  along  the  St. 
Croix  Dalles  as  4,000  feet.  Hall"^  estimates  a  thickness  of  20,000  feet  on  Snake  and  Kettle 
rivers  in  Minnesota. 

On  the  south  side  of  the  synclme  at  the  base  of  the  Keweenawan  m  Wisconsin  is  a  great  basal 
gabbro,  which  in  every  respect  is  equivalent  to  the  Duluth  gabbro  described  on  pages  372-373. 
Tliis  gabbro  has  been  traced  from  Black  River  in  Micliigan  as  far  west  as  R.  7  W.,  but  how  much 
farther  it  extends  is  unknown.  Thus  it  has  an  extent  northeast  and  southwest  of  60  miles  or 
more.  For  most  of  the  distance  the  belt  is  from,  2  to  5  miles  broad.  The  I'ocks  of  the  mider- 
lying  upper  Huronian  along  most  of  tliis  gabbro  belt  dip  about  75°  N.  If  the  thickness  of  the 
gabbro  mass  were  calculated  at  right  angles  to  the  dip  of  the  underl;y'ing  Huronian  rocks,  this 
would  give  a  thickness  of  9,500  to  25,000  feet. 

It  has  been  explamed  in  connection  with  the  Penokee-Gogebic  district  that  this  gabbro 
cuts  diagonally  across  all  the  formations  of  the  Huronian  series  and  down  into  the  Archean; 
also  that  adjacent  to  the  contact  the  upper  Huronian  rocks  are  profomidly  metamorphosed, 
the  Tyler  slate  into  mica  slates  and  mica  schists,  the  iron-bearmg  Ironwood  formation  into 
actinolite-magnetite  schists,  the  Bad  River  hmestone  into  a  coarsely  crystalline  tremolitic  lime- 
stone. Further,  witliin  the  Huronian  and  the  Archean  are  smaller  masses  of  intrusive  gabbro 
which  doubtless  are  offshoots  or  necks  of  the  main  mass.  Thus  in  every  respect  the  relations 
of  tliis  basal  gabbro  to  the  underlying  rocks  are  the  same  as  in  northern  Minnesota. 

a  Personal  communication. 

b  Irving,  R.  D.,  Tlie  copper-bearing  rocks  of  Lalce  Superior:  Mon.  U.  S.  Geol.  Survey,  vol.  5, 1883,  pp.  230-231. 

cBerkey,  C.  P.,  Geology  of  the  St.  Croi-x  Dalles:  Am.  Geologist,  vol.  20,  1897,  p.  382. 

d  Hall,  C.  W.,  Keweenawan  area  of  eastern  Miimesota:  Bull.  Geol.  Soc.  America,  vol.  12,  1901,  p.  331, 


378  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Unfortunately  the  relations  between  the  gabbro  mass  and  the  lavas  of  the  Keweenawan 
have  not  been  closely  studied.  Irving  °  represents  this  gabbro  as  feathering  out  into  a  series  of 
points  to  the  east,  suggesting  very  strongly  its  intiusive  character.  However,  iiis  descriptions 
scarcely  correspond  to  that  distribution.     He  says:" 

The  coarse  gray  gabbros  so  largely  developed  in  the  Bad  River  country  of  Wisconsin,  at  the  base  of  the  series,  present 
the  appearance  of  a  certain  sort  of  unconformity  with  the  overlying  beds.  The.se  gabbro.s,  which  lie  immediately  upon 
the  Iluroniaii  ."latos,  form  a  belt  which  tapers  out  rapidly  at  both  ends  and  seem.s  to  lie  right  in  the  course  of  the  diabase 
belts  to  the  east  and  west,  since  these  belts,  both  westward  toward  Lake  Numakagon  and  eastward  toward  the  Montreal 
River,  lie  directly  against  the  older  rocks,  without  any  of  the  coarse  gabbros  intervening. 

The  coarseness  of  grain,  the  perfection  of  the  crystallization,  the  abrupt  terminations  of  the 
belts,  the  complete  lack  of  structure,  and  the  presence  of  intersecting  areas  of  crystalline  grani- 
toid rocks  led  Irvrng**  to  the  beUef  that  these  rocks  were  not  ordinary  lavas,  but  had  solidified 
at  a  great  depth. 

The  acidic  rocks  cutting  these  coarse  gabbros  are  clearly  intrusive. 

The  gabbro  in  Wisconsin,  Uke  the  Duluth  gabbro,  is  behoved  to  be  a  great  laccolith,  which 
was  intruded  in  Keweenawan  time  after  a  considerable  thickness  of  Keweenawan  lava  beds 
had  been  built  up,  and,  as  in  Minnesota,  it  roughly  followed  the  contact  at  the  base  of  the  Kewee- 
nawan and  penetrated  diagonally  across  the  lower  formations  as  well  as  irregularly  across  the 
Keweenawan  beds  themselves.  It  has  since  been  turned  up  at  angles  of  75°  or  80°  and  trun- 
cated by  erosion. 

Gabijro  on  the  north  side  of  the  Keweenawan  trough  in  Douglas  County,  Wis.,  is  described 
by  Grant,"  but  its  extent  has  not  been  determined.  It  dips  to  the  south  and  its  relations  to 
the  lavas  are  similar  to  those  of  the  gabbro  on  the  south  side  of  the  Douglas  County  syncline. 
It  is  faulted  on  the  north  against  the  Cambrian  rocks,  which  are  on  the  downthrown  side.  It 
dips  in  the  same  direction  as  the  Duluth  gabbro,  and  the  displacement  of  the  fault  is  in  such  a 
direction  as  to  show  that  it  may  have  been  originally  continuous  with  the  Duluth  gabbro. 

UPPER    KEWEENAWAN. 

The  upper  division  of  the  Keweenawan  in  this  area  consists  of  red  sandstones,  shales,  and 
conglomerates,  divided,  in  the  eastern  part  of  the  district,  into  several  distinct  members. 
Beginning  at  the  base  are  found  conglomerate  300  to  1,200  feet  in  thickness,  black  shales  up 
to  400  feet,  about  19,000  feet  of  red  arkose  sandstone,  grading  up  to  more  siUceous  sandstone, 
red  and  green  shales,  and  coarse  arkose.  Above  tliis  is  quartz  sandstone, somewhat  feldspathic 
at  the  base,  nearly  4,000  feet  thick,  here  called  the  Lake  Superior  sandstone.  These  beds 
appear  to  thin  rapidly  toward  the  west.     These  figures  make  no  allowance  for  initial  dip. 

RELATIONS   OF  THE  KEWEENAWAN   TO   OTHER   SERIES. 

The  only  places  at  which  the  relations  between  the  Keweenawan  and  lower  series  are 
shown  are  in  Wisconsin.  Here,  as  has  been  seen,  the  lowest  formation  of  the  Keweenawan  is 
made  up  of  conglomerate  and  coarse  sandstone  and  is  overlain  by  the  lava  flows  of  the  middle 
Keweenawan.  The  coarse  conglomerate  of  Potato  River  is  evidence  of  the  erosion  interval 
between  the  Keweenawan  and  the  upper  Huronian,  but  the  magnitude  of  the  imconformity  is 
realized  only  bj^  a  study  of  the  relations  of  the  two  along  the  strike,  which  gives  evidence  of  a 
large  amount  of  erosion  of  the  Huronian  series  before  Keweenawan  time.  The  details  proving 
the  greatness  of  this  unconformity  are  given  in  the  chapter  on  the  Penokee-Gogebic  district 
(pp.  2.34-2.35). 

As  to  the  relations  of  the  middle  Keweenawan  %\ith  the  Upper  Cambrian  sandstone  along 
St.  Croix,  Kettle,  and  Copper  rivers  (of  Minnesota),  there  is  no  difference  of  opinion.  The 
Upper  Cambrian  sandstone,  in  horizontal  attitude,  rests  upon  the  steeply  tilted  and  eroded 

a  Mon.  U.  S.  Oeol.  Survey,  vol.  5,  1883,  pp.  155-156. 
t>  Idem,  p.  144. 

c  Grant,  U.  S  ,  Preliminary  report  on  the  copper-bearing  rocks  of  Douglas  County,  Wis.:  Bull.  Wisconsin  Geol.  and  Xat.  Hist.  Survey  Xo.  6 
(2ded.),  1901,  pp.  31-32. 


THE  KEWEENAWAN  SERIES.  379 

edges  of  the  middle  Kewecnawan  rocks  and  bears  abundant  detriius  from  them.  It  is  there- 
fore perfectly  clear  that  before  the  sandstone  was  laid  down  the  middle  Keweenawan  had  been 
placed  at  its  present  angles  and  had  been  profoundly  eroded.  The  relation  is  very  well  illus- 
trated at  Taylors  Falls  on  St.  Croix  River,  where  the  Cambrian  sandstone  is  fossiliferous  and 
has  been  certainly  determined  as  of  Upper  or  Middle  Cambrian  age.  The  relations  between 
the  diabases  and  the  Cambrian  here  are  shown  by  figure  57. 

The  relation  of  the  upper  Keweenawan  feldspathic  sandstone  and  the  quartz  sandstones, 
here  called  the  Lake  Superior  sandstone,  has  long  been  a  subject  of  dispute,  but  the  discovery 
by  Thwaites  in  1910  of  outcrops  on  Fish  Creek  near  Ashland  has  thrown  new  hght  on  the 
question.  At  this  point  the  layers  are  steeply  inclined  to  the  north,  exposing  about  1,400  feet 
of  strata  and  disclosing  a  transition  between  the  red  shales,  arkose  sandstones,  and  conglom- 
erates of  the  upper  Keweenawan  and  the  Lake  Superior  sandstone.  A  deep  well  at  Ashland 
passes  into  these  red  shales  at  a  depth  of  2,670  feet. 

A  reexamination  of  Middle  River  in  Douglas  County  north  of  the  great  fault  showed  that 
the  sandstone  beds  are  inverted."  About  3,100  feet  of  strata  have  been  turned  up  by  the 
faulting,  exposing  mud-cracked  and  ripple-marked  green  and  red  shales  and  arkose  sandstones 
of  the  usual  Keweenawan  aspect,  grailing  above  into  the  Lake  Superior  sandstone  such  as  is 
found  in  horizontal  attitude  along  the  shore  of  the  lake.  On  St.  Louis  River,  Minnesota,^  a 
similar  transition  occurs  between  red  shales  and  brown  sandstones.  Clinton  Point,  where 
somewhat  quartzose  sandstones  are  found,  does  not  belong  to 
the  Lake  Superior  sandstone  but  is  the  crest  of  a  minor  anti- 
cline in  the  lower  beds.  Nearly  2,000  feet  of  similar  rocks  he 
some  distance  beneath  the  red  shales  on  Fish  Creek.  Carnt 

The  contact  with  the  flat-lying  quartz  sandstones  (Lake 
Superior  sandstone)  along  the  north  side  of  the  area  of  middle 
Keweenawan  in  Douglas  County  has  long  been  known  to  be  a 
fault.  The  best  exposures  are  on  Black,  Copper,  Amicon, 
and  Middle  rivers.  That  on  Middle  River  has  been  described 
above.     At  all  other  points  the  sandstone  is  turned  up  sharply 

.  ,  ,.  ',  ii-jii-i  i-iT  1  FiGUEE   57.— Sketch    showing    unconformable 

tor  a  short  distance  to  the  north  Ot  the  fault,  which  dips,  where  contact  between  Keweenawan  diabase  por- 

expOSed,    38°    to    45°    S.       At    all    places    the    trap    is    intensely  P^^^y  '^'"^  Cambrian  sandstone  at  Taylors 

,  .  ,    1  ,  ,  •  1    1  rp  1        ,^      T.1       1  Falls,  Minn.    (After  Strong.) 

brecciated,  but  the  sandstone  is  much  less  attected.     On  Black 

and  Amicon  rivers  the  sandstone  is  conglomeratic  for  a  few  feet  from  the  contact.    The  pebbles 

are  usually  small  and  are  not  matched  in  the  neighboring  igneous  rocks. 

Within  the  trap  breccias  are  found  large  blocks  of  sandstone.  The  view  in  the  past  has 
been  that  this  contact  was  an  unconformable  one  along  a  fault  scarp,  and  that  movement  had 
taken  place  along  the  fault  since  the  deposition  of  the  sandstone,  thus  comphcating  the  simple 
unconformable  relations.  An  alternative  view,  supported  by  considerable  evidence,  is  that  the 
conglomerate  has  been  faulted  up  by  parallel  faults  from  conglomerate  found  at  lower  horizons 
in  the  sandstone,  and  in  jiart  dragged  up  along  the  fault  plane.  The  displacement  must  be  at 
least  equal  to  the  thickness  of  the  beds  turned  up  at  Middle  River — 3.100  feet. 

The  significance  of  the  relations  of  the  Keweenawan  to  the  Lake  Superior  sandstone  is 
discussed  on  pages  415-416. 

KEWEENAWAN  GRANITES   OF  FLORENCE  COUNTY,   NORTHEASTERN 

WISCONSIN. 

The  granite  along  the  south  side  of  the  Florence  district  of  northeastern  Wisconsin  is 
intrusive  into  green  schists  which  are  interbedded  with  upper  Huronian  slates.  These  granites 
are  probably  part  of  the  same  mass  that  intrudes  the  Quinnesec  schist  of  the  Menominee 
district,  where  the  relations  are  similar.     These  granites  of  northeastern  Wisconsin,  therefore, 

"  Grant,  U.  S.,  Junction  of  Lake  Superior  sandstone  and  Keweenawan  [raps  in  Wisconsin:  Bull.  Geol.  Soc.  America,  vol.  IS.  1902,  pp.  6-9. 
'  Winchell,  N.  H.,  A  rational  view  of  the  Keweenawan:  Am.  Geologist,  vol.  16,  1895,  p.  150;  Geology  of  Miimesota,  vol.  4,  1899,  p.  15. 


380  GEOLOGY  OF  THE  LMvE  SUPERIOR  REGION. 

like  those  south  of  the  Cuj'una  district  in  central  Minnesota,  arc  to  be  regarded  as  the  phitonic 
eciuivalents  of  igneous  flows.  In  both  areas  these  plutonic  masses  have  greatlj'  metamorphosed 
the  invaded  strata. 

NORTHERN  MICHIGAN. 

DISTRIBUTION. 

The  Keweenawan  rocks  of  northern  Michigan  ()ccu])y  a  broad  belt  running  continuously 
from  Montreal  River,  the  boundary  between  Michigan  and  Wisconsin,  along  the  lake  shore  to 
the  outer  extremity  of  Keweenaw  Point  and  including  Manitou  Island  and  Stannard  Rock. 
Tills  belt  ranges  in  breadth  from  15  or  20  miles  west  of  Lake  Gogebic  to  about  6  miles  at  the 
outer  part  of  Keweenaw  Point.  Approximately  one-half  of  Keweenaw  Point  is  occupied  by 
rocks  of  the  Keweenawan  series.  The  general  strike  roughly  follows  the  coast.  In  passing 
from  the  southwest  the  strikes  gradually  change  from  about  N.  45°  E.  to  east-west,  and  at  the 
extreme  outer  part  of  the  point  the  rocks  swing  south  of  east,  here  having  a  northwesterly 
strike.  This  curved  outer  area  of  the  end  of  Keweenaw  Point  beyond  Portage  Lake  corresponds 
almost  exactly  with  the  strike  of  the  rocks.  Except  in  one  fold  in  the  Porcupine  Mountains 
the  dips  are  always  to  the  north  or  northwest. 

The  dips  of  the  middle  and  lower  divisions  are  in  general  lower  toward  the  east  end  of 
Keweenaw  Point,  the  steepest  dips  ranging  from  nearly  vertical  on  the  Gogebic  Range  to  27° 
at  the  end  of  the  point.  There  is  a  somewhat  regular  decrease  in  the  dip  of  each  of  the  sections 
in  passing  from  lower  to  higher  horizons.  The  best  illustration  of  this  is  furnished  by  the 
section  at  Black  River  in  Michigan,  which  shows  a  continuous  succession  from  the  base  of  the 
series  to  and  including  a  part  of  the  upper  sandstone.  According  to  Gordon,"  at  the  base  of 
the  series  the  dips  are  from  75°  to  78°  N.,  whereas  the  highest  strata  show  a  dip  of  about  20°  N. 
The  change  in  dip  in  passing  from  the  lower  to  the  higher  members  is  gradual.  Further  illus- 
trations are  furnished  by  the  sections  on  Keweenaw  Point;  for  instance,  at  the  Portage  Lake 
section  the  dips  of  the  lower  beds  are  as  high  as  55°,  whereas  in  the  lower  part  of  the  upper 
series  they  have  dropped  as  low  as  7°.  At  the  outer  part  of  Keweenaw  Point  the  dips  of  the 
lowest  part  of  the  series  there  exposed  are  from  51°  to  57°,  but  according  to  Hubbard,*  the  dips 
of  the  higher  beds  constituting  the  outer  front  of  the  point  do  not  average  more  than  23°. 

In  this  region,  as  in  northern  Wisconsin,  the  lower,  middle,  and  upper  Keweenawan  are 
all  represented.  The  general  characterization  which  has  been  made  for  these  divisions  (see 
pp.  376-379)  applies  to  the  northern  Michigan  area. 

The  Keweenawan  of  Micliigan  will  be  more  specifically  discussed  below. 

KEWEENAW  POINT. 

SUCCESSION  AND  CORRELATION. 

On  account  of  the  occurrence  of  great  and  valuable  deposits  of  copper  on  Keweenaw  Point, 
more  detailed  studies  have  been  made  of  this  than  of  any  other  of  the  Keweenawan  districts, 
with  the  possible  exception  of  Lsle  Royal.  Areas  which  have  been  studied  with  consiiierable 
detail  are  the  outer  part  of  Keweenaw  Point,  especially  Eagle  River,  by  Mai-^ine  ''■  and  Hubbard;  <* 
Mount  Bohemia,  by  Wright;  <^  and  the  Portage  Lake  area,  where  the  important  deposits  of 
cop])er  occur,  by  Pumpelly-'^  and  Hubliard.'^  Studies  of  intermediate  areas  have  been  less 
detailed  but  still  suflicient  for  Irving,''  Seaman,''  and  others  to  attempt  to  correlate  the  differ- 
ent formations  for  Keweenaw  Point.     (See  PI.  XXVIII.) 

o  Gordon,  W.  C,  assisted  by  A.  C.  Lane,  A  geological  section  from  Bessemer  down  Black  River:  Rept.  Michigan  Geol.  Survey  for  1906, 1907, 
p.  iKS. 

t  Michigan  Geol.  Survey,  vol.  6,  pt.  2, 1898,  p.  53. 

c  Marvinc,  A.  U.,  Ocol.  Survey  Michigan,  1869-1873,  vol.  1,  pt.  2, 1873,  pp.  47-01,  95-140. 

"1  Hubbard,  L.  L.,  Keweenaw  Point,  with  particular  reference  to  the  felsites  and  their  associated  rocks:  Geol.  Survey  Michigan,  vol.  6,  p*.  2, 
1898. 

t  Wright,  F.  E.,  The  Intrusive  rocks  of  Mount  Bohemia,  Michigan :  Ann.  Rept.  Geol.  Survey  Michigan  for  1908,  1909,  pp.  361-402. 

/  Pnmpclly,  Raphael,  Geol.  Sur%'ey  Michigan,  1S()9-1873,  vol.  1,  pt.  2, 1873,  pp.  1-46,  02-94. 

e  Irving,  R.  D.,  Copper-bearing  rocks  of  Lake  Superior:  Mon.  II.  S.  Geol.  Survey,  vol.  5,  1SS3. 

"Jour.  Geology,  vol.  15, 1907,  pp.  8SO-095. 


u  s.  afOLoan:*!.  SURVEY 

JIEQBGE  OtiS  SUirx.  QIBECtOW 


MONOGmpH  ui  mie  I 


GEOLOGIC   MAP  OF  KEWEENAW  POINT  COPPER  DISTRICT,  MICHIGAN 

Rovis.-il  I'v  A  !•:  Seaman.  Mirhigan  Coflego  oPMmes 
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THE  KEWEENAWAN  SERIES. 


381 


Below  are  given  the  successions  of  Irving °  for  the  entire  point  and  of  Hubbard''  for  the 
outer  part  of  the  point,  with  their  corrchition. 

Sections  of  rocks  on  Keweenaw  Point. 


Irving. 

Hubbard. 

12.  Eastern  sandstone. 

Keweenaw  scries. 
Upper  division: 

U.  Red  sandstone. 

10.  Black  shale  and  gray  sandstone  ("Nonesuch  belt"). 

9.  Red  sandstone  and  conglomerate  ("Outer  conglomerate"). 

Outer  conglomerate. 

Lower  division: 

S.  Diabase  and  diabase  amygdaloid,  including  at  least  one  conglomerate  belt 
("Lake  Shore  trap"). 

Lake  Shore  trap  (upper). 
Middle  conglomerate. 
Lake  Shore  trap  (lower). 

7.  Red  sandstone  and  conglomerate  ("Great  conglomerate"). 

Great  conglomerate. 

6.  Diabase  and  diabase  amygdaloid,  including  several  sandstone  belts  (Mar- 
vine's  "  Group  C  "  of  the  Eagle  River  section). 

5.  Diabase  and  diabase  amygdaloid,  including  conglomerates. 

4.  Luster-mottled  melaphjTes  and  coarse-grained  gabbros  and  diabases  ("  Green- 
stone group"). 

Ophites  and  porphyrites  witli  interbedded  conglomerates 
and  sandstones. 

3.  Diabase,   diabase  amygdaloid,  and  luster-mottled  melaphyre,  including  a 
number  of  conglomerate  beds. 

Melaphyres  and  interbedded  conglomerates. 

2.  Quartz  porphyry  and  felsite. 

(a)  Bohemia conglomerate.iLocallv  Mount  Houghton  fel- 
(6)  Melaphyre.                     I    site  replaces  a  and  5. 
(c)  Porphyrite  and  felsite  porphyrite. 

-;l 
1    1.  Diabase,  diabase  amygdaloid,  melaphyre,  diabase  porphyry,  and  orthoclase 
'•'■            gabbro,  including  also  conglomerate  beds  and  beds  or  areas  of  quartz  porphyry 
and  granitic  porphyry  (■'  Bohemian  Range  group"). 

Ophite  belt. 

Lac  la  Belle  conglomerate. 

LOWER  AND  MIDDLE  KEWEENAWAN  OF  KEWEENAW  POINT. 


ORDER    OF    EXTRUSION. 

Hubbard ■=  lias  studied  the  order  of  extrusion  for  the  outer  part  of  Keweenaw  Point.  He 
finds  the  oldest  lavas  to  be  melaphyres  and  these  are  interstratified  with  melaphyre  conglomer- 
ates. Following  the  melaphyres  are  porphyrites  and  interstratified  with  the  porphyrites  are 
porphyrite  conglomerates.  Next  come  the  felsites  and  interstratified  with  these  and  above 
them  are  the  felsite  conglomerates.  All  these  rocks  are  at  very  low  horizons.  Above  them  lies 
a  great  mass  of  melaphyres,  ophites,  and  porphyrites  with  their  various  interbedded  conglom- 
erates and  sandstones.  Still  higher  are  the  "Great"  conglomerate  and  tlie  "Lake  Shore"  trap 
with  the  "Middle"  conglomerate.  Thus  Hubbard's  studies  of  Keweenaw  Point  led  him  to  the 
conclusion  that  there  was  a  regular  order  of  extrusion  of  the  igneous  rocks — (1)  basic  melaphyres, 
(2)  intermediate  porphyrites,  (3)  acidic  felsites  and  porphyries,  and  (4)  the  upper  basic  rocks 
represented  by  melaphyres,  opliites,  porphyrites,  etc. 

PRESENCE    OF    BASIC    INTRUSIVE    ROCKS. 

Curiously  the  descriptions  of  the  basic  rocks  of  Keweenaw  Point  mention  no  interstratified 
intrusive  sills,  all  the  basic  rocks  being  assumed  to  be  flows.  However,  certain  groups,  as  for 
instance  the  greenstone  group,  are  described  as  contrastmg  sharply  with  the  rocks  above  and 
below  them.  They  contain  no  mtercalated  amygdaloidal  beds.  They  consist  of  massive  laj^ers. 
In  texture  they  vary  from  diabases  to  gabbros.  Although  this  and  other  masses  were  not  sufii- 
ciently  examined  to  make  any  positive  assertion  possible,  it  is  our  impression  that  a  large  part 
of  the  greenstone  is  an  intrusive  sill.  The  other  masses  of  rocks  which  have  been  described  as 
gabbro  or  orthoclase  gabbro,  especially  those  on  the  southwestern  part  of  the  point,  are  intrusive. 


o  Mon.  U.  S.  Geol.  Survey,  vol.  5, 1883,  PI.  XVII. 


6  Geol.  Survey  Michigan,  vol.  6,  pt.  2,  1898,  PI.  IV. 


c  Op.  eit. 


382  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION'. 

On  Mount  Bohemia  the  intrusive  gabbro  has  produced  contact  effects  on  the  invaded  ophites. 
Tlic  prolilem  of  separating  the  intrusive  basic  rocks  from  the  extrusives  remains  partly  to  be 
accompUshcd.  ' 

ACIDIC    INTRUSIVE    ROCKS. 

Hubbard's'*  studies  show  that  the  felsites  of  Bare  Hill  and  West  Pond  at  ver\'  low  horizons 
are  intrusive.  Tiic  fclsite  of  Bare  Hill,  when  mapped  in  detail,  is  seen  to  cut  across  the  beds  of 
other  rocks,  although  in  a  single  section  near  its  center  it  would  seem  to  be  interstratified.  The 
felsite  of  West  Pond  has  disturbed  the  beds  in  its  immediate  area.  They  are  broken  into  frag- 
ments and  in  places  are  even  changed  into  typical  breccias,  some  of  which  are  almost  undistin- 
guishable  from  the  conglomerates.  These  intrusive  rocks  were  perhaps  correlative  with  the 
extrusive  felsites  of  Mount  Houghton  and  others  of  approximately  the  same  age  found  at  higher 
horizons.  The  intnisive  nature  of  these  felsites  explains  the  absence  of  pebbles  derived  from 
them  ill  the  melaphj-re  conglomerates  interstratified  with  the  melaphyres  adjacent  to  and  at 
horizons  above  the  felsites.  Wliile  some  of  the  felsites  and  por[)liyries  are  extrusive,  even  these 
have  a  very  minor  extent.  This  is  very  well  illustrated  at  Mount  Houghton,  where  the  felsite 
locally  replaces  the  "Bohemia"  conglomerate  and  the  melaphyre  flow  below.  (See  preceding 
table.) 

NATURE    AND    SOURCE    OF    DETRITAL   MATERIAL. 

It  is  well  known  that  the  felsite  and  porphyry  pebbles  are  ver}-  prevalent  and  in  places 
dominant  in  the  numerous  conglomerate  beds  interstratified  with  the  basic  rocks  at  the  higher 
horizons  of  Keweenaw  Point,  and  even  in  the  "Great"  conglomerate,  "Middle"  conglomerate, 
and  "Outer"  conglomerate.  There  seems  to  be  an  enormous  amount  of  felsite  and  porphj-ry 
detritus  in  the  sediments  as  compared  with  the  known  original  areas  from  which  it  may  have 
been  derived.  Doubtless  a  part  of  the  acidic  detritus  of  Keweenaw  Point  may  have  been  derived 
from  porphyries  farther  east  and  west  than  the  point,  as,  for  instance,  those  of  the  Stannard  Rock 
area  to  the  east  and  the  Porcupine  Mountains  to  the  west.  But  also  the  lack  of  large  areas  of 
felsites  may  be  due  to  the  exceptional  erosion  to  which  they  have  been  subjected  because  of  their 
viscous  and  bunchj'  character,  which  raised  them  and  made  them  the  objects  of  excessive 
attack.  Finally,  a  considerable  portion  of  the  acidic  detritus  may  have  been  in  the  form  of 
volcanic  fragmental  material  that  was  scattered  far  and  wide  from  the  original  cones  from 
which  it  was  ejected  and  therefore  never  formed  a  part  of  any  continuous  solid  intrusion  or 
extrusion. 

Lane*  states  that  the  detritus  of  several  conglomerates,  especially  of  the  "Great"  conglom- 
erate, includes  numerous  pebbles  of  intnisive  red  rock  and  gabbro.  He  says  that  if  he  is  correct 
in  his  identification  of  the  materials  there  is  evidence  of  an  erosion  of  sufficient  magnitude  dur- 
mg  middle  Keweenawan  time  to  expose  these  plutonic  rocks  at  the  surface.  He  also  finds  agate 
pebbles  which  he  believes  to  have  formed  in  the  lavas  lower  in  the  series,  and  thus  he  concludes 
that  extensive  metasomatie  changes  have  taken  place  in  this  part  of  the  series  before  the  liigher 
interstratified  conglomerates  were  laid  down. 

VARIATIONS    IN    THICKNESS    OF    SEDIMENTARY    BEDS. 

Close  studies  of  Keweenaw  Point  show  rapid  variations  m  the  thickness  and  character  of  the 
interstratified  sedimentary  beds.  These  have  been  especially  studied  in  the  mineraUzed  area. 
Many  illustrations  coukl  be  given,  but  perhaps  one  of  the  clearest  is  that  of  the  "Great"  conglom- 
erate wliich  Hubbard  '^  says  tliins  400  to  700  feet  in  passing  from  Copper  Harbor  to  a  pouit  7 
miles  farther  east.  Not  only  do  the  beds  change  in  their  character,  but  a  single  sedimentary 
1)0(1  may  be  split  into  several  beds  separated  by  lava  flows.  Thus  m  the  Bohemia  basin  a  con- 
glomerate is  first  split  mto  two  parts  by  a  bed  of  melaphyre  and  the  lower  part  is  in  turn  split 
into  two  beds  by  a  mass  of  felsite.  The  beds  are  in  general  lenticular,  broadly  consitlered,  but 
some  of  these  lenses  ma}-  be  onh*  a  few  miles  in  length,  as  illustrated  by  the  Calumet  and  Hecla 
conglomerate. 

a  Uuhbaril,  L.  L.,  Michi;;an  Geol.  Survey,  vol.  6,  pt.  2,  1898,  pp.  35,  43. 

ti  Lane,  .\.  C  Geology  of  Keweenaw  Point:  Proc.  Lake  Superior  Min.  Inst.,  vol.  12,  1907,  p.  93. 

c  Op.  ell.,  p.  «4. 


THE  KEWEENAWAN  SERIES.  383 

FAULTS. 

Hubbard's  "  detailed  studies  of  small  areas  have  led  also  to  the  conclusion  that  the  middle 
Keweenawan  has  been  displaced  by  a  very  large  number  of  dip  faults,  the  throws  of  which,  how- 
ever, are  of  minor  extent.  These  have  been  worked  out  in  great  detail  with  reference  to  the 
melaphyre  and  melaphyre  conglomerates  at  West  Pond.  Here  are  figured  no  less  than  twelve 
cross  faults,  the  throws  of  which,  however,  are  not  sufficiently  great  to  be  traced  into  the  thick 
overlying  formations,  and  hence  they  do  not  appear  on  liis  general  map.  Similarly  Lane  ^  states 
that  there  are  a  large  number  of  small  transverse  faults  in  the  mining  district.  The  throws  of 
most  of  these  faults  are  not  more  than  2.5  feet  and  very  few  exceed  .50  feet.  However,  the 
presence  of  many  faults  at  each  of  the  two  areas  that  have  been  closely  studied  on  Keweenaw 
Point  suggests  very  strongly  that  when  like  thorough  studies  are  made  of  other  areas  on  this 
point  similar  faulting  wUl  be  found. 

In  the  mining  district  there  are  also  many  slide  faults.  According  to  Lane,"^  the  dip  of 
many  of  these  slide  faults  is  somewhat  steeper  than  the  bedding,  so  as  to  cut  diagonally  across 
the  beds  at  acute  angles.  As  to  the  direction  of  movement  along  these  dip  faults,  he  thinks  it 
is  more  commonly  down  than  up  on  the  hanging-wall  side,  for  beds  are  more  likely  to  be  cut  out 
than  repeated.  Hubbard"  described  one  very  important  slide  fault,  the  major  movement  of 
wliich,  instead  of  being  parallel  to  the  dip,  is  nearly  parallel  to  the  strike.  Tliis  is  the  fault  at 
the  top  of  the  Kearsarge  conglomerate,  whicli  is  well  illustrated  in  tlie  Central  mine.  Hubbard 
makes  a  calculation  of  throw  and  reaches  the  conclusion  that  "the  part  of  the  Keweenawan 
series  that  lies  above  the  Kearsarge  conglomerate  has  moved  from  its  original  position,  in  a 
northerly  direction,  horizontally,  about  2.7  miles,  or  along  an  inclined  plane  its  equivalent  dis- 
tance of  about  2.9  miles."  Such  a  slide  fault  as  this  approaches  the  ordinary  strike  faults,  the 
chief  difTerence  being  that  of  hade,  the  bedding  fault  having  such  a  hade  as  not  to  intersect  the 
bedding,  whereas  ordinaiy  strike  faults  do  intersect  the  bedding.  Although  the  Kearsarge  shde 
fault  is  nearly  in  the  direction  of  the  strike,  it  is  believed  to  be  probable  that  the  most  common 
direction  of  movement  in  the  faults  of  this  area  is  parallel  to  the  dip.  In  this  case  the  move- 
ments are  largely  explained  by  the  natural  adjustments  which  are  necessary  when  a  set  of  beds, 
is  folded. 

UPPER  KEWEENAWAN. 

The  upper  Keweenawan  consists,  from  the  base  upward,  of  three  members — (1)  the  "  Outer" 
conglomerate,  (2)  the  Nonesuch  shale,  and  (3)  the  Freda  sandstone. 

The  "Outer"  conglomerate  is  found  at  the  north  side  of  the  east  end  of  Keweenaw  Point 
as  far  as  Gate  Harbor,  where  it  passes  under  the  water;  it  reappears  on  the  point  some  miles 
west  of  Eagle  River  and  continues  along  to  the  point  and  westward  through  Michigan  into 
Wisconsm.  It  is  in  no  respect  different  from  the  underlying  "Great"  conglomerate  or  other 
conglomerates  interstratified  with  the  Keweenawan,  except  that,  accordmg  to  Lane,'*  it  contains 
near  its  top  fragments  derived  from  the  jaspery  and  other  Huronian  formations. 

The  Nonesuch  formation  ranges  from  a  soft,  fine-grained,  highly  argillaceous  shale  to  a  sand- 
stone. The  shale  is  predominant,  the  sandstones  bemg  mterbedded.  In  color  the  shale  is  dark 
purplish  gray  to  nearly  black  and  the  sandstone  dark  gray  to  black.  The  thin  sections  of  the 
sandstones  show  detritus  from  the  porphyries  and  other  acidic  original  rocks  of  the  Keweenawan. 
With  these  materials  in  all  the  sections  is  mingled  more  or  less  basic  detritus.  Indeed,  the  basic 
material  is  usually  abundant  and  not  uncommonly  becomes  dominant.  The  basic  material  is 
more  abundant  in  the  darker-colored  rocks.     In  these  rocks  there  is  also  a  plentiful  calcite 


a  Hubbard,  L.  L.,  Michigan  Geol.  Survey,  vol.  6,  pt.  2,  1898,  pp.  87-91. 

!>  Lane.  A.  C,  Geology  of  Keweenaw  Point:  Proc.  Lake  Superior  Min.  Inst.,  vol.  12, 1907,  pp.  83-84. 

c  Idem,  pp.  S4-85. 

d  Lane,  A.  C,  Jour.  Geology,  vol.  15, 1907,  p.  690. 


384  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

cement  filling  all  interstices  between  the  fragments.  The  basic  detritus  appears  in  the  .shape  of 
particles  of  the  basic  rocks,  showing  more  or  less  plainly  the  several  ingredients,  always  much 
altered,  and  of  particles  of  the  single  minerals — augitc,  almost  wholly  altered  to  a  greenish  sub- 
stance triclinic  feldspar,  and  magnetite.  The  formation  also  contains  materials  which  must 
have  been  contributed  by  the  Ihironian,  Kewcenawan,  and  Laurent ian  rocks.  The  Nonesuch 
shale  therefore  differs  from  the  sediments  interstratilied  with  the  Kewcenawan  m  the  greatly 
decreastid  amount  of  acidic  material,  the  abundance  of  basic  material,  and  the  presence  of  detri- 
tus derived  from  other  formations  than  the  Kewcenawan. 

The  Freda  sandstone  is  in  no  respect  different  from  the  sandstone  of  the  liCfger  areas  in 
Wisconsin,  which  are  a  continuation  of  the  sandstone  in  Michigan.  It  need  here  be  only  remarked 
that  the  materials  have  the  same  varieties  of  sources  as  the  Nonesuch' shale,  but  the  materia 
derived  from  the  basic  lavas  seems  to  be  even  more  prominent. 

BELATIONS  TO  CAMBRIAN  BOCKS. 

On  the  north  and  west  sides  of  the  Kewcenawan  the  series  nowhere  comes  into  contact  with 
the  Cambrian.     The  possible  relations  between  the  two  are  discussed  in  another  place.     (See  pp. 

415-416.) 

On  the  southeast  side  the  Kewcenawan  is  in  contact  with  the  Cambrian.  Irving  and 
Chamberlin,"  in  their  bulletin  on  this  contact,  conclude  that  the  sandstone  was  deposited  uncon- 
formably  against  an  ancient  fault  scarp  of  Kewcenawan  rocks  and  that  it  was  subsequently 
faulted  dowTi  along  the  old  fault  plane.  This  relation  is  apparently  similar  to  those  observed  at 
the  fault  on  the  north  side  of  the  Keweenawan  syncline  in  Douglas  County,  Wis.,  and  thence 
southwestward  into  Minnesota. 

MAIN    AREA    WEST    OF    KEWEENAW    POINT,    INCLUDING    BLACK    KIVER    AND 

THE  PORCUPINE  MOUNTAINS. 

A  very  detailed  study  of  the  entire  Keweenawan  section  at  Black  River  has  been  made  by 
Gordon.''  According  to  him,  this  river  shows  the  following  descending  succession,*^  the  classifi- 
cation into  middle  and  lower  Keweenawan  being  added  by  us. 

Section  of  Keweenawan  rocks  at  Black  River,  Mich. 

Upper  Keweenawan: 

I.  Upper  sandstone  lacking.  leet. 

II.  Nonesuch  formation 500 

III.  Outer  conglomerate o,  000 

5,500 

Middle  Keweenawan: 

IV.  Lake  Shore  trap,  consisting  of  five  flows,  having  from  the  top  downward  the 

following  thicknesses:   35,  35,  115,  85,  130  feet,  respectively 400 

V.  Conglomerate 350 

VI.  Mixed  eruptives  and  sedimentaries ■">,  ■'>00 

VII.  Felsite •. ^50 

VIII.  Eruptives  with  very  few  sedimentaries 2G,  000 

IX.  Mixed  eruptives  among  which  are  conspicuous  labradoritc  porphyrites 4, 800 

X.  Gabbro 200 

XI.  Melaphyres  and  labradorite  porphyrites  that  are  not  conspicuous 4, 500 

42, 200 

Lower  Keweenawan: 

XII.  Basal  sandstone 300 

48,000 

"In'ing,  R.  D.,and  Chimlwrlin,  T.  C,  Observations  on  the  junction  between  the  Eastern  sandstone  and  the  Keweenaw  series  on  Keweenaw 
Point.  Lake  Superior:  Bull.  U.  S.  Ocol.  Survey  No.  23, 18.S5. 

i>  Gordon,  W.  C,  assisted  by  A.  C.  Lane,  A  geological  section  from  Bessemer  down  Black  River:  Kept  Michigan  Geol.  Survey  tor  1900,  1907, 
pp.  .197-507. 

cidem,  p.  421. 


THE  KEWEENAWAN  SERIES.  385 

Throughout  the  Black  River  section  there  is  no  evidence  of  a  physical  break  in  the  Kewee- 
nawan.  Lane,"  because  of  the  character  of  the  formation,  suggests  that  possibly  there  might 
be  a  slight  break  at  the  base  of  the  Nonesuch  shale,  but  Gordon's  detailed  descriptions  give  no 
evidence  in  support  of  this  view. 

It  is  known  that  in  this  district  dip  faults  occur.  According  to  Gordon,*"  at  least  four  such 
faults  traverse  the  Trap  Range  north  of  Bessemer,  the  throws  of  three  of  which  are  80,  .350,  and 
1,500  feet,  the  throw  of  the  fourth  not  being  determinable.  It  is  very  likely  that  strike  faults 
occur,  for  great  strike  faults  occur  elsewhere  in  the  Keweenawan.  (See  p.  38.3.)  Though 
such  faults  have  not  been  detected,  they  may  very  readily  occur  at  any  of  the  very  numerous 
stretches  of  the  river  where  exposures  are  lacking.  The  presence  of  faults  at  these  places  is  very 
probable  because  of  •  the  brecciation  and  consequent  more  easily  erosible  condition  of  rocks 
along  fault  planes. 

In  the  Porcupine  Mountains  the  same  divisions  of  rocks  occur  as  in  Keweenaw  Point,  but 
the  order  is  only  in  a  general  way  similar  to  that  on  the  point,  the  difference  being  that  compara- 
tively high  in  the  series  are  large  masses  of  quartz  porphyry  and  felsite,  and  the  acidic  rocks  at 
these  horizons  perhaps  largely  explain  the  source  of  the  abundant  felsite,  quartz  porphyry,  and 
augite  syenite  pebbles  in  the  "Great,"  "Middle,"  and  "Outer"  conglomerates. 

In  the  Porcupine  Mountains  the  great  synclinal  basin  of  Lake  Superior,  which  controls 
the  general  dip  of  the  Keweenawan  rocks  about  the  lake,  is  disturbed  by  a  subordinate  fold, 
so  that  in  a  section  diagonally  northeast  and  southwest  across  the  mountains  the  lower  beds 
are  regarded  by  Irving  "^  as  repeated.  He  shows  a  subordinate  anticline  and  sj'ncline  between 
the  monoclinal  beds  north  of  Lake  Gogebic,  at  tlie  south  side  of  tlie  middle  division  and  the 
northward-dippmg  beds  at  tlie  lake.  This  area  is  a  forest-covered  one  in  which  the  exposures 
are  somewhat  imperfect,  and  it  is  hinted  by  Hubbard  <^  that  possibly  the  abundant  felsite  and 
porphyry  here  are  intrusive,  as  they  are  at  Bare  Hill  and  West  Pond,  and  that  the  unusual 
structure  may  be  explamed  by  these  intrusive  masses  rather  than  by  exceptional  orogenic 
movement.  This  suggestion  is  made  because  of  very  considerable  disturbances  in  the  regidar 
bedding  of  the  rocks  about  the  intrusive  felsite  of  Bare  Hill.  The  Porcupine  Mountains  are 
now  being  studied  in  detail  by  F.  E.  Wright  for  the  Micliigan  Geological  Survey,  but  the  results 
of  his  work  have  not  been  available  in  the  preparation  of  this  monograph. 

The  upper  Keweenawan  of  this  area  is  the  same  in  all  respects  as  that  described  for  Kewee- 
naw Pomt. 

THE  SOUTH  RANGE. 

Beginning  in  T.  47  N.,  R.  44  W.,  Michigan,  from  the  lower  Keweenawan,  which  there 
consists  of  diabase,  diabase  amygdaloid,  melaphyre,  and  a  few  coarse  interbedded  thin  con- 
glomerates, an  arm  projects  to  the  east  and  south  nearly  to  Gogebic  Lake  and  east  of  this 
lake  again  for  some  distance.  This  is  the  so-called  South  Range.  It  is  separated  fi-om  the 
main  range  of  the  Keweenawan  by  the  Jacobsville  or  "Eastern"  sandstone.  At  the  eastern 
point  the  South  Range  is  18  miles  south  of  the  northern  area  of  the  Keweenawan.  This  range 
varies  from  less  than  half  a  mile  to  2  miles  or  more  in  breadth.  The  rocks  of  the  South  Range 
dip  to  the  north  at  angles  of  30°  to  50°.  At  some  places  at  the  base  of  the  Keweenawan  series 
in  the  South  Range  there  is  a  coarse  sandstone.  At  other  places  the  lowest  rock  is  a  basic  lava. 
Locally  sediments  are  hiterstratified  with  the  lavas.  Thus  the  conditions  prevalent  in  early 
Keweenawan  time,  as  indicated  by  the  rocks  at  the  base  of  the  Keweenawan  of  the  South  Range, 
are  similar  to  those  of  other  tlistricts.  In  no  respects  do  these  rocks  differ  from  those  near  the 
base  of  the  Keweenawan  to  the  west.  West  of  Gogebic  Lake  the  Keweenawan  rocks  rest 
directly  upon  the  upper  Huronian.  The  western  part  of  this  belt  of  Keweenawan  rests  directly 
upon  the  Tyler  slate.  When  followed  to  the  east  it  is  seen  to  pass  diagonally  to  lower  and 
lower  horizons,  imtil  at  Sunday  Lake  it  is  in  contact  with  the  Ironwood  formation.  These 
relations  have  been  more  fully  described  in  connection  with  the  Penokee  district. 

a  Lane,  A.  C,  Jour.  Geology,  vol.  15,  1907,  p.  091.  "  Irving,  R.  D.,  Men.  U.  S.  Geol.  Survey,  vol.  5, 1883,  pp.  209-225. 

i>  Op.  oit.,  pp.  464-465.  d  Hubbard,  L.  L.,  Geol.  Survey  Micbigan,  vol.  6,  pt.  2, 1898,  pp.  5-8. 

47517°— VOL  52—11 25 


386  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

It  is  believed  that  the  separation  of  the  South  Range  from  the  mam  range  is  due  to  a  great 
strike  fault  between  the  two  which  results  in  a  repetition  of  the  beds  of  the  main  range  in  the 
South  Range. 

ROCKS  OF  POSSIBLE  KEWEENAWAN  AGE  IN  OUTLYING  AREAS. 

Certam  reddisli  feklspatliic  and  little-consolidated  sandstones  of  low  dip,  lying  uncon- 
formably  across  the  end  of  the  upper  Iluronian  of  the  Felch  Mountain  trough,  may  possibly  be 
classed  as  Keweenawan.     Similar  rocks  are  known  also  in  the  Sturgeon  trough  to  the  north. 

THICKNESS   OF  THE  KEAVEENAWAN   OF  MICHIGAN. 

Irving"  gives  an  estimate  of  the  thickness  of  the  Keweenawan  of  northern  Michigan  at 
Eagle  River  and  Portage  Lake,  and  Gordon'  estimates  a  section  on  Black  River. 

EAGLE  RIVER  SECTION.' 

Irving's  section  at  Eagle  River,''  based  largely  on  the  detailed  work  of  Marvine,  is  as  follows: 

Section  of  Keweenawan  rods  at  Eagle  River,  Michigan. 
Upper  division:  Feet. 

Outer  conglomerate;,  porphyry  conglomerate  and  sandstone;  about 1,  000 

Lower  division : 

Lake  Shore  trap;  very  plainly  bedded  fine-grained  diabases,  strongly  marked  amygda- 
loids,  and  one  or  more  thin  porphyry  conglomerates;  about 1,  500 

Great  conglomerate ;  jjorphyiy  conglomerate  and  sandstone 2,  200 

Marvine's  group  "c;"  plainly  bedded  and  separable  fine-grained  diabases,  with  strongly 
marked  amygdaloids,  predominatingly  calcitic;  and  some  850  to  900  feet,  in  all,  of  inter- 
stratified  sandstones 1,  417 

Marvine's  group  "b,"  or  the  Ashbed  group;  made  up  mostly  of  thin,  fine-grained  diabases, 
which  vary  a  good  deal  in  appearance,  but  are  generally  provided  with  distinct  amyg- 
daloids; including  some  beds  of  the  peculiar  tj'pe  known  as  ashbed  diabase;  also  several 
Bcoriaceous  amygdaloids,  being  intermingled  sandstone  and  amygdaloid;  also  one  thin 
sandstone  seam 618 

Marvine's  group  "a;"  made  up  of  relatively  heavy  beds  without  strongly  developed 

amygdaloids;  including  one  thin  seam  of  .sandstone 925 

Greenstone  group;  made  up  of  relatively  heavy  beds,  without  amygdaloids,  of  rocks  for 
the  most  part  relatively  coarse  grained;,  these  belong  mostly  to  the  coarse-grained 
olivine-free  diabases  and  gabbrosand  to  the  luster-mottled  melaphyres,  or  fine-grained 
olivine-diabases,  the  greenstone  at  the  base  of  the  group  being  of  the  last-named  class. .     1,  200 

Subgreenstone  group,  in  which  all  of  the  fissure-vein  mines  are  working;  having  at  top  a 
thin  conglomerate,  the  equivalent  of  the  "Allouez"  and  "Albany  and  Boston''  con- 
glomerates in  the  Portage  Lake  district;  composed  of  fine-grained  diabases,  with  not 
very  strongly  developed  amygdaloids;  about 1,  COO 

Central  Valley  beds;  the  layers  not  well  exposed,  but  evidently  chiefly  fine-grained  dia- 
bases and  amygdaloids,  with  a  number  of  thin  porphyry  conglomerates,  in  all  respects 
like  the  overlying  group;  about 5,  540 

Bohemian  Range  beds;  made  up  chiefly  of  diabases  and  melaphyres  in  all  respects  like 
the  higher  layers,  and  including  .some  of  the  usual  porphjTy  conglomerates;  but  also  in 
part  made  up  of  quartziferous  porphyry,  felsite,  nonquartziferous  porphyry,  and  coarse- 
grained orthoclase  gabbro;  in  all.  al)c)ut 10,  000 

26, 000 

Of  this  thickness,  the  "Great"  conglomerate  and  the  ten  conglomerates  and  sandstones  in 
"group  c"  together  constitute  3,100  feet.  These  sediments  are  all  in  the  upper  5,000  feet  of 
the  lower  Keweenawan.  The  lower  part  contains  only  a  few  seams  of  detrital  material.  The 
lower  five-si-\ths  of  the  lower  Keweenawan  for  this  section  is  therefore  almost  exclusively  vol- 
canic, and  of  the  total  lower  Keweenawan  somewhat  less  than  one-ninth  is  sediment arA",  the 
remaining  eight-ninths  being  igneous. 

"  In-ing,  R.  D.,  The  copper-bearing  rocks  of  Lake  Superior:  Mon.  U.  S.  Geol.  .Survey,  vol.  5,  1883.  pp.  lGf>-I97. 

b  Gordon,  W.  C,  assisted  by  .\.  C.  Lane,  A  geoiogica!  section  from  Bessemer  down  Black  River;  Rept.  Michigan  Geol.  Survey  for  1906,  1907, 
p.  421. 

clrvlng,  K.  D.,  op.  cit.,  pp.  lso-187. 


THE  KEWEENAWAN  SERIES.  387 

PORTAGE  LAKE  SECTION. 
At  Portage  Lake  the  section  is  as  follows: 

Section  of  Keweenawan  rocks  at  Portngc  Luke. a 

Upper  division:                                                                                                                       ■  peet. 
Larijely  covered,  but  apparently  for  the  most  part  red  shales  and  sandstone;  toward  the 
base  there  is  a  considerable  thickness  (upward  of  200  feetj  of  dark-colored,  fine-grained 
sandstone  and  black  shale,  in  which  the  usual  porphyry  detritus  is  mingled  with  more 

or  less  basic  detritus;  the  lowest  layers  are  also  conglomeratic;  in  all  about 9, 000 

Lower  division: 

Covered  space  of  some  1,200  feet,  in  which  must  be  the  equivalents  of  the  outer  trap  of 

the  eastern  part  of  Keweenaw  Point,  corresponding  to  a  thickness  of  about 500 

The  Great  conglomerate,  including  the  sandstone  and  conglomerate  at  the  Atlantic  mill 
and  conglomerate  22  on  the  south  side  of  Portage  Lake,  with  some  intervening  ex- 
posures, about I  ^  500- 

Diabase Og 

Conglomerate  21 15 

Diabase  and  amygdaloid 51 

Conglomerate  20 19' 

Diabase 100' 

Conglomerate  19 13- 

Diabase 94 

Conglomerate  18 I55. 

Diabases  and  amygdaloids 34O 

Conglomerate  17  (Hancock  West) 32: 

Diabases  and  amygdaloids;  including  the  South  Pewabic  cupriferous  amygdaloid  at  50 

feet  below  17 55O 

Conglomerate  16  (not  seen  on  south  side  of  Portage  Lake) lO- 

Diabases  and  amygdaloids;  including,  at  400  feet  above  conglomerate  15,  the  Pewabic 
cupriferous  amygdaloid  or  "lode"  so  largely  worked  for  copper  on  the  west  side  of 

Portage  Lake 900 

Conglomerate  15  (Albany  and  Boston  conglomerate  on  the  north  side  of  Portage  Lake). .  3S 

Diabases  and  amygdaloids 33O 

Conglomerate  14  (the  Houghton  conglomerate  of  the  north  shore) 2 

Diabases  and  amygdaloids 1_  400 

Conglomerate  12  (north  side  of  Portage  Lake) 3 

Diabases  and  amygdaloids 680 

Conglomerate  11 20 

Diabases  and  amygdaloids 200 

Conglomerate  10 gO 

Diabases  and  amygdaloids 46Q 

Conglomerate  9  (sandstone  seam). 

Diabases  and  amygdaloids;  including,  at  670  feet  above  conglomerate  8,  the  Grand  Port- 
age cupriferous  amygdaloid,  and  at  510  feet  the  Isle  Royal  cupriferous  amygdaloid, 

largely  worked  on  the  south  shore  of  Portage  Lake 2, 05O 

Conglomerate  8 12 

Diabases  and  amygdaloids 420 

Conglomerate  7 24 

Diabases  and  amygdaloids 260 

Conglomerate  6 3 

Diabases  and  amygdaloids IgX 

Conglomerate  5 24 

Diabases  and  amygdaloids 240 

Conglomerate  4 12 

Diabases  and  amygdaloids 1  149 

Conglomerate  3 5g 

Diabases  anct  amygdaloids 37O 

Conglomerate  2 35 

Diabases  and  amygdaloids 1  140 

Conglomerate  1 97 

Amygdaloid 14 


22, 680 


'  Irving,  R.  D.,  The  copper-bearing  rocks  of  Lake  Superior:  Mon.  U.  S.  Geol.  Survey,  vol.  5,  1S83,  pp.  194-195. 


388  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

In  tlio  above  section  of  tlio  lower  Keweenaw  an  the  thickness  of  tiie  c()n<;lomeiates  amounts 
to  2,125  feet,  leaving  11,555  feet  for  the  igneous  rocks.  Thus  the  lower  Keweenawan  is  about 
one-sixtli  sediment  and  about  five-sixtlis  igneous. 

The  Portage  Lake  section  differs  in  one  important  respect  from  the  Eagle  River  section. 
At  Portage  Lake  the  interstratified  conglomerates  extend  to  the  bottom  of  the  section,  whereas 
at  Eagle  River  the  conglomerates  and  sandstones  do  not  occui'  in  the  lower  five-sixths  of  the 
section,  tlie  thickness  of  which  as  a  whole  is  al>out  the  same  as  at  Portage  Lake. 

BLACK  RIVER  SECTION. 

In  the  Black  River  section  the  total  thickness,  according  to  Goi-don,"  is  48,000  feet. 
Irving*  estimates  the  thickness  of  the  upper  sandstone  at  Montreal  River,  a  few  miles  west  of 
Black  River,  at  12,000  feet.  This  part  of  the  section  is  absent  on  Black  River,  and  if  it  were 
ailded  to  the  Black  River  section  this  would  give  for  this  district  a  thickness  of  60,000  feet  for 
the  entire  Keweenawan  series. 

In  the  middle  Keweenawan  of  the  Black  River  section  (p.  384)  the  sediments  are  mainly  in 
the  upper  6,000  feet,  and  of  this  amount  sedunents  are  known  to  make  up  575  feet,  distributed 
as  follows: 

Feet. 

In  V,  conglomerate 350 

In  VI,  mixed  eruptive  and  sedimentary  rocks: 

Sandstone .30 

Conglomerate 20 

Sandstone 2.5 

Sandstone 30 

Sandstone 20 

Conglomerate 100 

225 

575 

As  a  space  corresponding  to  .3,000  feet  is  not  exposed,  doubtless  the  total  thickness  of  the 
sediments  is  much  greater  than  tliis,  though  a  part  of  this  .3,000  feet  is  certain  to  be  volcanic. 
However,  the  addition  of  all  of  it  would  make  the  maximum  possible  thickness  of  sediment 
3,575  feet.  Thus  the  sediments  at  most  make  up  only  about  one-twelfth  of  the  middle  Kewee- 
naw^an  and  are  largely  concentrated  in  the  upper  sixth  of  the  division. 

The  question  now  arises  whether  this  apparent  thickness  for  the  several  sections  repre- 
sents the  real  thickness  of  the  series  as  laid  down.  It  is  believed  to  be  probable  that  the  real 
thickness  is  less  than  the  apparent  thickness.  The  reasons  for  this  belief  apply  as  well  to  the 
estimated  thicknesses  of  other  districts,  and  therefore  they  are  given  later.     (See  pp.  418-419.) 

RELATIONS  OF  THE  KEWEENAWAN  OF  MICHIGAN  TO  UNDERLYING  AND 

OVERLYING  FORMATIONS. 

The  only  locality  in  which  the  relations  of  the  Keweenawan  with  the  underlying  forma- 
tions are  shown  is  in  the  Penokee-Gogebic  district.  It  has  been  stated  (pp.  234-235)  that  these 
relations  are  those  of  unconformity,  erosion  amoimting  to  several  thousand  feet  having  taken 
])lace  after  Iltironian  time  and  before  the  deposition  of  the  Keweenawan.  Still,  the  strike 
and  di|)  of  the  two  series  are  very  nearly  the  same,  and  the  greatness  of  the  break  between  the 
two  appears  only  b}"  their  stratigraphic  relations. 

The  Upper  Cambrian  ("Eastern")  sandstone  comes  against  the  lower  part  of  the  Keweena- 
wan from  the  outer  end  of  Keweenaw  Point  to  the  region  west  of  Gogebic  Lake.  It  is  agreed 
by  all  who  have  studied  this  contact  that  it  marks  a  great  fault.  The  Keweenawan  along 
the  contact  has  its  usual  steep  northern  dips.  The  sanilstone  at  the  contact  is  bent  and  locally 
broken,  so  that  it  strikes  and  dips  in  various  directions,  in  some  places  dipping  away  from  the 

o  Gordon,  W.  0.,  assisted  by  A.  C.  Lane,  A  geological  section  from  Bessemer  down  Black  River:  Kept.  Michigan  Geol.  Survey  for  1906, 1907, 
p.  421. 

l>  Irving,  U.  D.,  The  copper-bcnring  rocks  ol  L:iko  Superior:  Mon.  U.  S.  Geol.  Survey,  vol.  5,  1S83,  p.  230. 


THE  KEWEENAWAN  SERIES.  389 

Keweenawan  and  in  others  apparently  dipping  under  it.  A  short  (Hstance  away  from  the 
Keweenawan,  usually  within  a  few  hundred  feet,  the  sandstone  assumes  its  normal  horizontal 
attitude. 

At  only  a  few  localities  has  the  Upper  Cambrian  sandstone  been  found  in  close  relations 
with  the  rocks  of  the  South  Range.  Irving  '^  concluded  that  in  the  South  Range  this  sand- 
stone rests  unconformably  against  the  Keweenawan  rocks.  However,  the  particular  locality 
he  described  as  showing  unconformable  relations  has  been  interpreted  differently  by  Seaman,* 
who  finds  there  a  dike  of  igneous  rock  penetrating  the  so-called  "Eastern"  sandstone  and 
spreading  out  above.  Seaman  regards  the  "Eastern  "  sandstone  here  as  probably  Keweenawan 
and  believes  that  there  is  no  way  of  proving  that  it  is  of  different  age  from  the  "Western" 
sandstone  (upper  Keweenawan). 

ISLE  ROYAL. 

Isle  Royal  is  45  miles  in  length  and  varies  in  width  from  3  to  8  miles.  From  the  Rock 
of  Ages,  the  farthest  outlying  reef  to  the  southwest,  to  the  Gull  Island  rocks  on  the  northeast, 
the  distance  is  57  miles.  The  island  lies  off  Thunder  Bay,  northwest  of  the  outer  part  of  Kewee- 
naw Point.  The  strike  of  Isle  Royal  and  Keweenaw  Point  are  substantially  the  same,  north- 
east and  southwest.  This  island  has  been  mapped  geologically  by  Lane.''  His  succession 
in  descending  order  is  as  follows: 

Section  of  Keweenawan  rocks  on  Isle  Royal. 

Sandstone  and  conglomerate  ("the  Great  conglomerate"?). 

Ophites  dowfl  to  Island  mine  conglomerate  (Marvine's  group  C). 

Intercalated  sandstones  and  conglomerates. 

Melaphyre  porphyrites  and  scoriaceous  conglomerates  ("Ashbed"  group). 

"The  greenstone" — thickest  ophite. 

Amygdaloids  and  thin  ophites  down  to  Minong  breccia  (Kearsarge  conglomerate). 

Minong  porphyrite  and  Minong  trap. 

Ophites  and  conglomerates,  including  Huginnin  porphyrite,  down  to  felsite. 

It  is  clear  from  the  general  character  of  the  succession  that  it  is  like  that  of  the  middle 
Keweenawan  of  the  remainder  of  the  Lake  Superior  region;  that  is  to  say,  it  consists  of  igneous 
rocks  and  sediments.  The  igneous  rocks  are  dominantly  basic.  They  are  all  regarded  as 
extrusive  by  Lane.*^ 

However,  the  same  question  may  be  raised  with  reference  to  the  greenstone,  wliich  is 
given  a  tliickness  of  2.33  feet,  as  was  raised  concerning  that  of  Keweenaw  Point.  Is  it  an  extru- 
sive or  is  it  a  later  intrusive  ?  Certainly  it  has  all  the  characteristics  of  the  diabase  of  Beaver 
Bay  on  the  Minnesota  coast,  which  is  almost  certainly  intrusive. 

The  intercalated  sandstones  and  conglomerates,  from  lowest  to  highest,  contain  a  much 
greater  proportion  of  material  from  acidic  rocks  than  would  be  expected  from  the  small  pro- 
portion of  original  acidic  rocks.  The  sandstones  and  conglomerates  are  subordinate  in  amount 
in  the  major  portion  of  the  section  and  only  become  of  great  volume  with  the  appearance  of 
the  "Great"  conglomerate.  The  field  terms  for  the  igneous  rocks  and  their  relations  are 
expressed  by  Lane<*  as  follows: 

Felsite  Melaphyre 

Trap — nonamygdaloidal  dark  rocks 
Amygdaloid 


Porphyry Porphyrite  Ophite 

Lane'  gives  one  very  detailed  section  based  largely  on  drill  records.  Its  thickness  is  9,000 
feet.  In  this  section  the  felsite  flows  are  confined  to  the  lower  150  feet,  but  at  a  high  horizon 
one  bed  of  porphyry  tuff  10  feet  thick  is  noted.     This  tuff  may  be  regarded  as  a  confirmation 

"Irnug,  R.  D.,  The  copper-bearing  rocks  of  Lake  Superior;  Men.  U.  S.  Geol.  Survey,  vol.  5, 1883,  pp.  360-361. 

6  Personal  communication. 

c  Lane,  A.  C,  Geological  report  on  Isle  Royale,  Michigan:  Geol.  Survey  Michigan,  vol.  6,  pt.  1, 1898,  281  pp. 

dldem,  p.  53. 

c  Idem,  pp.  27  et  seq. 


390  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

of  the  suggestion  made  in  another  place  (p.  382)  that  volcanic  fragmental  rocks  of  the  acidic 
type  are  nuich  more  abundant  in  the  Keweenawan  than  had  been  supposi'd.  Most  of  the 
interstratifiod  sedimentary  beds  are  conglomerates  and,  with  three  exceptions,  they  range  from 
a  knife-edge  to  50  feet  in  thickness.  Two  of  the  thicker  beds  are  mainly  sandstone.  In  addition 
to  a  number  of  seams  which  were  too  small  to  be  measured,  the  total  number  of  sedimentary 
beds  in  the  district  is  24  and  the  total  thickness  is  430  feet.  To  the  "Great"  conglomerate 
is  given  a  thickness  of  2,600  feet,  making  a  total  thickness  of  sediments  of  3.030  feet.  This 
leaves  5,970  feet  for  the  lavas. 

In  the  matter  of  correlation.  Lane  "  assumes  that  the  thick  conglomerate  at  the  top  of 
the  series  is  a  continuation  of  the  "Great"  conglomerate  of  Keweenaw  Point,  and  with  this 
horizon  as  a  starting  point  he  attempts  to  correlate  somewhat  closely  the  h6x\s  of  Isle  Royal 
with  those  of  Keweenaw  Point,  as  is  indicated  by  the  succession  given  on  page  389,  the  names 
in  parentheses  being  those  of  formations  on  Keweenaw  Point.  Although  it  is  probable  that 
the  top  conglomerate  corresponds  to  the  "Great"  conglomerate  of  Keweenaw  Point,  and 
although  it  may  be  possible  that  the  formations  are  to  some  extent  equivalent,  it  maj-  perhaps 
be  doubted  whether  the  correlation  of  individual  thin  beds,  such  as  the  interstratified  conglom- 
erates, is  justified,  especially  as.  there  is  so  remarkable  a  likeness  in  the  petrography  of  the  beds 
of  the  Keweenawan  at  difterent  horizons  in  the  several  districts  of  Lake  Superior.  If  the  bed 
of  greenstone  more  than  200  feet  thick  is  really  intrusive,  as  suggested,  its  correlation  with  the 
greenstone  of  Keweenaw  Point  on  a  stratigraphic  basis  is  very  questionable. 

In  any  section  on  Isle  Royal  there  is  a  lessening  of  the  dip  in  passing  from  lower  to  higher 
horizons,  just  as  at  Keweenaw  Point  and  at  Michipicoten.  For  instance,  at  the  west  end  of 
the  island  on  the  north  side  the  dips  are  16°  S.  and  on  the  south  side  in  the  "Great"  conglom- 
erate 8°  S.,  a  difference  of  8°.  Toward  the  east  end  of  the  island  the  dips  on  the  north  side  are 
26°  and  on  the  south  side  18°,  again  a  difference  of  8°. 

MICHIPICOTEN  ISLAND. 

The  folio  whig  account  of  Michipicoten  Island  is  taken  almost  wholly  from  Burwash,' 
who  alone  has  made  a  close  study  of  this  area.  However,  it  should  be  said  that  Logan's  general 
accoinit  of  this  district "  is  remarkably  accurate. 

The  island  is  roughly  ellipsoidal  in  shape,  about  16|  miles  long  by  6  miles  in  greatest  width. 
Its  longer  axis  lies  east  and  west  parallel  to  the  coast,  and  its  west  end  is  south  and  a  httle  west 
of  Pukaskwa  River. 

The  Keweenawan  rocks  occupy  the  entire  island  as  well  as  the  row  of  smaller  islands  off 
its  south  shore.  They  are  confined  wholly  to  the  middle  Keweenawan.  The  igneous  rocks 
overwhelmingly  dominate  in  mass.  They  are  described  as  extrusive,  no  intrusive  rocks  being 
mentionetl.  Lithologically  they  include  all  the  varieties  of  the  ordinary  extnisive  rocks, 
ophitic  and  diabasic  melaphyres,  amygdaloids,  porphyrites,  felsites,  and  quartz  jiorphyries. 
The  acidic  rocks  are  much  more  readily  eroded  than  the  basic  rocks.  In  consequence  they 
usually  occupy  depressions,  whereas  the  basic  rocks  constitute  the  ridges.  In  this  respect 
there  is  a  contrast  between  Micliipicoten  Island  and  Keweenaw  Point,  where  the  acidic  rocks 
constitute  elevations.  It  may  be  suggested  that  the  difference  is  due  to  the  fact  that  the- 
Michipicoten  acidic  rocks  are  largely  extrusive,  while  those  of  Keweenaw  Point  are  largelj' 
intrusive.  No  order  of  extrusion  of  the  lavas  is  suggested,  the  acidic  and  basic  rocks  both 
occurring  from  the  higliest  to  the  lowest  horizons.  As  to  volume,  there  does  not  seem  to  be 
much  difference  between  tiie  basic  and  acidic  varieties.  Selwyn  and  Coleman  state  that  pyro- 
clastic  rocks  occur  on  Michipicoten  Island,  but  such  rocks  were  not  observed  by  Burwash  <* 
and  if  ])resent  are  certainly  extremely  insignificant  in  amount.  The  lava  ])eds  attain  their 
maximum  thiclaiess  in  the  eastern  and  central  parts  of  the  island  and  are  thinner  toward  the 

a  Lane,  A.  C,  Geological  report  on  Isle  Royale,  Michigan:  Geol.  Survey  Michigan,  vol.  6,  pt.  1, 1898,  pp.  99  et  seq. 

6  Bnrwa.sh,  E.  N.,  The  geology  of  Michipicoten  Island:  Univ.  Toronto  Studies,  Geol.  ser.,  No.  3,  Toronto,  1905,  with  map. 

« Logan,  W.  E.,  Report  of  progress  to  18G3,  Geol.  Survey  Canada,  1863. 

dOp.  clt.,  pp.  27,  47. 


THE  KEWEENAWAN  SERIES.  391 

west,  where  they  are  interstratified  with  the  conglomerates.  The  lower  beds  strike  approxi- 
mately northeast  and  southwest.  In  passing  to  higher  horizons  the  strike  approaches  east  and 
west.     Thus  there  is  an  appearance  of  minor  unconformity  between  the  lower  and  upper  beds. 

The  debris  of  the  conglomerates  is  as  usual  derived  largely  from  the  acidic  rocks,  but  with 
them  are  included  granites,  greenstones,  and  biotite  gneisses  derived  from  pre-Keweenawan 
formations.  Abundant  material  derived  from  the  basic  rocks  is  also  recognized.  The  sedi- 
mentary rocks  occur  mainly  at  lower  horizons,  although  one  conglomerate  is  foimd  at  a  com- 
paratively high  horizon.  These  conglomerates  are  confined  to  the  nortliwestem  part  of  the 
island,  being  thickest  at  the  west  and  thinning  out  to  the  northeast. 

These  facts  suggest  that  the  central  and  eastern  parts  of  the  island  formed  a  center  of 
volcanic  dispersion,  that  the  lavas  flowed  toward  the  west,  and  that  in  the  part  of  the  area 
somewhat  removed  from  the  main  volcanic  outbursts  there  was  opportunity  to  build  con- 
glomerates between  the  successive  lava  flows. 

The  dip  of  the  beds  on  the  north  and  northwest  sides  of  the  island  is  55°  S.  From  this 
there  is  a  steady  decrease  in  dip  until  on  the  islands  off  the  south  shore  of  Michipicoten  the 
dips  are  about  14°  S.,  the  lessening  of  dip  across  the  series  being  therefore  40°. 

Burwash"  gives  the  following  descending  succession: 

Section  of  Keweenawan  rods  on  Michipicoten  Island. 

Feet. 

1.  Felsite  of  islands  off  the  south  shore 1, 000 

2.  Pitchstone  bed 530 

3 .  Quartzless  porphyry  of  Quebec  Harbor 695 

4.  Melaphyre  porphyrites  of  Channel  Lake 1,  660 

5.  Quartz  porphyries ;  1 355 

2 • 1,160 

3 1,493 

6.  Beds  exposed  at  lake  on  road 1, 575 

7.  Felsite 513 

8.  Diabase  porphyrite •. 463 

9.  Beds  underlying  farm  (three) 1, 140 

10.  Several  beds  at  mine 645 


11,230 


This  result,  obtamed  by  accurate  measurement  of  three  sections  and  by  careful  studies,  is 
a  remarkable  confirmation  of  the  judgment  of  Logan,''  who  states  that  the  thickness  of  tlie 
formations  developed  in  Michipicoten  Island,  at  the  most  moderate  dips  observed,  would  not 
fall  far  short  of  12,000  feet. 

It  is  stated  that  on  the  mainland  near  the  mouth  of  Pukaskwa  River  there  are  rocks  of 
Keweenawan  age,  and  this  leads  to  the  suggestion  that  the  Keweenawan  constitutes  a  mono- 
clinal  succession  from  the  north  shore  of  Lake  Superior  to  the  south  side  of  Michipicoten. 
For  the  intervening  distance  between  the  mainland  and  the  island  an  estimated  thickness  is 
given  of  34,000  feet,  and  thus  a  suggested  thickness  for  the  entire  Keweenawan  series  of  45,000 
feet.  But  it  seems  to  us  more  probable  that  between  Michipicoten  Island  and  the  main  shore 
there  is  a  strike  fault  and  that  therefore  the  Micliipicoten  rocks  may  be  near  the  bottom  of 
the  Keweenawan  series.  This  idea  is  perhaps  confirmed  by  the  presence  in  the  conglomerates 
of  the  Michipicoten  district  of  material  from  pre-Keweenawan  sources. 

EAST  COAST  OF  LAKE  SUPERIOR. 

Several  prominent  points  along  the  east  coast  of  Lake  Superior  exliibit  Keweenawan  rocks. 
While  none  of  these  areas  are  large,  they  are  significant,  extending  along  nearly  the  entire  east 
coast  of  Lake  Superior  from  Cape  Choyye,  near  Micliipicoten  Harbor,  to  Gros  Cap,  intervening 
locaHties  being  Cape  Gargantua,  Pointe  aux  Mines,  and  Mamainse  Peninsula.     At  all  these  local- 

n  Op.  lit.,  pp.  40-41. 

6  Logan,  W.  E.,  Report  of  progress  to  1803,  Geol.  Survey  Canada,  1863,  p.  82. 


392  GEOLOGY  OF  THE  LAKE  SUPERIOR- REGION. 

ities  the  rocl«  belong  to  the  middle  Keweenawan.  They  consist  of  basic  lavas,  including  mela- 
pliyres,  porphyritos,  and  amygdaloids,  and  interstratified  sandstones  and  conglomerates.  The 
sandstones  and  conglomerates  tlillVr  from  the  ordinary  sculimentary  rcjcks  interstratified  with  the 
lavas  in  that  they  contain  a  consitlerable  amount  of  detritus  derived  from  the  subjacent  Archean 
rocks.  This  is  particularly  noticeable  at  Mamainse.  For  the  most  part  the  masses  exposed 
are  small,  but  Logan"  estimates  the  tliickness  of  the  series  at  Pointe  aux  Mines  to  be  3,000  feet. 
At  Mamainse  Peninsula  the  Keweenawan  rocks  occupy  much  the  largest  area  along  the  east 
coast.  Macfarlan(\ *  calculates  a  total  thickness  in  this  locality  of  16,208  feet,  of  whicli  inter- 
stratified conglomerates  make  up  2,138  feet.  Macfarlane's  section,  from  the  base  upward,  is 
as  follows: 

Section  of  Keweenawan  rods  on  Mamainse  Peninsula. 

Feet. 

1.  Granular  melaphyre,  coilsisting  of  a  small-grained  mixture  of  dark-brown  feldspar  with 

angular  grains  of  a  dark-green  chloritic  mineral.     It  varie.s  frequently  in  its  structure, 

and  in  the  upper  part  contains  amygdules  of  calc  spar  and  delessite  (iron  chlorite) 3, 930 

2.  Brown  argillaceous  sandstone,  striking  N.  20°  W.  and  dipping  35°  SW 12 

3.  Compact  greenish-gray  melaphyre,  with  grains  of  feldspar,  iron  chlorite,  and  hematite; 

strike  N.  10°  W.;  dip  32°  SW 1,787 

4.  Conglomerate  holding  granitic  or  gneissoid  bowlders 852 

5.  Granular  melaphyre,  containing  feldspar,  which  weathers  white,  and  dark-green  chlorite.  426 

6.  Sandstone 20 

7.  Dark-brown  compact  trap 71 

8.  Conglomerate 70 

9.  Dark-green  melaphyre,  slightly  amygdaluidal 710 

10.  Conglomerate 43 

11.  MelaphjTe,  striking  N.  5°  W.,  dip  30°  W. ;  fine  grained  and  of  a  dark-brownish  color 1,  207 

12.  Conglomerate 71 

13.  Granular  melaphyre,  containing  brownish-red  feldspar  and  abundance  of  delessite 355 

14.  Conglomerate 35 

15.  Fine-grained  greenish-red  melaphyre,  becoming  amygdaloidal  in  the  upper  part  of  the 

bed.     Strike  N.  20°  W.,  dip  35°  SW.,  where  it  adjoins  conglomerate  N.  15°  W.>45°  SW.  489 

16.  Conglomerate,  with  a  small  layer  of  sandstone,  the  latter  striking  N.  17°  W.,  dip  40°  SW..  163 

17.  Compact  dark-brown  crystalline  trap 340 

18.  Conglomerate 170 

19.  MelaphjTe 100 

20.  Conglomerate,  striking  N.  5°  W.  and  dipping  42°  W.  at  junction  with  overlying  rocks 204 

21.  Melaphyre 240 

22.  Conglomerate,  striking  N.  12°  W.     In  this  bed  the  bowlders  are  smaller  than  in  those 

hitherto  mentioned 34 

23.  MelaphyTe,  striking  N.  23°  W.  and  dipping  37°  SW 682 

24.  Conglomerate  and  sandstone,  striking  N.  14°  W.  and  dipping  44°  SW 12 

25.  Melaphyre;  strike  N.  33°  W.;  dip  28°  SW 250 

26.  Measures  concealed 160 

27.  Melaphyre,  granular  and  of  a  reddish-green  color,  striking  N.  30°  W.  and  dipping  18°  SW.  25 

28.  Measures  concealed 125 

29.  Melaphyre;  strike  N.  33°  W.;  dip  28°  SW 272 

30.  Mea-sures  concealed 180 

31.  Melaphyre,  amygdaloidal  in  part 436 

32.  Measures  concealed 400 

33.  Conglomerate,  consisting  of  bowlders  of  Laurentian  rocks  in  matrix  of  red  sandstone 330 

34.  Measures  concealed 172 

35.  Melaphyre,  strikingN.  35°  W.  and  dipping  20°  SW 100 

36.  Conglomerate,  in  which  the  bowlders  consist  to  a  much  greater  extent  than  heretofore  of 

amygdaloidal  and  other  varieties  of  melaphyre.     Strike  N.  20°  W.;  dip  25°  SW.  at  the 
junction  with  the  overhang  rock 50 

37.  Reddish-gray  granular  melaphyre,  becoming  amygdaloidal  in  the  upper  part 200 

38.  Sandstone,  strikingN.  30°  W.  and  dipping  24°  SW : 12 

39.  Conglomerate,  containing  here  and  there  layers  of  sandstone,  striking  N.  40°  W.  and 

dipping  15°  SW 30 

u  I.ognn,  W.  E.,  Report  of  progrc-is  to  lSf.3,  Oeol.  Siin-ey  Canada,  1S63,  p.  82. 

1>  Mactarlaiie,  Tlioimis,  Report  of  progress  from  1SG3  to  1860,  Geol.  Sur\'ey  Canada,  1866,  pp.  132-134. 


THE  KEWEENAWAN  SERIES.  393 

Feet. 

40.  Dark-green  glittering  nielaphyre,  striking,  at  its  junction  mth  the  underlying  conglom- 

erate, N.  50°  W.  and  dipping  30°  SW 114 

41.  Measures  concealed 137 

42.  Melaphyre,  striking  N.  50°  W.  and  dipping  29°  SW ;.  16 

43 .  Measures  concealed 114 

44.  Melaphyre,  dark  reddish  green,  striking  N.  50°  to  55°  W.  and  dipping  21°  to  25°  SW 300 

45.  Dark-green  and  glittering  melaphjTe;  N.  25°  W.>20°  SW 250 

46.  Compact  fine-grained  trap,  containing  geodes  of  agate,  in  which  calc  spar  frequently 

occupies  the  center 350 

47.  Porphyritic  conglomerate  and  sandstone;  N.  8°  W.>21°  W 30 

48.  Compact  fine-grained  trap,  containing  agates  in  many  places 72 


16, 208 

This  thickness  does  not  inchide  the  basal  sandstone  to  be  mentioned  below.  It  is  to  be 
noted  that  in  the  5,729  feet  at  the  bottom  of  the  section  there  is  only  one  layer  of  sediment — a 
sandstone  12  feet  thick.  In  the  remainder  of  the  section,  10,479  feet,  conglomerates  and  sand- 
stones are  interstratified  at  several  places,  the  thickest  bed  being  S.52  feet  thick  and  lying  at  the 
bottom  of  the  part  containing  sandstones  and  conglomerates.  Thus  the  lower  third  of  the 
middle  Keweenawan  is  essentially  igneous  and  the  upper  two-thirds  consists  of  igneous  and 
sedimentary  rocks. 

At  Mamainse,  Pointe  aux  Mines,  and  Cape  Choyye  the  lower  Keweenawan  beds  are  con- 
glomerates and  sandstones.  At  Mamainse  these  basal  beds  of  sandstone,  according  to  Mac- 
farlane,"  seem  to  have  a  very  considerable  thickness.  At  Pointe  aux  Mines,  according  to 
Logan,*  there  are  sandstones  at  the  base  of  the  series  nearly  in  contact  with  the  gneiss.  At 
Cape  Choyye  the  basal  bed  is  a  red  sandstone  of  considerable  thickness.  However,  at  Cape 
Gargantua  and  at  Batchewanung  Bay  the  amygdaloidal  trap  rests  unconformably  upon  the 
Archean,  and  thus  at  these  points  igneous  rocks  are  at  the  lowest  horizon  of  the  Keweenawan 
series. 

Thus  for  eastern  Lake  Superior  the  Keweenawan  may  be  divided  into  lower  Keweenawan 
and  middle  Keweenawan,  the  former  being  represented  by  the  sediments  at  the  bottom  of  the 
series  and  the  latter  by  the  lavas  and  interstratified  sediments. 

The  dips  at  Mamainse  are  20°  to  30°  lakewartl,  and  fi'om  these  amounts  on  the  east  coast 
they  range  up  to  60°,  as  at  Gros  Cap.  In  general  direction  the  strike  of  the  strata  of  the  Kewee- 
nawan of  the  east  coast  curves  in  and  out,  corresponding  to  the  minor  folds  of  the  synchnorium, 
but  the  average  strike  is  somewhat  west  of  north,  corresponding  with  the  general  direction  of 
the  east  coast,  and  the  dips  are  to  the  west,  varying  from  as  low  as  10°  at  Cape  Choyye  to  as 
high  as  45°  or  even  60°  at  Gros  Cap.     The  usual  dips,  however,  run  between  20°  and  35°. 

From  the  general  relation  of  the  Cambrian  sandstone  (Sault  Ste.  Marie,  "Eastern"  or  Pots- 
dam sandstone  of  several  wTitcrs)  and  its  extensions  adjacent  to  the  Keweenawan,  Logan  con- 
cluded that  there  was  an  unconformity  between  the  two.     He  says:'' 

The  contrast  between  the  general  moderate  dips  of  these  sandstones  and  the  higher  inclination  of  the  igneous 
strata  at  Gargantua,  Mamainse,  and  Gros  Cap,  combined  with  the  fact  that  the  sandstones  always  keep  to  the  lake  side 
of  these,  while  none  of  the  many  dikes  which  cut  the  trappean  strata,  it  is  believed,  are  known  to  intersect  the  sand- 
stones (at  any  rate  on  the  Canadian  side  of  the  lake),  seems  to  support  the  suspicion  that  the  sandstones  may  overlie 
unconformably  those  rocks  which,  associated  with  the  trap,  constitute  the  copper-bearing  series. 

GENEEAL  CONSIDEEATION  OF  THE  KEWEENAWAN  SERIES. 

LOWER  KEWEENAWAN. 

In  reference  to  the  lower  Keweenawan,  it  need  here  only  be  remarked  that  these  sediments 
are  in  no  way  pecuHar.  They  are  derived  from  the  preexisting  Huronian  and  Archean  precisely 
as  similar  detrital  formations  are  built  up.  At  the  bottom  are  conglomerates ;  over  these  lie 
sandstones;  and  in  the  Black  and  Nipigon  bay  districts  above  these  are  interstratified  marls, 
hmestones,  shales,  and  sandstones. 

a  Report  of  progress  from  1863  to  1S06,  Geol.  Survey  Canada,  ISGO,  p.  134. 
t>  Report  of  progress  to  1803,  Geol.  Survey  Canada,  18G3,  p.  82. 
c  Idem.  p.  85. 


394  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Thoui^li  it  is  not  known  that  sediments  were  everywhere  deposited  at  the  base  of  the  Kewee- 
nawan,  it  is  a  remarkable  fact  that  in  most  places  where  the  actual  contact  between  the  non- 
intrusive  ])arts  of  (ho  Keweenawan  and  the  next  underlyinfj  rocks  can  l>e  seen  such  sediments 
occur.  Those  deposits  liave  their  greatest  vohnne  and  widest  extent  in  tlie  n-gion  about  Black 
and  Nipigon  bays,  where  the  tliickness  is  variously  estimated  from  550  to  1,400  feet.  In  north- 
eastern Minnesota,  at  the  base  of  the  series  is  the  Puckwunge  conglomerate.  In  Micliigan,  at 
Black  River,  at  the  bottom  of  the  succession  is  a  basal  sandstone  known  to  be  300  feet  thick, 
and  it  may  be  considerably  thicker  than  this,  occupying  a  part  of  the  unexposed  area  to  the 
soutli.  How  far  this  sandstone  extends  east  and  west  is  not  known,  as  the  formations  next 
underlying  tlie  Keweenawan  are  not  usually  exposed.  However,  the  formation  is  known  to  be 
present  north  of  Ironwood  and  also  in  sec.  11,  T.  45  N.,  R.  1  W.,  near  Potato  River,  in  Wisconsin, 
more  than  20  miles  west  of  Black  River  (Michigan).  At  the  latter  place  the  conglomerate  and 
quartzite  below  the  lavas  are  probably  as  tliick  as  at  Black  River.  On  the  east  side  of  Lake 
Superior  the  actual  contacts  between  the  pre-Keweenawan  and  the  Keweenawan  are  found 
at  a  number  of  localities,  and  at  the  more  extensive  of  these  exposures  the  lowest  formation 
of  the  Keweenawan  is  a  conglomerate,  although  at  other  locahties  the  lavas  he  directly  against 
the  gneiss.  Where  the  lowest  Keweenawan  rock  is  an  intrusive,  as  for  instance  the  Duluth 
gabbro,  this  must  of  course  be  excluded  from  all  consid<>ration  in  connection  with  the  oldest 
formation  of  the  Keweenawan.  Also  there  must  be  excluded  from  consideration  the  localities, 
such  as  Keweenaw  Point  and  western  Wisconsin,  where  the  base  of  the  Keweenawan  is  not 
exposed. 

MIDDLE  KEWEENAWAN. 

The  middle  Keweenawan  was  the  great  epoch  of  combined  igneous  and  aqueous  activities. 
There  are  two  divisions  of  its  rocks — original  igneous  and  derived  sedimentary. 

IGNEOUS  ROCKS. 
VARIETIES. 

The  igneous  rocks  constitute  a  province  of  rather  remarkable  uniformity.  The  different 
kinds  anil  their  relations  are  substantially  the  same  in  each  of  the  important  districts.  Chem- 
ically the  igneous  rocks  include  basic,  acidic,  and  intermediate  varieties.  The  basic  materials 
overwhelmingly  dominate,  the  acidic  rocks  are  considerable  in  quantity,  and  the  intermediate 
rocks  are  few  and  local.  Each  variety  of  rocks  includes  both  intrusive  and  extrusive  facies, 
so  that  the  basic,  acidic,  and  intermediate  gi'oups  all  have  textures  characteristic  for  plutonic 
and  volcanic  rocks.  Barring  the  work  of  KIoos  and  Streng,"  which  was  limitetl  in  scope,  Pum- 
pelly*  made  the  first  careful  petrographic  study  of  the  Keweenawan  rocks.  In  general  Irving' 
followed  PumpeUy  in  the  use  of  terms,  but  his  studies  were  more  extensive  and  disclosed  new 
variations. 

According  to  Irving,  the  basic  plutonic  igneous  rocks  comprise  olivinitic  and  nonolivinitic 
gabbros,  olivinitic  and  nonolivinitic  diabases,  and  "anorthite  rock."  Tiie  surface  varieties 
include  melaphyres,  porphyrites,  and  amygdaloitls.  The  coarser-grained  melapluTes  have 
often  been  called  dolerites,  diabases,  or  ophites,  depending  on  their  texture.  The  deep-seated 
phase  of  the  acidic  rocks  is  granite,  augitic,  or  Jiornblendic,  and  the  extrusive  phase  is  made  up 
of  porpliyry,  cjuart  ziferous  and  nonquartziferous,  and  felsite.  The  intermediate  rocks  occur  in 
subordinate  amounts.  The  most  important  intrusive  phases  of  them  are  described  by  Irving 
as  augite  syenites  and  orthoclase  gabbros,  and  the  extrusive  varieties  as  porjiliyrites.  The  term 
"trap"  is  used  by  Irving  in  its  usual  sense  to  include  both  basic  and  intermediate  fine-grained 
rocks. 

<■  Slreng,  A. ,  and  KIoos,  J.  H. ,  tjber  die  krystallinisohcn  Gcsteine  von  Minnesola  in  Nord-Amerilta:  Neiies  Jahrb.,  1877. 
ii  Pnmpelly,  Rapliael,  Copper-bearing  rocl<s:  Gcol.  Survey  Mii-tilgan,  vol.  1,  pt.  2.  1873,  pp.  l-)(i,  62-94. 
c  Irving,  R.  D. ,  Tlie  copper-bearing  rocks  of  Lake  Superior:  Mon.  U.  S.  Oool.  Survey,  vol.  5, 1SS3. 


THE  KEWEENAWAN  SERIES.  395 

The  plutonic  ijineous  rocks  arc.  very  little  altered.  The  very  readily  changeable  ohvine 
may  be  altered  to  cldorite,  serpentine,  etc.,  to  a  small  extent.  The  augite  and  plagioclase  are 
locally  chloritized,  but  still  these  alterations  are  purely  sul)ordinate. 

The  volcanic  rocks  are  much  altered.  This  is  especially  true  of  the  vesicular  amygdaioidal 
basic  lavas.  In  these  rocks  the  original  minerals,  which  were  dominantly  augite,  olivine, 
plagioclase  feldspars,  magnetite,  and  glassy  base,  have  been  extensively  altered  and  the  vesicules 
of  the  amygdaloids  filled  with  secondary  j)roducts.  These  are  mainly  alterations  of  the  belt  of 
cementation  m  the  zone  of  katamorphism.  A  complicated  set  of  secondary  minerals  has  been 
produced,  of  which  the  following  are  very  common:  Various  zeolites,  such  as  laumontite, 
thomsonite,  stilbite,  and  mesolite;  also  calcite,  chlorite,  epidote,  quartz,  prehnite,  orthoclase, 
hematite,  and  limonite.  ' 

The  acidic  rocks,  the  origmal  minerals  of  which  were  mainly  quartz,  orthoclase,  plagioclase, 
and  glass,  have  also  been  extensively  decomposed,  with  the  development  of  much  secondaiy 
quartz  and  other  alteration  jiroducts,  which  are  not  always  completely  determinable  but  which 
certaml}'  include  epidote  and  chlorite.  Hematite  and  limonite  are  common.  Many  microliths 
Jiave  formed,  the  exact  nature  of  which  it  is  difficult  to  determine. 

REVIEW    OF    NOMENCLATURE    OF    KEWEENAWAN    IGNEOUS    ROCKS." 

By  Alexander  N.  Winohell. 

The  Keweenawan  igneous  rocks  of  the  Lake  Superior  region  have  been  studied  and  dis- 
cussed by  many  geologists  during  the  past  thirty  years.  At  the  beginnmg  of  that  period 
microscopic  petrography'  was  in  its  infancy  and  mmor  errors,  due  to  faulty  methods,  inevitably 
resulted.  In  the  course  of  the  years  these  have  been  gradually  corrected,  involving  changes 
of  nomenclature.  Some  variations  in  nomenclature  have  resulted  from  the  varying  points 
■of  views  of  the  authors.  But  the  general  progress  of  petrography  has  brought  more  numerous 
and  important  modifications. 

In  order  to  make  the  names  used  by  the  prominent  writers  on  the  subject  more  readily 
intelligible,  a  correlation  of  these  names  is  presented  herewith.  It  must  be  remembered  that, 
since  the  basis  of  petrographic  classification  used  by  the  authors  has  varied  somewhat,  such  a 
correlation  can  be  only  an  approximation,  but  it  will  nevertheless  serve  the  purpose  of  showing 
the  various  changes  that  have  occurred  and  of  presenting,  at  least  in  its  outlines,  the  main  facts 
of  nomenclature  of  each  writer. 

In  order  to  give  precision  to  such  a  correlation,  it  is  desirable  that  the  nomenclature  of 
each  writer  be  compared,  not  simply  with  that  used  by  other  authors  but  also  with  an  expressed 
and  definite  classification.  Therefore  the  following  classification  has  been  prepared,  on  the 
basis  of  textures  and  mineral  composition.  It  is  not  a  general  classification  of  igneous  rocks 
but  is  intended  to  include  merely  the  tyj^es  represented  in  the  Keweenawan  of  the  Lake  Supe- 
rior region. 

a  Revision  of  article  published  in  Jour.  Geology,  vol.  16, 1908,  pp.  765-774.    Originally  prepared  for  this  monograph. 


396  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Mineralogical  classification  of  Keweenawan  igneom  rocks  of  the  Lnl-e  Superior  region. 


Texture. 


Chief  feldspar  orthoclase. 


Orthoclase  with 
equal  plagioclase. 


h  Quartz. 


-Quartz. 


±  Mica±  Amphibolei  Pyroxene. 


±  Microcllne. 


Granitic. 


Granite. 


Ophitic. 


Porphyritic     (pheno- 
crysts  prominent). 


Felsitic  or  porphyritic 
(phenocrysts  tew). 


Fragraental. 


Glassy. 


Rhyollte  porphyry 
(quartz  por- 
phyry). 


Rhyolite. 


+Anorthoclase. 


Soda  granite. 


Quartz  kerato- 
phyre. 


Quartz  kerato- 
phyre. 


±  Microcllne. 


Syenite. 


Trachyte   por- 
phyry. 


Trachyte. 


Monzonlte. 


Chief  feldspar  plagioclase. 


With  quartz. 


+MonocIinic  pyroxene. 


+ Orthoclase. 


Orthoclase  gabbro 


Orthoclase  diabase 


-Orthoclase. 


Quartz  gabbro. 


Quartz  diabase. 


Acidic  tufls. 


Obsidian. 


Texture. 


Granitic. 


Ophitic. 


Chief  feldspar  plagioclase — Continued. 


With  quartz— Continued. 


Without  quartz. 


+  Orthorhombic 
pyroxene. 


Quartz  norlte. 


Quartz-enstatite 
diabase. 


Porphyritic      (pheno- 
crysts prominent). 


Felsitic  or  porphyritic 
(phenocrysts  few). 


Fragmental. 


-l-.\mphiboIe.      ,  No  ferromagnesian 
±  Biotite.  '  mineral. 


Quartz  diorite. 


Dacite. 


Plagloclasite. 


-(-.\mphibole. 
±  Biotite. 


Diorite. 


Andesite  por- 
phyry. 


Andesite. 


-I-Monocllnic  pyroxene. 


-Olivine. 


Gabbro. 


Diabase. 


Augite      andesite 
porphyry. 


Augite  andesite. 


+  Olivine. 


Olivine  gabbro. 


Olivine  diabase. 


Basalt  porphyry. 


Basalt. 


Basalt  tuffs. 


Glassy.                  .        | 

Tachylyte. 

Chief  feldspar  plagioclase— Continued. 

No  feldspar. 

Without  quartz— Continued. 

-l-OUvme. 

±  Pvroxenei  .\mphibole±  Biotite. 

Texture. 

-1-  Orthorhombic  pyro-xene. 

+  Monoclinic  py- 
roxene. 

-Olivine. 

-1-  OIi\-lne. 

-OUvine. 

-1- OUvine. 

Granitic. 

Augite  norlte. 

Norlte. 

Olivine  norlte. 

Troctollte. 

Pyroxenite. 

Peridotlte. 

Ophitic. 

Hypcrsthcne  diabase 

PorphyrKlc     (pheno- 
crysts prominent). 

Felsitic  or  porphyritic 
(phenocrysts  few). 

1 

Fragmental. 

Basalt  tuffs. 

Glassy. 

Tachylyte. 

THE  KEWEENAWAN  SERIES.  397 

Macfarlane,"  in  1S66,  described  the  Keweenawan  rocks  of  Michipicoten  Island.  He  found 
melaphyre,  trap,  amygdaloid,  quartz  porpliyry,  porphyrite,  and  trachytic  phonolite.  His 
"quartz  porphyry,"  which  occurred  at  the  contact  of  the  sandstone  and  trap,  was  doubtless 
a  modified  quartzite.  His  "trachytic  phonolite"  is  not  fully  described,  and  correlation  is 
uncertain. 

Kloos,''  in  1871,  described  gabbro  or  hypersthenite,  black  porphyry  or  melaphyre,  por- 
phyry, and  amygdaloid.     The  first  named  was  probably  a  gabbro  and  the  second  a  diabase. 

Pumpelly,'^  in  1873,  described  melaphyre,  trap,  and  amygdaloid  without  microscopic 
study;  ho  distinguished  three  kinds  of  melaphyre — coarse  grained,  fine  grained,  and  melaphyre 
porphyry.  Correlations  of  these  names  are  impracticable  and  woukl  bo  misleatiing  rather  than 
helpful. 

Marvine,'*  in  the  same  year,  described  melaphyre,  trap,  diorite,  and  amygdaloid.  Pum- 
pelly  later  claimed,  probably  correctly,  that  Marvine's  diorite  mcluded  samples  of  diabase, 
melaphyre,  and  gabbro,  but  no  true  diorite. 

Strong,*^  in  1877,  described  melaphyre,  melaphyre  porphyry,  and  hornblende  gabbro  from, 
the  Keweenawan  of  Minnesota.  He  published  chemical  analyses  of  two  of  these,  which  permit 
their  correlation  on  the  quantitative  basis.     (See  table  on  p.  402.) 

Pumpelly,/  m  1878,  described  the  alterations  which  some  of  the  Keweenawan  rocks  had 
siiffered  in  great  detail,  but  brought  to  light  no  additional  varieties  of  the  unaltered  rocks. 

The  same  author,!'  in  1880,  identified  eight  or  ten  kinds  of  igneous  rocks  in  the  Keweena- 
wan. (See  table  on  p.  400.)  He  distinguished  diallage  from  augite  by  means  of  the  parting  in 
the  former,  and,  in  accordance  with  the  usage  at  that  time,  called  a  massive  igneous  rock  con- 
taining plagioclase  and  diallage  a  gabbro,  wliile  one  containing  plagioclase  and  augite  he  called 
a  diabase.  But  all  the  diabase  covered  by  his  descriptions  and  illustrations  seems  to  have  an 
opliitic  texture.  liis  identifications  of  the  plagioclase  feldsijars  were  all  based  on  incorrect 
methods,  so  that  his  so-called  albite  and  oligoclase  are  actually  andesine-oligoclase,  his  lab- 
radorite  is  andesine,  and  his  anorthite  is  chiefly  labradorite  -with  some  bytownite. 

Irving'*  followed  the  practice  of  Pumpelly,  but  described  about  twice  as  many  petrographic 
varieties.  He  protested  agamst  the  practice  of  basing  rock  names  on  any  such  distinction  as 
that  between  diallage  and  augite,  but  followed  the  custom ,  nevertheless,  in  the  main,  although 
he  tried  to  discriminate  between  diabase  and  gabbro  on  the  basis  of  coarseness  of  crystalhzation, 
assigning  the  name  gabbro  to  the  coarser  grained  varieties.  Irvuig's  orthoclase  gabbro  has 
been  called  hornblende  gabbro  by  Wadsworth  and  porphyritic  gabbro  by  N.  II.  Winchell;  it 
is  nearly  the  same  as  Lane's  gabbro-aplite ;  recently  it  has  been  called  oligoclase  gabbro  by 
F.  E.  Wright.*' 

N.  H.  Winchell,-'  m  1881,  described  thin  sections  of  dolerite,  labradorite  rock,  hyperite, 
and  gabbro.  He  made  the  name  "dolerite"  so  general  in  meanmg  as  to  include  gabbro,  diabase, 
olivine  gabbro,  olivine  diabase,  augite  andesite,  and  basalt.  His  "labradorite  rock"  was  called 
"anorthite  rock"  by  Irving  and  is  now  called  plagioclasite  (or  anorthosite) ;  his  hyperite  is 
now  known  as  norite. 

Wadsworth,*  in  1887,  proposed  a  new  classification  of  the  Keweenawan  igneous  rocks  on 
the  basis  of  the  alterations  wluch  a  given  type  has  undergone.  Thus  a  gabbro  whose  augite 
had  altered  to  hornblende  he  would  call  a  gabbro-diorite.  A  peridotite  may  by  alteration 
become  a  serpentine  or  a  talc  schist ;  in  either  case  Wadsworth  would  call  it  still  a  peridotite, 
addmg  a  name  to  indicate  its  present  condition.     Consequently,  a  rock  called,  for  example,  a 

a  Macfarlane,  Thomas,  Report  of  progress  from  1863  to  18fi6,  Geol.  Survey  Canada,  1868. 

b  Kloos.  J.  II.,  Zeitschr.  Deutsch.  geol.  Gesell.,  1871,  p.  417. 

c  Tumpelly,  Raphael,  Geology  of  Michigan,  vol.  1,  pt.  2, 1S73. 

d  Marvine,  A.  R.,  idem. 

'  Streng,  A.,  Neues  Jahrb.,  1877. 

/  Pumpelly,  Raphael,  Proc.  Am.  Acad.  Arts  and  Sci.,  vol.  1.3, 1878,  p.  285. 

g  Pumpelly,  Raphael,  Geology  of  Wisconsin,  vol.  3, 1880,  pp.  27-49. 

Ii  Irving,  R.  D.,  Geology  of  Wisconsin,  vol.  3,  1880,  pp.  107-200;  Mon.  U.  S.  Geol.  Survey,  vol.  ,'>,  1883;  Geology  of  Wisconsin,  vol.  1,1883,  p.  340. 

>■  Wright,  F.  E.,  Science,  vol.  27,  June,  IMS,  p.  892.     . 

i  Winchell,  N.  n.,  Proc.  Am.  Assoc.  Adv.  Sci.,  vol.  30, 1881,  p.  100. 

i  Wadsworth,  M.  E.,  Bull.  Geol.  and  Nat.  Hist.  Survey  Minnesota  No.  2, 1887. 


398  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

gabbro  by  Wadswortli  may  belong  to  any  one  of  a  dozen  types  as  commonly  recognized.  Never- 
theless, Wadsworth's  names  as  actually  applied  in  tliis  case  may  be  correlated  approximately 
with  the  names  of  otlier  writers,  as  shown  in  the  tal)le  on  page  400. 

Wadswortli  huiorsed  Irving's  protest  against  using  the  distinction  between  augite  and 
diallage  as  a  basis  of  rock  classification,  and  yet,  like  Irving,  he  used  it.  He  did  not  discrimi- 
nate sharply  between  the  ophitic  and  the  poikilitic  textures,  both  of  which  may  be  found, 
sometimes  together,  m  jMimiesota  diabases. 

Bayley,"  in  1889-1897,  described  the  gabbro  batholith  of  Minnesota  in  considerable  detail 
and  also  studietl  tlie  peripheral  phases  of  the  ga})bro.  To  emphasize  the  close  connection  in 
origin  between  the  peridotite  and  the  gabbro  of  tlie  district,  he  called  the  former  nonfeldspathic 
gabbro.  Although  some  of  the  peripheral  phases  described  by  Bayley  may  be  of  later  date 
than  the  gabbro,  if  we  assume  that  they  all  belong  in  the  Keweenawan,  we  fbid  that  Bayley 
recognizes  not  only  the  augite  syenite  of  Irving,  but  also  a  porphyritic  equivalent  which  he  calls 
quartz  keratophyre  on  account  of  the  presence  of  anorthoclase.  He  speaks  of  ohvine-pyroxene 
aggi'egates  which  should  apparently  be  correlated  with  wehrhte,  dimite,  and  pyroxenite. 

In  tlie  peripheral  phases  he  finds  a  texture  which  he  considers  somewhat  characteristic; 
it  consists  of  the  presence  of  many  rormded  grains  of  the  more  important  constituents  inclosed 
by  other  minerals.  Bayley  calls  it  the  granulitic  texture.  It  has  been  called  the  contact 
structure  by  Salomon  and  the  globular  by  Fouque.  It  is  well  described  by  the  term  globular 
or  globuhtic. 

Grant,'  m  1893  and  1894,  described  gabbro,  diabase,  granite,  and  fuie-grained  rocks  pre- 
viously called  muscovadites  in  the  Minnesota  reports.  Grant's  granite  is  the  equivalent  of 
Irving's  augite  syenite,  later  called  soda-augite  granite  by  Bayley.  (See  table  on  p.  400.)  The 
fuie-grained  rocks,  called  muscovadites,  mclude  border  facies  of  the  gabbro  mass  of  various 
types,  but  especially  norite,  fuie-graiiied  gabbro  often  with  hypersthene,  ohvuie  norite,  cordierite 
norite,  etc. 

Hubbard,<^  in  1898,  described  various  types  of  the  Keweenawan  of  Keweenaw  Point.  His 
melapliyre  is  cliiefly  andesite  or  basalt;  Ms  doleritic  melaphyre  is  a  coarser  basalt  or  a  gabbro; 
his  ophitic  nielaphyre  is  a  poikilitic  and  luster-mottled  diabase;  and  liis  porphyrite  is  cliiefijr 
andesite  and  trachyte. 

Lane,"^  in  1898-1906,  described  the  Keweenawan  rocks  of  Isle  Royal  and  northern  Micliigan. 
His  melapliyre  porphyrite  is  the  equivalent  of  Pumpelly's  "Ashbed"  diabase  and  Ii-ving's 
diabase  porphyrite.  Lane's  melapliyre  ojjhite  is  an  olivine  diabase,  luster-mottled  by  means  of 
poikilitic  textures;  his  doleritic  melaphyre  is  a  basalt  porphyry.  Lane  would  confine  the  name 
diabase  to  dike  rocks.  His  augite  syenite  is  said  to  be  at  least  in  part  an  equivalent  of  Bayley's 
quartz  diabase.  He  uses  the  term  ophitic  in  a  narrow  sense,  not  justified  b}''  the  original  defi- 
nition of  Michel  Levy,*  nor  by  his  usage./  He  applies  it  to  those  luster-mottled  rocks  in  which 
single  pyroxene  individuals  inclose  several  plagioclase  crystals,  usually  lath-shaped  and  irregularly 
placed.  It  denotes  thus,  for  Lane,  a  variety  of  the  poikilitic  texture.  In  its  original  meaning, 
still  commonly  used  by  many  and  adopted  here,  it  refers  to  that  texture  of  a  basic  igneous 
rock  produced  when  the  plagioclase  crystallizes  in  lath-shaped  forms  before  the  pyroxene 
solidifies. 

A.  N.  Winchell,^  in  1900,  described  in  detail  a  few  samples  of  the  Keweenawan  rocks  of 
Minnesota.  He  used  the  new  term  jdagioclasite  for  the  rocks  previously  known  usually  as 
anorthosites. 

a  Bayley,  W.  S.,  Am.  Jour.  Sol.,  3ci  ser.,  vol.  37, 18S9,  p.  54;  vol.  39, 1890,  p.  273;  Bull.  U.  S.  Oeol.  Surrey  No.  109, 1893;  Jour.  Geology,  vol.  1, 
1893,  p.  433;  vol.  2, 1894,  p.  S14;  vol.  3, 1895,  p.  1 ;  Mon.  U.  S.  C.eol.  Survey,  vol.  2S,  1S97,  p.  ,il9. 

ft  Grant,  U.  S.,  Twenty-first  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  1893,  p.  5:  Twenty-second  Ann.  Rept.,  1894,  p.  70. 

I-  lluhbani,  L.  L.,  Oeol.  Survey  Michigan,  vol.  (1,  pt.  2, 1898. 

d  Lane,  A.  C,  Geol.  Survey  Michigan,  vol.  6,  pt.  1, 1898;  Bull.  Geol.  Soc.  America,  vol.  14, 1903,  pp.  369, 385;  Jour.  Geology,  vol.  12,  1904,  p.  83;. 
Ann.  Kept.  Geol.  Survey  Michigan  for  liKB,  1905,  pp.  205,  239;  idem  for  1904, 1905,  p.  113;  I'roc.  Lake  Superior  Min.  Inst.,  vol.  12, 1906,  p.  85. 

c  Bull,  Soc.  gSol.  I'"rance,  vol.  0,  1878.  p.  158. 

/  Min(?raIogie  micrographuine.  1879,  ?1.  XXXVI.    See  also  p.  153. 

0  Winchell,  .\.  N.,  Am.  Geologist,  vol.  20,  1900,  pp.  151  (197),  261,  348. 


THE  KEWEENAWAN  SERIES.  399 

N.  H.  Winchell  and  U.  S.  Grant"  publislied  in  1  noo  hy  far  the  most  complete  accounts  of 
tlie  peti'ogiaphy  of  the  Keweenawan  igneous  rocks.  Theii-  nomenclature  varies  very  little 
from  that  commonly  in  use  at  present.  They  described  practically  all  the  petrographic  types 
of  the  Keweenawan  ])reviously  known  and  added  some  half  dozen  new  varieties.  They  used 
diorite-porphyrite  or  diabase-porphyrite  to  designate  more  or  less  ophitic  types  of  andesite 
porphyry  or  augite  andesite  porphyry.  They  used  Wadsworth's  name  zirkelite  for  a  devitri- 
fied  basalt,  basaltic  tuff,  or  tachylyte;  devitrified  obsidian  they  called  an  apobsidian,  and  a 
devitriftod  rhyolite  an  apoi-hyolite,  as  suggested  by  Bascom.  Wadsworth's  quartz-biotite  dio- 
rite  is  called  syenite  by  Grant.     It  is  an  intermediate  type  corresponding  to  a  monzonite. 

o  Winchell,  N.  H.,  and  Grant,  U.  S.,  Final  Rept.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  vol.  5, 1900. 


400 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


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GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


The  quantitative  classification  of  igneous  rocks  as  proposed  by  Cross,  Iddings,  Pirsson, 
and  Wasliington  may  be  used  as  the  basis  of  a  correlation  of  the  Keweenawan  igneous  rocks. 
With  respect  to  chemical  comi^osition  such  a  correlation  (see  table  on  pp.  402-403)  is  more 
exact  than  one  based  on  the  mineral  composition  and  texture,  but  it  can  include  only  those  rock 
types  of  which  satisfactory  quantitative  analyses  are  available. 

An  exammation  of  the  table  of  correlation  on  this  basis  will  reveal  the  fact  that  the  num- 
ber of  satisfactory  analyses  available  is  not  great,  especially  when  compared  with  the  descrip- 
tions previously  mentioned.  Several  of  the  early  analyses  arc  not  included  in  the  tabulation 
because  of  manifest  inaccuracy  or  incompleteness. 

The  analyses  of  Streng  and  Pumpelly  are  good  for  the  time  at  which  tlicy  were  made. 
Calculation  of  the  norms  of  the  analyses  made  for  Pumpelly  by  R.  W.  Woodward  yields  the 
results  tabulated  below  in  columns  1,  2,  and  3: 


1. 

Andose, 
middle  of 
bedS7. 

2. 

Camptonosc 

bottom  of 

bed  S7. 

3. 

Auvergnose, 
lower  part 
of  bed  64. 

4. 

Hessose, 

Cleveland 

mine. 

5. 

Vaalose, 

Houghton 

County,  Mich. 

5.10 
8.34 
16.77 
32.25 

8.46 

7.78 
42.  a 
17.24 

7.23 
22.01 
23.91 

4.26 
22.55 

0.56 
16.24 
36.97 

8.90 

23.06 

10.40 



di                                            

15.88 

20.  .■M 

l.U 

3.71 

1.98 

11.02 
9.90 

13.75 

V,v                                                                

1.30 
17.97 
4.41 
4.41 

20.82 

°f 

7.47 
4.18 
5.32 

mt                                       

8.45 
5.17 

5.80 

.34 

Wio'.'.'.'.'.'.'.'.'..'. 

4.51 

3.25 

2.73 

2.W 

1.39 

100.06 

100.18 

99.52 

99.64 

98.92 

Sweet  published  two  analyses  of  Keweenawan  rocks.  One,  of  diabase  from  the  Ashland 
mine,  Ashland  County,  Wis.,  is  wholly  unsatisfactory;  the  other,  which  represents  a  "greenish- 
gray  diabase"  from  the  Fond  du  Lac  copper  mine,  Douglas  County,  Wis.,  seems  to  be  approxi- 
mately correct.  As  it  stands  it  classifies  as  bandose,  but  this  is  on  the  basis  of  a  content  of  13 
per  cent  of  magnetite  and  10  per  cent  of  quartz.  Both  of  these  figures  are  extremely  improbable 
for  this  rock,  and  point  to  an  error  in  the  determination  of  the  state  of  oxidation  of  the  iron. 
If  the  analysis  is  corrected  in  tliis  particular  it  classifies  as  hessose. 

The  analyses  of  gabbros  published  by  Wadsworth  are  recalculated  in  Washington's  tables 
of  chemical  analyses  of  igneous  rocks ;°  the  norms  of  his  diabase-granophyrites  from  the  Cleve- 
land mme  and  from  Houghton  County  are  given  in  columns  4  and  5  of  the  table  above.  Wash- 
mgton's  tables  give  full  details  regarding  the  recalculation  of  the  analyses  of  Keweenawan 
rocks  pubUshed  by  Van  Hise,  N.  H.  Winchell,  and  Bayley. 

The  norms  of  the  analyses  reported  by  Hubbard  may  be  summarized  as  foUows: 


Magdebur- 
gose. 

Tehamose. 

Lebachose. 

Umptekose. 

Akerose. 

No.  17039. 

No.  17007. 

Q                                                                                                                                  

38.58 

1.94 

39.48 

10.77 

48.90 

18.60 

1.14 

c 

26.13 
18.34 
3.61 

70.61 
.52 

20.57 

57.64 

1.67 

20.  .57 
57.  M 

15.01 

ab.                 

48.21 

7.78 

2.27 
3.70 

3.69 

4.16 
3.66 
1.43 

.86 

di 

.12 

5.62 
2.70 

.43 

5.18 

hv 

of 

3.22 
5.57 
5.92 
1.23 

5.32 

.46 

1.92 

.41 

.70 
1.28 
1.03 

d.ra 

3.04 
2.23 

8.12 

4.00 

HjO 

.42 

2.76 

99.56 

lOO.U 

100.27 

100.64 

.       100.55 

100.07 

o  Washington,  H.  S.,  Prof.  Paper  U.  S.  Geol.  Survey  No.  14, 1903. 


THE  KEWEENAW  AN  SERIES. 


405 


It  is  to  be  remarked  that  not  one  of  these  rock  types  described  by  Hubbard  corresponds 
chemically  with  any  variety  described  by  any  otlier  author.  The  fact  suggests  possible  inaccu- 
racies in  Hubbard's  analyses. 

Lane's  analyses,  as  well  as  Hubbard's,  were  overlooked  and  omitted  from  Washington's 
tables.     Recalculations  of  the  anatyses  given  by  Lane  yield  tlic  following  norms: 


Tonalose- 
dacose. 

Andose. 

Beerba- 
chose. 

Hessose. 

Auvergnose. 

No.  V. 

No.  VI. 

No.  IV. 

No.  VII. 

No.  8 
Light- 
house 
Point. 

St.  Mary 
Land  Co. 

Mount 
Bohemia. 

o 

13.08 
1.02 
17.79 
36.15 
16.90 

1.80 

6.12 
28.82 
24. 19  . 
2.84 
3.80 

6.12 
23.58 
30.30 

4.26 
18.41 

2.78 
46.59 
20.02 

1.67 
28.82 
34.19 

2.78 
21.48 
41.14 

3.89 
18.35 
36.14 

1.C7 
20.96 
31.41 

10.  OL 

ab 

18.34- 

31.69' 

di                                          

1.30 
2.76 
12.58 
11.37 

10.  06 
1.56 

12.97 
6.50 

10. 64 
5.76 
6.01 

10.44 

13.62 
10.77 
13.  06 
3.71 

i2.74 
12.20 

1.36. 

hv 

11.00 

21.62 

o\  

23.25 
3.94 

2.10 
11.14 

2.08 

2.32 

10.67 

2.24 

4.10 

.34 

1.10 

.10 

.67 

7.42 

il 

1.98 

4.71 

.34 

.90 

2.30 

2.00 

.70 

.36 

HjO                                              

5.01 

3.49 

2.82 

3. 90 

1.83 

100.30 

98.87 

101.62 

101.22 

100.27 

100.78 

100.14 

100.00 

97. 2J 

Lane's  gabbro-aphte  differs  in  its  norm  from  the  orthoclase  gabbro  of  Duluth  in  having 
a  greater  abundance  of  quartz  and  also  a  greater  proportion  of  alkalies  as  compared  with  salic 
lime.  His  porphyrite  VI,  on  the  contrary,  belongs  to  the  same  type  (andose)  as  the  orthoclase 
gabbro.  His  porphyrite  No.  1  is  a  beerbachose;  the  others  belong  to  the  classes  hessose  and 
auvergnose,  so  well  represented  in  the  Keweenawan. 

The  analyses  published  by  A.  N.  Winchell  in  1900  were  recalculated  by  Washington  with  the 
exception  of  that  of  the  troctolite,  the  nOrm  of  which  is  given  with  those  derived  from  the  new 
analyses  of  diabase  in  the  secoml  table  below. 

In  view  of  the  scarcity  of  analyses  of  the  typical  volcanic  rocks  of  the  Keweenawan  the  fol- 
lowing new  analyses  are  of  much  interest.     They  were  made  by  George  Steiger  in  the  laboratory 

of  the  Survey. 

Analyses  of  Keweenawan  diabase. 


1. 

2. 

1. 

2. 

Si02 

47.  69 

16.02 

2.41 

8.70 

8.31 

10.54 

2.44 

None. 

.44 

2.04 

1.38 

50.07 

12.  63 

3.  84 

10.30 

5.23 

6.65 

3.  .53 

1.90 

.86 

1.96 

2.50 

ZrOj 

None. 
None. 

.06 
None. 
None. 

.26 
None. 
None. 

None. 

MjOj                                                                   

CO2 

FejOs 

PoOj      

02 

FeO                                                                

SO3 

M^O 

s   

None. 

CaO                                                                  

MuO 

.42 

lsja.,0 

BaO 

.02 

KsO                                                                       .   .. 

SrO 

TI2O 

H2O+                                                       

100. 29 

100.03 

TiOs         

1.  Olivine  diabase  from  bed  108,  Eagle  River  section,  Greenstone  Cliff,  Keweenaw  Point,  Mich.    Sample  No.  5  of  Rohn's  collection  of  Lake 
Superior  rocks.    Rock  powdered  to  pass  a  100-mesh  sieve  before  analysis. 

2.  "Ashhed"  diabase  from  bed  i'..i,  Ea^Ie  River  section,   Keweenaw  Point,  Mich.     Sample  No.  7  of  Rohn's  collection  of  Lake  Superior 
rocks.    Rock  powdered  to  pass  a  100-mesh  sieve  before  analysis,  thus  improving  the  accuracy  of  the  figures  for  ferrous  iron  and  water. 


406 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


Recalculation  of  these  analyses,  together  with  that  of  the  troctolite,  on  the  basis  of  the  quan- 
titative classification  gives  the  following  norms: 


Olivine 
dial)ase. 

"Ashbed" 
diabase. 

Troctolite. 

or 

11.12 
29.87 
13.07 

2.22 

ab           

20.44 
32.80 

7.86 

an                   

28.63 

5.11 

dl               

15.60 
15.04 
7.08 
3.48 
2.74 
.14 

15.12 
15.32 
2.00 
.5.  57 
4.71 
.50 

5.91 

hy • 

of               

30.21 

10.67 

il              

4.41 

MnO          

.18 

HjO 1. 

2.48 

2.82 

6.23 

100.38 

lOO.lO 

100.43 

The  olivine  diabase  belongs  to  the  same  class  as  the  olivine  gabbros  of  Birch  Lake,  the  dia- 
base of  Lighthouse  Point,  ami  several  others — that  is,  to  the  auvergnose  type,  which  seems  to  be 
the  dominant  ty])e  of  the  Keweenawan,  although  the  hessose  type,  which  differs  only  in  having 
a  greater  proportion  of  salic  minerals,  is  also  fairly  abundant.  But  the  "Ashbed  "  diabase  classi- 
fies as  a  camptonose,  very  near  a  kilauose.  It  is  therefore  related  to  Irving's  melaphyre  of  bed 
87  of  the  Eagle  River  section,  and  to  the  more  basic  phases  of  the  orthoclase  gabbro  of  Duluth. 

On  summarizing  the  results  of  this  correlation  of  chemical  analyses  of  Keweenawan  igneous 
rocks  on  the  basis  of  the  quantitative  classification,  it  appears  that  eight  analyses  belong  toClass  I, 
the  persalanes,  in  wliich  less  than  one-eighth  of  the  rock  consists  of  ferric  minerals;  22  analyses 
fall  in  Class  II,  the  dosalanes,  in  wliich  more  than  one-eighth  and  less  than  three-eighths  of  the 
rock  consists  of  ferric  minerals;  14  analyses  belong  in  Class  III,  the  salfemanes,  in  wliich  the  salic 
and  ferric  constituents  are  in  nearly  equal  proportions;  while  the  single  remaining  rock  represents 
Class  IV,  the  dofemanes,  in  wliich  the  ferric  constituents  make  up  about  three-fourths  of  the 
whole. 

Barring  the  peculiar  analyses  of  Hubbard  and  the  hypersthene  gabbro  of  Bayley,  which  is 
kno^\^l  to  be  a  border  facies,  several  general  characteristics  of  the  Keweenawan  igneous  rocks 
appear.  The  salic  constituents  always  make  up  at  least  one-half  of  the  rock;  they  are  usually 
all  feldspar  and  everywhere  are  dominantly  feldspar;  quartz  is  nowhere  very  abundant:  and  the 
lenads  are  almost  unknown  in  the  norms,  as  they  are  also  in  the  modes.  In  all  except  the  Pigeon 
Point  rocks  and  one  sample  from  Mount  Bohemia,  the  anortliite  molecules  either  equal  or  domi- 
nate over  the  alkali  feldspar  molecules  and  the  albite  molecules  dominate  over  orthoclase. 

Still  other  relations  may  be  brought  out  by  considering  separately  the  analyses  belonging  to 
each  class. 

Class  I.  The  eight  persalanes  fall  in  six  subrangs,  half  of  wliich  are  due  to  Hubbard's 
analyses.  They  range  from  the  quartz-rich  felsites  of  Hubbard  to  tlie  quartz-free  plagioclasite. 
The  four  intermediate  types,  belonging  to  two  subrangs,  are  all  derived  from  Pigeon  Pouit. 
In  all  the  rocks  of  tliis  class  except  the  plagioclasite,  alkali  feldspar  molecules  greatly  pre- 
dominate over  anortliite,  and  orthoclase  is  notably  abundant  in  only  two  of  the  rocks. 

Class  II.  The  22  dosalanes  fall  in  nine  subrangs,  but  these  would  be  reduced  to  seven  if 
compound  names  like  beerbachose-andose  were  omitted.  These  rocks  are  chiefly  perfelic,  con- 
taining no  quartz,  but  four  samples  are  quardofelic,  from  the  presence  of  small  amounts  of 
quartz.  The  silica  in  no  case  falls  so  low  as  to  produce  lenails.  Here,  as  in  (Mass  I,  the  anortliite 
molecules  dominate  over  the  alkali  feldspars  in  only  one  subrang,  in  (his  case  hessose.  The 
albite  molecules  always  dominate  over  the  orthoclase. 

Class  in.  The  14  salfemanes  fall  in  four  sui)rangs,  and  10  of  them  fall  in  a  single  subrang, 
namely  auvergnose,  wliich  undoubtedly  represents  the  prevailing  rock  tA'pe  of  the  Keweena- 
wan of  the  district.  The  commonest  variation  from  this  type  is  an  increase  of  salic  constitu- 
ents, with  no  other  change.  Tliis  produces  hessose,  of  which  eight  analyses  are  recorded.  The 
silica  content  of  the  rocks  of  Class  III  is  high  enough  in  all  cases  to  prevent  normative  lenads; 


THE  KEWEENAWAN  SERIES.  407 

6nly  one  analysis  shows  any  normative  quartz.  In  tlic  felilspars  tlie  albitc  molecules  again 
dominate  clearly  over  the  orthoclase. 

Class  IV.  The  single  analysis  falling  in  Class  IV  is  clearly  not  representative;  it  is  a  border 
facies  of  the  great  gabbro  intrusion.  It  is  characterized  by  dominance  of  ferric  constituents,  of 
which  pyroxene  is  the  most  important.  It  is  low  in  soda  and  lime  and  liigh  in  ferrous  iron,  and 
especially  in  magnesia. 

It  is  to  be  expected  that  additional  analyses  of  the  Keweenawan  volcanic  rocks  would  disclose 
still  other  types,  especially  such  as  would  parallel  the  Ivnown  plutonic  types.  The  parallelism  in 
composition  already  established  is  remarkable,  considering  the  relatively  small  number  of  analyses 
available.  Thus  it  appears  that  Lane's  porphyrite  (No.  IV)  and  opliite  (No.  VII),  as  well  as 
Sweet's  Douglas  County  diabase  and  Wadsworth's  diabase-granophyrite  from  the  Cleveland 
mine,  are  the  chemical  equivalents  among  the  volcanic  and  dike  rocks  of  Bayley's  olivine  gabbro 
from  Pigeon  Point  and  from  T.  61  N.,  R.  12  W.,  and  of  A.  N.  Winchell's  ohvine  gabbro  and 
diabase  from  Birch  Lake  among  the  plutonic  rocks.  Again,  Pumpelly's  melaphyre  from  the 
middle  of  bed  87  and  Lane's  porphj'rite  (Nos.  V  and  VII)  from  Isle  Royal  correspond  chemically 
with  the  coarse  hornblende  gabbro  and  orthoclase  gabbro  from  Duluth.  Finally,  the  same 
chemical  type,  auvergnose,  includes  plutonic  rocks  such  as  Bayley's  gabbro  and  olivine  gabbro 
from  Birch  Lake,  N.  H.  Winchell's  gabbro  from  Bashitanaquab  Lake,  and  A.  N.  Winchell's 
troctolite,  together  with  volcanic  or  dike  rocks,  such  as  Pumpelly's  melaphyre  from  bed  64,  Van 
Hise's  Gogebic  County  diabase,  and  Lane's  ophite  from  the  property  of  the  St.  Mary  Land  Com- 
pany and  from  Mount  Bohemia. 

THE  GRAIN  OF  KEWEENAWAN  IGNEOUS  ROCKS THE  PRACTICAL  USE  OF  OBSERVATIONS.  " 

A.  C.  Lane  has  found  that  the  grain  of  the  Keweenawan  igneous  rocks  has  sufficiently  close 
relation  to  the  size  and  tliickness  of  the  masses  to  be  of  some  jiractical  importance  in  explora- 
tory work.  Lane's  theory  of  the  causes  of  the  variations  in  grain  is  not  here  discussed;  it  may 
bo  found  in  his  papers.'     The  facts  which  he  has  developed  are  briefly  as  follows: 

The  Keweenawan  igneous  rocks  most  commonly  occur  in  dikes,  sills,  sheets,  and  lava  floods. 
It  is  found  in  most  places  in  the  Lake  Superior  region  that  crystallization  in  such  forms  has 
resulted  in  finer  grain  near  the  margin  and  coarser  grain  near  the  center.  "The  relative  coarse- 
ness of  crystaUization  may  be  determined  by  measuring  grains  of  any  mineral,  but  experience 
shows  that  in  the  Keweenawan  rocks  of  the  Lake  Superior  region  measurement  of  the  grains  of 
pyroxene  (augite)  is  usually  most  feasible  and  most  satisfactory.  Such  measurements  show 
that  the  size  of  grain  in  narrow  dikes  increases  from  the  margin  to  the  center,  the  size  at  any 
point  being  approximately  proportional  to  the  distance  from  the  margin.  In  wider  dikes 
measurements  show  a  coarse  central  zone  of  variable  width  in  which  the  pj^roxene  grains  have 
approximately  equal  size  or  increase  much  more  slowly.  On  each  side  of  this  central  zone  the 
law  already  stated  is  found  to  hold  approximately.  In  surface  flows,  where  convection  was 
probably  active,  the  size  of  grain  of  the  augite  increases  usually  all  the  way  from  the  margin 
to  the  center.  In  wide  dikes  and  thick  flows  the  augite  is  found  to  increase  in  size  at  a  rapid 
rate  near  the  margin  and  at  a  much  slower  rate  near  the  center.  The  rapid  rate  of  increase  is 
confined  to  a  marginal  zone,  usually  less  than  10  feet  wide.  The  slower  rate  of  increase  is 
fairly  uniform  for  any  one  flow,  and  in  the  luster-mottled  melaphyres  or  ophites  is  usually 
between  1  milUmeter  in  10  feet  and  1  millimeter  in  20  feet,  or  say  1  millimeter  in  3  to  5  meters. 
In  more  feldspathic  flows  it  is  less.  The  very  highest  rate  of  increase  in  the  inner  zone  is 
1  millimeter  for  each  8.5  feet,  but  in  most  cases  the  rate  comes  out  as  1  millimeter  in  11  to 
16  feet.  This  refers  to  the  linear  diameters  of  the  augite  grains,  as  indicated  by  the  mottlings 
on  the  drill  cores.  It  makes  some  difference  whether  they  are  measured  in  this  way,  or  by  the 
luster  mottUngs  due  to  the  Hashes  in  the  sunlight,  or  by  the  knobs  in  the  weathered  surface. 

a  Adapted  from  article  by  A.  C.  Lane. 

liBull.  Geol.  Soc.  America,  vol.  8. 1S9G,  pp.  403—107;  Geological  report  on  Isle  Royalei  Geol.  Survey  Michigan,  vol.  6,  189S,  pp.  IDiVlol;  Am. 
Jom-.  Sci.,  4th  ser.,  vol.  14,  1902,  pp.  39.3-395;  Bull.  Geol.  Soc.  America,  vol.  14,  1903,  pp.  309-400;  Ann.  Kept.  Geol.  Survey  Michigan  for  1903, 
1904,  pp.  205-237;  idem  for  1904, 1905,  pp.  147-153, 163;  idem  for  1908,  1909,  pp.  380-384;  Am.  Geologist,  vol.  35, 1905,  pp.  05-72;  Jour.  Canadian  Miu. 
Inst.,  vol  ?,  Die  Korngrosse  der  .\uvergnosen:  Suppl.  to  Rosenbusch  Festschrift,  1900;  Tufts'  College  Studies,  ITI,  pp;  41-42. 


408  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

The  practical  applications  of  this  study  of  the  size  of  augite  grains  in  mining  have  been 
numerous,  especially  where  contacts  of  igneous  rock  are  an  important  factor,  as  in  the  Keweenaw 
copper  mines,  where  the  desire  is  to  find  the  amygdaloids  at  the  tops  of  massive  lava  flows. 

1.  One  may  distinguish  in  drill  cores  between  amygdaloid  streaks  or  inclusions  in  the  body 
of  a  flow  and  the  main  amygdaloid  top  by  the  fining  of  the  grain  of  the  rock  as  a  whole  toward 
the  latter.  There  is  also  a  finer  grain  just  around  individual  amygdules,  but  in  a  zone  of  only 
microscopic  breadth. 

2.  Extra  wide  flows  may  be  itlentified  by  the  relative  coarseness  in  all  parts,  the  maximum 
orain,  and  the  rate  of  increase  of  grain.  Of  course  such  flows  will  run  out  or  grow  tliinner  in  a 
sufiicient  distance.  For  instance,  an  ophite  attaining  an  augite  grain  of  7  millimeters  is  rather 
persistent  just  about  200  feet  above  the  Baltic  lode.  The  coarsest  ophite  of  all,  attaining  a 
maximum  augite  grain  of  76  miUimeters  (3  inches),  is  several  hundred  feet  thick  out  on  Kewee- 
naw Point  and  runs  over  to  Isle  Royal,  but  diminishes  in  thickness  to  50  feet  at  Portage  Lake. 
It  lies  just  above  the  Allouez  conglomerate  and  a  group  of  former  mines,  and  where  thickest 
is  easily  identified  by  its  grain. 

3.  With  due  regard  to  the  possibihty  of  being  deceived  by  an  extra  feldspatliic  bed,  it  is 
possible  from  a  slow  increase  in  coarseness  of  grain  in  the  thamond-drill  cores  to  infer  that  the 
bed  is  being  traversed  obUquely  and  to  obtain  an  idea  of  its  true  dip.  L.  L.  Hubbard  had  to 
open  the  Challenge  exploration  through  a  heavy  covering  of  drift  in  a  region  where  no  outcrops 
were  near.  The  first  drill  hole  was  put  down  vertically.  The  rate  of  coarsening  of  grain  in 
normal-looking  ophites  being  about  half  what  would  be  expected,  a  dip  of  about  60°  was 
inferred — correctly,  as  it  proved  later. 

4.  A  shaft  sinking  through  drift  entered  massive  trap.  The  question  arose,  "Which  way 
lies  the  nearest  amygdaloid  ?     A  drift  in  the  direction  of  the  finer  grain  soon  found  it. 

5.  A  crosscut  encountered  a  clay  seam  in  a  heavy  trap,  wliich  was  proved  to  be  more  than 
a  mere  seam,  a  displacement,  by  a  marked  difference  in  the  coarseness  of  grain  on  the  two  sides. 
This  difference  acted  as  a  guide  until  the  displaced  lode  was  found. 

6.  It  is  possible  to  tell  how  far  one  has  to  go  through  a  bed  already  more  than  half  pene- 
trated. For  instance,  Kearsarge  shaft  21  of  the  Calumet  and  Hecla  did  not  strike  the  lode 
where  it  was  expected,  owing  to  an  unknown  displacement.  Search  was  made  in  two  opposite 
directions.  Neither  drill  hole  reached  the  lode,  but  it  was  possible  from  the  grain  to  say 
that  the  lode  was  probably  about  30  feet  beyond  the  end  of  one  hole,  which  had  penetrated 
the  foot-wall  trap. 

7.  A  regularity  and  harmony  in  grain  may  distinguish  obscure  outcrops  from  casual  bowl- 
ders. In  the  case  just  mentioned  some  insignificant  outcrops  were  found  which  passed  this  test 
and  indicated  from  their  fineness  that  they  were  in  the  hanging  wall  of  the  lode,  a  little  above 
it.  The  lode  was  thus  found  in  a  week,  when  it  might  have  taken  months  without  this  means 
of  testing. 

8.  Conversely  a  very  coarse  gram  indicates  a  hea\y  bed  of  trap,  and  thus  gaps  and  covered 
spots  in  a  section  may  be  bridged. 

9.  The  ways  of  recognizing  extremely  feldspathic  beds  and  intrusives  have  already  been 
mentioned. 

THE    EXTRUSIVE    MASSES. 

The  extrusive  rocks  are  almost  altogether  lava  beds,  piled  one  upon  another,  the  volcanic 
clastic  rocks  being  insignificant  in  quantity.     The  total  volume  of  the  extrusives  is  vast. 

The  basic  lavas  greatly  predonunate.  They  occupy  about  6,000  square  miles.  It  is 
scarcely  necessary  to  describe  them  in  full,  for  notwithstanding  their  age  they  show  all  the 
textures  and  structures  characteristic  of  the  Tertiary  volcanic  basalts  of  the  West,  the  only 
important  difference  between  the  two  being  that  the  Keweenawan  lavas  have  suffered  exten- 
sive metasomatic  alterations.  The  beds  vary  from  those  less  than  2  feet  to  those  which  are 
100  feet  or  more  hi  thickness,  although  lava  beds  thicker  than  100  feet  are  rather  rare  and  those 


THE  KEWEENAWAN  SERIES.  409 

200  feet  thick  very  rare.  Lane''  mentioned  two  ophites  in  the  Bhick  River  section,  each  of  wliicli, 
accoriling  to  liim,  has  a  thickjiess  of  at  least  500  feet,  and  Wilson  *  describes  a  known  tliickness 
of  more  than  500  feet  of  apparently  one  mass  in  the  Nipigon  basin. 

The  textm-es  exhibited  by  the  lavas  are  to  a  considerable  extent  a  function  of  the  thick- 
ness of  the  flows.  The  surfaces  of  the  flows  show  an  aplianitic  or  glassy  texture.  In  the  thin 
beds  the  aphanitic  texture  may  prevail  to  the  center  of  the  flow.  In  many  of  the  beds  of  mod- 
erate thickness,  from  10  to  20  feet,  there  is  a  well-developed  opliitic  texture.  As  already  noted, 
Lane ''has  worked  out  very  carefidly  the  relations  between  the  te.xtures  exhibited  by  the  lava 
flows  and  their  thickness,  and  he  holds  that  the  textures  are  defhiite  functions  of  the  tliickness. 

The  borders  of  the  flows  are  commonly  amygdaloidal.  As  a  rule  the  amygtlaloidal  borders 
are  thicker  at  the  upper  parts  of  the  flows  than  at  the  lower  parts.  This  texture  may  extend 
2  to  10  feet,  or  even  to  20  feet,  from  the  tops  of  the  flows.  The  amygdules  decrease  in  size  in 
passing  fi'om  the  surface  inward.  In  many  places  the  lower  borders  showing  this  texture 
exhibit  the  peculiar  type  known  as  the  spike  amygdule.  Where  the  lava  beds  are  thm  the 
amj'gdaloidal  texture  may  extend  to  the  centers,  but  this  is  not  common. 

The  lavas  show  in  places  the  usual  volcanic  structures.  Many  of  the  betls  are  columnar. 
Some  flows  present  ropy  surfaces.  The  iipper  parts  of  many  flows  have  a  fragmental  appear- 
ance. In  some  flows  this  is  due  to  the  brealdiig  up  of  the  upper  part  of  the  lava  mass  while 
it  was  still  liquid  or  semiliquid,  and  in  such  places  the  debris  is  likely  to  be  cemented  by  the 
other  parts  of  the  lava.  Here  and  there  the  broken  material  at  the  top  of  the  lava  beds  seems 
to  be  truly  volcanic.  In  other  places  the  broken  fragments,  whether  formeil  by  flowage  or  by 
the  action  of  air  and  water,  are  cemented  by  their  own  debris.  More  rarely  volcanic  fragmental 
deposits,  bombs  and  ashes,  seem  to  have  been  laid  down  upon  the  surface  of  one  flow  between 
the  time  of  its  consohdation  and  the  extrusion  of  the  next  flow.  While  there  is  undoubtedly 
some  volcanic  fi-agmental  material,  as,  for  instance,  the  tuff  at  Taylors  Falls  on  St.  Croix  River 
described  by  WincheU,''  on  the  whole  for  the  basic  lavas  this  is  extraordinarily  small  in  amount, 
consitlering  the  great  extent  and  volume  of  the  igneous  series. 

The  distance  for  which  a  single  bed  can  be  followed  is  to  a  considerable  extent  a  frmction 
of  its  thickness.  The  thin  flows  have  a  very  moderate  extent,  and  even  the  thicker  flows  for 
the  most  part  have  not  been  traced  for  any  great  distance.  The  greatest  distances  for  single 
flows  which  have  been  recorded  are  30  miles,  by  Irving,^  for  the  greenstone,  and  22  miles,  by 
Grant,  ^  for  one  of  the  melaphyres  of  the  St.  Croix  Range.  Irving''  says  that  he  has  traced 
individual  flows  with  certainty  on  the  Minnesota  coast  for  10  to  15  miles.  Groups  of  lava  beds 
have  been  traced  for  much  greater  distances.  For  instance,  the  thin  belt  of  amygdaloids  and 
diabases  above  the  "Great"  conglomerate  of  Keweenaw  Point  has  been  traced  uninterruptedly 
for  150  miles.  Although  a  group  of  lava  beds  may  be  traced  throughout  a  district,  no  group 
is  known  to  be  regional  in  extent;  so  that  in  general  it  is  impossible  to  correlate  lava  groups 
from  district  to  district,  as,  for  instance,  those  of  Keweenaw  Point  with  those  of  the  Mmnesota 
coast.  The  only  such  correlation  yet  attemjjted  is  that  by  Lane  9  between  the  flows  of  Keweenaw 
Point  and  those  of  Isle  Royal. 

The  acidic  flows  differ  physically  from  the  basic  flows.  In  general  they  appear  to  have 
been  much  less  fluid,  and  therefore  have  a  much  shorter  lateral  extent  in  proportion  to  their 
thickness.  In  fact,  a  bunchy  or  lenticular  form  is  characteristic  of  these  flows,  as  is  illustrated 
by  Mounts  Houghton  and  Bohemia  on  Keweenaw  Point.  Amygdaloidal  textures  are  not  so 
common  in  the  acitlic  as  in  the  basic  lavas.  A  flowage  structure,  on  the  other  hand,  is  much 
more  prevalent  in  them  than  in  the  basic  lavas,  and  glassy  textures  are  exceedingly  common. 

a  Gordon,  W,  C,  assisted  by  A.  C.  Lane,  A  geological  section  from  Bessemer  down  Black  River;  Kept.  Geol.  Survey  Michigan  for  1906, 
1907,  p.  461. 

b  Wilson,  A.  W.  G.,  Trap  sheets  of  the  Lake  Nipigon  basin:  Bull.  Geol.  Soc.  America,  vol.  20,  1909,  p.  198. 

c  Geol.  Survey  Michigan,  vol.  6,  pt.  1,  1898,  pp.  123  et  seq. 

i  Winchell,  N.  H.,  The  significance  of  the  fragmental  eruptive  debris  at  Taylors  Falls,  Minn'.:  Am.  Geologist,  vol.  22,  1898,  pp.  72-78. 

f  Irving,  R.  D.,  The  copper-bearing  rocks  of  Lake  Superior:  Mon.  U.  S.  Geol.  Survey,  vol.  5,  1883,  p.  140. 

/  Grant,  U.  S.,  Preliminary  report  on  the  copper-bearing  rocks  of  Douglas  County,  Wis.:  Bull.  Geol.  and  Nat.  Hist.  Survey  Wisconsin 
No.  0,  2d  ed.,  1901,  p.  12. 

9  Lane,  .V.  C,  Geological  report  on  Isle  Royale,  Mich.:  Geol.  Survey  Michigan,  vol.  6,  pt.  1, 1898,  pp.  99-102. 


410  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

The  basic  and  acidic  extrusives  may  correspond  roughly  to  the  gabbro-like  intrusives,  which 
are  regarded  by  Wriglit  "  and  others  as  differentiates  from  the  same  magma. 

As  a  rule  the  dijjs  of  the  lava  beds  range  between  10°  and  4.5°,  but  locally  they  go  beyond 
these  extremes,  sinking  to  liorizontal  and  rising  to  80°  or  even  to  vertical.  A  mass  of  rock 
made  up  of  lava  beds  having  moderate  dips  gives  a  very  characteristic  topography,  which  may 
bo  described  as  ste])likc  or  savvtoothed.  Where  the  beds  are  thin  the  steps  are  low;  where 
thick,  they  are  high.  When  one  walks  across  a  set  of  lava  beds  in  the  direction  of  the  dip, 
as  he  approaches  a  lava  flow  he  finds  a  steep  slope,  or  even  a  precipitous  wall,  which  indicates 
approximately  the  thickness  of  the  flow.  As  he  climbs  to  the  top  of  this  wall  he  finds  a  gentle 
slope,  which  corresponds  I'oughly  with  the  dip  of  the  rocks,  and  down  wluch  he  may  travel  until 
he  comes  to  the  next  flow,  where  he  will  encounter  another  steep  wall,  and  so  on.  The  char- 
acter of  this  topography  has  been  very  well  figured  by  Irving ''  for  the  sawtooth  range  of  Mm- 
nesota  and  for  the  Eagle  River  section  of  Keweenaw  Point. 

THE    INTRUSIVE    MASSES. 

Chemically  the  intrusive  rocks  include  basic,  acidic,  and  intermediate  varieties.  Struc- 
turally they  comprise  every  laiown  form  of  intrusive  rocks  except  bathohths.  There  are  great 
laccoliths,  many  large  bosses,  numerous  and  extensive  sills,  and  abimdant  dikes,  fi-om  those  of 
small  size  to  those  hundreds  of  feet  across.  Many  of  the  dikes  and  sills  beautifully  show  a 
columnar  structure.  In  some  of  the  earUer  studies  the  sills  were  not  separated  from  the  lava 
flows. 

As  to  magnitude,  the  masses  vary  from  the  Duluth  gabbro  of  Minnesota,  wliich,  as  showr 
in  another  place,  has  an  exposed  area  of  2,000  square  miles  and  a  possible  diameter  of  100 
miles,  to  emanations  so  small  as  to  be  lost  in  the  intruded  rocks.  It  appears  probable  that  the 
volume  of  the  intrusive  rocks  within  the  previously  formed  extrusive  lavas  and  conglomerates 
is  really  greater  than  the  volume  of  the  lavas  themselves. 

The  greatest  of  the  intrusions  of  late  Keweenawan  time  are  basic.  These  are  represented 
by  the  gabbro  laccoliths  of  Minnesota  and  Wisconsin.  Some  of  the  acidic  masses  also  are 
large,  but  they  are  likely  to  occur  in  bosshke  forms.  Representatives  of  these  are  the  masses 
at  Bare  Hill  and  West  Pond  on  Keweenaw  Point. 

In  the  description  of  the  areas  of  Huronian  and  Archean  rocks  it  has  been  shown  that 
varying  quantities  of  the  Keweenawan  igneous  rocks  intrude  all  the  previous  formations.  In 
these  great  series  throughout  the  Lake  Superior  region  is  a  mass  of  Keweenawan  rocks  wliich 
is  perhaps  as  great  as  the  lavas.  The  most  conspicuous  examples  of  this  class  are  the  intrusive 
siUs  of  the  Ammikie  group,  called  the  Logan  sills,  which  are  conspicuously  illustrated  at  Thunder 
Bay.  Larger  masses  of  granite  intrude  and  highly  metamorphose  the  Animikie  rocks  west- 
ward from  St.  Louis  River  in  central  Minnesota  through  the  Cu^^una  and  Little  Falls  areas. 
The  granite  of  northeastern  Wisconsin  intrudes  green  schists,  wliich  are  interbedded  ^\■ith  the 
Animikie  group,  and  is  probably  of  the  same  age  as  the  central  Minnesota  granites.  Both  are 
regarded  as  Keweenawan. 

Wright  "  regards  the  aplite  of  Mount  Bohemia  as  differentiation  from  the  gabbro  magma. 
The  aphte  antedates  the  gabbro  sUghtly  in  its  period  of  crystallization.  The  apUte  occurs  not 
ordy  as  a  central  large  mass  but  also  as  small  dikes  and  patches  in  the  gabbro  mass  itself  and 
as  small  apophyses  of  gabbro  in  the  adjacent  ophites.  Wright  suggests  that  the  gal)bro  and 
aphte  are  respectively  the  deep-seated  equivalents  of  the  basic  and  acidic  flows  of  the  Keweena- 
wan series. 

The  close  genetic  association  of  the  aphtes  and  gabbros  has  been  recognized  at  many 
places  in  Minnesota,  AVisconsin,  Micliigan,  and  Ontario.  The  a])lite  was  regarded  as  efl"used 
acidic  sediment  by  Bayley '  in  his  studies  on  Pigeon  Point  and  by  Bowen  <*  in  his  studies  on 

a  Wriglil.  F.  E.,  The  inlrnsivc  rocks  of  .MoiHil  Bohemia.  Michigan:  Ann,  Rcpt.  Geol.  Survey  Michigan  for  loas,  1909,  p.  393. 
b  Irving,  R.  D.,  The  copper-bearing  rocks  of  Lake  Superior:  Mon.  U.  S.  Ctol.  Survey,  vol.  ,5,  18S3,  lig.  1,  p.  142:  fig.  2,  p.  178. 
c  Hayley,  W.  S.,  The  eruptive  and  seilinu-ntary  rocks  on  I'igeon  Point,  Minnesota,  ami  their  contact  plienoroena:  lUiU.  C  S.  Geol.  Survey 
No.  109, 1893. 

■f  Bowen,  N.  L.,  Diabase  and  granophyre  of  the  Gowganda  district,  Ontario:  Jour.  Geology,  vol.  18, 1910,  pp.  (V)S-C74. 


THE  KEWEENAWAN  SERIES.  411 

the  Cobalt  district,  but  practically  all  others  who  have  studied  the  subject,  including  Wright, 
Collins,"  and  the  authors,  regard  the  aphtes  and  gabbros  as  magmatic  differentiations  from  a 
single  magma. 

In  general,  the  later  intrusives  have  not  greatly  metamorphosed  the  early  Keweenawan 
rocks  intruded  by  them,  but  there  are  some  exceptions.  The  far-reaching  metamorpliic  effect 
of  the  great  laccohths  and  bosses  upon  the  lower  scries  has  already  been  described  in  connection 
with  the  Penokee,  Vermilion,  and  Animikie  distiicts.  It  is  probable  that  future  studies  will 
also  show  pronounced  metamorpliic  effects  of  these  laccoliths  on  the  intruded  Keweenawan 
rocks.     Already  tliis  has  been  found  to  be  true  for  the  gabbro  of  Black  River. 

In  several  places  the  acidic  and  especially  the  granitic  rocks  have  produced  notable  meta- 
morpliic effects  on  the  Keweenawan  as  well  as  on  the  lower  series.  Indeed  it  is  believed  that 
the  so-called  orthoclase  gabbros  of  Irving  *"  at  several  places,  at  least,  along  the  Minnesota 
coast  are  due  to  the  granitization  of  ordinary  gabbros  by  the  acidic  rocks. 

SOURCE    OF    LAVAS. 

As  to  the  location  of  the  fissures  from  wliich  the  lavas  issued  it  is  not  possible  to  make 
any  very  definite  statement.  It  has  been  suggested  that  they  were  situated  along  the  south 
shore  of  Lake  Superior.  It  seems  to  us  that  a  much  more  probable  suggestion  is  that  the 
entire  border  of  Lake  Superior,  with  the  possible  exception  of  the  south  side  of  the  east  end, 
was  the  locus  of  a  series  of  great  fissures  which  extended  inland  from  the  lake  for  a  very  con- 
siderable distance,  certainly  in  Wisconsin  and  Minnesota  for  at  least  100  miles.  That  such 
fissures  existed  on  an  extensive  scale  is  shown  by  the  numerous  dikes  cutting  the  Huronian  of 
the  Gogebic  district,  presumably  constituting  necks  for  the  flows  of  the  Keweenawan  just  to 
the  north.  Some  upper  Hui-onian  dikes  in  the  ]\Iarquette  district  may  also  be  so  classed. 
Along  the  north  shore  of  Lake  Superior  are  many  dikes  wliich  may  well  be  related  to  the  flows 
as  necks.  Farther  to  the  west  both  basic  and  acidic  intrusive  dikes  cut  the  flows  of  the  Mesabi 
and  Cuyuiia  districts.  The  convex  outline  of  the  Dulutli  gabbro  laccolith  away  from  the  Lake 
Superior  shore  suggests  a  source  somewhere  in  the  direction  of  Lake  Superior.  It  is  not  certain 
that  similar  vents  may  not  underhe  the  lake. 

No  evidence  has  been  found  of  volcanic  vents.  Fragmental  ejectamenta  of  volcanoes  are 
very  subordinate  in  the  Keweenawan  lavas,  and  the  extent  of  the  lavas  is  greater  than  is  usual 
for  lavas  coming  from  ordinary  volcanoes. 

So  far  as  evidence  is  available,  the  lavas  welled  through  widely  distributed  fissures  cer- 
tainly bordering  and  possibly  underlying  the  present  area  of  the  lake. 

It  is  well  known  that  volcanism  is  a  function  of  orogenic  movements.  In  tliis  connection 
it  is  to  be  noted  that  Keweenawan  volcanism  followed  in  general  the  axis  of  the  Lake  Superior 
synchnorium.  Plutonic  intrusives,  probably  equivalent  in  age  to  the  Keweenawan  flows,  are 
the  large  granite  masses  cutting  the  slates  of  the  Animikie  group  in  the  Cuyuna  and  St.  Louis 
districts  in  central  Mimiesota,  near  the  principal  axis  of  deformation  of  the  Lake  Superior 
syncUne.  The  lavas  issuing  from  the  many  laiown  fissures  bordering  the  synclinorium  doubtless 
flowed  down  the  slopes  toward  the  Lake  Superior  basin.  The  movement  would  be  to  the  south 
from  the  north  side,  to  the  northwest  from  the  south  side,  and  to  the  west  from  the  east  side 
and  not  improbably  northeastward  from  the  west  end  of  the  basin. 

How  far  out  into  the  basin  of  what  is  now  the  lake  the  orifices  went  is  uncertain,  but  Stan- 
nards  Rock,  Micliipicoten,  and  Isle  Royal  show  that  if  they  did  not  extend  some  distance  beyond 
the  shore  the  lavas  have  flowed  a  considerable  distance.  As  the  orogenic  movements  which 
produced  the  Lake  Superior  basin  occurred  largely  during  middle  Keweenawan  time  the  con- 
ditions would  continue  to  be  favorable  for  the  further  issuance  of  lava  and  the  slopes  would 

n  Collins,  W.  11.,  The  quartz  diabases  of  Nipissing  district,  Ontario:  Econ.  Geology,  vol.  5, 1910,  pp.  538-550. 
b  Irving,  R.  D.,  The  copper-bearing  rocks  of  Lake  Superior:  Mon.  U.  S.  Geol.  Survey,  vol.  5, 1883,  pp.  50-56. 


412  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

remain  adequate  to  coiilrol  its  flows,  notwithstanding  the  tendenc\^  for  tlic  earlier  lavas  to- 
lessen  the  slope.  It  is  bcUeved  that  the  analogy  of  the  Kewceiiawari  lavas  is  with  the  Tertiary 
volcanic  rocks  of  the  West,  such  as  those  of  the  Snake  and  Columl)ia  River  plateaus,  which  were 
poured  out  from  parallel  and  intersecting  lines  of  fissures  scattered  over  a  broad  area.  In  short 
the  middle  Keweenawan  is  believed  to  have  been  a  time  of  fissure  erui)tion,  comparabh;  with  the 
great  Tertiary  outbreaks  rather  than  with  local  volcanism,  such  as  occurs  at  the  present  time. 

SEDIMENTARY  ROCKS. 

SOURCE    AND    NATURE    OF    MATERIAL. 

The  sedimentary  rocks  of  the  middle  Keweenawan  are  dominantly  conglomerates  and 
sandstones.  Shales  are  subordinate.  A  light-red  to  dark-red  color  is  very  characteristic  for 
the  Keweenawan  detritus  mterstratified  with  the  lava  beds.  Among  these  rocks  gray  sand- 
stones are  unknown.  The  conglomerates  range  m  coarseness  from  great  bowlder  conglomerates 
to  fine  conglomerates  and  these  grade  into  sandstones  and  the  sandstones  into  shales.  All  the- 
sediments  are  interstratified  with  the  lava  beds. 

The  detritus  of  the  sandstones  and  conglomerates  is  dominantly  derived  from  the  Keweena- 
wan igneous  rocks  themselves.  It  comprises  bowlders,  pebbles,  and  grams  of  sand  and  includes 
materials  from  all  varieties  of  the  basic^  intermediate,  and  acidic  rocks.  The  ease  of  recognizing 
the  fragments,  wliicli  are  of  considerable  size,  has  led  to  a  closer  study  of  the  conglomerates  than 
of  the  sandstones  and  shales. 

Usuall}'  the  coarse  detritus  of  the  conglomerates  is  largely  or  even  dominanth'  from  the 
acidic  group  of  lavas — felsites,  porphyries,  and  granites — and  in  places  also  from  the  intermediate 
rock,  augite  syenite,  even  where  the  sedimentary  beds  are  between  basic  lavas.  This  is  doubtless 
explained  in  a  measure  by  the  more  resistant  character  of  these  formations  as  compared  with 
the  basic  rocks,  but  it  is  also  probable  that  the  explanation  rests  partly  in  the  fact  that  the 
acidic  lavas  were  viscous  and  therefore  they  built  up  mountains  wliich  rose  to  great  height  and 
were  subject  to  exceptional  erosion,  wliile  the  basic  lavas  formed  areas  of  relatively  low  relief. 
Not  uncommonly,  however,  where  the  conglomerates  immediately  overlie  basic  or  intermediate 
rocks,  detritus  from  tliis  source  is  especially  likely  to  be  present  and  may  be  dominant.  In 
some  places,  as  in  the  localities  near  the  mouth  of  Little  Montreal  River  on  Keweenaw  Point, 
described  by  Hubbard,"  the  pebbles  and  bowlders  are  derived  wholly  from  the  earlier  beds  of 
lava.  Thus  between  beds  of  melaphyre  are  melaphyre  conglomerates  and  between  beds  of 
porjihyrite  are  porphyrite  conglomerates.  Similarly  between  beds  of  felsite  are  felsite  con- 
glomerates. There  are,  however,  all  gradations  from  conglomerates  whose  pebbles  and  bowlders 
are  derived  largely  or  exclusively^  from  the  immediately  subjacent  flow  to  those  in  which  the 
pebbles  are  from  various  sources  and  thus  comprise  basic,  acidic,  and  intermediate  materials 
all  mingled  in  different  proportions. 

In  the  conglomerates  the  finer  material  between  the  pebbles  is  usually  composed  of  detritus 
from  the  same  rocks  as  the  pebbles  themselves.  However,  in  a  particular  conglomerate  the 
matrix  may  include  material  from  different  sources  and  in  diiTerent  proportions  fnun  that  of 
the  pebbles.  Thus  even  in  the  melaphyre  and  porphyrite  conglomerates  described  b}'  Hubbard* 
the  matrix  is  derived  largely  from  acidic  rocks. 

Commonly  man^'  of  the  particles,  even  in  the  matrix,  are  sufTieiently  coarse  to  be  composed 
of  more  than  one  mineral.  But  where  the  mechanical  subdivision  of  the  material  has  gone  far, 
the  original  rocks  are  broken  into  their  constituent  minerals  and  thus  in  the  matrix  of  the  con- 
glomerates (here  are  likely  to  be  minerals  from  the  cliief  varieties  of  the  original  igneous  rocks. 
Generally  the  original  minerals  from  the  acidic  rocks  are  more  noticeable,  as  the  basic  con- 
stituents are  more  subject  to  alteration.  Still  it  is  usually  easy  to  recognize  constituents  from 
the  basic  rocks.  Of  these,  magnetite,  being  the  least  destructible,  is  especially  hkel}-  to  be 
conspicuous. 

a  Geol.  Survey  Michigan,  vol.  6,  pt.  2, 1898,  p.  38.  K  Idem,  p.  21. 


THE  KEWEENAWAN  SERIES.  413 

As  a  rule  there  have  been  extensive  alterations  of  the  (constituents  of  the  conglomerates. 
These  are  more  pervasive  in  the  matrix  than  in  the  original  pebbles,  l)ut  may  extend  throughout 
even  large  bowlders.  As  a  result  of  tliis  alteration  there  are  commonly  present  the  secondaiy 
minerals  zeolite,  chlorite,  epidote,  calcite,  cjuartz,  and  in  places  copper,  which  have  been  brought 
in  by  infiltrating  waters  or  have  formed  in  place  by  metasomatic  changes.  Lane  "  has  discussed 
the  chemical  features  of  one  of  these  alterations. 

The  sandstones,  so  far  as  they  have  been  studied,  seem  to  have  aljout  the  same  variations 
as  the  conglomerates.  In  general  the  particles  are  composc(L  largely  of  fragments  of  the  same 
acidic  rocks  whose  fragments  compose  the  conglomerate.  Tliis  means  that  their  dominant 
constituents  are  feldspar  and  r|uartz,  with  which  there  is  always  more  or  less  clayey  material  and 
abundant  ferrite.  Ordinarily  also  there  are  subordinate  contributions  from  the  basic  rocks, 
wliich  furnish  feldspar,  augite,  and  magnetite.  Hematite  staining  the  grains  is  also  pervasive. 
Perhaps  their  most  characteristic  feature  as  compared  with  common  quartzose  sandstones  is 
the  fact  that  quartz  is  not  a  dominating  constituent.  As  in  the  conglomerates,  so  also  in  the 
sandstone  beds,  secondary  calcite,  chlorite,  epidote,  and  the  other  alteration  products  of  the 
original  rocks  are  common,  and  even  copper  is  to  be  found  here  and  there. 

EXTENT    OF    SEDIMENTS. 

As  to  the  extent  of  the  sediments  interstratified  with  the  lavas,  tlie  same  statements  may 
be  made  as  with  reference  to  the  lavas;  none  of  them  are  regional.  In  proportion  as  they  are 
tliick  they  naturally  have  a  greater  lateral  extent.  The  thickest  of  these  formations,  the  "Great" 
conglomerate  of  Keweenaw  Point,  which  has  a  maximum  thickness  of  2,300  feet,  has  been 
traced  for  over  100  miles,  and  one  of  the  comparatively  thin  conglomerates  lying  immediately 
under  the  greenstone  of  Keweenaw  Point  has  been  traced  for  a  distance  of  50  miles.  A  con- 
glomerate bed  may  vary  greatly  along  the  strike  in  the  proportion  of  the  constituents  from  a 
particular  source;  also  in  thickness  and  coarseness.  At  many  places  where  conglomerate  beds 
thin  they  run  laterally  into  sandstones  or  shales,  the  coarser  fragments  failing  altogether. 
Finally,  a  single  sedimentary  betl  along  the  strike  may  be  split  into  more  than  one  bed  by  inter- 
leaved lavas. 

UPPER  KEWEENAWAN. 

The  upper  Keweenawan  is  confined  to  northern  Wisconsin  and  ]\Iichigan,  where  it  con- 
stitutes a  great  sedimentarv  division,  consisting  of  conglomerates,  sandstones,  and  shales. 
It  extends  from  Manitou  Island,  east  of  Keweenaw  Point,  along  the  border  of  the  outer  end  of 
Keweenaw  Point,  where  its  strike  carries  it  out  into  the  waters  of  Lake  Superior  at  Gate  Hai^bor. 
It  reappears  about  6  miles  west  of  Eagle  Harbor  and  extends  contmuously  as  a  northwestward- 
dipping  monocline  to  the  head  of  Chequamegon  Bay  in  Wisconsin,  the  other  side  of  the  syn- 
clinal fold  being  under  the  waters  of  Lake  Superior.  The  peninsula  north  of  Chequamegon 
Bay  brings  to  the  surface  the  north  side  of  the  syncline,  so  that  inland  to  the  southwest  in 
Wisconsin  the  full  fold  is  present  in  a  canoe-shaped  area. 

The  upper  Keweenawan  consists  from  the  base  up  of  the  "Outer"  conglomerate,  the 
Nonesuch  shale,  and  the  Freda  sandstone. 

The  "Outer"  conglomerate  on  Keweenaw  Point  has  a  thickness  of  1,000  feet.  To  the 
west  it  increases  in  thickness  and  at  Black  River  apparently  attains  5,000  feet.  Farther  Avest 
it  becomes  tliinner,  the  tliickness  at  Potato  River  being  800  to  1,200  feet  and  at  Bad  River  only 
350  feet.  The  "Outer"  conglomerate  has  thus  been  traced  from  the  east  side  of  Manitou 
Island  to  Penokee  Gap,  a  distance  between  175  and  200  miles.  Petrographically  tliis  con- 
glomerate is  like  the  conglomerate  interstratified  with  the  lavas  and  is  therefore  composed 
mainly  of  detritus  from  the  acidic  rocks. 

Above  the  "Outer"  conglomerate  is  the  Nonesuch  shale,  which  has  been  traced  from 
Portage  Lake  to  Bad  River,  a  distance  of  125  mUes.     Its  thickness  at  Portage  Lake  is  about 

a  Lane,  A.  C,  The  decomposition  of  a  bowlder  in  the  Calumet  and  Hecla  conglomerate:  Econ.  Geology,  vol.  4, 1909,  pp.  158-173. 


414  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

200  feet,  at  Montreal  River  500  feet,  at  Potato  River  from  250  to  400  feet,  and  at  Bad  River 
125  feet.  Altliou<;h  this  formation  is  diiefly  shiilo  it  lias  interstratifie<l  sandstone  hu'ers,  and 
unlike  tiae  sandstones  and  conglomerates  interstratified  with  the  lavas  it  contains  large  amounts 
of  basic  detritus.  In  places,  indeed,  the  basic  material  is  so  abundant  as  almost  to  exclude  the 
acidic.  Thus  at  the  base  of  the  Xonesuch  shale  there  is  an  important  change  in  the  character 
of  the  material  of  the  Keweenawan  sediments. 

The  Freda  sandstone  composes  much  the  larger  portion  of  th(^  up]>er  division  of  the  Kewee- 
nawan.  The  apparent  thickness  of  the  entire  formation  is  not  less  than  19,000  feet.  Irving  " 
gives  the  thickness  of  the  sandstone  exposed  at  Montreal  River  as  12,000  feet,  and  7,000  feet 
of  overl3dng  beds  are  seen  near  Ashland.  Accortling  to  Irving  it  is  a  characteristic  feature  of 
this  sandstone  tjiat  quartz  is  ver\'  subordinate.  Indeed,  in  jilaccs  it  is  nearly  quart zless.  The 
detritus  has  therefore  been  derived  tlominantly  fi'om  the  basic  igneous  rocks  and  only  subordi- 
nately  from  the  acidic  igneous  rocks  of  the  Keweenawan,  and  apparently  the  pre-Keweenawan 
rocks  have  contributed  but  small  amovuits  of  material.  However,  Lane*  states  that  pebbles 
of  banded  jaspery  hematite  and  other  .iron-bearing  rocks  occur  abundantly  in  the  "Outer" 
conglomerate  and  further  that  the  detritus  of  the  sandstones  themselves  is  derived  predomi- 
nantly from  the  Iluronian  and  Keewatin  rocks.  Proljably  the  statements  of  Irving  and  Lane 
were  made  with  different  areas  in  mind,  and  more  ex,tensive  studies  of  the  upper  Kew  cenawan 
are  perhaps  necessary  in  ordjer  to  make  exact  general  statements  concerning  the  sources  of  its 
detritus. 

As  the  upper  Keweenawan  is  confined  to  Michigan  and  Wisconsm,  it,  like  the  middle  and 
lower  Keweenawan,  fails  to  be  regional  in  extent,  although  it  has  a  greater  linear  and  surface 
extent  than  the  other  two  divisions.  It  is  probable,  however,  that  the  upper  Keweenawan 
origmally  occupied  a  large  part  of  the  Lake  Superior  basin.  It  is  the  softest  division  of  the 
series  and  was  therefore  more  deeply  eroded  than  the  others.  At  present  the  area  once  prob- 
ably covered  by  tliis  sandstone  is  occupied  by  the  Cambrian  sandstone  or  the  waters  of  the 
lake. 

RELATIONS   TO   UNDERLYING   SERIES. 

The  Keweenawan  rests  unconformably  on  all  of  the  lower  series  with  wliich  it  comes  into 
contact.  This  unconformity  is  so  perfectly  cle^ar  for  the  Archean  gneisses  that  it  has  been 
recognized  since  the  days  of  Logan,  "^  that  great  geologist  having  noted  tliis  relation  at  Granite 
Island,  on  the  north  side  of  I^ake  Superior,  and  at  several  points  on  the  east  shore  of  the  lake. 
The  Keweenawan  has  unconformable  relations  vritli  each  of  the  Iluronian  liivisions  with  which 
it  comes  into  contact,  but  in  earlier  days  the  unconformity  between  the  Keweenawan  and  the 
upper  Iluronian  was  not  recognized. 

The  relations  of  the  Keweenawan  series  and  the  Animikie  group  have  been  especialh' 
studied  north  of  Thunder  Bay,  and  here  the  Animikie  was  indurated  and  yielded  well-rounded 
fragments  to  the  Keweenawan  basal  conglomerate  at  many  points.  Details  as  to  these  rela- 
tions are  more  fuUy  given  on  pages  207—208.  In  the  Penokee  district  the  Keweenawan  extends 
for  many  miles  along  the  upper  Huronian,  and  here  there  is  evidence  of  even  a  greater  erosion 
interval  between  the  two  series  than  on  the  north  shore. 

It  has  been  noted  that  the  Duluth  gabbro  at  its  bottom  is  in  contact  at  many  places  with 
the  Iluronian  and  with  the  Archean.  Near  its  bolder,  in  areas  occupied  by  the  rocks  of  these 
periods,  are  numerous  dilces  and  bosses  which  are  identical  in  chemical  composition  and  even 
correspond  very  closely  in  mineralogical  character  with  the  Duluth  gabbro.  Indeed,  some  of 
the  masses  may  be  actually  connected  with  the  Duluth  gabbro.  There  can  scarcely  be  any 
doubt  that  these  intrusive  rocks  in  the  lower  series  are  of  Keweenawan  age. 

The  Keweenawan  ago  of  the  great  dikes  and  sills  of  diabase,  which  are  so  abundant  in  the 
Arumikie  group,  is  scarcely  less  clear.     These  ilikes  and  sills  are  Identical  in  their  chemical  and 

o  Mon.  U.  S.  Geol.  Survey,  vol.  5, 1883,  p.  230.  c  Logan,  W.  E.,  Report  oJ  progress  to  1863,  Geol.  Survey  Canada,  1863,  p.  "S. 

6  Jour.  Geology,  vol.  15, 1907,  p.  090. 


THE  KEWEENAWAN  SERIES.  415 

mineralogical  composition  and  in  their  structural  and  textural  characters  with  those  which 
are  found  in  the  Keweenawan  itself  east  of  the  Animikie  at  Thunder  anil  Black  bays  and  west 
of  the  Animikie  in  ]\Iinnesota.  Some  of  the  capping  diabases  of  the  Nipigon  basin  may  be 
flows  resting  unconformably  upon  lower  Keweenawan,  Huronian,  and  Archean  rocks. 

In  the  Penokee-Gogebic  district  numerous  diabase  dikes  cut  the  iron-bearing  formation. 
These  have  attitudes  at  right  angles  to  the  dips  and  in  chemical  composition  are  like  the  basic 
lavas  on  the  overlying  Keweenawan  traps.  It  can  hardly  be  doubted  that  these  are  the  pipes 
through  which  the  lavas  issued. 

The  Animikie  group,  including  the  latest  Huronian  formations,  is  cut  by  acidic  intrusive 
rocks  which  are  almost  certainly  Keweenawan.  The  largest  of  these  that  has  been  recognized 
is  the  Embarrass  granite  of  the  Giants  Range,  the  granites  south  of  the  Cuyuna  district  of 
Minnesota,  and  the  granite  intrusive  into  the  Quinnesec  schist  of  northeastern  Wisconsin. 
Dikes  of  granite  are  known  to  cut  the  Animikie  group  along  the  Giants  Range. 

RELATIONS   TO   OVERLYING   SERIES. 

The  lowest  fossiliferous  Cambrian  rocks  in  the  Lake  Superior  region  are  of  Upper  Cambrian 
age.  These  rest  unconiormably  upon  the  middle  Keweenawan  in  the  St.  Croix  Valley  and 
on  the  southeast  side  of  Keweenaw  Point.  In  the  former  locality  an  actual  unconformable 
contact  is  observed,  but  in  the  latter  the  relations  are  complicated  by  faulting.  The  middle 
Keweenawan  throughout  is  considerably  tilted,  wliile  the  Upper  Cambrian  beds  are  uniformly 
flat-lying.     These  facts  prove  only  that  the  middle  Keweenawan  is  pre-Upper  Cambrian. 

The  upper  Keweenawan  is  in  contact  only  with  the  Lake  Superior  sandstone  (supposedly 
Upper  Cambrian),  a  red,  quartzose  sandstone  outcropping  along  the  southwest  shore  of  Lake 
Superior.  The  feldspathic  sandstones  and  shales  of  the  upper  Keweenawan  grade  conformably 
up  into  the  red  quartzose  Lake  Superior  sandstone.  Exposures  of  the  gradation  are  observed 
on  Fish  Creek,  on  Middle  River,  and  on  St.  Louis  River.  The  only  possible  doubt  about  the 
gradation  is  the  fact  that  the  feldspathic  sandstones  and  mud-cracked  shales  have  not  been 
absolutely  proved  to  be  Keweenawan,  although  from  their  character,  distribution,  and  rela- 
tions to  the  Keweenawan  there  is  every  reason  to  believe  that  they  are  the  uppermost  Kewee- 
nawan. At  no  place  are  there  fragments  of  the  Keweenawan  sandstone  within  the  Lake 
Superior  sandstone.  Finally,  the  upper  Keweenawan  sandstone  and  the  Lake  Superior  saml- 
stone  are  closely  related  in  their  deformation,  for  whUe  the  upper  Keweenawan  as  a  whole  is 
folded,  and  the  Lake  Superior  sandstone  as  a  whole  is  flat-lying,  along  the  axis  of  the  synclino- 
rium  in  the  vicinity  of  Asliland  and  eastward,  both  are  tUted.  The  western  Lake  Superior 
sandstone  seems  to  be  areally  connected  with  the  known  Upper  Cambrian  of  the  St.  CroLx 
River  valley  and  has  been  correlated  with  the  Upper  Cambrian.  However,  it  is  nonfossiliferous, 
areal  continuity  with  the  known  Cambrian  is  not  established,  and  it  is  entirely  possible  that  the 
western  Lake  Superior  sandstone  as  a  whole  may  be  older  than  the  Upper  Cambrian.  If  the 
Lake  Superior  sandstone  is  Upper  Cambrian,  as  it  is  now  correlated,  then  the  upper  Keweenawan 
is  pre-Upper  Cambrian. 

In  the  absence  of  the  Middle  and  Lower  Cambrian,  it  is  difficult  decisively  to  prove  that 
the  Keweenawan  is  pre-Cambrian  rather  than  Middle  or  Lower  Cambrian.  It  has  seemed 
to  us,  as  it  has  to  Irving,"  to  ChamberUn,''  and,  in  fact,  to  most  of  tlie  geologists  who  have 
studied  this  area,  that  in  hthology,  lack  of  fossils,  deformation,  and  separation  of  the  middle 
Keweenawan  from  the  Upper  Cambrian  by  unconformity  the  Keweenawan  series  as  a  whole 
is  much  more  closely  allied  to  the  pre-Cambrian  than  to  the  Cambrian.  Another  group  of 
geologists,  while  admitting  all  these  differences,  nevertheless  hold  that  the  Keweenawan  is 
probably  Cambrian. 

Our  reasons  for  assigning  the  Keweenawan  as  a  whole  to  the  pre-Cambrian  rather  than  to 
the  Middle  or  Lower  Cambrian  are  summarized  below.     While  we  assume  the  Upper  Cambrian 

"Irving,  R.  D.,  Mod.  U.  S.  Geol.  Survey,  vol.  5,  1883.  tChamberlin,  T.  C,  Bull.  U.  S.  Geol.  Survey  No.  23, 1885. 


416  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

ao-o  of  tlio  Lake  Superior  sandstone,  these  conclusions  are  no^  wiiolly  dcnendcnt  upon  such 
interpretation  of  age  of  the  Lake  Superior  sandstone. 

The  Cajnl)rian  is  fossihferous-.  the  Keweenawan  is  not. 

The  Canihrian  is  largely  a  subacpieous  deposit;  the  Keweenawan  is  largely  subaorial. 

The  Cambrian  contrasts  with  the  Keweenawan  in  lacking  volcanisni. 

The  known  Upper  Cambrian  is  almost  flat-lying.  The  same  is  true  for  niost  of  the  Lake 
Superior  sandstone.  The  Keweenawan  as  a  whole  is  tilted.  In  the  few  localities  where  the 
Lake  Superior  sandstone  and  upper  Keweenawan  are  tilted  together,  this  may  be  due  partly  to 
movements  as  late  as  the  Cretaceous.  Also,  as  already  noted,  there  is  possible  doubt  about  the 
Upi)er  Cambrian  age  of  the  Lake  Superior  sandstone.  It  is  agreed  by  all  that  the  known 
Upper  Cambrian  rests  unconformably  upon  middle  Keweenawan  beds. 

The  Cambrian  rests  upon  a  peneplain  of  continental  extent,  over  which  the  Paleozoic  sea 
swept  and  deposited  Paleozoic  sediments,  with  overlap  relations  to  the  pre-Cambrian  rocks. 
This  sea  did  not  reach  the  Lake  Superior  country  until  Upper  Cambrian  time,  and  parts  of 
Canada  were  not  reached  until  Ordovician  time.  If  the  Keweenawan  is  Cambrian  it  constitutes 
a  marked  local  variation  from  the  general  uniform  conditions  of  overlap.  The  upper  Kewee- 
nawan sediments  rest  on  a  plane  which  cuts  the  pre-Cambrian  peneplain  at  a  considerable 
angle,  as  is  well  shown  on  Keweenaw  Point.  (See  p.  97.)  If  the  Keweenawan  were  to  be 
regarded  as  MidiUe  or  Lower  Cambrian,  it  would  be  necessary  to  conclude  that  the  Middle  or 
Lower  Cambrian  in  this  district  had  taken  on  remarka])le  local  characteristics  different  from 
those  of  the  Middle  and  Lower  Cambrian  elsewhere.  On  the  other  hand  these  local  character- 
istics are  accordant  with  those  of  the  pre-Cambrian  rocks  of  this  area. 

The  similarity  of  lithology  and  accordance  of  structure  between  upper  Keweenawan  and 
Cambrian  are  the  natural  sequence  of  transgression  of  a  sea  over  fiat-lying  sediments.  The 
conditions  are  not  different  from  those  that  would  prevail  if  the  ocean  were  to  transgress  to-day 
from  the  Gulf  of  Mexico  across  the  flat-lying  and  little-consolidated  Paleozoic  sediments  of  the 
upi)cr  Mississippi  Valley.  It  would  be  extremely  difflcult  to  prove  the  unconformity  in  any 
limited  area,  especially  where  exposures  are  not  numerous.  In  fact,  it  is  known  that  the 
Lake  Superior  basin  was  formed  during  Keweenawan  time,  and  it  is  entirely  probable  that 
local  sedimentation  within  this  basiii  would  merge  upwards  into  tlie  sedimentation  from  the 
overlapping  Upper  Cambrian  ocean,  while  upper  Keweenawan  beds  may  locaUy  have  uncon- 
formably overlapped  the  lower-middle  member,  from  whose  detritus  they  are  in  large  part 
built  up.     It  is  concluded  that  the  Keweenawan  is  mainly  pre-Cambrian. 

Our  view  of  the  sequence  of  deposition  is  this:  The  main  portion  of  the  Keweenawan  was 
put  down  in  pre-Cambrian  time.  During  and  subsequent  to  its  deposition  folding  developed 
the  Lake  Superior  basin.  In  late  Keweenawan  time  erosion  of  the  lower  beds  near  the  rim  of 
the  basin  and  deposition  of  the  upper  beds  within  the  basin  were  going  on  simultaneously. 
The  deposition  within  the  basin  continued  nearly  or  quite  to  the  time  that  the  Paleozoic  sea, 
encroaching  from  the  south,  reached  the  basin.  The  Paleozoic  sea  then  deposited  its  beds 
with  marked  structural  discordance  upon  the  lower-middle  Keweenawan,  and  with  substantial 
accordance  upon  upper  Keweenawan  beds  in  parts  of  the  Lake  Superior  basin  in  wliich  deposi- 
tion was  continuous  up  to  the  time  of  the  arrival  of  this  sea. 

CONDITIONS   OF  DEPOSITION. 

The  r|uestion  now  arises  as  to  the  i)hysical  conditions  under  whicli  the  Keweenawan  was 
laid  down.  According  to  the  standard  interpretation  the  widespread  sandstones  and  con- 
glomerates at  the  bottom  of  the  Keweenawan  would  be  taken  a,s  evidence  that  at  the  beginning 
of  Kc'weenawan  time  this  region  was  submerged.  Under  this  interpretation  the  occurrence  of 
sandstones  and  conglomerates  between  the  lavas  has  been  taken  as  evitlence  that  the  ellusive 
rocks  were  largely  submarine.  The  persistence  of  sedimentary  beds  such  as  those  that  occur 
at  the  up])er  iiorizons  and  es|)ecially  the  "Great"  conglomerate  of  the  middle  Keweenawan  has 
usually  been  taken  as  decisive  evidence  of  this  conclusion.     However,  work  by  Medhcott  and 


THE  KE  WEEN  A  WAN  SERIES.  417 

Blanford,"  Walther,''  Passaige/  Davis,''  Huntington/  Johnson/  Barrell/  Chamberliii  and 
Salisbuiy,^  and  others  has  emphasized  the  importance  of  continental  sedimentary  deposits. 
As  yet  the  criteria  for  discriminating  continental  and  submarine  deposits  have  not  been  fully 
worked  out,  and  therefore  there  must  be  considerable  uncertainty  as  to  our  conclusions  upon 
this  matter  concerning  the  Keweenawan,  especially  as  the  Keweenawan  sediments  have  never 
been  studied  with  reference  to  this  particular  point. 

The  following  evidence  we  take  to  favor  the  terrestrial  oi-igin  of  at  least  a  part  of  the 
Keweenawan : 

1.  The  thickness  of  the  sediments. 

2.  The  repetition  of  conglomerate  beds  at  many  horizons  through  several  thousand  feet. 
This  would  involve  too  rapid  fluctuation  of  water  level  for  the  beds  to  be  satisfactorily  explained 
as  aqueous  deposits.  The  continuity  of  thick  beds  of  conglomerate  also  is  in  accord  with  ter- 
restrial sedimentation,  for  subaqueous  sedimentation  is  more  likely  to  develop  thick  beds  over 
only  local  areas,  as  about  steep  shores. 

3.  The  feldspathic,  poorly  assorted,  and  almost  completely  oxidized  character  of  the 
Keweenawan  sediments,  as  shown  by  their  prevailing  red  colors  and  lack  of  graphitic  material. 
They  also  show  locally  alternating  beds  of  red,  yellow,  and  purple,  suggestive  of  seasonal  varia- 
tions. 

4.  Many  ripple  marks  in  the  Freda  sandstone  are  of  the  horseshoe  shape  made  by  rills  of 
water  at  the  surface.     These  contrast  with  the  ripple  marks  made  by  wave  action. 

5.  The  fact  that  except  for  alterations,  the  basic  flows  are  in  all  essential  respects  like  the 
subaerial  basaltic  lava  flows  of  Tertiary  time.  Their  upper  and  lower  surfaces  are  amygdaloidal. 
Although  in  places  their  surfaces  have  a  broken  or  pseudoconglomerate  appearance,  they  usually 
lack  the  peculiar  ellipsoitlal  structure  wliich  is  "characteristic  of  the  Keewatin  and  Huronian 
basic  lavas  described  in  another  place  (pp.  510-512)  and  which  has  been  shown  to  be  especially 
characteristic  of  subaqueous  basic  lava  flows. 

6.  The  fact  that  the  matrix  of  the  basal  conglomerate  on  the  north  shore  is  in  places  a  lime- 
stone, suggesting  deposition  of  evaporation  under  surface  arid  or  semiarid  conditions,  as  may  be 
observed  to-day  in  the  Bighorn  Mountains  and  elsewhere  in  the  West. 

7.  The  lack  of  fossils. 

8.  The  general  contrast  with  the  underlying  Huronian  sediments,  in  which  evidence  of 
water  deposition  is  faii'ly  good. 

9.  Mud  cracks  are  common  in  some  shales. 

10.  The  rapid  alternation  of  thin  beds  of  coarse  unweathered  debris  with  fine  red  mud- 
cracked  and  ripple-marked  shales. 

We  are  therefore  inclined  to  believe  that  terrestrial  deposition  has  played  an  important 
part  in  the  development  of  this  portion  of  the  Keweenawan,  but  with  the  information  now  avail- 
able we  are  unable  to  say  how  much  of  a  part  it  has  played. 

The  truth  probablj'  lies  between  the  two  extremes  of  the  subaqueous  and  subaerial  Iiypothe- 
ses;  that  is,  the  Keweenawan  lavas  and  sediments  were  neither  exclusively  terrestrial  nor  exclu- 
sively subaqueous,  though  too  little  is  known  to  warrant  definite  statements  concerning  their 
origin.  For  the  middle  and  upper  Keweenawan  it  is  believed  to  be  largely  subaerial,  but  also  in 
considerable  measure  subaqueous.  When  the  orogenic  movement  and  the  period  of  volcanism 
of  middle  Keweenawan  time  were  well  under  way  it  would  be  very  natural  that  the  areas  where 

oMedlicott,  H.  B.,  and  Blanford,  W.  T.,  Geology  of  India,  2d  ed.,  revised  by  E.  D.  Oldham,  1S79,  pp.  149-150,  391-458. 

1)  Walther,  Johannes,  Das  Gesetz  der  Wiistenljildung,  Berlin,  1900. 

c  Passarge,  Siegfried,  Die  Kalahari,  Berlin,  1904. 

d  Davis,  W.  M.,  The  fresh-water  Tertiary  formations  of  the  Eocky  Mountain  region:  Proc.  Am.  Acad.  Arts  and  Sci.,  vol.  35, 1900,  pp.  345-373; 
Bull.  Geol.  Soc.  America,  vol.  11,  1900,  pp.  590-COl,  603-604;  A  journey  across  Turkestan:  Carnegie  Inst.  Washington,  Pub.  26, 1905. 

« Huntington,  Ellsworth,  Pulse  of  Asia,  1907. 

/Johnson,  W.  D.,  The  High  Plains  and  their  utilization:  Twenty-first  Ann.  Eept.  U.  S.  Geol.  Survey,  pt.  4, 1901,  pp.  C09-741. 

9  Barrell,  Joseph,  Origin  and  significance  of  the  Mauch  Chunk  shale;  Bull.  Geol.  Soc.  America,  vol.  18,  1907,  pp.  449-476;  Belations  tetween 
climate  and  terrestrial  deposits:  Jour.  Geology,  vol.  16.  1908.  pp.  159-190,  255-295,  363-384. 

liChamberlin,  T.  C,  and  Salisl)ur>-,  E.  D.,  Geology,  vol.  2, 1906. 

.   47517°— VOL  52— 11 27 


418  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

tlie  flexures  were  large  and  where  the  lavas  were  issuing  rapidly,  that  is,  along  the  border  (jf  the 
lake,  should  be  above  the  water.  However,  the  movement  producing  the  synclinal  Imsin  would 
certaiidy  make  a  depression  in  the  center  of  the  lake  whicli  would  naturally  be  Idled  with  water. 
Thus  along  the  borders  of  the  Keweenawan  the  conditions  may  have  favored  terrestrial  deposits 
and  in  the  basin  of  the  lake  the  conditions  may  have  favored  subaqueous  deposits,  and  at  the 
shore  zone  there  were  various  combinations  of  the  two. 

If  these  tentative  conclusions  are  correct,  the  question  still  remains  open  as  to  whether  the 
water-deposited  parts  of  the  Keweenawan  were  submarine  or  continental,  for  deposits  laid  down 
in  great  lakes  are  usually  classed  as  continental.  Wliethcr  tJiis  basin  connected  with  a  sea  or 
was  inclosed  there  is  now  no  means  of  knowing,  unless  the  possible  extension  of  the  Keweenawan 
into  central  Minnesota,  cited  on  ])ages  376-379,  may  indicate  such  a  connection. 

THICKNESS    OF  THE  KEWEENAWAN  ROCKS. 

In  the  descriptions  of  the  individual  districts  the  estimated  thicknesses  of  the  Keweenawan 
have  been  given.  Wherever  there  is  a  fidl  section  the  estimated  thickness  is  verj'  large.  For 
northern  Minnesota  it  is  17,000  or  18,000  feet  exclusive  of  the  gabbro  laccolith,  for  northern 
Wisconsin  and  Michigan  a  maximum  of  60,000  feet,  and  for  Mamainse,  at  the  east  end  of  Lake 
Superior,  16,000  feet.  Only  relatively  small  parts  of  these  thicknesses  are  made  up  by  the 
sediments.  There  are  a  number  of  factors  which  make  all  these  estimates  of  very  uncertain 
accuracy.     The  more  important  of  these  factors  are  faults,  intrusive  rocks,  arid  initial  dips. 

It  has  been  seen  that  during  the  formation  of  the  Lake  Superior  syncline  strike,  dip,  and 
bedding  joints  and  faults  were  produced,  and  that  some  of  the  strike  faults  are  of  great  magni- 
tude. The  different  conglomerates  and  lava  beds  of  the  middle  Keweenawan  are  very  similar 
litliologically  and  it  is  therefore  extremely  difficult,  indeed  usually  impossible,  to  recognize  the 
individual  beds  except  those  of  large  size,  like  the  "Great"  conglomerate.  Hence,  it  has  only 
been  in  the  vicinity  of  the  mining  areas,  where  studies  of  the  most  detailed  nature  have  been 
made,  that  the  extent  of  the  faulting  is  appreciated.  There  can  be  no  doubt  that  strike  faults 
have  repeated  the  beds  at  numerous  localities.  It  is  to  be  said  that  the  close  studies  of  Hubbard  ° 
on  Keweenaw  Point,  those  of  Gordon''  at  Black  River,  those  of  Lane*^  on  Isle  Royal,  and  those 
of  Burwash"*  at  Michijjicoten  have  not  discovered  faults  which  have  repeated  the  beds  of 
these  areas  to  any  considerable  extent.  It  has  been  seen,  however,  that  the  strike  fault  between 
the  north  and  south  ranges  of  Keweenaw  Point  reproduces  the  lower  parts  of  the  rocks  of 
the  Keweenawan  in  the  south  range.  Similarly  it  is  probable  that  1)etween  Isle  Royal  and 
Black  and  Nipigon  bays  is  a  great  strike  fault  which  results  in  the  repetition  of  the  Black  and 
Nipigon  bays  Keweenawan  on  Isle  Royal. 

In  the  estimates  of  the  thickness  of  the  Keweenawan  the  intnisive  rocks  have  been  ignored. 
It  is  certam  that  in  northern  Minnesota  the  intrusive  lavas  constitute  a  considerable  proportion 
of  the  igneous  rocks  of  the  Minnesota  coast.  Also  it  is  suspected  that  closer  studies  will  show 
that  the  intrusive  rocks  are  more  extensive  in  other  areas,  as,  for  instance,  at  Keweenaw  Point, 
than  has  been  supposed.  Indeed,  the  recent  studies  of  Hubbard"  have  shown  tlus  to  be  true 
for  the  acidic  rocks,  but  as  yet  studies  have  not  been  made  along  the  same  lines  for  the  basic 
rocks. 

In  estimating  the  thiclcness  of  these  rocks  no  account  has  been  taken  of  initial  dips.  It 
is  well  known  that  the  initial  dips  of  basic  lavas  and  all  coarse  conglomerates  are  in  many 
places  higher  than  10°,  and  they  may  be  more  than  20°.  This  statement  applies  both  to  sub- 
aqueous and  to  subaerial  deposits. 

oHiibbard,  L.L.,  Keweenaw  Toiat,  with  particular  reference  to  the  felsites  and  their  associated  rocks:  Gcol.  Survey  Michigan,  vol.  6,  pt.  2, 1S98. 
l>  Gordon,  W.  C,  assisted  by  A.  C.  Lane,  A.  geological  section  from  Bessemer  down  Black  River:  Rept.  Geol.  Survey  Michigan  for  1906,  1907, 
pp.  397-507. 

cLanc.  .\.  C.  OeoloKical  report  on  Isle  Royale,  Michigan:  Geol.  Snrvey  Michigan,  vol.  R,  pt.  1,  1S9S. 

li  Burwash,  E.  N.,  The  geology  of  Michipicotcn  Island:  Univ.  Toronto  Studies  (Geol.  scr.),  No.  3, 190S;  with  map. 


THE  KEWEENAWAN  SERIES. 


419 


_^i1_^^ 

r 

'^V;vv:l"v„v^^^^^W 

^^ 

^'vv???:-\ 

c' 

FiGUBE  58.— Diagrammatic  section  illustrating  the  assigned  change  of  attitude  of  a  series  of  beds, 
like  the  Keweenawan,  from  an  original  depositional  inclination  (B-C)  toa  more  highly  inclined 
attitude  (B'-C),  a  comparatively  simple  change.  If  the  beds  were  laid  down  horizontally  in 
a  sinking  basin,  as  illustrated  at  the  right  ( F-G),  it  is  obvious  that  a  greater  and  a  more  com- 
plicated movement  would  be  necessary  to  bring  the  Ijcds  into  the  attitude  represented  in  the 
lower  figure  at  the  left,  which  represents  the  present  attitude  of  the  Keweenawan  beds.  (After 
Chamberlin,  T.  C,  and  Salisbury,  R.  D.,  Geology,  vol.  2, 1900,  fig.  110.) 


There  thus  arises,  in  connection  with  the  middle  Keweenawan  especially,  the  same  problem 
that  arises  in  determming  the  thiclaiess  of  a  delta  deposit,  the  larger  portion  of  which  (the 
foreset  beds)  in  a  great  delta  has  rather  steep  initial  dips.  If  such  a  delta  coulil  be  truncated 
through  its  central  part  and  the  thickness  of  the  beds  determined  on  the  basis  of  dii>  it  might 
be  calculated  that  the  delta  represents  many  thousands  of  feet  of  strata,  although  as  a  matter 
of  fact  the  deposit  might  not  be  vertically  more  than  a  few  hundred  feet  thick.  (See  fig.  58.) 
Plowever,  there  are  reasons 
for  believing  that  a  large 
angle  of  dip  is  due  to  erogenic 
movements,  and  such  an 
angle  is  sufficient  to  allow  a 
large  thickness. 

Because  of  the  factors 
named  above  it  is  extremely 
probable  that  aU  the  esti- 
mates of  the  thickness  of 
the  Keweenawan  based  on 
appearances  are  excessive. 
To  what  extent  they  are  ex- 
cessive is  a  matter  of  con- 
jecture, but  we  suspect  that  the  vertical  thickness  of  the  Keweenawan  at  the  tune  it  was 
formed  was  probably  not  more  than  half  and  possibly  only  a  third  of  the  apparent  thickness. 

AREAS  OF  KEWEENAWAN  ROCKS. 

The  areas  of  the  different  phases  of  the  Keweenawan  in  square  miles  are  as  follows: 

North  shore: 

Basic  intrusive  rocks 2, 170 

Acidic  intrusive  rocks 550 

Basic  extrusive  rocks 1, 950 

4, 670 

Sediments 752 

5, 422 

South  shore: 

Basic  intrusive  rocks 95 

Acidic  intrusive  rocks 145 

Basic  extrusive  rocks 4,  500 

4, 740 

Sediments ' 2, 070 

6, 810 

East  shore: 

Basic  extrusive  rocks 145 

Grand  total 12,  377 

Total  area  of  basic  intrusive  rocks 2,  265 

Total  area  of  acidic  intrusive  rocks 695 

Total  area  of  basic  extrusive  rocks 6,  595 

Total  area  of  sediments 2, 822 


VOLUME  OF  KEWEENAWAN  ROCKS. 

From  the  foregoing  figures  of  tliicltncss  and  area  it  is  apparent  tliat  the  volume  of  the 
Keweenawan  rocks  is  very  large.  For  the  extrusive  rocks  an  area  of  6,000  square  miles  and  a 
thicliness  of  4  miles  would  give  a  volume  of  24,000  cubic  miles.  For  the  sediments  an  area  of 
2,800  square  miles  and  a  thickness  of  4  miles  would  give  a  volume  of  11,200  cubic  miles. 

These  figures  leave  out  of  account  the  enormous  masses  of  intrusive  rocks.  If  the 
gabbro  has  a  circular  outline,  as  indicated  by  the  convex  border  of  Minnesota,  and  if  its  southern 
border  is  indicated  by  the  Gogebic  district,  the  diameter  would  be  about  100  miles.     With  the 


420  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

ratio  of  thiclmcss  to  diameter  given  by  Gilbert "  for  the  Henry  Mountains  the  maximum  tliick- 
ness  would  be  15  miles.  On  calculating  the  thickness  in  another  way,  by  assuming  an  average 
dip  of  10°  for  a  distance  of  50  miles  on  the  north  shore,  si  maximum  tluckncss  of  8t  miles  is 
obtained.  With  a  thickness  of  8^^  miles  at  the  center  and  a  diameter  of  100  miles  approximately 
30,000  cubic  miles  may  be  figured  for  these  intrusive  rocks. 

Althougii  these  figures  merit  little  consideration  as  actual  measurements,  it  is  beheved  that 
they  are  of  value  in  showing  the  enormous  donunance  in  volume  of  tiie  igneous  rocks  over  the 
sediments  and  of  the  mtrusive  igneous  rocks  over  the  extrusive  igneous  rocks.  Reduced  to 
terms  of  mass,  these  figures  would  be  somewhat  changed,  but  the  essential  conclusions  would 
not  be  altered. 

LENGTH   or  KEWEENAWAN   TIME. 

Because  of  the  facts  discussed  in  the  foregoing  section  on  thickness  it  is  of  course  impossible 
to  give  any  estimate  of  the  time  involved  in  the  deposition  of  the  Keweenawan  series,  but 
allowing  a  wide  margin  for  overestimates  of  thickness  we  can  hardly  escape  the  conclusion  that 
the  Keweenawan  probably  required  as  long  a  time  for  its  formation  as  the  average  geologic 
period,  such  as  the  Silurian,  Devonian,  and  Carboniferous,  and  it  may  have  been  as  long  as  the 
Cambrian. 

JOINTING  AND   FAULTING. 

Commonly,  where  the  dip  of  the  lava  beds  is  considerable,  the  beds  are  cut  by  two  sets  of 
joints,  one  of  strike  joints  and  the  other  of  dip  joints.  Both  sets  are  approximately  at  right 
angles  to  the  beds,  but  the  plane  of  the  strike  joints  contains  or  does  not  vary  greatly  from  the 
line  of  strike,  and  the  plane  of  the  dip  joints  contains  or  does  not  vary  greath"  from  the  line  of 
dip.  These  positions  for  the  joints  have  been  noticed  by  Grant  ^  for  northern  Wisconsin  and  by 
Hubbard '^  for  northern  Michigan.  In  many  places  there  are  also  joints  parallel  to  the  beds  or 
between  them,  and  these  may  be  called  bedding  jomts.  Where  the  intrusive  rocks  have  dis- 
turbed the  lava  beds  the  jomtmg  is  very  much  less  regular. 

As  would  be  expected  in  a  fractured  series  of  rocks,  there  is  also  somewhat  extensive  faulting. 
Indeed,  faulting  has  been  discovered  in  almost  every  locahty  where  close  studies  have  been 
miade,  but  usually  the  greater  number  of  the  faults  are  not  of  sufficient  magnitude  to  be  an 
important  factor  in  the  stratigraphy.  Like  the  joints,  the  common  faults  may  be  divided  into 
strike  faults  and  dip  faults,  there  being  a  general  correspondence  between  the  planes  of  the 
faults  and  those  of  the  joints.  Most  of  the  dip  faults  have  no  great  throw,  although  locally 
the  displacement  may  be  very  considerable.  A  beautiful  illustration  of  the  dip  faults  is  fur- 
nished by  Hubbard ''  for  the  West  Pond  area  on  the  south  side  of  Keweenaw  Pomt.  (See 
p.  383.)  F.  E.  Wright's  detailed  mapping  of  the  Porcupine  Mountains  and  vicinity* 
discloses  a  large  number  of  both  strike  and  dip  faults. 

Some  of  the  strike  faults  are  of  great  magnitude  and  extent.  The  greatest  of  these  known 
is  that  at  the  southeast  side  of  the  Keweenawan  series,  extending  from  the  end  of  Keweenaw 
Point  along  the  border  of  the  Keweenawan  to  Gogebic  Lake.  Another  great  strike  fault  is 
known  in  Douglas  Coimty,  m  northern  Wisconsin,  and  in  Minnesota  along  the  northern  border 
of  the  Keweenawan.  Both  of  these  faults  are  at  the  contacts  of  the  Keweenawan  and  the 
Lake  Superior  sandstone,  a,nd  it  is  beheved  that  the  newer  series  represents  the  downthrow  side. 
If  so,  this  downthrow  was  to  the  south  of  the  Keweenawan  at  Keweenaw  Point  and  to  the  north 
of  it  in  Douglas  County.  The  latter  fault  plane  dips  38°  to  45°  S.  and  in  Wisconsin  at  least  has 
aspects  of  an  overthrust  fault. 

Martin  (see  pp.  112-115)  concludes  on  physiographic  groimds  that  there  is  a  fault  along  tiie 
Minnesota  coast  havmg  a  throw  of  at  least  1,000  feet.     There  is  notliing  to  show  that  the  throw 

a  Gilbert,  G.  K.,  The  geology  o(  the  Uenry  Mountains,  2d  ed.:  U.  S.  Oeog.  and  Geol.  Survey  Rocky  Mtn.  Region,  1880,  p.  55. 

6Granl,  U.  S.,  Preliminary  report  on  the  copper-bearing  rocks  of  Douglas  County,  Wis.:  Bull.  Wisconsin  Geol.  and  Kat.  Hist.  Survey  Xo.  6, 
2ded.,  1901,  p.  21. 

cHubbard,  L.  L.,  Keweenaw  Point,  with  particular  reference  to  the  felsites  and  their  associated  rocks:  Geol.  Survey  Michigan,  vol.  6,  pt.  2, 
1898,  pp.  19,  2>1,  35. 

didem,  pp.  87,  91. 

«  Ann.  Rept.  Geol.  Survey  Michigan  for  1908,  1909,  PI.  I. 


THE  KEWEENAWAN  SERIES.  421 

is  not  much  greater  than  tliis  amount.  The  evidence  given  by  Martin  confirms  what  was  before 
a  behef  as  to  the  existence  of  this  fault,  based  on  the  fact  that  if  there  were  not  such  a  fault 
between  Isle  Royal  and  the  mainland,  repeating  the  beds,  it  would  be  necessary  to  accept  an 
almost  iiicredible  tliickness  for  the  Keweenawan.  The  faults  in  the  zone  between  Isle  Royal 
and  the  Miimesota  coast  are  probably  an  extension  of  that  in  Douglas  County,  Wis.,  or,  if  not, 
they  accomphsh  for  the  jVIumesota  area  correspomling  adjustment  of  the  Keweenawan  during 
deformation. 

Just  as  there  are  bedcUng  joints  there  are  also  bedding  faults.  These  are  especially  likely 
to  occur  between  the  diiferent  beds  of  lava  or  of  lava  and  conglomerate.  In  many  of  them  the 
dip  is  slightly  steeper  than  the  beddmg.  The  direction  of  movement  along  these  beddmg  faults 
may  be  parallel  to  the  strike,  parallel  to  the  dip,  or  at  any  angle  between  them.  Although  this 
is  true,  it  woukl  be  natural  to  expect  that  the  most  common  movement  along  the  beddmg  faults 
would  be  approximately  parallel  to  the  dip,  this  being  the  natural  direction  of  differential 
movement  between  beds  in  a  folded  series.  As  to  the  direction  of  movement  along  the  dip,  by 
differential  movement  in  a  fold  of  ordinary  magnitude  the  higher  bed  moves  upward  as  compared 
with  the  lower  bed,  but  it  is  far  from  certain  that  tliis  rule  would  hold  in  a  great  simple  syn- 
clinorium  like  that  of  Lake  Superior.  It  might  be  that  gravity  would  be  more  important  than 
the  strength  of  the  beds  and  that  the  upper  members  woidd  move  downward  as  compared  with 
the  lower. 

Hubbard "  and  Lane  *  conclude  from  their  close  study  of  the  Keweenawan  district  that 
bedduag  faultmg  or  slide  faulting  is  very  common.  Hubbard  finds  that  at  least  one  slide  fault 
substantially  parallel  to  the  dip  has  a  very  large  movement.  Lane''  says  that  many  of  the 
shdo  faults  have  a  slightly  steeper  hade  than  the  dip.  The  details  of  these  occurrences  are  given 
in  the  section  on  Keweenaw  Point  (p.  383). 

Along  any  of  the  faults  there  may  be  slickensides  or  even  brecciation.  Such  brecciation 
is  especially  prevalent  at  the  bedding  faults,  wliich  follow  an  amygdaloidal  lava  surface,  one 
of  their  most  common  positions,  because  the  amygdaloidal  belts  are  planes  of  weakness. 

It  will  be  seen  on  pages  575-576  that  the  several  classes  of  fractures  and  faults  have  a  very 
important  bearing  on  the  development  of  ore  bodies. 

The  time  of  the  fracturmg  is  partly  contemporaneous  with  the  folding  of  the  series  and 
partly  later;  how  much  later  is  not  known.  Some  of  the  faults,  notably  the  great  faults 
bounding  the  Keweenawan  series  on  Keweenaw  Point  and  in  Douglas  County,  Wis.,  are  partly 
post-Cambi'ian.  It  has  been  suggested  by  Wilson"^  and  Weidman,''  from  work  m  other  areas, 
that  faulting  may  have  affected  these  rocks  as  late  as  Cretaceous  time. 

THE  LAKE  SUPERIOR  SYNCLINAL  BASIN. 

It  is  little  short  of  certam  that  the  great  Lake  Superior  synclinal  basin  beg>  n  to  form 
during  middle  Keweenawan  time.  The  general  character  of  tlris  syncline  is  admirably  exhib- 
ited in  figure  59,  from  Irving,  and  by  the  sections  on  the  general  map,  Plate  I.  This  synclinal 
basin  is  rather  remarkable  for  its  simplicity.  Indeed  only  at  one  place  does  Irving  figure  a 
subordinate  fold,  that  at  Porcupine  Mountains.  The  strikes  and  dips  of  the  rocks  show  several 
prominent  flexures,  however,  as,  for  instance,  along  St.  Croix  River  of  Wisconsin,  near  Ashland  and 
Clinton  Point  at  the  head  of  Lake  Superior,  and  at  Michipicoten  Harbor.  Later  strike  faults 
have  considerably  modified  the  syncline.  Doubtless  future  close  studies  will  show  that  the 
Lake  Superior  synclmorium  has  a  greater  complexity  in  detail  than  has  been  supposed.  Cer- 
tainly one  very  important  subordmate  basin,  that  of  Lake  Nipigon,  must  be  attached  to  the 
major  synclinorium.  It  is  to  be  remembered  that  along  the  main  shore  line  and  outer  islands 
of  Black  and  Nipigon  bays  the  middle  Keweenawan  is  found  with  lakeward  dips  at  angles  of 

nOp.  cit.,  pp.  87-91. 

t>  Lane,  A.  C,  Geology  of  Keweenaw  Point,  a  l)rief  description:  Proc.  Lake  Superior  Miu.  Inst.,  vol.  12,  1907,  pp.  S3-S4. 

c  Geol.  Soe.  America,  winter  meeting,  December,  1908. 

d  Personal  communication. 


422 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


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THE  KEWEENAWAN  SERIES.  423 

8°  to  10°.  In  the  peninsulas  between  Thunder,  Black,  and  Nipigon  bays  the  lower  Keweena- 
wan  hes  substantially  flat.  Farther  to  the  north  the  middle  Keweenawan  reappears,  overlying 
the  lower  division  \\'ith  northern  dips.  It  thus  appears  that  at  Black  and  Nipigon  bays  there 
is  a  subordinate  anticlinal  arch,'  which  separates  the  great  synclinal  fold  of  Lake  Superior  from 
the  subordmate  synclinal  fold  of  Lake  Nipigon.  The  latter  lake  is  in  a  subordinate  basin  of 
Keweenawan  rocks,  just  as  Lake  Superior  is  in  a  great  basin  of  that  series. 

Similarly  Batchewanung  Bay,  at  the  east  side  of  the  Lake  Superior  basin,  is  a  subordinate 
synclinal  fold.  A  part  of  the  shore  is  Archean.  Inside  of  this  is  a  fragmentary  border  of  Huronian 
almost  cut  away;  inside  of  this  a  partial  border  of  Keweenawan,  and  the  center  of  tlic  basin  is 
fiUetl  wath  Cambrian.  In  short  this  bay  is  a  miniature  of  the  Lake  Superior  basin,  containing 
the  four  great  divisions  of  rocks  of  the  region — the  Ai'chean,  Huronian,  Keweenawan,  and 
Cambrian — in  a  synclmal  basin. 

It  has  been  seen  that  in  general  in  any  one  section  the  dips  are  much  steeper  at  the  lower 
horizons  than  at  the  higher  horizons.  It  is  certain  that  the  present  dips  at  the  lower  horizons 
are  largely  due  to  the  folding  wliich  formed  the  Lake  Superior  basin.  To  illustrate:  The 
Keweenawan  lava  flows  and  sediments  north  of  the  Gogebic  range  have  the  same  dip  as  the 
upper  Huronian  sediments,  and  therefore  the  main  dips  of  both  must  have  been  produced  by 
orogenic  movements.  Indeed  it  is  thought  probable  that  in  general  the  major  portion  of  the 
dips  of  tlie  most  steeply'  mclined  lavas  is  due  to  orogenic  movements,  for  the  natural  position 
of  repose  for  basalts,  such  as  those  of  Ivilauea,  is  with  dips  of  10°  to  18°.  It  is  reasonably 
certain  that  if  15°  is  subtracted  from  the  lakeward  dip  of  the  basic  lavas  the  remainder  of  the 
dip  is  due  to  orogenic  movement.  The  steadily  lessening  dips  of  the  lavas  at  liigher  horizons 
are  therefore  to  be  largely  explained  by  the  progress  of  the  orogenic  movement  which  pro- 
duced the  Lake  Superior  basin,  although  they  are  doubtless  in  part  explained  by  the  natural 
lessening  of  the  dip  toward  the  center  of  a  syncUnal  fold. 

To  Ulustrate  again:  In  the  Black  River  section  the  dips  at  the  base  are  from  75°  to  78°  N., 
and  at  the  highest  strata  exposed  on  the  "Outer"  conglomerate  only  20°.  In  the  Keweenaw 
Point  section  the  lavas  at  the  south  side  dip  55°  N.  and  those  of  the  middle  division  at  the  north 
side  cUp  25°,  and  it  may  be  supposed  that  during  the  time  in  wliich  the  lavas  and  conglomerates 
of  the  middle  Keweenawan  in  this  area  were  built  up  the  synclinal  movement  had  tilted  the 
lower  beds  30°  as  a  maximum,  but  from  this  amount  to  obtain  the  actual  tilting  there  must 
be  subtracted  the  unknown  amount  which  is  due  to  the  normal  decrease  in  dip  toward  the 
center  of  a  syncline.  Similarly  at  IMichipicoten,  on  the  northwest  side  of  the  synclinorium, 
the  basal  beds  have  a  dip  of  55°  SE.,  and  at  the  top  of  the  exposed  sections  on  the  islands 
south  of  Micliipicoten  the  dip  is  14°,  a  inaximum  difference  of  41°,  wliich  may  be  attributed 
to  orogenic  movement  during  the  formation  of  the  middle  Keweenawan  in  this  part  of  the 
region.  The  same  thing  is  illustrated  at  Isle  Roj^al,  where  at  the  southwest  end  of  the  island 
the  dips  on  the  north  side  are  16°  and  on  the  south  side  8°,  and  at  the  east  end  of  the  island 
the  dips  on  the  north  side  are  26°  and  on  the  south  side  18°.  It  thus  appears  that  the  decrease 
in  dip  from  the  north  to  the  south  side  is  8°,  without  reference  to  the  steepness.  Tliis  fact 
strongly  suggests  that  the  steeper  dips  at  the  northeastern  part  of  the  island  as  compared  with 
the  southwestern  part  are  to  be  explained  by  greater  orogenic  movements  in  that  part  of  the 
island,  and  thus  gives  a  confirmation  to  the  suggestion  made  that  the  steep  dips  are  mainly 
due  to  orogenic  movement  rather  than  to  the  original  angle  of  deposition.  The  foldmg  of  the 
basin  was  practically  complete  at  the  end  of  Keweenawan  time,  but  in  post-Cambrian  time 
and  possibly  in  post-Cretaceous  time  the  region  suffered  the  great  strike  faulting  already  noted. 

METAMORPHISM. 

For  the  most  part  the  metamorphism  of  the  Keweenawan  igneous  rocks  is  that  of  the 
zone  of  katamorphism.  The  alterations,  fully  described  byPumpelly"  and  Irving,*  have  pro- 
duced very  extensive  changes  in  the  lavas,   especially   those  wliich  were  scoriaceous.     The 

a  Pumpelly,  Raphael,  Metasomatic  development  of  the  copper-bearlBg  rocks  of  Lake  Superior;  Proc.  Am.  Acad.  Arts  and  Sci.,  vol.  13.  1878, 
pp.  253-309. 

I> Irving,  R.  D.,  Mon.  U.  S.  Geol.  Survey,  vol.  5, 1883. 


424  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

important  secondary  minerals  produced  in  the  basic  rocks  are  the  zeolites,  epidotes,  chlorites, 
calcite,  quartz,  laumoutito,  prclinite,  datoUte,  etc.  Man}'  of  tlio  thin  vesicular  })eds  are  largely 
transformed  to  these  substances  and  the  vesicles  have  been  filled  with  them,  forming amygdules. 
Although  the  porous  beds  are  extensively  altered,  the  massive  centers  of  the  thick  lava  flows, 
the  dike  rocks,  and  the  sills  and  laccoliths  are  ver}'  fresh;  indeed  some  of  them  are  almost  as 
little  altered  as  similar  rocks  of  Tertiary  age.  The  felsitc  and  (|uartz  {)orphyries  have  undergone 
the  usual  metasomatic  alterations  for  ancient  acidic  lavas.  The  glasses  have  devitrified.  A  wide 
variety  of  secondary  minerals  have  formed,  but  they  occur  usually  in  such  minute  particles  as 
to  be  determinable  with  difliculty. 

The  alterations  of  the  Keweenawan  lavas  doubtless  began  as  soon  as  they  were  consohdated. 
The  process  continued  tlirough  Keweenawan  time  and  the  great  erosion  period  between  the 
Keweenawan  and  Cambrian,  and  indeed  is  still  going  on. 

The  alterations  of  the  sedimentary  rocks  vary  greatly  in  degree.  The  lower  and  middle 
Keweenawan  sediments  are  much  more  changed  than  those  of  the  upper  Keweenawan.  In  the 
sandstones  and  conglomerates  interstratified  with  the  lavas  the  same  metasomatic  change  took 
place  as  in  the  lavas,  resulting  in  the  formation  of  a  hke  group  of  secondary  minerals.  The 
filling  of  the  openings  between  the  grains  and  pebbles,  strictly  analogous  to  the  filling  of 
the  openings  in  the  vesicular  lavas,  has  been  nearly  complete,  thus  thoroughly  indurating  the 
rocks.  The  cementing  materials  in  the  sandstones  and  conglomerates  interstratified  with  the 
lavas  are  much  more  varied  than  those  of  ordinar}^  cementation.  It  was  in  these  rocks  tliat 
the  senior  author  first  noted  the  secondary  enlargement  of  detrital  feldspar.  So  thoroughly  have 
the  clastic  materials  been  cemented  that  where  the  rocks  have  not  been  weathered  fractures 
commonly  pass  across  both  pebbles  and  matrix.  The  sandstones  are  intermediate  between 
sandstones  and  quartzites  in  their  cementation.  Though  these  sediments  are  well  indurated 
they  certainly  are  less  metamorphosed  than  similar  secUments  of  the  Animikie  group.  Inasmuch 
as  the  conditions  since  they  have  been  laid  dowTi  have  been  practically  the  same  as  those  that 
have  affected  the  Animikie  beds,  upon  which  they  rest,  this  difl'erence  in  metamorpliism  confirms 
the  conclusion  as  to  a  considerable  time  break  between  the  two  series. 

The  cementation  in  the  sandstones  of  the  upper  Keweenawan  has  not  proceeded  so  far  as 
m  the  detrital  rocks  of  the  middle  tlivision.  Indeed  these  sanilstones  are  very  similar  to  those 
of  Cambrian  age.  The  individual  particles  of  these  sandstones,  bemg  largely  basic,  are  usually 
much  altered,  but  it  is  difficult  to  say  what  part  of  these  changes  have  taken  place  since  they  were 
deposited  as  sandstones  and  what  part  took  j^lace  before  they  were  broken  from  the  lavas  from 
which  they  came. 

The  segregation  producing  copper  ores  was  an  incident  of  the  metasomatic  changes  above 
summarized,  and  the  details  of  it  are  considered  in  another  place  (pp.  580  et  seq.) 

The  intrusive  rocks,  especially  the  great  basal  gabbros,  and  the  large  masses  of  acidic  rock, 
as  has  been  noted  in  another  place  (p.  411),  produced  profound  anamorphic  changes  in  the 
pre-Keweenawan  rocks  which  they  cut.  It  is  believed  that  later  studies  will  show  that  in  con- 
nection with  the  deep-seated  bathohths  of  Minnesota  and  Wisconsin  anamorphic  changes  will 
be  found  in  the  intruded  Keweenawan  lavas  and  sediments,  but  as  yet  studies  have  not  been  made 
along  the  border  of  the  gabbros  in  order  to  ascertain  whether  or  not  this  conjecture  is  correct. 
Tliis  suggestion  gains  much  probability  from  the  fact  that  along  the  borders  of  the  much  smaller 
laccolith  of  Black  River  in  Michigan  F.  E.  Wright  has  found  the  intruded  Keweenawan  lavas 
and  sediments  to  be  greatly  metamorphosed. 

RESUME  OF  KEWEENAWAN   HISTORY. 

From  the  facts  which  have  been  presented  we  ma^-  make  the  following  general  statements: 
After  the  great  epoch  of  upper  Iluronian  deposition  the  Lake  Superior  region  was  raised 
above  the  sea  and  was  sul)jected  to  denudation  for  a  long  time,  duiing  which  the  erosion  amounted 
to  thousands  of  feet.     The  Keweenawan  period  was  begun  by  the  deposition  of  sediments,  con- 
sisting of  conglomerates,  sandstones,  shales,  and  limestones,  now  found  generally  at  the  base  of 


THE  KEWEENAWAN  SERIES.  425 

the  known  intrusive  part  of  the  Keweenawan  where  it  has  been  looked  for.  These  may  be 
subaeiial  deposits. 

After  the  deposition  of  sediments  of  very  moderate  tliickness  occurred  the  events  of  the 
middle  Keweenawan,  which  especially  characterize  the  series.  The  chief  event  was  the  out- 
break of  regional  volcanism  in  the  larger  part  of  the  Lake  Superior  basin. 

In  a  large  part  of  the  region,  and  perhaps  all  of  it,  igneous  rocks  practically  excluded  sedi- 
ments in  the  lower  portion  of  the  middle  Keweenawan.  Igneous  rocks,  with  an  almost  inappreci- 
able proportion  of  sediments,  constitute  the  Minnesota  coast,  the  lower  eight-ninths  of  the  Eagle 
River  section,  nine-tenths  of  the  Portage  Lake  section,  all  of  the  Douglas  County  range  of  Wis- 
consin, all  of  the  4,000  feet  of  the  Taylors  Falls  section,  more  than  eleven-twelfths  of  the  section 
at  Black  River,  and  about  4,000  feet,  or  one-fourth  of  the  section,  at  Mamainse.  It  does  not 
follow  that  the  time  represented  by  the  sediments  may  not  be  as  long  as  or  even  longer  than  that 
represented  by  the  lavas.  After  the  period  of  dominating  volcanism  had  continued  until 
thousands  of  feet  of  lava  had  been  built  up,  there  was  a  decrease  in  volcanic  activity  and  the 
sediments  again  became  of  sufficient  importance  to  be  recognized  in  the  section.  This  was  the 
later  part  of  the  middle  Keweenawan. 

The  change  in  conditions  in  the  niiddle  Keweenawan  by  wliich  the  sediments,  insignificant 
in  the  lower  part,  became  important  in  the  upper  j)art  is  not  supposed  to  have  occurred  at 
the  same  time  over  the  entire  Lake  Superior  basm.  Indeed,  it  seems  extremely  probable 
that  the  change  was  not  simultaneous  in  all  parts  of  the  region.  This  niay  be  illustrated  by 
the  Portage  Lake  and  Eagle  River  sections  on  Keweenaw  Pomt.  The  alterations  of  notable 
masses  of  sediments  with  the  lavas  seem  to  have  become  important  in  the  Portage  Lake  section 
before  they  did  in  the  Eagle  River  section,  for  at  Eagle  River  lavas,  to  the  practical  exclusion 
of  sediments,  constitute  all  but  the  upper  5,000  feet  of  the  middle  Keweenawan,  whereas  at 
Portage  Lake  the  portion  containing  sediments  is  much  thicker. 

As  the  middle  Keweenawan  epoch  neared  its  close  igneous  activity  ceased.  In  northern 
Michigan  the  longest  cessation  of  volcanism  was  marked  by  the  deposition  of  the  "Great" 
conglomerate,  which  is  locally  more  than  2,000  feet  thick.  After  this  conglomerate  was  laid 
down  there  were  further  outbreaks  of  volcanic  activity,  which  resulted  in  the  "Lake  Shore" 
trap.  But  tlie  outbreaks  represented  by  tliis  formation  were  relatively  feeble,  as  is  indicated 
by  the  fact  that  the  lava  beds  are  separated  by  conglomerates  of  considerable  thickness.  For 
Michigan  tliis  "Lake  Shore"  trap  represents  the  last  dying  effort  of  the  epoch  of  regional  volcanic 
activity. 

Thus  middle  Keweenawan  time  witnessed  a  sudden  begmning  of  volcanic  activity,  which 
was  dominant  for  a  long  time,  then  intermittent  volcanic  activity,  then  total  cessation.  Evi- 
dence has  been  presented  which  seems  to  favor  the  view  that  the  midtUe  Keweenawan  was 
deposited  largely  imder  subaerial  rather  than  subaqueous  conditions. 

The  present  distribution  of  the  middle  Keweenawan  shows  that  much,  if  not  all,  of  the 
Lake  Superior  basm  must  have  been  covered  by  volcanic  flows,  for  the  igneous  material, 
besides  occurring  along  the  rim  of  the  lake,  constitutes  Isle  Royal,  Micliipicoten,  and  Stannard 
Rock,  off  Marquette. 

During  middle  Keweenawan  time  there  were  at  least  two  alternations  of  basic  and  acidic 
rocks,  and  locally  between  basic  and  acidic  rocks  of  the  first  cycle  there  were  intermediate 
rocks,  as  on  Keweenaw  Point  and  Isle  Royal.  Whether  these  cycles  were  general  for  the 
Keweenawan  over  the  Lake  Superior  region  and  whether  there  were  more  cycles  than  two 
is  as  yet  undetermined. 

As  already  stated  (p.  410),  during  middle  Keweenawan  time,  contemporaneous  with  and 
followmg  the  extrusions  of  the  lavas,  there  were  also  mtrusions,  and  these  mtrusive  rocks 
are  of  very  great  quantitative  importance.  In  many  places  m  the  lava  series  the  mtrusions 
in  the  form  of  beds  and  dikes  compose  a  considerable  percentage  of  the  mass.  Although  the 
mtrusives  to  a  large  extent  rose  into  the  middle  Keweenawan  beds,  still  greater  masses  spread 
out  approximately  along  the  contact  between  the  Keweenawan  and  the  lower  I'ocks,  and  also 


426  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

between  the  laj^ers  of  the  lower  formations.  The  vastest  intrusive  body  of  this  class  is  the 
great  Duluth  laccohth,  which  extends  from  Duhith  to  the  international  boundary  and  has  a 
breadth  reaclung  30  miles.  Another  of  these  great  intrusive  masses  is  that  at  Bad  River. 
The  bodies  intruded  between  the  beds  of  the  Animikie  group  are  so  prominent  that  they  have 
been  called  the  Logan  sills.  The  so-called  crowning  overflow  of  Thunder  Cape  may  fall  here. 
The  peculiar  topography  of  the  steep  cUfFs  about  Thunder  Bay  and  Pie  Island  is  due  largely 
to  these  mtrusive  flat-lying  sills.  The  acidic  rocks  intrusive  in  the  lower  Keweenawan  are  also 
important.  Granite  bosses  of  considerable  size  intrude  upper  Huronian  rocks  in  central  Minne- 
sota and  northeastern  Wisconsin. 

During  middle  Keweenawan  time  progressive  folding  of  the  Lake  Superior  basin  went  on, 
with  the  result  that  the  upper  beds  have  a  lower  dip  than  the  lower  ones. 

Conformably  upon  the  rocks  of  the  middle  division  were  built  up  the  sediments  of  the 
upper  Keweenawan.  These  sediments  consist,  in  ascending  order,  of  the  "Outer"  conglomerate, 
havmg  a  maximum  thickness  of  5,000  feet;  the  Nonesuch  shale,  having  a  maximum  tliickness 
of  500  feet;  and  the  Freda  sandstone,  having  a  maximum  tliickness  of  19,000  feet.  As  the 
"Outer"  conglomerate  hes  directly  upon  the  basic  lavas  and  in  its  main  mass  is  lithologically 
like  the  conglomerates  interstratified  with  the  lavas  there  is  no  reason  to  suppose  that  the 
conditions  at  the  time  tliis  conglomerate  was  deposited  were  in  any  way  different  from  those 
prevailuig  at  the  time  of  the  earher  conglomerates,  except  that  late  m  the  epoch  detritus 
from  pre-Keweenawan  rocks  appeared.  ^Beginning  with  the  Nonesuch  shale,  the  sediments  are 
of  a  different  character  from  those  lower  in  the  Keweenawan  series.  This  formation  and 
the  Freda  sandstone  are  largely  and  in  places  mainly  composed  of  detritus  derived  from  the 
basic  lavas.  ^Vlso,  they  contain  contributions  from  the  Huronian,  Keewatin,  and  Laurentian 
rocks.  This  means  that  by  the  erosion  of  the  basic  lavas,  or  by  tliis  cause  combined  with 
uplift,  the  pre-Keweenawan  became  the  subject  of  attack  by  atmospheric  agents.  The 
relative  lack  of  abundance  of  material  from  the  acidic  lavas  may  also  mean  that  the  volcanic 
mountains  composed  of  acidic  rocks  had  by  late  Keweenawan  time  become  so  reduced  as 
to  yield  only  a  small  amount  of  material. 

As  the  change  in  the  nature  of  the  materials  of  the  sediments  from  those  mterstratified 
with  the  lavas  to  the  Freda  sandstone  was  gradual,  there  is  no  reason  to  place  a  break  at  any 
defuiite  horizon.  Volcanic  activity  gradually  died  out,  orogenic  movement  and  erosion  con- 
tinued, and  these  afford  sufficient  explanations  for  the  increasing  variety  of  the  detritus  of 
the  upper  Keweenawan. 

As  the  Nonesuch  shale  and  Freda  sandstone  together  are  of  very  great  thickness  and  are 
made  up  of  fine-grained  sediments,  there  must  have  been  steady  and  long-continued  subsidence 
of  the  basin  where  these  formations  were  deposited.  Also,  their  volume  is  so  great  as  to  indicate 
steady  upfift  in  some  other  part  of  the  region,  exposing  the  lavas  and  other  rocks  to  erosion. 

The  development  of  the  Lake  Superior  syncline  continued  to  the  end  of  Keweenawan  time 
and  w'as  then  substantially  complete.  The  basm  was  modified  afterwards  only  by  post-Cambrian 
faulting. 

Keweenawan  sedimentation  was  largely  subaerial,  but  it  may  have  become  subaqueous 
toward  the  close  of  the  period  in  the  water-filled  Keweenawan  syncline  and  may  have  ultimately 
merged  into  Upper  Cambrian  subaqueous  deposition. 


CHAPTER  XVI.     THE  PLEISTOCENE 


By  Lawrence  IMartix. 


'  THE  GLACIAL  EPOCH. 

PLAN   OF  PRESENTATION. 

The  statement  that  the  Lake  Superior  region  has  been  invaded  and  profoundly  modified 
by  a  continental  glacier  or  ice  sheet  docs  not  require  proof.  It  will- suffice  to  name  some  of  the 
locahties  in  wliich  the  proofs  are  found  and  to  describe  the  glacial  phenomena  and  their  effects 
on  the  present  topography  and  the  Ufe  of  the  region. ° 

The  ice,  wliich  advanced  from  two  centers,  one  east  and  one  west  of  Hudson  Bay,  in  a  series 
of  lobes,  oscillated  so  that  glacial  deposits  thought  to  be  of  two  or  more  ages  were  produced. 
The  latest  of  these  are  called  the  deposits  of  the  Wisconsin  stage  of  glaciation  and  cover  the 
greater  part  of  the  area  here  discussed.  In  advancing,  the  ice  produced  striae,  roches  moutonnees, 
cirques,  broadened,  deepened,  and  hanging  valleys,  etc.  It  transported  great  quantities  of  the 
materials  eroded  in  producing  these  forms.  As  the  ice  melted,  these  materials  were  deposited 
as  an  overmantle  of  glacial  drift.  The  drift,  which  is  partly  stratified,  was  formerly  known 
as  modified  and  unmodified  drift.  Later  studies  show,  however,  that  the  largely  unstratified 
(unmodified)  drift,  including  terminal  or  recessional  moraines,  ground  moraine,  and  drumhns, 
was  deposited  directly  by  the  ice.  The  drift  deposited  by  rumiing' water  either  under  or  in  front 
of  the  ice  or  in  standing  water  is  stratified,  though  not  essentially  modified,  and  includes  out- 
wash  deposits,  lake  deposits,  loess,  kames,  eskers,  etc.  Most  of  these  varieties  of  drift  are 
found  both  in  the  older  and  in  the  latest  glacial  drift,  as  will  be  discussed.  In  all  the  glaciated 
area  the  drainage  was  greatly  modified  by  the  erosion  and  deposition  due  to  the  ice.  During 
deglaciation  there  was  a  great  series  of  marginal  glacial  lakes,  the  ancestors  of  the  present 
Great  Lakes.  Since  the  glacial  period  there  has  been  warping  in  the  region,  resulting  in  tilting 
of  the  shore  Unes  of  the  former  lakes.  Streams  have  made  sfight  modifications  of  the  glacial 
drift  and  of  the  topography  of  the  land.  The  lake  shores,  especially  those  of  Lakes  Superior  and 
Michigan,  are  the  seat  of  active  work,  and  in  these  lakes  the  detritus  carried  from  the  land  by  the 
rivers  and  from  the  shores  by  waves  and  currents  is  being  deposited. 

ICE  ADVANCES. 

The  scratches  and  grooves  upon  the  ledges  in  the  Lake  Superior  region  aH'ord  the  ])rincipal 
evidence  of  the  direction  of  movement  of  the  glaciers,  and  the  sketch  map  (fig.  60)  is  a  generaliza- 
tion based  on  these  marks.  It  will  be  seen  that  in  general  the  ice  moved  in  a  series  of  lobes 
of  which  those  in  the  Lake  Michigan  basin,  the  Lake  Superior  basin,  and  the  valley  of  Red 
River  were  the  most  important,  the  lobes  between  these,  especially  one  extending  from  the 
highland  region  of  northern  Wisconsin,  known  as  the  Chippewa-Keweenaw  lobe,  and  one  extend- 
ing from  the  highland  region  of  northern  Minnesota,  known  as  the  Rainy  Lake  lobe,  being  less 
extensive. 

a  The  author  is  indebted  to  Messrs.  Frank  Leverett  aad  W.  C.  Alden,  of  the  United  States  Geological  Survey,  who  have  more  recently  done 
detailed  work  on  the  glacial  features  of  the  south  coast  of  Lake  Superior  and  in  eastern  Wisconsin,  respectively,  for  critical  suggestions  concerning 
this  chapter.    The  author,  however,  assumes  responsibility  for  any  errors  in  interpretation. 

427 


428 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


The  ice  whicli  overspread  the  Lake  Superior  region  came  from  two  principal  sources,  one  in 
the  highlands  of  eastern  Canada,  generally  called  the  Labrador  glacier,  and  one  in  the  region 
west  of  Hudson  Bay,  usually  kno%\Ti  as  the  Keewatin  glacier.  It  seems  probable  that  fully 
two-thirds  of  the  ice  which  covered  the  Lake  Superior  basin  came  from  the  Labrador  glacier. 
It  is  supposed,  however,  that  this  glacier  was  not  the  first  to  spread  over  the  region,  but  that  the 
Keewatin  glacier,  while  largely  synchronous  and  confluent  with  the  Labrador  glacier,  arrived 
earher  and  stayed  longer,  probably  advancing  over  parts  of  the  region  formerly  covered  by  lobes 
of  the  Labrador  glacier  after  these  lobes  had  retreated  to  the  northeast.  WTiether  or  not  the 
ice  advance  from  the  northwest  covered  all  the  Lake  Superior  region  is  unknown. 

In  the  area  covered  by  this  monograph  the  glacial  lobes  were  profoundly  affected  by  the 
areas  of  highland  and  lowland,  and,  as  would  naturally  be  expected,  the  ice  was  the  thickest 
and  moved  fastest  in  the  tleepest  depressions;  consequently,  the  Lake  Michigan  lobe  of  the 
Labrador  glacier  (figs.  4,  p.  87,  and  60)  extended  farther  south  than  any  of  the  others,  and  the 


FiGVRE  GO. — Sketch  map  showing  the  glaciation  of  the  Lake  Superior  region,  giving  names  of  lobes  and  probable  directions  of  ice  flow.     There 
may  have  been  an  earlier  stage  with  ice  advance  from  the  northwest  through  a  large  part  of  tbe  area. 

Green  Bay  lobe  of  the  Labrador  glacier,  also  having  a  deep  axis  of  flow,  extended  nearly  as 
far  south  as  the  Lake  Micliigan  lobe.  The  Keweenaw  and  Chippewa  lobes  of  the  Labrador 
glacier,  being  obhged  to  advance  over  the  highland  region  of  upper  Micliigan  and  northern 
Wisconsin,  cUd  not  advance  as  far  south  as  the  lobes  to  the  east,  though  the  Chippewa  lobe  over- 
rode the  i)art  of  Keweenaw  Peninsula  west  of  Ontonagon  River  and  advanced  farth(>r  south  than 
the  atljaceut  Keweenaw  lobe.  The  Lake  Superior  lobe  of  the  Labrador  glacier,  turned  west- 
ward by  the  topography,  advanced  to  the  west  end  of  Lake  Superior,  where  it  escajied  from  the 
confining  walls  of  the  rift  valley  or  trough  near  Duluth  and  spread  out  in  a  much  broader  lobe 
(fig.  60),  part  of  which  advanced  nearly  westward  in  the  region  south  of  Leech  Lake,  probably 
moAnng  southwest  in  the  region  of  Mille  Lacs  and  swinging  round  to  the  south,  and  even  to  the 
southeast  in  the  vicinity  of  St.  Croix  Falls.  The  Rain}'  Lake  lobe,  which  seems  to  have  come 
partly  from  the  Labrador  and  partly  from  the  Keewatin  center,  moved  south  and  southwest 


THE  PLEISTOCENE.  429 

over  the  hills  of  northern  Minnesota  (fig.  4).  The  Red  River  lobe,  the  principal  division  of  the 
Keewatin  glacier,  often  referred  to  as  the  Minnesota  lobe,  advanced  southward  in  the  valley  of 
Red  River.  Although  these  lobes  are  described  and  discussed  as  somewhat  separate  glaciers, 
too  much  emphasis  should  not  be  placed  on  their  separate  existence.  It  would  naturally  be 
true  that  as  the  Labrador  glacier  advanced  from  the  northeast  it  would  project  farthest  where 
the  deepest  valleys  existed  and  would  have  reentrants  where  the  hills  caused  obstruction  to 
free  glacial  advance.  It  therefore  seems  probable  that  the  Lake  Michigan  and  Lake  Superior 
lobes  actually  did  advance  independently  over  the  regions  described ;  but  it  must  also  be  remem- 
bered that  with  farther  advance  to  the  south  the  lobes  in  the  Great  Lakes  basins  and  those  on 
the  hills  would  coalesce  until  the  hilly  region  was  completely  covered  by  one  confluent  ice  sheet. 
For  example,  after  the  Lake  Superior  lobe  had  advanced  westward  from  Duluth  and  the  Rainy 
Lake  lobe  had  advanced  over  the  highland  area  of  northern  Minnesota  and  the  international 
boundary,  their  farther  advance  would  cause  these  short  lobes  to  become  confluent  and  form 
one  great  ice  cap. 

DRIFTLESS   AREA. 

If  there  was  not  time  enougli  for  two  lobes  to  become  confluent  before  the  retreat  of  the 
ice,  there  would  be  left  between  them  an  area  where  the  soil,  the  ledges,  and  the  drainage  bore 
no  evidence  of  the  glacial  advance.  Such  an  area  might  have  been  formed  in  northern  Minne- 
sota if  the  Lake  Superior  lobe  and  the  Ramy  Lake  lobe  had  never  coalesced.  They  did  coalesce, 
however,  but  in  one  small  area  at  the  extreme  northeastern  part  of  Minnesota,  described  by 
N.  H.  Winchell "  and  U.  S.  Grant,  *  the  drift  is  so  thin  that,  although  the  topography,  striated 
rock  surfaces,  and  scattered  foreign  bowlders  definitely  prove  glaciation  of  the  area,  the  fact 
that  the  residual  soil  has  not  all  been  removed  and  the  absence  of  nearly  all  glacial  deposits 
have  led  to  the  description  of  the  locality  as  "  a  possibly  driftless  area."  Part  of  the  Marquette 
district  is  an  area  of  very  thin  drift,  as  near  the  Mansfield  mine,  on  Michigamme  River.  Similar 
areas  of  tliin  drift  are  described  as  occurring  in  Canada. 

In  western  Wisconsin  and  the  adjacent  parts  of  Minnesota  and  Iowa  there  is  a  true  driftless 
area,  and  this  was  recognized  in  1852  or  earher  by  D.  D.  Owen"  and  has  been  studied  and  fully 
described  by  Chamberlin  and  Salisbury.'*  A  portion  of  the  Driftless  Area  (fig.  68,  p.  45.3)  is 
included  in  the  southwestern  part  of  the  region  described  in  this  report.  Recent  studies  by 
Weidman^  and  by  Leverett  and  Alden  are  somewhat  modifying  the  ideas  previously  held  as  to 
the  shape  and  boundaries  of  this  area,  although  the  main  fact  of  its  existence  and  the  assign- 
ment of  its  cause  to  insufficient  time  for  the  reduced  supply  of  ice  from  the  north,  retarded  by 
the  higlilands,  to  reach  this  driftless  region  still  stand  approved. 

RETREATING  ICE. 

The  so-called  retreat  of  the  ice  sheet  was  not  an  actual  backward  motion,  the  opposite  of 
the  forward  motion  of  the  advance,  but  a  melting  back  of  the  front  of  the  ice  sheet.  Wliile  the 
front  of  the  continental  ice  sheet  was  retreating  from  tliis  region,  the  highlands  first  emerged 
from  the  ice  cover  because  the  ice  was  thinnest  above  their  tops,  and  valley  glaciers  or  lobes 
lingered  longest  in  the  valleys  because  it  was  there  that  the  ice  was  thickest  and,  after  tliinnuig 
by  ablation,  most  protected  by  the  load  of  soil  and  stones  which  it  was  carrying.  Accordingly 
during  the  retreat  the  ice  front  was  always  lobate.  The  lobes  in  the  Lake  Michigan  and  Lake 
Superior  basins  were  much  more  extensive  than  those  in  the  northern  Minnesota  and  northern 
Wisconsin  liiglilands,  as  the  glacial  deposits  that  have  been  left  in  the  region  prove.  There 
were  probably  slight  readvances  during  the  retreat  of  the  ice  sheet  south  of  Lake  Superior. 

a  Filteenth  Aim.  Rept.  Minnesota  Geo!,  and  Nat.  Hist.  Survey,  1S87,  p.  350. 

6  Am.  Geologist,  vol.  24,  1899,  pp.  377-381;  Final  Rept.  Minnesota  Geol.  and  Nat.  Hist.  Survey,  1899,  pp.  421,  437-438. 
c  Geological  survey  of  Wisconsin,  Iowa,  and  Minnesota,  1S52. 

d  Chamberlin,  T.  C,  and  Salisbiu-y,  R.  D.,  Tlie  driftless  area  of  the  upper  Mississippi  Valley:  Sixth  Ann.  Rept.  U,  S.  Geol.  Survey,  1884,  pp. 
199-322. 

«  Bull.  Wisconsin  Geol.  and  Nat.  Hist.  Survey  No.  16,  1907,  pp.  548-565. 


430  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

A  study  of  thcso  deposits  also  suggests  that  the  ice  lobe  whic^h  advanced  down  the  Red 
River  valley,  moving  southward  and  southeastward  in  the  area  discussed  in  this  monograph, 
came  after  the  Lake  Superior  and  Lake  Micliigan  lobes  had  retreated  for  some  distance,"  perhaps 
into  the  basins  of  the  present  lakes.  Moreover,  glacial  grooves  and  stria;  on  the  ledges  seem  to 
show  the  same  thing.  In  the  St.  Croix  Dalles  region  glacial  scratches  on  the  rock  are  associated 
with  the  deposits  made  by  the  Lake  Superior  lobe  of  the  Labrador  glacier  in  such  a  way  as  to 
suo'i-est  that  they  wore  made  during  a  first  glacial  advance,  while  striations  associated  with 
overlying  glacial  deposits  made  by  the  Red  River  lobe  of  the  Keewatin  glacier  differ  in  direction 
and  were  probably  made  after  the  first  set.*"  The  relation  of  moraines  of  red  and  of  gray  drift 
near  the  south  boundary  of  the  upper  peninsula  of  Michigan,  west  of  Crystal  Falls,  suggested 
the  possibility  to  I.  C.  Russell  <^  that  the  Chippewa  (or  Keweenaw)  lobe  of  the  Superior  glacier 
was  still  advancing  after  the  Green  Bay  lobe  of  the  Lake  Michigan  glacier  had  partly  retired 
from  the- area. 

That  there  were  slight  rcadvances  of  the  ice  during  its  general  recession  is  indicated  in 
several  places,  as  in  eastern  Wisconsin,  where  red  till  moraines  of  the  Green  Bay  and  Lake 
Michigan  lobes  overlie  the  earlier  moraines  of  the  Wisconsm  glaciation.  Certain  stages  of  the 
marginal  glacial  lakes  discussed  later  also  indicate  a  halt  in  Lake  Michigan  in  the  latitude  of 
Manistee  and  a  subsequent  slight  readvance.  These  readvances  durmg  the  deglaciation  of 
the  region,  however,  do  not  seem  to  have  been  very  many  or  very  great,  so  far  as  the  preliminary 
studies  thus  far  made  give  evidence. 

CONTRASTED    GENERAL  EFFECTS   OF   GLACIATION. 

In  general  the  glacial  invasion  stripped  the  peneplain  of  its  soil  in  the  area  north  of  Lake 
Superior,  while  south  of  the  lake,  in  the  highland  region  of  northern  Wisconsin,  it  removed 
the  soil  but  left  a  heavy  mantle  of  glacial  deposits.  Nevertheless,  tliroughout  this  area  the 
influence  of  glaciation  on  topography  was  minor,  while  the  effects  on  soU,  drainage,  forests,  and 
the  subsequent  pursuits  of  man  were  most  profound.  WTiat  was  a  hill  in  this  upland  area 
north  of  the  lake  before  the  glacial  advance  is  still  a  hUl;  what  was  a  vaUey  is  almost  without 
exception  still  a  valley,  but  it  may  be  marsh  or  lake,  or  stony  soil,  and  so  useless  for  agriculture. 
It  may  have  had  a  fertile  soil  before  glaciation,  or  may  have  contained  some  evidence  of  an 
adjacent  body  of  u-on  ore,  and  this  the  glacier  has  taken  away,  leavmg  as  compensation  per- 
haps a  sandy  soil  supportmg  a  splendid  pine  forest,  possibly  a  ledge  from  which  the  location  of 
the  ore  body  may  be  inferred,  perhaps  only  a  clogged  valley,  a  chain  of  lakes,  and  broad,  loiter- 
mg  stream  courses  along  which  the  prospector  or  geologist  may  travel  by  canoe,  and  so  reach 
regions  of  mineral  wealth  that  otherwise  might  have  lain  hidden  to  this  day.  Quite  in  con- 
trast to  tliis  pre-Cambrian  area,  the  horizontal  Cambrian  rocks  of  the  south  shore  of  Lake  Superior 
near  Duluth  and  Ashland  and  eastward  from  Marquette  to  Sault  Ste.  Marie,  the  belted  plain 
of  Wisconsin  and  Michigan,  and  the  flat-lymg  Cretaceous  deposits  of  east-central  Minnesota 
are  deejDly  obscured  by  glacial  drift.  Throughout  nearly  all  these  areas  the  rocks  were  so 
readily  abraded  by  the  ice  and  the  hills  were  so  little  higher  than  the  adjacent  valleys  that  the 
glacial  deposits  have  entu-ely  covered  the  preglacial  topography  ami  molded  a  new  topography 
of  their  own.  Moreover,  the  draming  of  the  glacial  lakes  which  occupied  the  basin  of  the 
present  Lake  Superior  and  overlapped  its  shores  has  permitted  streams  to  produce  a  peculiar 
topograpliy  of  scidptured  lake  clays.'* 

DESTRUCTIVE  WORK  OF  THE   GLACIERS. 

Removal  of  weathered  rock. — Glacial  erosion  removed  quantities  of  weathered  rock,  includ- 
ing the  nonrcsistant  iron  ores,  perhaps  truncating  the  iron-bearing  roclvs  to  a  lower  level  ui 
Canada  than  in  tlie  United  States,  and  hence  maldng  the  Canadian  mines  less  productive,  as 

a  Weidman,  Samuel,  Bull.  Wisconsin  Geol.  and  Nat.  Hist.  Survey  No.  16, 1907,  fig.  21,  p.  434,  and  map  in  pocket.    Berkey,  C.  P.,  Jour.  Geology, 
vol.  13,  ISIII.'i.  pp.  .15,  39. 

6  rhamberlin,  R.  T.,  Jour.  Geology,  vol.  13,  190),  pp.  249-251. 
c  Ann.  Kept.  Cieol.  Survey  Michigan  for  1906,  1907,  pp.  47-52. 
d  Irving,  R.  D.,  Geology  ot  Wisconsin,  1873-1879,  vol.  3,  1880,  p.  69. 


b 


THE  PLEISTOCENE.  431 

Van  Hise°   has  suggested.     Lawson,*  however,  brmgs   evidence  to  show  tliat   there   was  no 
very  material  reduction  of  level. 

Striee  and  roches  moutonnees. — The  scratches  or  strias  and  the  smoothly  polished  surfaces 
which  were  made  by  the  ice  advancing  over  the  region  are  to  be  found  throughout  the  Lake 
Superior  region  wherever  there  are  ledges  of  hard  rock  which  will  preserve  them.  On  the 
Archean  and  Algonkian  ledges  these  striae  are  exceeduagly  common.  The  advance  of  the 
glaciers  over  these  ledges  modified  them,  producmg  the  rountled  forms  known  as  roches  mouton- 
nees. Some  of  these  have  longer  axes  in  the  direction  of  ice  movement  and  steep  or  even  pre- 
cipitous slopes  on  the  lee  side,  due  to  a  process  called  plucking,  in  which  large  blocks  of  ice 
are  rasped  or  torn  away  by  the  glacier.  In  the  pre-Cambrian  areas  these  roches  moutonnees 
are  exceedingly  common,  although  in  the  main  rather  low  and  not  very  prominent. 

•  Broadened  and  deepened,  valleys. — In  certain  favorable  localities,  either  where  the  ice  flow 
is  very  strong  or  where  the  rock  is  exceptionally  weak,  glaciers  broaden  and  deepen  their 
valleys.  Clements''  suggests  the  possibility  that  in  the  Vermilion  district  "glacial  erosion 
was  also  active  in  widening  and  deepenmg  these  preglacial  valleys,  changing  V-shaped  into 
U-shaped  valleys."  The  overdeepening  of  certam  parts  of  the  bottoms  of  valleys  results 
in  the  production  of  basins  in  the  solid  rock,  and  these  are  afterward  occupied  by  lakes. 
(See  PI.  VI,  p.  118.)  The  rock  basins  of  this  description  are  very  common  m  the  Lake  Superior 
region,  and  glacial  erosion  has  probably  caused  the  deepenmg  of  many  of  the  lakes  in  the 
granite  area  of  northern  Minnesota,  where  it  is  possible  to  go  all  around  the  lake  shores  on 
ledges,  demonstratmg  that  the  lake  basins  are  lower  than  the  surrounding  country.  Lake 
Superior  was  somewhat  deepened  by  glacial  erosion  at  the  time  when  the  ice  was  advancing 
through  it  (PI.  II,  p.  86),  and  Lake  Michigan  and  Green  Bay,'^  like  the  Wumebago  Valley, 
were  also  somewhat  deepened  in  this  way,  although,  as  previously  stated,  these  depressions 
must  have  existed  before  the  glacial  ice  advanced  through  them. 

Glacial  erosion  also  broadened  and  rounded  out  the  great  transverse  valley  of  Portage 
Lake,  which  crosses  Keweenaw  Pomt  at  Houghton,  as  well  as  many  other  valleys  in  the  region, 
especially  in  the  more  hilly  areas.  The  overdeepening  may  be  seen  west  of  Houghton,  where 
Huron  Creek  occupies  a  hanging  valley  (PI.  XXX,  B,  p.  434). 

The  effect  of  glacial  erosion  on  the  Duluth  escarpment  northwest  of  Lake  Superior,  where 
Thimder,  Black,  and  Nipigon  bays  occupy  submerged  hanging  valleys,  has  already  been  dis- 
cussed (p.  114). 

Glacial  rock  basins. — The  rock-basLn  lakes  occupymg  depressions  produced  by  glacial 
erosion  are  numerous  in  the  areas  of  pre-Cambrian  rocks.  (See  PI.  VIII,  in  pocket.)  Their 
character  and  origm  may  be  inferred  from  one  specific  illustration.  In  the  Michipicoten 
district  a  series  of  lake  basins  entirely  rimmed  by  rock  has  been  studied  by  Coleman,^  who 
concluded  that  these  basins  have  been  formed  by  chemical  action  and  are  not  due  to  glacial 
erosion. 

The  writer  visited  the  Michipicoten  district  durmg  the  summer  of  1907  and  after  a  study 
of  these  rock  basins  came  to  a  conclusion  different  from  that  of  Coleman.  For  a  number  of 
reasons  it  seems  probable  that  Hematite  Mountain,  at  whose  base  is  the  Helen  iron  mine  and 
one  of  the  rock  basins,  was  the  seat  of  a  local  glacier  that  probably  came  into  existence  as  the 
ice  was  advancing  over  southern  Ontario  and  lingered  as  the  ice  sheet  was  retreating,  because 
of  the  height" of  the  hill  (1,700  feet).  The  north  and  northwest  slopes  of  the  hill  would  receive 
less  sunlight  and  heat  than  the  south  slope  and  the  snow  and  ice  would  therefore  Imger  there 
longest.  The  local  glacier  would  naturally  be  on  that  side.  The  shape  of  the  depression  in 
which  the  Helen  mine  is  situated  is  such  as  to  suggest  that  it  is  a  glacial  cirque  (fig.  61),  and 
the  rock  basin  is   of   exactly  the  kind  which  is  made  by  small  glaciers   m  tlieii'  cirques.     A 

a  Van  nise,  C.  R.,  Twenty-flrst  Ann.  Kept.  U.  S.  Geol.  Survey,  pt..3,  1901,  pp.  411-412. 
6  Laws»n.  A.  C,  Bull.  Geol.  Soc.  America,  vol.  1,  1890,  p.  169. 
c  Clements,  J.  M.,  Men.  U.  S.  Geol.  Survey,  vol.  45,  1903,  p.  43. 

d  Winchell,  N.  H.,  Am.  Jour.  Sci.,  3tl  ser.,  vol.  2,  1871.  pp.  15-19.  ' 

e  Coleman,  A.  P.,  Rock  basins  of  Helen  mine.  Michipicoten,  Canada:  Bull.  Geol.  Soc.  America,  vol.  13.  1902,  pp.  293-304;  Univ.  Toronto 
Studies,  1902,  pp.  5-6,  26;  Rept.  Bur.  Mines  Ontario,  vol.  15,  pt.  1,  1906,  pp.  187,  ISS;  Econ.  Geology,  vol.  1,  1906,  p.  522. 


432 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


ledge  separates  it  from  an  adjoining  rock  basin  a  little  farther  down  (a  normal  glacial  rock  basin 
relation  of  which  many  examples  are  known)  and  a  rather  marked  hanging  valley  (PI.  XXIX,  ^) 
connects  the  depression  in  which  these  two  lakes  are  situated  with  a  lower  trunk  valley  in  wliich 
lies  still  another  lake  (fig.  61).  The  existence  of  this  hanging  valley  indicates  glacial  erosion 
in  the  region.  The  glacial  striae  in  the  upper  part  of  the  valley,  which  occasioned  one  of  Cole- 
man's difficulties  in  believing  this  a  glacial  rock  basin,  are  oblique  to  the  trend  of  the  valley, 
as  would  be  natural  during  the  higher  stages  of  the  continental  glacier,  but  the  lowcrstriajrun 
in  the  proper  direction  for  the  later  stages  of  a  local  glacier.     Ice  would  naturally  excavate 

HefTjatitc  Mtn 
Glacial  rock  basins  I700 

Hanging         SnvrsZ- 
Grade  .of  main   valley  to  which  hanging  valley  is  tributary  valley      ^^-■ISSS'' 

Talbot  Lake  BOO' 


0  1000 2000 3000  -WOO  FEET 

FiGUKE  61.— Sketch  showing  the  glacial  cirque,  the  rock  basins,  and  the  hanging  valley  near  the  Helen  mine,  Michipieoten. 

along  the  zone  of  weak  iron-bearing  rocks,  which  were  possibly  somewhat  prepared  for  the 
excavation  by  chemical  action  of  the  sort  that  Coleman  suggests. ° 

The  real  crux  of  the  determination  of  these  lake  basins  as  of  chemical  or  glacial  origin  lies 
in  the  fact  that  the  iron  ore  remaining  in  the  basins  is  found  in  just  that  locality  where  a  small 
glacier  in  a  cirque  would  protect  it,  although  removing  the  rest  of  the  iron  ore,  whereas  if  a 
chemical  origin  is  thought  plausible,  the  selective  chemical  action  in  preserving  the  ore  at  just 
this  point  and  removing  it  elsewhere  in  the  basin  must  be  accounted  for. 


TRANSPORTING   WORK   OF   GLACIERS. 

It  is  well  established  that  the  deposits  carried  by  the  glaciers  have  been  worn  by  the  ice 
from  the  .ridges  over  which  the  ice  sheet  advanced  and  that  in  any  place  where  glaciers  have 
been  the  rocks  brought  by  them  are  apt  to  be  of  an  entirely  difjFerent  sort  from  the  ledges  which 
underlie  them,  although  a  large  part  of  the  material  in  the  drift  may  be  of  local  derivation. 
This  transportation  of  foreign  material  was  early  observed  in  tliis  region,  though  explained 
by  Bigsby ''  as  due  to  "an  earthquake  sea  wave"  or  "loaded  icebergs."  When  rocks  of  a 
distinctive  kind  are  found  in  an  area  where  no  similar  rocks  normally  occur  and  the  striae 
indicate  that  the  glaciers  moved  in  the  proper  direction  to  carry  these  rocks,  it  may  be  con- 
sidered demonstrated  that  glacial  ice  has  moved  the  material  from  one  place  to  the  other.  The 
early  students  lacked  this  conception  of  moving  glaciers.  Devonian  limestone  Math  fossils 
was  thus  brought  into  the  Micliipicoten  district  from  a  locality  some  150  miles  to  the  northeast, 
and  iron  ore  was  thus  transported  in  the  upper  peninsula  of  Micliigan."^  Cambrian  or  Silurian 
limestone  pebbles  "*  from  ledges  in  Manitoba  seem  to  have  been  brought  to  the  Lake  of  the 
Woods  region  of  old  crystalhne  rocks  bj"  a  later  movement  of  the  Keewatin  glacier  after  the 
chief  northeast-southwest  movement  of  the  Labrador  ice  sheet.  Many  fragments  of  the  granites 
anil  gneisses  of  the  Archean  and  the  por]:)hyrites  and  quartzites  and  jaspers  of  the  Lake  Superior 
region  were  transported  by  the  glaciers  and  are  now  foimd  in  the  region  of  horizontal  Paleozoic 
rocks  to  the  south,  fragments  of  tliis  kind  coming  from  both  the  north  and  the  south  shores  of 
Lake  Superior.  It  is  sometimes  an  aid  to  the  iron  prospector  to  study  the  stones  in  the  glacial 
drift  in  order  to  determine  where  possible  ledges  of  iron-bearing  formations  may  be  found. 
The  most  notable  case  of  glacial  transportation  of  iron  ore  is  that  of  the  30,00()-ton  mass  south 
of  the  Fayal  mine,  on  the  Mesabi  range,  which  Leith «  describes  as  being  entirely  inclosetl  in 
the  glacial  drift  and  hence  evidently  transported  bodily  from  the  ledges  to  the  north. 

<■  Coleman,  .\.  P.,  Rept.  Bur.  Mines  Ontario,  vol.  8,  pt.2,  1899,  pp.  156-157 

l>  Bigsby,  J.  J.,  On  the  erratics  of  Canada:  Quart.  Jour.  Geol.  Soc.,  vol.  7, 1S51,  pp.  215-238.  ' 

c  Brooks,  T.  B.,  Geol.  Survey  Michigan,  vol.  1,  1873,  pp.  76-79. 

dLawson,  A.  C..  Gcnl.  anrt  Nat.  Hist.  Survey  Canada,  vol.  1.  1885,  p.  132cc. 

'  Leith,  C.  K.,  The  Mesabi  iron-bfearing  district  of  Minnesota:  Men.  U .  S.  Geol.  Survey,  vol.  43,  1903,  p.  263. 


U.  S-  GEOLOGICAL  SURVEY 


MONOGRAPH    Lll     PL.  XXIX 


A.     HANGING    VALLEY    NEAR     HELEN     MINE,     MICHIPIGOTEN. 
Talbot  Lake  in  foreground.     See  page  432. 


B.     LAKE    CLAY    OVERLYING    STONY     GLACIAL    TILL    IN     MOUNTAIN     IRON     OPEN     PIT,     MESABI 

RANGE,     MINN. 

See  page  443. 


THE  PLEISTOCENE.  433 

Among  the  distinctive  materials  wliich  are  found  in  the  glacial  drift  are  diamonds  and 
native  copper.  The  copper  is  of  course  traceable  to  the  copper-bearing  rocks  of  northern 
Wisconsin  and  Michigan  and  Michipicoten  Island,  but  the  source  of  the  diamonds  is  not  loiown." 

CONSTRUCTIVE  WORK   OF   GLACIERS. 

GROUND  MORAINE. 

Much  of  the  material  carried  by  the  ice  sheet  is  ground  finer  and  finer  until  it  is  reduced  to 
clay,  and  this  clay  with  the  included  stones  of  various  sizes  which  were  not  ground  up  so  fine 
forms  the  most  widespread  of  the  deposits  left  by  the  glaciers.  It  is  generally  called  till  or 
bowlder  clay  and  was  formerly  known  as  unmodified  glacial  drift.  It  reached  its  present 
position  simply  by  being  dropped  from  the  melting  ice,  and  forms  the  great  mantle  of  ground 
morame  and  parts  of  the  ridges  of  terminal  or  recessional  moraines.  The  present  thickness 
varies  with  the  former  tliickness  of  the  ice,  the  amount  of  such  debris  which  was  contained  in 
the  ice,  and  the  amount  of  erosion  by  running  water  either  in  connection  with  the  melting  ice 
or  subsequently.  Tliis  glacial  till  is  found  with  varying  tliicknesses  in  every  part  of  the  Lake 
Superior  region,  overlymg  the  Archean,  Algonldan,  Paleozoic,  and  Cretaceous  rocks,  being 
entirely  absent  or  represented  only  by  scattered  stones  in  some  rock  ledges,  and  covering  other 
areas  and  completely  obscuring  the  bed  rock  by  an  overburden  200  to  300  feet  thick. 

The  type  of  topography  produced  by  the  glacial  till  in  the  ground-moraine  areas  depends 
largely  on  whether  enough  of  it  accumulated  to  bury  the  preglacial  topography  or  not.  Many 
hills  in  the  glaciated  area  still  have  the  form  of  their  bed-rock  cores  or  are  merely  thiidy  veneered 
with  the  bowlder  clay.  Many  vaUeys  also  are  only  partly  filled  by  the  tdl  (fig.  55,  p.  364)  and 
remain  as  vaUeys,  though  not  now  as  deep  as  before  the  glacial  advance.  On  the  other  hand, 
more  commonly  the  topography  was  so  mild  before  the  glacial  advance  and  the  accumulation 
of  glacial  deposits  was  so  thick  that  an  entirely  new  topography  is  modeled  by  the  ice.  (See 
Pis.  XI,  p.  180,  and  XXXI,  A,  p.  436.)  This  topography  is  generally  of  the  "moderately  rolhng," 
"undulating  or  rolling,"  and  "flat  or  undulating"  types  described  by  Warren  Upham  and  others.'' 

DRUMLINS. 

A  class  of  till,  or  unassorted  ground  moraine,  which  deserves  special  mention  is  the  drum- 
lin.  Drumlins  in  only  one  or  two  areas  within  the  field  of  this  report  have  yet  been  described, 
but  they  doubtless  exist  at  numerous  other  points.  The  drumlins  of  the  Lake  Superior  region 
are  lenticular  hills  of  bowlder  clay  or  till,  varying  m  shape  from  that  of  half  of  an  egg  that  has 
been  bisected  lengthwise  to  that  of  half  of  a  cigar  cut  in  two  in  the  same  way.  They  character- 
istically have  one  rather  steep  side  and  one  gentle  slope,  the  steep  slope  being  on  the  side  from 
wliich  the  ice  came.  The  long  axis  of  the  drumlin  is  invariably  parallel  to  the  direction  of  the 
latest  ice  movement. 

Three  areas  of  drumlins  in  Micliigan  have  been  described.  The  first  is  in  the  Menominee 
district,'^  where  the  drumUns  are  found  over  an  area  of  about  150  square  miles  and  have  an 
average  height  of  about  40  feet.  The  second  area  is  also  in  the  upper  peninsula  of  Michigan, 
including  Les  Cheneaux  Islands  and  a  portion  of  the  adjokdng  mainland  on  the  north  shore  of 
Lake  Huron.*^  The  tliird  drumhn  area  is  in  the  Grand  Traverse  region,^  in  the  northM'estern 
part  of  the  southern  peninsula  of  Michigan. 

oSalishury,  R.  D.,  Notes  on  the  dispersion  of  drift  copper:  Trans.  Wisconsin  Acad.  Sci.,  Arts  and  Letters,  vol.  6,  1SS6,  pp.  42-50.  Ilobbs, 
W.  H.,  Emigrant  diamonds  in  America:  Ann.  Rept.  Smitlisonian  Inst.,  1901,  pp.  359-366;  Am.  Geologist,  vol.  16,  1894,  pp.  31-35;  Jour.  Geology, 
vol.  7,  1899,  pp.  375-388.  Farrington,  O.  C,  (  orrelation  of  distribution  of  copper  and  diamonds  in  tbe  glacial  drift  ol  the  Great  Lakes  region:  Proc. 
Am.  Assoc.  Adv.  Sci.  vol.  58.  190S,  p.  288. 

i>  Final  Rept.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  te.xt  accompanying  county  maps. 

"•Russell,  I.  C,  The  surface  geology  of  portions  of  Menominee,  Dickinson,  and  Iron  counties,  Mich.:  Ann.  Rept.  Geol.  Survey  Michigan  for 
1906,  1907,  pp.  8-91. 

d  Russell,  I.  C,  A  geological  reconnaissance  along  the  north  shore  of  Lakes  Huron  and  Michigan:  .\nn.  Rept.  Geol.  Survey  Michigan  for  1904, 
1905,  pp.  39-150. 

elxverett,  Frank,  Science,  new  ser.,  vol.  21,  1905,  p.  220;  Water-Supply  Paper  U.  S.  Geol.  Survey  No.  183,  1907,  pp.  333-335. 

47517°— VOL  52—11 28 


434  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Geologists  have,  not  tliorouglily  agreed  as  to  the  origin  of  ch-umlins.  Two  theories  have 
been  held.  One  holds  that  the  drumlins  are  constructed  under  the  ice  by  the  accumulation  of 
material  there,  the  material  being  derived  by  the  ice  sheet  from  the  land  from  wliich  it  is 
advancing  and  the  drumlins  being  built  somewhat  like  bars  in  a  river.  The  alternate  hypoth- 
esis ascribes  drumUns  to  a  dcstnactive  action,  the  ice  sheet  being  supposed  to  carve  drumlins 
from  a  preexisting  mass  of  tUl  laid  down  by  a  previous  ice  sheet.  The  drumlins  of  the  first 
two  areas  described  seem  to  have  been  formed  by  the  destructive  process,  as  very  decisive 
evidence  by  Russell  proves,  but  Leverett  tliinks  that  some  of  the  drumlins  in  the  Grand 
Traverse  region  are  constructional  rather  than  destructional. 

ESKERS. 

Another  glacial  feature  to  be  described,  the  esker,  is  a  fossil  stream  course  formed  in  or  under 
the  ice  by  a  stream  flowing  in  a  tunnel  and  depositing  its  load  of  sediment,  wldch  is  preserved 
on  the  surface  as  a  low  winding  ridge  after  the  ice  has  melted  away.  Eskers  in  many  parts  of 
the  Lake  Superior  region,  as  in  northeastern  Miimesota  "  and  the  Menominee  district,*  have 
been  described.  Russell  describes  them  as  low,  serpentine  gravel  ridges  in  the  valleys  between 
the  drumlins.  They  are  doubtless  also  present  in  many  other  areas.  They  are  mentioned  here 
rather  than  with  the  other  stratified  drift  dejjosits,  like  outwash  plains,  because  in  this  area 
they  are  commonly  associated  with  the  ground  moraine  rather  than  with  the  outwash  of  the 
valleys. 

TERMINAL  MORAINES. 

The  deposit  piled  up  at  the  end  of  the  ice  tongue  or  lobe  is  called  a  terminal  moraine,  and 
the  name  ds  applied  not  only  to  the  deposit  made  at  the  farthest  advance  of  the  ice  but  also  to 
those  made  at  any  point  where  the  ice  halts.  The  latter  are  also  sometimes  called  recessional 
moraines.  The  only  terminal  moraines  in  the  Lake  Superior  region  wliich  mark  the  farthest 
advance  of  the  ice  lie  aroimd  the  borders  of  the  Driftless  Area,  but  recessional  moraines  are  more 
abundant.  Some  of  them,  so  far  as  mapped,  are  shown  in  figure  68  (p.  453).  These  recessional 
moraines  may  be  made  up  of  two  rather  different  kinds  of  material — the  glacial  till,  or  unmodi- 
fied drift,  and  the  drift  which  is  assorted  and  stratified  by  running  or  standing  water.  A  termi- 
nal or  recessional  moraine  in  the  Lake  Superior  region  usually  consists  of  a  series  of  ridges  or 
knolls  (PI.  XXX,  A),  in  general  constituting  a  long,  narrow  zone  of  hilly  country,  which  may 
be  in  a  single  ridge,  but  is  more  commonly  an  irregular  belt  of  ridges  and  valleys.  The  charac- 
teristic terminal  moraine  is  made  up  largely  of  laiobs  and  kettles.  The  belts  of  terminal  morauie 
range  from  several  hundred  yards  to  several  miles  in  width  but  are  rarely  over  4  or  5  miles  wide 
and  generally  a  mile  or  less.  A  great  terminal  moraine  of  course  indicates  that  the  edge  of  the 
ice  remained  at  one  point  for  a  considerable  length  of  time.  During  this  time,  if  the  glacier  was 
moving,  it  would  be  constantly  bringing  material  up  to  tliis  point,  dropping  the  material 
there,  and  perhaps,  by  slight  readvances,  shoraig  ahead  the  material  wliich  had  prcA-iously 
been  deposited  by  the  melting  ice,  and  all  this  material  would  be  subject  to  constant  removal 
or  rearrangement  by  the  running  water  that  issued  from  the  ice  as  the  glacier  was  melting. 
These  terminal  morames  are  therefore  made  up  of  a  mixture  of  unmodified  till  and  stratified 
sand,  gravel,  and  clay  deposited  by  running  water,  with  variations  of  the  two  as  the  ice  may 
have  advanced,  or  as  the  water  may  have  cut  chamiels  in  the  deposits,  or  as  portions  of  the  ice 
may  have  been  buried  beneath  the  deposits  made  by  the  melting  of  the  upper  ice  layers  or  laid 
down  by  the  streams.  The  subsequent  melting  of  these  buried  ice  blocks  has  caused  the  glacial 
drift  to  slump  down,  forming  broad  hollows  and  steep-sided  pits.  This  is  the  general  origin  of 
the  kettles  which  are  found  in  terminal  moraines. 

o  Elftman,  A.  H.,  Am.  Geologist,  vol.  21, 1898,  p.  97. 

b  Russell,  I.  C.  The  siirface  geology  of  portions  of  Menominee,  Dickinson,  and  Iron  counties,  Mich. :  Ann.  Kept.  Geol.  Survey  Michigan  for 

190(5, 1907,  pp.  8-91;  .Vm.  Geologist,  vol.  35, 1905,  pp.  177-179;  Science,  new  ser.,  vol.  21,  1905,  pp.  220,  221. 


(5     IV^o 
O      « 


THE  PLEISTOCENE.  435 

KAMES. 

Karnes,  or  irregular  hummocks  of  waterworn  sand  and  gravel,  are  present  throughout  the 
morame  belts  of  the  Lake  Superior  region,  many  of  them  at  the  borders  of  valleys,  as  if  formerly 
at  the  margin  of  an  ice  sheet  whose  melting  has  caused  the  edges  of  marginal  terraces  to  slump 
down  into  irregular  hummocks  and  kettles.  Russell  describes  irregular  hillocks  of  rounded  kame 
gravels  in  the  Menominee  area  and  ascribes  them  to  accumulation  beneath  wells,  or  moulins,  in 
the  ice  sheet,  where  streams  on  or  in  the  glacier  fell  vertically  and  deposited  their  load. 

BECESSIONAL  AND  INTERLOBATE  MORAINES. 

The  recessional  moraines  formed  at  temporary  terminal  points  of  the  ice  sheets  during  the 
Wisconsin  stage  are  seen  from  the  map  (fig.  68,  p.  453)  to  be  definitely  related  to  the  larger 
lowland  and  highland  areas,  and  it  is  by  a  study  of  these  moraines  that  some  of  the  conclusions 
as  to  the  behavior  of  the  different  ice  lobes  in  the  Lake  Superior  region  have  been  reached. 

As  the  ice  retreated  from  the  maximum  stage  of  a  confluent  ice  cap  and  once  more  resolved 
itself  into  lobes,  some  very  distinctive  deposits  were  formed  between  the  adjacent  lobes,  and 
these  are  called  interlobate  moraines.  An  example  of  the  moraines  of  this  kind  is  found  in  the 
interlobate  (kettle)  moraine  of  eastern  Wisconsin,  which  was  accumulated  between  the  Green 
Bay  lobe  and  the  Lake  Michigan  lobe.  Other  interlobate  moraines  were  formed  between  the 
Chippewa  lobe  and  the  Superior  lobe  in  Bayfield  and  Douglas  counties,  Wis.,  west  of  Ashland, 
and  between  the  Superior  and  the  Rainy  Lake  lobes  m  northeastern  Minnesota. 

DRAINAGE  OF  DRIFT-COVERED  AREAS. 

The  accumulation  of  till  over  this  great  area  has  modified  the  drainage,  and  one  of  the  most 
prominent  effects  of  this  accumulation  is  the  destruction  of  mature  or  submature  preglacial 
drainage  and  the  superposition  of  young  drainage  on  the  drift,  causing  gorges,  waterfalls,  and 
the  great  numbers  of  lakes  and  swamps  for  which  the  region  is  noted.  (See  PI.  XXII,  in  pocket.) 
These  lakes  and  swamps  are  due  to  a  common  cause — mterference  with  the  free  run-off  of  rain 
by  the  irregular  deposition  of  the  drift.  Among  the  most  common  kinds  of  lakes  and  swamps  or 
muskegs  (PI.  XXXI,  B)  are  those  wliich  are  produced  by  the  accumulation  of  water  m  shallow 
depressions  in  the  undulating  or  mildly  irregular  till  sheet.  As  the  material  of  the  till  was 
largely  clay,  it  would  naturally  be  difficult  for  the  water  to  escape  tlirough  it.  Another  com- 
mon cause  of  lakes  is  the  accumulation  of  a  greater  thickness  of  the  glacial  till  in  one  part  of  the 
valley  than  in  another,  producing  an  obstn.iction  to  drainage.  Many  of  the  streams  were  also 
forced  out  of  their  preglacial  courses  by  the  deposits  of  glacial  till,  and  numerous  rapids  and 
waterfalls  are  due  to  this  cUsplacement.  Clements "  has  described  Deer  River,  Michigan 
(PI.  XXII),  as  typical  of  a  stream  with  associated  swamps  and  lakes  in  a  till-covered  area  and 
has  outHned  the  life  history  of  such  a  dramage  system.  The  normal  type  of  preglacial  drain- 
age of  the  entire  Lake  Superior  region  is  illustrated  in  Plate  XXXI,  A,  showing  part  of  the 
Driftless  Area.  Plate  XXXI,  B,  shows  the  young  drainage  of  the  glacial  drift  which  now  covers 
the  greater  part  of  the  region. 

DIFFERENCES  BETWEEN  YOUNGER  AND  OLDER  DRIFT. 

There  is  evidence  in  the  central  United  States  which  has  been  interpreted  as  indicating  that 
the  glacial  period,  instead  of  being  simple,  was  decidedly  complex.  It  is  thought  that  the 
ice  did  not  advance  from  the  Labrador  and  Keewatin  centers  once  and  retreat  once,  but  that 
instead  it  underwent  a  series  of  oscillations  so  that  glacial  deposits  were  laid  down  under  or  in 
front  of  the  ice,  the  ice  retreated  from  them,  and  then  the  weathering  and  erosional  agencies 
acted  upon  these  deposits.  This  is  the  reason  why  the  lakes  among  the  older  glacial  deposits  are 
largely  either  filled  or  drained,  the  till-veneered  liillsides  are  cut  by  streams,  the  stones  in  the 
drift  are  weathered  and  disintegrated,  and  the  soluble  constituents  have  been  leached  out  of  the 
soil  by  percolating  water.  After  all  this  had  taken  place  the  glaciers  are  thought  to  have  read- 
vanced  and  covered  the  older  drift  with  a  sheet  of  new  till,  etc.,  wliich  in  some  places  extends 

■■  Clements,  J.  SI.,  Mon.  U.  S.  Geol.  Surrey,  vol.  30, 1899,  pp.  32-30;  Am.  Geologist,  vol.  17, 1890,  pp.  120-127. 


436  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

farther  out  tliaii  tlio  older  drift  and  in  olliers  has  left  a  Ijroad  zone  of  it  exposed.     Tliis  fresh, 
unweathered,  young  till  forms  a  decided  contrast  to  the  older  drift. 

Only  the  extreme  southwestern  part  of  this  region  contains  any  of  what  has  been  inter- 
preted as  older  drift.  In  the  greater  part  of  the  region  the  drift  seems  to  be  solely  the  work  of 
the  Wisconsin  ice  sheet.  The  drift  near  the  borders  of  the  Driftless  Area  has  been  ascribed  to 
two  or  three  earlier  glacial  epochs,  but  most  of  the  Lake  Superior  region  furnishes  no  evidence 
whatever  of  more  than  one  glacial  advance,  eitiier  in  the  deposits  or  in  the  topograph}'. 

EFFECT  OF  NUNATAK  STAGES  ON  DISTKIBUTICN  OF  DRIFT. 

In  spite  of  the  lack  of  detailed  studies  in  a  large  part  of  this  region,  it  seems  probable  that 
the  behavior  of  the  ice  in  retreating  can  be  somewhat  discriminated,  ^^^len  an  ice  sheet  covers 
an  irregular  land  surface,  there  are  two  ways  in  which  it  may  retreat.  It  may  disappear  grad- 
ually from  the  lowlands  and  linger  longest  in'  the  upland  regions,  as  is  the  case  in  the  Rockies, 
in  Norway,  in  Alaska,  and  in  Switzerland  to-day.  It  does  this,  however,  only  where  the  elevated 
areas  are  high  enough  to  become  centers  of  local  glaciation  and  to  supply  new  ice.  The  con- 
trasting condition  is  found  where  the  highland  areas  are  not  sufficiently  elevated  to  retain  snow 
through  the  summers  and  therefore  to  supply  ice.  Where  the  latter  condition  prevails,  the 
glacier  does  not  continue  to  be  active  up  to  the  very  time  of  its  extinction,  as  in  the  Rocky 
Mountains  at  present,  but  becomes  stagnant  because  there  is  no  fresh  supply  of  ice.  When  an 
ice  sheet  becomes  stagnant,  the  high  areas  are  first  exposed  by  melting,  because  over  them  the 
ice  is  thinnest,  and  they  rise  out  of  the  ice  sheet  as  nunataks.  These  nunataks  gradually 
increase  in  size,  and  eventually  the  ice  shrinks  until  it  is  found  only  in  the  valleys,  where  it 
was  thickest. 

The  conditions  just  described  seem  to  have  prevailed  in  parts  of  the  Lake  Superior  region. 
Northwest  of  Lake  Superior  the  Giants  Range  was  a  nunatak  (figs.  60  and  6;?),  emerging  in  the 
interiobate  area  between  the  Rainy  Lake  glacier  and  the  Lake  Superior  glacier.  These  lobes 
gradually  retreated  to  the  Lake  Superior  basin  and  to  the  valley  of  Red  River,  respectivclvj 
marguial  lakes  being  formed  as  described  in  another  section  (p.  441).  North  and  northeast  of 
Lake  Superior,  in  Ontario,  the  conditions  maj^  possibly  have  been  similar,  the  ice  shrinking 
away  from  an  interiobate  area  near  the  Height  of  Land  and  occupying  the  basin  of  Lake  Superior 
largely  as  a  stagnant  mass. 

South  of  Lake  Superior,  however,  the  highland  area  seems  to  have  had  a  sonjewhat  different 
history.  The  ice  from  the  Chippewa  and  Keweenaw  lobes,  which  advanced  over  the  highland 
region  of  northern  Wisconsin  and  somewhat  down  its  southward  slope,  probably  retreated 
northward  over  the  same  slope  without  the  emergence  of  the  northern  Wisconsin  highland  as  a 
nunatak  area,  although  the  Porcupine  Mountains  were  probably  uncovered  as  a  nunatak  region 
about  the  time  the  glacier  became  lobate  in  the  valle3's  east  and  west  of  Keweenaw  Point.  The 
Huron  Mountains  aeem  also  to  have  first  emerged  as  a  nunatak  area,"  lying  between  the  Kewee- 
naw and  Green  Bay  lobes.  Some  of  the  earliest  drift  deposits  were  developed  about  these 
emerging  nunataks. 

VARIATION  OF  DEPOSITS  WITH  SLOPES. 

When  a  glacier  is  retreating — that  is,  melting  back  faster  than  the  ice  advances,  or  melting 
back  with  no  advance,  as  in  a  stagnant. ice  sheet — two  rather  different  kinds  of  deposits  are 
made  in  association  with  two  diverse  topographic  conditions.  One  kind  is  formed  where  the 
land  slopes  away  from  the  ice,  allowing  a  free  run-off  of  the  glacial  streams  which  are  fed  by  the 
melting  ice.  The  other  kind  is  formed  where  the  land  slopes  toward  the  ice  and  the  drainage 
from  the  ice  is  detained  in  a  glacial  lake  imtil  it  rises  to  a  sudiciently  high  level  to  flow  over  a 
neighboring  divide.  The  first  condition  was  well  exemplified  by  the  t'hippewa-Keweenaw  lobe 
as  it  retreated  from  the  highland  region  of  northern  Wisconsin,  when  its  streams  flowed  freely 
awaj-,  carrj'ing  great  quantities  of  gravel,  sand,  and  cla}'^  that  were  deposited  in  outwash  plains 

»  Davis,  C.  \..  N'inth  Rept.  Michigan  Acad.  Scl.,  1907,  pp.  132-135. 


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THE  PLEISTOCENE.  437 

or  valley  trains,  a  number  of  which  cross  the  Driftless  Area  of  Wisconsin.  At  later  stages  such 
outwash  gravels  are  likely  to  be  so  dissected  by  stream  erosion  that  terraces  are  formed  at 
higher  levels  than  the  present  stream.  This  is  believed  to  be  the  origin  of  the  terraces  in  the 
valley  of  Wisconsin  River  near  Wausau,  in  that  of  the  St.  Croix  near  the  Dalles,  and  along 
several  other  stream  courses  of  the  region. 

OUTWASH  DEPOSITS. 

Wlien  several  streams  flowing  out  side  by  side  build  up  a  broad  plain  of  the  same  kind  as  the 
valley  trains,  but  not  confined  to  a  valley,  the  deposit  is  called  an  outwash  plain  (PI.  XXX,  A). 
Outwash  plains  of  this  type  are  found  in  the  Upper  Peninsula  of  Michigan,  in  Ontario,  in  Minne- 
sota, and  in  northern  Wisconsin.  Weidman  "  has  described  some  of  them  as  "alluvium"  and 
believes  that  these  deposits  are  associated  with  (a)  uplift  of  the  land,  rejuvenating  the  streams 
and  causing  intrenchment;  (6)  lowering  of  the  land,  permitting  aggradation,  during  which 
these  so-called  alluvial  deposits  were  laid  down;  and  (c)  later  uplift,  permitting  reintrenchment 
of  the  streams,  and  terrace  cutting.  The  age  of  this  alluvium  he  is- inclined  to  place  as  perhaps 
pre-Iowan,  between  his  "Second"  and  "Third"  drift  sheets.  It  may  be  pointed  out  that  the 
alluvium  is  in  places  directly  associated  with  terminal  moraines,  and  Weidman  has  not  brought 
forward  evidence  to  show  that  it  extends  beneath  them  or  is  plowed  up  by  them.  After  short 
field  studies  by  the  writer  it  seems  more  probable  that  nearly  all  of  this  material  is  normal 
outwash. 

In  view  of  some  of  the  most  recent  conclusions  concerning  the  conditions  that  determine 
stream  work,  it  may  be  conceived  that  the  volume  and  load  of  the  streams  have  varied,  rather 
than  the  grade.  The  advance  of  ice  sheets,  with  increased  supply  of  water,  would  perform  the 
same  work  of  intrenchment  as  the  uplift  postulated  by  Weidman,  if  indeed  this  intrenchment 
is  not  preglacial.  Later  the  increased  load  of  the  streams,  supplied  with  debris  from  the  melting 
ice,  would  necessitate  aggradation  and  the  formation  of  outwash  deposits,  exactly  similar  to 
Weidman's  alluvium  and  such  as  are  knowTi  in  association  with  existing  ice  fronts  the  world 
over.  Still  later  the  diminution  of  the  debris  furnished  to  the  streams  by  melting  ice  would 
result  in  their  relief  from  overloading  and  in  a  return  to  processes  of  intrenchment  and  terrace 
cutting.  More  than  this,  Weidman's  alluvium,  where  supposedly  overriden  by  the  ice  depositing 
the  "Third"  drift  in  the  Wisconsin  River  valley,  seems  to  lack  entirely  the  broad  truncation 
and  grooving  characteristic  of  gravels  overridden  and  eroded  by  ice,  as  they  are  known  in  Alaska. 
Again,  Weidman  has  not  shown  that  the  terraces  are  gullied  or  the  drift  in  them  weathered  and 
leached  as  it  should  be  if  they  are  pre-Wisconsin  in  age.  Lastly,  if  these  so-called  alluvial 
deposits  are  not  outwash  and  mostly  of  Wisconsin  age,  it  may  be  asked.  What  became  of  the 
water  and  debris  from  the  melting  Wisconsin  ice  sheet  ? 

I.  C.  Russell  *>  described  a  series  of  interesting  outwash  deposits  in  the  valley  of  Menominee 
River  (PL  XXVI,  in  pocket).  They  he  in  a  series  of  steplike  levels  associated  with  moraines, 
marking  recedmg  stages  of  the  border  of  the  Green  Bay  lobe.  The  angular  turns  of  Menominee 
River  seem  also  to  be  related  to  these  receding  stages. 

At  Grantsburg,  Wis.,  in  the  valley  of  the  St.  Croix,  C.  P.  Berkey"  has  studied  a  series  of 
laminated  red  and  gray  clays,  judged  to  have  been  formed  in  a  glacial  lake  whose  deposits  over- 
lie Wisconsin  till.  He  reaches  the  conclusion  that  the  clays  were  derived  fi-om  the  melting  of 
an  oscillating  ice  sheet  and  estimates  a  greater  length  of  time  than  is  usually  thought  of  since  the 
retreat  of  the  ice,  on  the  theory  that  each  of  the  laminae  represents  a  year  of  melting  interrupted 
by  freezmg  and  supply  of  finer  sediment.  He  has  also  compiled  an  excellent  sketch  map<^ 
showing  the  relation  of  recessional  moraines  west  and  south  of  the  end  of  Lake  Superior  in 
Wisconsin  and  Muinesota. 


1  Weidman,  Samuel,  Bull.  Wisconsin  Geol.  and  Nat.  Hist.  Survey,  vol.  16,  1907,  pp.  418-421,  425,  477,  497-498,  5Dl,  504,  506,  514-547,  569-571, 
609-610,  622-624. 

b  Ann.  Rept.  Michigan  Geol.  Survey  for  1906, 1907,  p.  65. 
c  Jour.  Geology,  vol.  13,  1905,  pp.  35-44. 
dldem,  flg.  1,  p.  43. 


438  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

PITTED  PLAINS. 

There  is  one  phase  of  the  btiikling  of  outwash  gravel  deposits  or  valley  trains  which  deserves 
special  mention.  In  numerous  places  these  gravel  deposits  arc  deeply  pitted.  Such  pits  or 
kettles  are  well  dcvel()])e(l,  for  example,  near  Negaunee,  m  the  Marquette  <Ustrict;  all  through 
the  valley  of  Michigamme  River;  m  the  Perch  Lake  district  (Pi.  XXI,  in  pocket) ;  in  the  Crystal 
Falls  district,  between  Randville  and  Witbeck  (PI.  XXII,  in  pocket) ;  in  the  valley  of  Menominee 
River  south  of  Iron  Mountaui  (PI.  XXVI,  in  pocket);  m  the  lowland  region  of  the  northern 
peninsula  of  Micliigan,  east  of  Marcjuette;  in  the  Michipicoten  district  of  Canada;  and  doubt- 
less elsewhere.  As  the  glaciers  in  these  regions  retreated  small  tongues  or  isolated  blocks  of  ice 
were  buried  beneath  the  gravels  of  the  glacial  streams.  Subseijuently,  when  these  detached  ice 
blocks  melted,  the  gravel  layers  slumped  and  the  kettles  which  pit  the  surface  of  the  gravel  plain 
were  formed.  Many  of  the  gravel  kettles  contain  lakes  (PI.  XXX,  A,  p.  4.34)  and  a  considerable 
number  of  the  small  lakes  of  northern  Micliigan  and  Wisconsin  are  of  tliis  origin. 

LOESS. 

In  the  southwestern  part  of  the  area  is  a  fuie  clayey  or  sandy  material  called  loess,  formed 
possibly  from  the  rock  flour  carried  by  the  streams  flowing  from  the  retreating  glaciers  or 
transported  by  winds.     Its  distribution  witlun  tliis  area  is  not  well  known  as  yet. 

VALLEY  LAKES  DUE  TO  VARIATION  IN  STREAM  LOAD. 

There  is  a  striking  contrast  between  the  streams  that  were  the  outlets  of  marginal  glacial 
lakes  and  the  streams  that  flowed  directly  from  the  ice,  the  former  being  relatively  clear  streams 
and  the  latter  bemg  heavily  loaded  with  sediment.  Accordmgly  it  was  possible  for  Chippewa 
River,  with  its  heavy  load  of  glacial  material,  supplied  directly  by  the  melting  ice,  to  build  its 
outwash  plain  right  across  Mississippi  River  in  western  Wisconsin  in  spite  of  the  fact  that  the 
volume  of  the  Mississippi  was  probably  much  larger,  so  that  it  should  have  been  able  to  carry  away 
the  sediment  supplied  by  a  small  tributary  like  the  Cliippewa.  Many  of  the  streams  feeding  the 
upper  Mississippi  were,  like  the  outlets  of  Lake  Agassiz,  Lake  Nemadji,  and  Lake  Duluth,  out- 
lets of  glacial  lakes  in  which  the  sand,  gravel,  and  clay  had  all  been  strained  out.  Accordingly 
the  small  Chippewa,  with  its  heavy  load,  aggraded  at  its  confluence  with  the  Mississippi  and 
was  able  to  dam  back  the  Mississippi  itself  in  a  narrow,  lakeUke  expansion  more  than  25  miles 
long,  called  Lake  Pepin  (PL  II,  p.  86). 

Farther  up  the  ^^ississippi,  on  the  Wisconsin-Minnesota  boimdary  near  St.  Paul,  the  process 
just  outlined  was  reversetl,  the  maua  stream  havuig  more  load  as  well  as  more  volume  than  its 
tributary,  the  St.  Croix  (PI.  II).  Accordmgly  the  Mississippi  outwash  plam  and  more  recently 
the  modern  flood  plain  have  retarded  the  outflow  of  the  St.  Croix,  so  that  a  lake  is  formed  in  its 
valley  fi-om  the  mouth,  where  a  modern  sandbar  surmounts  the  flood  plam,  to  a  point  about 30 
miles  upstream,  the  head  of  the  present  Lake  St.  Croix. 

Similar  valley  lakes  on  the  Minnesota  side  of  the  Mississippi  have  been  described  by  Win- 
chell."  During  the  summer  of  1908  the  writer  observed  a  similar  series  of  lakes  in  the  tributary 
valley  mouths  on  the  Wisconsin  shore  of  the  Mississippi.  These  are  in  the  Driftless  ^\j'ea.  They 
were  formed  during  glacial  time  by  the  greater  buiklmg  up  of  the  mam  glacier-fed  Mississippi 
(through  outwash)  than  of  its  rain-fed  tributaries.  The  assumption  by  Wmchell  of  a  long,  nar- 
row ice  tongue  m  the  Mississippi  Valley,  however,  seems  to  the  writer  uiuiecessary.  The  out- 
wash  itself,  carried  by  the  great  volume  of  water  from  the  melting  glaciers  and  not  by  the  more 
slender  stream  of  the  motlern  Mississippi,  could  perfectly  well  accomit  for  these  glacial  materials 
in  the  Driftless  Area  and  for  the  shallow  lateral  lakes,  like  Waumandee  Lake  in  Wisconsui, 
across  the  river  from  Wmona,  and  numerous  imnamed  ponds  and  swamps  in  side  valley  mouths. 

aWinchell,  N.  H.,  BuU.  Geol.  Soc.  America,  vol.  12, 1901,  pp.  127-128. 


THE  PLEISTOCENE.  439 

DISTRIBUTION  OF  GLACIAL  DRIFT. 

The  detailed  work  on  the  distribution  of  the  morainic  deposits  in  this  region  has  not  covered 
anything  like  the  whole  area.  It  is  of  mterest  to  note  that  the  first  man  to  present  a  correct 
explanation  of  the  glacial  phenomena  in  America,  Louis  Agassiz,"  was  one  of  the  first  to  make 
observations  in  the  Lake  Superior  region,  as  did  James  Hector,*  Sir  William  Logan, '^  J.  J. 
Bigsby,''  J.  W.  Foster  and  J.  D.  Whitney,*  E.  Desor,/  D.  D.  Owen,ff  J.  G.Norwood,''  C.  Whit- 
tlesey,* B.  F.  Shumard,-*  G.  M.  Dawson,*  C.  T.  Jackson,'  W.  A.  Burt,™  and  many  other  early 
observers  who  observed  many  of  the  facts  of  transported  bowlders  and  soil,  waterworn  mate- 
rials, smoothed  and  striated  rocks,  etc.,  without  recognizing  or  being  willing  to  accept  their 
glacial  origin,  as  Agassiz  and  some  others  had  done. 

The  Pleistocene  deposits  of  the  Ashland  region,  Penokee  range,  etc.,  in  Wisconsin,  were 
early  described  by  R.  D.  Irving,"  who  distinguished  the  glacial  drift  and  the  lacustrine  clay  and 
showed  their  distribution  on  Ms  map.  E.  T.  Sweet  "  briefly  refers  to  the  unstratified  glacial 
deposits,  the  moraines,  and  the  stratified  drift  (lake  clays)  farther  west,  in  Bayfield  and  Douglas 
counties.  T.  C.  Chamberlin  v  made  a  report  based  on  notes  of  Moses  Strong,P  concerning  the 
glacial  features  in  the  upper  St.  Croix  district,  including  the  strise,  the  kettle  moraine,  the 
bowlder  clay,  and  the  "barrens."  The  glacial  deposits,  lakes,  morainic  belts,  etc.,  in  the  upper 
Flambeau  Vallej'  of  Wisconsin  are  described  by  F.  H.  King.?  The  glacial  deposits  in  eastern 
Wisconsin  are  described  by  T.  C.  Chamberlin.'"  The  glacial  features  of  an  area  in  the  upper 
Wisconsin  Valley  are  briefly  described  by  T.  C.  Chamberlin  from  notes  by  A.  C.  Clark.'  R.  D. 
Irving  *  described  and  mapped  the  glacial  deposits  in  central  Wisconsin  and  part  of  the  Drift- 
less  Area.  The  glacial  phenomena  of  all  Wisconsin  are  reviewed  by  T.  C.  Chamberlin,"  who 
has  also  correlated  the  glacial  features  of  the  southern  part  of  the  Lake  Superior  area  in  Minne- 
sota, Wisconsin,  and  Michigan."  Samuel  Weidman  has  recently  done  detailed  work  over  an 
area  of  about  7,200  square  miles  in  north-central  Wisconsin,  and  has  published  descriptions  ^ 
of  the  terminal  moraines,  the  ground  moraine,  the  older  drift,  etc.  He  has  also  surveyed  the 
glacial  geology  of  a  nearly  equal  area  west  of  this,  within  the  region  discussed  in  this  monograph, 
but  his  report  on  it  is  not  yet  published.  C.  P.  Berkey^  and  R.  T.  Chamberlin 2'  have  each 
discussed  the  glacial  geology  of  a  small  area  near  the  St.  Croix  Dalles.  The  detailed  mapping 
of  the  glacial  deposits  of  the  south  half  of  the  Green  Bay  glacier  by  W.  C.  Alden,  of  the  United 
States  Geological  Survey,  not  yet  published,  extends  up  to  the  south  boundary  of  the  area  here 
discussed. 

The  glacial  deposits  in  Michigan  were  examined  in  the  early  surveys  by  T.  B.  Brooks,^ 
Carl  Rominger/"  and  others.     More  recently  A.C.  Lane*"*  has  described  the  glacial  deposits  on 

0  Agassiz,  Louis,  Lalte  Superior,  its  physical  character,  vegetation,  and  animals,  1850,  pp.  395-416. 
^  Quart.  Jour.  Geol.  Soc,  vol.  17, 1861,  p.  393. 

I-  Geology  of  Canada,  18C3,  pp.  888-893,  904-908,  912-913,  and  plate  in  atlas  showing  superficial  deposits. 

d  On  the  erratics  of  Canada;  Quart.  Jour.  Geol.  Soc,  vol.  7, 1851,  pp.  215-238. 

e  Report  on  the  geology  and  topography  of  a  portion  of  the  Lake  Superior  land  district,  vol.  1, 1850,  pp.  186-218. 

/Idem,  vol.  2,  1851,  pp.  2.32-247. 

g  Report  of  a  geological  survey  of  Wisconsin,  Iowa,  and  Minnesota,  1852,  pp.  32,  36,  141-145,  etc. 

h  Idem,  pp.  298,  329-330,  348,  etc. 

ildem,  pp.  426-429,  435-436,  462-466,  etc. 

;  Idem,  pp.  515,  517,  etc. 

*  Geology  and  resources  of  the  region  in  the  vicinity  of  the  forty-ninth  parallel,  1875,  pp.  217-254. 

1  House  Ex.  Doc.  No.  5,  31st  Cong.,  1st  sess.,  pt.  3, 1849,  pp.  388-389. 
m  Idem,  p.  820. 

"  Geology  of  Wisconsin,  1873-1879,  vol.  3, 1880,  pp.  211-214,  PI.  XX. 

o Idem,  pp.  352-356. 

V  Idem,  pp.  382-387,  PI.  XXXVH. 

9 Idem,  vol.  4, 1882,  pp.  611-613. 

rldem,  1S73-1877,  vol.  2,  1877.  pp.  199-246. 

sidem,  1873-1879,  vol.  4,  1882,  pp.  717-721. 

1  Idem,  1873-1877,  vol.  2, 1877,  pp.  608-635. 

u  Idem,  1873-1879,  vol.  1,  1883,  pp.  261-298. 

f  Terminal  moraine  of  the  second  glacial  epoch:  Third  Ann.  Rept.  U.  S.  Geol.  Survey,  1883,  pp.  315-330,  381-393. 

KiBull.  Wisconsin  Geol.  and  Nat.  Hist.  Survey  No.  16, 1907,  pp.  433-513. 

1  Am.  Geologist,  vol.  20.  1897,  pp.  355-369. 
1/ Jour.  Geology,  vol.  13, 1905,  pp.  238-256. 

2  Geol.  Survey  Michigan,  vol.  1.  pt.  1,  1873,  pp.  72,  76-79. 

aa  Idem,  vol.  1,  pt.  3,  1873,  pp.  15-20;  vol.  4,  1881,  pp.  1-2,  40-41. 
bb  Idem,  vol.  6,  pt.  1, 1898,  pp.  183-184, 193. 


440  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION'. 

Isle  Royal  and  has  published  some  brief  notes  on  the  glacial  deposits  of  parts  of  Keweenaw 
Point."  Besides  this  ho  lias  written  a  sliort  description  and  ]nil)lished  a  }2;hicial  map  of  the 
deposits  in  the  Lower  Peninsula,''  the  nortliwestcrn  i)art  of  whicli  comes  within  the  area  of  this 
report,  from  published  and  unpul)lislied  data  by  Messrs.  Gordon,  Leverett,  Sherzer,  and  Lane. 
He  also  treats  the  drift  in  las  summary  of  the  surface  fijeolofry  of  Michigan.'^  I.  C.  Russell  has 
stu<licd  the  glacial  features  of  the  south  border  of  the  Upper  Peninsula  from  St.  Mar\-  River 
to  a  point  west  of  Crystal  Falls.  Ilis  map  shows  the  distribution  of  the  moraines  in  this  region, 
and  ids  work  lias  been  continued  by  C.  A.  Davis  "^  west  of  Marquette  and  south  of  the  Huron 
Mountams.  Frank  T^everett  has  studied  in  detail  the  glacial  deposits  there  and  in  the  eastern 
lowland  portion  of  the  Upper  Peninsula,  but  has  pubhshed  no  report  as  yet  except  a  brief  review. ' 

The  glacial  dei)osits  north  ami  northeast  of  Lake  Superior  in  Ontario  are  not  known  in 
detail,  though  A.  B.  Willmott,^  A.  P.  Coleman,!'  E.  S.  Moore,''  and  J.  M.  Bell,'  have  made  obser- 
vations in  the  Michipicoten  district.  Coleman  also  briefly  refers  to  the  glacial  deposits  near 
Lake  Nipigon,.'  as  does  E.  S.  Moore  *  to  those  in  the  Windegokan  district  east  of  Lake  Nipigon  * 
and  W.  H.  Collms '  to  those  west  of  Lake  Nipigon. 

Northwest  of  Lake  Superior  the  glacial  phenomena  in  the  Ijake  of  the  Woods  region  have 
been  described  by  G.  M.  Dawson,™  and  the  glacial  features  there  and  in  the  Rainy  Lake  region 
have  been  treated  fully  by  A.  C.  Lawson." 

To  the  east,  north  of  the  international  boundar\',  the  glacial  geology  of  Hunters  Island 
has  been  descril)ed  by  W.  H.  C.  Smith"  and  that  of  the  area  covered  by  the  Seine  River  and 
Lake  Shebandowan  map  sheets  by  William  McInnes.P 

In  Minnesota  the  glacial  deposits  have  been  studied  extensively  by  Warren  Upham,  N.  H. 
Winchell,  L^.  S.  Grant,  J.  E.  Todd,  A.  H.  Elftman,  and  others.  Their  discussions  are  found 
in  the  annual  reports  of  the  Minnesota  Geological  Survey  and  in  the  volumes  of  the  final  report, 
including  a  series  of  detailed  county  maps  and  descriptions.  This  is  the  most  detailed  series 
of  studies  of  the  glacial  deposits  thus  far  made  within  the  area  here  considered,  though  without 
sufficient  correlation.  II.  V.  Winchell  and  U.  S.  Grant  have  described  some  of  the  glacial  phe- 
nomena in  Miimesota,  near  Rainy  Lake.? 

In  a  preliminary  report  "■  Warren  Upham  has  described  the  moraines  of  northeastern 
Minnesota  and  published  a  map  of  part  of  the  Lake  Superior  area.  The  location  of  the  cliief 
morainic  deposits  on  this  map  seems  to  have  been  accurate,  but  there  have  been  some  dilTer- 
ences  of  opinion  as  to  the  interpolation  between  morainic  belts  and  the  correlation  and  inter- 
pretation of  the  moraines.  Upiiam  indicated  by  his  map  that  the  ice  all  retreated  northward, 
no  special  influence  being  exerted  by  the  Lake  Superior  basin,  the  valley  of  Red  River,  or  the 
liighlands  of  northern  Minnesota  and  the  international  boundary.  But  this  would  mean  that 
the  ^lesabi,  Itasca,  and  Leaf  Hills  moraines  had  the  ice  on  the  wTong  side,  as  is  proved  by  the 
superposition  of  lake  clay  on  glacial  till  south  of  the  Mesabi  range,  a  relation  that  would  not 
exist  if  the  ice  had  retreated  northward  over  the  range.  (See  figs.  62,  p.  443:  68,  p.  45.3:  and 
PI.  XXIX,  B,  p.  432.)     J.  E.  Toild "  subsecjuently  pointed  out  this  discrepancy  anil  A.  H. 

o  Proc.  Lake  Superior  Min.  Inst.,  vol.  12, 1907,  pp.  101-104. 
6  Water-supply  Paper  U.  S.  Geol.  Survey  No.  .TO,  1899,  PI.  II,  pp.  58-67,  75-77. 
<;  Ann.  Rept.  Geol.  Survey  Michigan  (or  1907, 190.S,  pp.  97-143. 
i  Ninth  Rept  Michigan  Acad.  Sci.,  1907,  pp.  132-1.15. 

«  Sixth  Kept.  Michiga    Acad  Sci.,  1904,  pp.  100-110;  Water-Supply  Paper  U.  S.  Geol.  Survey  No.  UiO,  1900,  pp.  29-33.  with  contour  map;  Water- 
Supply  Paper  U.  S.  Geol.  Survey  No.  183,  1907,  pp.  4-0. 
/  Rept.  Bur.  Mines  Ontario,  vol.  7, 1898,  pp.  204-205. 
■  B  Idem,  vol.  15,  pt.  1,  1905,  pp.  192-193. 
» Idem,  p.  200. 

"Idem,  vol.  14,  pt.  1,  1905,  p.  288. 
/Idem,  vol.  10,  pt.  1,  1907,  p.  135. 
*Idem,  pp.  147-148. 

'Summary  Rept.  Geol.  Survey  Canada,  1906,  pp.  103-104, 108. 
m  Quart.  Jour.  Geol.  Soc,  vol.  31,  1S75,  pp.  007-008. 

»  Geol.  and  Nat.  Hist.  Survey  Canada,  vol.  1,  new  ser.,  pt.  CC,  1886,  pp.  25-26,  130-140;  vol.  3,  pt,  F.  1890.  pp.  10.  20-21, 163-176. 
o  Gcol.  Survey  Canada,  new  ser.,  vol.  5,  pt.  1, 1893,  pp.  71G-74G. 
pidem,  vol.  10, 1899,  pp.  5in-54II. 

jTwenly-lhird  Ann.  Rept.  Minnesota  Geol.  and  Nat.  Hist.  Sur\'ey,  1895,  pp.  08-09. 
r  Twcnty-se('ond  Ann.  Rept.  Minnesota  Geol.  and  Nat.  Hist.  Survey,  1S94,  pp.  31-54. 
•  Am.  Geologist,  vol.  IS,  1890,  pp.  225-226;  Am.  Jour.  Sci.,  4th  ser.,  vol.  6, 189S,  pp.  409-477  (with  map). 


THE  PLEISTOCENE.  441 

Elftman  "  has  discussed  it  furtlier  and  published  a  revised  map  of  the  moraines  northwest  of 
Lake  Superior. 

The  geologists  of  the  United  States  Geological  Survey  have  referred  briefly  to  tiie  glacial 
deposits,*  and  their  work  is  cited  more  specifically  in  other  parts  of  this  report.  On  only  two 
of  their  geologic  maps  '^  are  glacial  deposits  separately  shown  (Pis.  XVII  and  XXII,  in  pocket), 
though  a  special  map'^  of  a  third  region  shows  the  distribution  of  the  recessional  moraines,  and 
there  are  detailed  maps  for  the  Marquette  district. 

The  map  of  the  Marquette  district  (PI.  XVII)  gives  a  separate  color  to  "undivided  Pleis- 
tocene" without  specifically  stating  of  what  tliis  consists.  The  areas  so  mapped  are  those  in 
which  no  ledges  whatever  are  found  because  of  the  thickness  of  the  glacial  drift  and  modem 
stream  and  swamp  deposits.  Accordingly  it  is  evident  that  these  areas  do  not  include  all  the 
Pleistocene  deposits  of  the  district,  but  merely  the  places  where  they  are  continuous  and  thick, 
completely  obscuring  the  older  rocks.  Pleistocene  deposits  are  found  throughout  the  district, 
but  in  other  places  are  discontinuous  or  very  thin.  The  undivided  Pleistocene  of  the  Mar- 
quette area,  which  is  confined  cliiefly  to  the  lowland  south  and  east  of  Marquette,  includes, 
where  mapped,  glacial  till,  morainic  deposits,  stream-assorted  glacial  outwash  deposits,  beaches 
of  higher  levels  of  Lake  Superior,  and  lake-bottom  clays,  besides  small  areas  of  modern  swamp 
accumulations,  like  peat  and  marl,  and  stream  deposits. 

The  undivided  Pleistocene  of  the  Crystal  Falls  area  (PI.  XXII)  where  mapped  in  the 
Micliigamme  River  valley  west  of  Floodwood  and  farther  south  near  Channing  and  Sagola 
includes  glacial  till,  recessional  moraines,  flat  sandy  outWash-plain  deposits,  and  various  swamp 
and  stream  deposits. 

On  the  sketch  map  (fig.  68,  p.  4.53)  it  has  been  thought  wise  to  distinguish  three  facts  con- 
cerning the  distribution  of  the  drift  and  the  terminal  moraines — (1)  the  distribution  of  the 
outermost  moraine,  whether  of  the  last  glacial  advance  or  of  an  earlier  one;  this  is  the  boundary 
of  the  Driftless  Area;  (2)  the  boundary  of  the  Wisconsin  stage  of  glaciation,  the  latest  stage; 
(3)  some  of  the  more  prominent  recessional  moraines,  so  far  as  their  location  is  known.  The 
locations  assigned  to  the  more  important  recessional  moraines  inside  the  border  of  the  terminal 
moraine  of  the  Wisconsin  stage  are  of  varying  degrees  of  accuracy,  because,  although  the  reces- 
sional moraines  in  Minnesota  are  fairly  well  known  and  well  mapped,  those  in  Wisconsin,  Mich- 
igan, and  Ontario  have  been  mapped  only  in  small  areas.  In  fact,  comparatively  Uttle  is 
known  of  the  episodes  accomjianying  the  withdrawal  of  the  ice  sheet  from  the  portions  of  the 
Lake  Superior  region  not  l3'irtg  in  IMinnesota,  save  Ln  regard  to  the  association  of  the  ice  with 
the  marginal  lakes  that  were  the  predecessors  of  Lake  Superior  and  Lake  Michigan. 

MARGINAL  LAKES. 

In  places  where  the  land  slopes  toward  the  ice  so  that  glacial  lakes  are  formed,  deposits 
of  a  quite  different  tyj^e  from  the  outwash  are  accumulated,  and  deposits  of  this  kind  were 
formed  in  the  glacial  lakes  now  to  be  described. 

WTiile  the  ice  sheet  was  retreating  into  the  basin  of  I^ake  Superior  marginal  lakes  were 
formed  between  the  ice  front  and  the  adjacent  higher  land.  Such  lakes  were  of  course  formed 
also  during  the  advance  of  the  glacier,  but  the  evidence  of  them  was  later  destroyed.  An 
early  nunatak,  already  referred  to  as  rising  through  the  ice,  was  the  long,  narrow  Giants  Range 
(figs.  4,  p.  87;  5,  p.  88;  62,  p.  443),  which  had  been  completely  buried  by  the  glacier,  but  because 
it  stood  highest  in  the  ice  was  the  first  to  emerge  after  the  ice  became  stagnant  and  began  to 
melt.  The  emergence  of  this  range  divided  the  ice  sheet  into  two  separate  glaciers — the  Kee- 
watin  or  western  continental  glacier  (Rainy  Lake  or  Red  River  or  Minnesota  lobe)  and  the 

a  Am.  Geologist,  vol.  21, 1898,  pp.  91-109. 

bUon.  U.  S.  Geol.  Siirvej-,  vol.  36  (Crystal  Falls  district),  1S99,  pp.  29-30,  332-333:  vol.  43  (Mesabi  district),  1903,  pp.  22,  24,  191-194,  199;  vol. 
45  (Vermilion  district),  1903,  pp.  .39,  425-130;  vol.  46  (Menominee  district),  1904,  p.  500;  Nineteenth  Ann.  Rept.  U.  S.  Geol.  Survey,  pt.  3,  1899, 
pp.  25-26;  Menominee  special  folio  (No.  62),  Geol.  Atlas  .U.  S.,  1900,  p.  12. 

c  Van  Hise,  C.  R.,  Bayley,  W.  S.,  and  Smyth,  H.  L.,  The  Marqnette  iron-bearing  district  of  Michigan:  Mon.  U.  S.  Geol.  Survey,  vol.  28, 1.897, 
atlas,  sheets  4,  25-.39.  Clements,  J.  M.,  Smyth,  H.  L.,  and  Bayley,  W.  S.,  The  Crystal  Falls  iron-bearing  district  of  Michigan:  Mon.  U.  S.  Geol. 
Survey,  vol.  36, 1899,  PI.  lit. 

"i  Clements,  J.  M.,  The  Vermilion  iron-bearing  district  of  Minnesota:  Mon.  U.  S.  Geol.  Survey,  vol.  45,  1903,  fig.  23,  p.  427. 


442  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Lauren tian  glacier  CLake  Superior  lobe),  the  two  probably  coalescing  some  distance  to  the 
northeast,  perhaps  north  and  oast  of  Gunflint  Lake.  The  Lake  Superior  lobe  filled  all  of  the 
Lake  Superior  basin,  extending  down  over  part  of  the  Archean  area  of  northern  Wisconsin  and 
southwestward  beyond  Carlton,  Minn.  The  Minnesota  or  Red  River  lobe  extended  north  and 
west  from  the  Giants  Range. 

GLACIAL  LAKE  AGASSIZ. 

In  the  valley  of  Red  River,  where  the  Red  River  lobe  of  the  Keewatin  glacier  probably 
retreated  some  time  after  the  Lake  Superior  lobe  had  gone  back,  the  topographic  conditions 
were  such  that  a  great  marginal  glacial  lake  was  formed.  These  conditions  consisted  in  the 
presence  of  a  broad  valle}'  with  gently  sloping  sides  and  a  slight  slope  toward  the  north,  and 
of  a  low  divide  between  its  headwater  region  and  the  headwaters  of  the  Mississippi.  Until 
the  ice  had  retreated  up  to  this  low  divide,  which  was  in  the  vicinity  of  Bigstone  and  Traverse 
lakes,  northwest  of  St.  Paul,  near  latitude  45°  30',  the  streams  from  the  melting  Red  River 
glacier  had  a  free  outflow  to  the  south  (glacial  River  Warren)  and  built  up  valley-train  deposits 
of  the  kind  already  described.  As  soon  as  the  ice  had  retreated  to  this  divide,  however,  an 
entirely  ditferent  condition  was  introduced.  The  ice  sheet  was  now  retreating  down  the  valley 
and  the  waters  emerging  from  it  were  temporarily  detained  in  a  marginal  glacial  lake.  With 
successive  stages  of  retreat  of  this  glacier  the  lake  became  enlarged,  although  probably  con- 
tinuing to  overflow  southward  through  the  valley  into  Mississippi  River  until  some  lower  outlet 
to  the  northeast,  whose  location  is  as  yet  unknown,  had  been  uncovered.  To  this  great  glacial 
lake  (fig.  65,  p.  446}  the  name  Lake  Agassiz  has  been  given.  Warren  Upham  has  described  the 
lake  and  its  abandoned,  tilted  shore  lines,  etc.,  in  a  monograph"  that  contains  a  fxdl  bibli- 
ography of  earlier  publications  on  the  lake.  The  whole  lake  as  commonly  shown  on  maps 
probably  never  existed  at  one  time.  It  is  not  definitely  known  to  have  been  contemporary 
with  glacial  Lakes  Duluth  and  Chicago,  as  the  sketch  map  (fig.  65)  shows  it  for  convenience. 

The  features  associated  with  the  several  stages  of  Lake  Agassiz  were  beaches  and  lake- 
bottom  clays.  The  beaches  are  found  in  the  Lake  Superior  region  as  far  east  as  Red  Lake  and 
Rainy  Lake,*  northwest  of  Lake  Superior;  the  lake  clays  overspread  all  the  areas  below  these 
beaches  and  form  the  fertile  lowland  in  the  wheat  lands  of  the  Red  River  valley  in  Minnesota, 
North  Dakota,  and  Manitoba. 

MARGINAL  GLACIAL  LAKES.c 

Before  or  durmg  the  early  stages  of  Lake  Agassiz  in  the  area  just  north  of  the  Giants  Range 
the  glacial  Lakes  Norwood,  Dunka,  Elftman,  and  Onnamani  were  the  first  ones  held  between 
the  east  end  of  the  Giants  Range  and  the  Rainy  River  lobe  of  the  Red  River  glacier,  outflowing 
southward  and  cutting  channels  across  the  Giants  Range.  (See  fig.  4,  p.  87;  5,  p.  SS;  PI.  11^ 
p.  86.)  The  present  Lake  Vermihon  is  a  smalLremnant  of  the  last  of  these  glacial  lakes.  Glacial 
Lake  Nicollet  <*  was  held  in  by  the  Red  River  glacier  and  the  encircling  land.  Leech,  Cass,  and 
Winnibigoshish  lakes  are  remnants  of  it.  To  the  north  glacial  Lakes  Big  Fork,  Beltrami,  and 
Thompson  were  small  margmal  stages  of  glacial  I^ake  Agassiz.  Rainy  Lake  and  Red  Lake 
probably  occupy  parts  of  the  basin  of  Lake  Thompson  and  of  the  later  Lake  Agassiz,  as  do  also 
the  Lake  of  the  Woods,  etc.  All  these  glacial  lakes  were  held  between  a  northwestward- 
retreatmg  ice  front  and  the -Height  of  I^and,  overflowmg  southwartl  to  the  Mississippi  drauiage 
basin. 

South  of  the  Giants  Range  the  Superior  lobe  similarly  held  up  glacial  lakes,  the  fu'st  notable 
one  beuig  a  long,  narrow  margmal  lake,  as  yet  imnamed,  parallel  to  the  Giants  Range  (fig.  62). 
This  lake  received  the  drainage  from  glacial  lakes  north  of  it,  as  described  by  Leith,«  and  in 

o  The  glacial  Lake  Agassi?.:  Moii.  U.  S.  Geol.  Survey,  vol.  25, 1890. 

t  Lawson,  ,\.  C,  Report  on  the  geology  o(  the  Lake  of  the  Woods  region,  with  special  reference  to  the  Keewatin  (Uuronian?)  belt  of  the  Archean 
rocks:  Ann.  Kept.  Geol.  and  Nat.  Hist.  Survey  Canada  for  1885,  vol.  1,  new  ser.,  1880,  pp.  139-HOCC;  Report  on  the  geology  of  the  Rainy  Lake 
region:  Idem  for  1887-88,  vol.  3,  new  ser.,  1S90,  pp.  109-17GF. 

c  Winchcll,  N.  H.,  Bull.  Geol.  Soc.  America,  vol.  12, 1901,  pp.  109-12S. 

d  Not  to  be  confused  with  glacial  Lake  Jean  Nicolet  in  Wisconsin. 

«  Mon.  U.  S.  Oeol.  Survey,  vol,  43,  1903,  pp.  193-194. 


THE  PLEISTOCENE. 


443 


it  were  deposited  the  lake  clays  that  overlie  the  stony  drift  in  most  of  the  open-pit  mines  on 
the  Mcsabi  rann;c.  The  relation  of.  stony  till  and  lake  clay  shown  in  Plate  XXIX,  B  (p.  432),  is 
explained  by  the  halt  of  the  ice  front  south  of  the  Giants  Range  and  the  bidlding  of  the  Mesabi 
moraine  (fig.  62,  a),  after  which  a  withdrawal  of  the  ice  toward  the  south  made  possible  the 
formation  of  the  glacial  lake  and  the  deposition  of  the  clay  overlying  the  till  (fig.  62,  h). 


VERTICAL     SCALE 
500  1000  1500 


H0F?IZONTAL    SCALE 


4  MILES 


Glacial  till  Debris-laden  ice  Clear  glacier  ice  Stratified  lake  deposits 

FiGiJHE  C2.— Sketch  shoning  the  origin  of  the  drift  deposits  overlying  the  ore  in  the  Mesabi  iron  range. 

With  farther  retreat  of  the  Lake  Superior  glacier  southeastward,  the  unnamed  marginal 
lake  mentioned  above  was  drained  and  a  new  glacial  lake,  Lake  Upham,  was  formed,  its  south- 
ernmost ice  barrier  being  near  the  upper  bend  of  the  present  St.  Louis  River,  while  the  gabbro 
highland  to  the  east,  the  granite  range  to  the  north,  and  the  morainic  highland  to  the  west  and 
south  held  it  in.  Lake  Upham  had  an  elevation  of  about  1,300  feet  and  its  bottom  forms  the 
flats  traversed  by  the  Duluth^  Missabe  and  Northern  Railway  in  the  great  muskeg  area  where 
the  railway  is  so  straight.  At  about  this  same  time  glacial  Lake  Aitkin  was  formed  farther 
west.  The  obstruction  on  the  site  of  Mille  Lacs  produced  glacial  Lake  Issati.  Afterward 
glacial  Lake  St.  Louis  was  formed  in  the  St.  Louis  Valley,  draining  out  over  a  low  col  near  Bar- 
num  and  Carlton,  at  an  elevation  of  about  1,135  feet,  and  having  an  area  of  about  40  square 
miles. 

Many  glacial  lakes,  includmg  Lake  Minnesota,  were  formed  in  southern  Mmnesota  in  asso- 
ciation with  the  Red  River  lobe.  Lake  Agassiz,  already  referred  to,  was  similarly  formed  in 
the  Red  River  valley  at  a  little  later  stage,  and  glacial  Lake  Jean  Nicolet  **  occupied  Green 
Bay  and  the  Fox  River  valley  in  Wisconsin,  drauiing  westward  into  Wisconsin  River  at  Portage. 
The  present  Lake  Winnebago  lies  in  its  basin. 


LAKE  NEMADJI.6 

Glacial  Lake  Nemadji  (fig.  63)  was  formed  between  the  ice  bai'rier  of  the  Lake  Superior 
lobe  on  the  northeast  and  east  and  the  higher  land  west  of  Lake  Superior  in  Minnesota.  Tliis 
lake,  which  was  about  65  feet  lower  than  Lake  St.  Louis  and  may  have  had  a  slightly  greater 
area,  drained  through  another  col  near  Barnum  and  Pickering,  southwest  of  Carlton,  mto  the 
Mississippi. 

As  the  ice  retreated  still  farther  to  the  northeast  '^  there  were  changes  in  the  levels  and  in 
the  outlets  of  the  glacial  lakes  that  he  between  ice  dams  and  the  surrounding  land.     The  first 

a  Upham,  Warren,  Am.  Geologist,  vol.  32, 1903,  pp.  105-115,  330-331. 

!>  Winchell,  N.  H.,  Final  Kept.  Geol.  and  Nat.  Hist,  Survey  Minnesota,  vol.  4, 1899,  pp.  2-3, 18-20. 

cT.  B.  Taylor  (A  short  history  of  Oie  Great  Lakes:  Studies  ip  Indiana  geography,  1897,  chapter  10,  pp.  1-21)  has  written  a  review  of  the 
various  lake  stages  and  the  outlets,  etc.,  associated  with  the  different  positions  of  ice  fronts  and  levels  of  the  land. 


444 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


consequence  of  the  retreat  of  the  ice  barrier  would  he  tliat  lower  valleys  across  the  hills  to  the 
south  or  east  might  h(;  exposed,  and  as  a  result  of  tliis  the  waters  of  the  lake  would  fuid  a  way 
out  throujfh  the  new  divide  and  the  lake  would  fall  to  a  new  level.  The  earliest  glacial  lakes 
in  northern  Wisconsin,  hkc;  tlie  predecessor  of  Lake  Gogebic  and  the  great  marginal  lake  in  the 
Ontonagon  Valley,"  probably  began  to  exist  before  or  tluring  the  Lake  Xemadji  stage. 


0  25  50  75  100  125 150  MILES 


Figure  C3.— Glacial  Lake  Nemadji. 


LAKE  DTJLUTH. 

As  the  ice  retreated  northeastward,  after  the  Lake  Nemadji  stage,  it  soon  retired  to  a  point 
far  enough  to  the  northeast  to  expose  the  col  now  crossed  by  the  Chicago,  Minneapolis,  St. 
Paul  and  Omaha  Railway.  As  a  result  the  outlet  near  Carlton  was  abandoned  and  the  waters  of 
this  lake  outflowed  directly  southward  through  the  St.  Croix  to  the  Mississippi  (fig.  64)  through 
a  channel  ''419  feet  above  the  present  Jjake  Superior,  between  the  headwaters  of  the  Brule  and 
those  of  the  St.  Croix.  Exactly  where  the  ice  front  of  the  Lake  Superior  glacier  stood  at  this 
stage  can  not  be  stated,  but  it  probably  halted  at  several  points  east  of  the  Apostle  Islands  and 
perhaps  as  far  east  as  Keweenaw  Point,  the  other  margin  resting  against  the  north  shore  of  Lake 
Superior  at  several  points  in  Minnesota,  smaller  marginal  lakes  being  held  on  each  shore  between 
the  ice  and  the  land  in  Mmnesota,  Wisconsin,  and  Michigan. 

The  great  glacial  lake  of  this  stage  is  called  Lake  Duluth,'=  although  Upham  "^  had  previously 
named  it  the  West  Superior  glarial  lake.  It  is  evident  that  this  lake  existed  for  a  longtime,  and 
there  are  three  kinds  of  dejiosits  which  indicate  that  this  was  so.  One  kmd  consists  of  the 
elevated  beaches  which  are  still  found  along  the  liillsides  at  the  level  of  the  St.  Croix  outlet  and 
which  are  so  broad  and  well  developed  on  the  escarpment  face  above  Duluth  that  the  Boulevard 

«  Lane,  A.  C,  Summary  of  the  surface  geolopy  of  Michigan:  Ann.  Kept.  Geol.  Survey  Michigan  for  19(17.  190.S,  pp.  Hl-l-l?. 

&  The  elevation  of  this  channel  is  (liven  as  1,070  feet  by  Warren  I'pham  (Twenty-second  .Vnn.  Kept.  Oeol.  and  Xat.  Hist.  .Purvey  Minnesota. 
1893,  p.  55:  Final  Kept.  Geoi.  and  Nat.  Hist.  .Purvey  Minnesota,  vol.  2,  1SS8,  pp.  (>-I2-G43).  The  aUitiule  of  the  stunmit  in  this  channel  is  stated 
by  I.cverett  to  be  1,021  feet,  as  shown  in  a  profile  in  House!  Doc.  330.  ,Wth  Cong.,  1st  scss.,  1890. 

c  Taylor,  F.  B.,  Studies  in  Indiana  geography,  1897,  fig.  1,  p.  10. 

i  Twenty-second  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Minnesota.  1894.  pp.  54-55. 


THE  PLEISTOCENE. 


445 


Drive  follows  one  or  two  of  them  for  miles.  This  aliore  can  be  traced  from  a  point  east  of 
Ashland  westward  to  Brnle  River  and  on  the  other  side  around  the  head  of  the  lake  to  a  point 
some  distance  east  of  Dulutli.  Similar  beaches  or  terraces  in  the  Lake  Superior  basin  were 
observed  early  in  the  ex])loration  of  the  region  "  and  were  explained  as  wave-wroutrht  forms. 

The  second  class  of  deposits  indicating  that  the  glacial  lake  at  Duluth  existed  for  a  long 
time  comprises  the  deltas  that  were  l)uilt  where  streams  flowed  into  the  lake  at  the  level  of  the 
Boulevartl  beaches,  as  at  Thomi)son  east  of  St.  Ijouis  River,  on  Tischers  Creek,  and  on  Chester 
Creek  at  Duluth. » 

The  third  class  of  these  deposits  consists  of  the  lake  clays,  which  without  question  accumu- 
lated in  later  periods  as  well  as  in  this,  but  which  would  of  course  have  formed  to  a  considerable 
depth  when  the  ice  front  stood  across  the  lake  and  was  discharging  icebergs  with  glacial  material, 
and  when  streams  from  the  hills  to  the  north,  south,  and  west  contributed  their  load  of  sediment. 


0  25 50  75  100  125  150  MILES 


Figure  04.— Glacial  Lake  Duluth. 


INTERMEDIATE  GLACIAL  LAKES. 

As  would  naturally  be  expected,  with  the  continued  retreat  of  the  Lake  Superior  and  Lake 
Michigan  ice  lobes,  the  lake  levels  were  falling  lower  and  lower.  One  of  the  next  levels  at 
which  there  was  a  notable  stand  of  the  ice  was  when  the  waters  of  the  western  Lake  Superior 
basin  escaped  past  Chicago  through  Illinois  River  to  the  Mississippi.  This  was  probably  some 
time  after  the  early  Lake  Duluth  stage  (fig.  65).  Whether  there  were  intermediate  outlets 
between  the  two  stages  referred  to  is  not  known,  but  it  seems  probable  that  the  ice  in  retreating 
northeastward  gradually  exposed  the  highland  of  northern  Wisconsin  and  Micliigan  so  that 

"  Logan,  W.  E.,  Report  on  the  geology  of  the  north  shore  of  Lake  Superior:  Geol.  Survey  Canada,  1847,  p.  31.  Hubbard,  Bela,  House  Ex. 
Doc.  No.  1,  31st  Cong.,  1st  sess.,  pt.  3, 1849,  pp.  910-911.  Foster,  J.  W.,  and  Wlutney,  J.  D.,  Report  on  the  geology  and  topography  of  a  portion  of 
the  Lake  Superior  land  district,  vol.  1,  1850,  pp.  194-197,  211-213.  Desor,  E..  idem,  vol.  2,  1S51.  pp.  248-255,  268-270.  Whittlesey,  Charles,  idem, 
pp.  270-273.     Agassiz,  Louis,  Lake  Superior,  1S50,  pp.  00,  GO,  lOO-lOl,  and  frontispiece. 

i  Upham,  Warren,  Twenty-second  Ann.  Rept.  Geol.  and  Nat.  Uist.  Survey  Minnesota,  1893,  pp.  C5-06. 


446 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


eventually  the  waters  from  the  enlarged  Lake  Duluth  abandoned  the  St.  Croix  outlet  for  some 
lower  ones  in  northern  Wisconsin  and  Michigan,  and  still  later  outflowed  southward  along  the 
margin  of  the  ice  sheet  into  Lake  Jean  Nicolet,  in  eastern  Wisconsin,  which  drained  into  Wis- 
consin and  Mississippi  rivers.  Still  later  the  drainage  went  into  the  enlarged  Lake  Chicago. 
It  is  Icnown  that  there  were  a  number  of  intermediate  stages  due  either  to  lowering  of  the  ice 
barrier,  to  discovery  of  lower  outlets,  or  to  tilting  of  the  land,  because  the  beaches  preserved 
on  the  hillsitles  below  the  upper  Lake  Duluth  beach  indicate  other  stands  of  the  lake  waters 
for  considerable  periods  of  time.  The  beaches  associated  with  these  intermediate  stages  are 
found  at  several  levels  below  the  Boulevard  Beach,  as  shown  in  the  ta])le  (p.  4.51). 

It  seems  likely  that  some  of  the  intermediate  stages,  like  the  Lake  Duluth  stage,  were  of 
considerable  duration,  because  the  beaches  that  were  built  are  broad,  the  cliffs  that  were  cut 
are  well  marked,  and  good-sized  deltas  were  formed  at  the  mouths  of  the  streams.  Of  these 
deltas  that  of  Dead  River  at  Forestville  near  Marquette  and  those  of  Swedetown  and  Huron 
creeks  near  Houghton  are  good  examples."     The  fine  material  carried  beyond  the  deltas  into 


Figure  65.— Hypothetical  intermediate  stage  with  the  expansion  of  glacial  Lalce  Chicago  and  the  later  stage  of  glacial  Lake  Duluth;  part  of 
glacial  Lake  Agassiz  near  the  northwest  corner.    Xn  isolated  stagnant  ice  block  is  shown  in  the  Lake  Superior  basin. 

the  lake  formed  thick  deposits  of  glacial  clays,  of  which  some  are  now  exposed  and  others  are 
still  below  lake  level. 

LAKE  ALGONQUIN. 

After  the  episode  of  the  Chicago  outlet  the  glacial  barrier  continued  to  retreat  to  the  north- 
east, and  the  glacial  lake,  which  came  into  existence  gradually,  occupied  all  of  the  basin  of  the 
present  Lake  Superior,  its  waters  covering  parts  of  the  peninsula  of  upper  Michigan  west  of 
Marquette  and  being  confluent  with  those  in  the  basins  of  the  present  Lakes  Michigan  and  Huron 
(fig.  66).  This  is  called  the  Lake  Algonqum  stage. .  At  this  time  the  ice  barrier  stood  east  of 
North  Bay  in  the  Ottawa  Valley,  and  had  retreated  from  Lake  Superior  north  of  the  Height  of 

n  Lane,  A.  C,  Sunuuary  of  the  surface  geology  of  Michigan:  .Vmi.  Kept.  Michigan  Geol.  Surrey  (or  1907,  190S,  p.  142. 


THE  PLEISTOCENE. 


447 


Land.  Possibly  there  was  a  stagnant  isolated  ice  block  in  the  Lake  Superior  basin  at  this  time 
or  just  before.  During  the  Lake  Algonquin  stage,  which  of  course  came  after  a  series  of  inter- 
mediate stages  in  which  Lakes  Chicago  and  Duluth  were  enlarged  as  recorded  by  the  successive 
beach  levels  one  below  the  other,  the  waters  deserted  the  outlet  past  Chicago  to  Illinois  and 
Mississippi  rivers  because  lower  outlets  were  uncovered  to  the  east.  Lake  Algonquin  had  two 
such  outlets.  The  first  led  past  Port  Huron  through  the  present  Lake  St.  Clair  and  Lake  Erie 
into  glacial  Lake  Iroquois,  which  covered  more  than  the  basin  of  the  present  Lake  Ontario; 
the  second  outlet  also  led  into  Lake  Iroquois  tlirough  the  Trent  River  valley  from  Georgian 
Bay.  There  were  several  oscillations  with  one  or  both  of  these  outlets  running  during  the  Algon- 
quin stage.  The  Lake  Iroquois  waters  flowed  eastward  through  Mohawk  River  to  Hudson 
River  and  New  York  Harbor.  All  around  the  Lake  Superior  basin  the  strongest  Lake  Algon- 
quin beaches  are  well-marked  shore  lines  elevated  high  alcove  the  waters  of  the  present  lake. 
At  this  stage  glacial  lakes  probably  occupied  the  Kaministikwia  and  Nipigon  River  valleys, 
including  all  the  basin  of  the  present  Lake  Nipigon. 


Figure  66.— Glacial  Lake  Algonquin. 


NIPISSING  GREAT  LAKES. 

With  the  continued  retreat  of  the  ice  sheet  to  the  northeast,  a  still  lower  outlet  than  that 
tlu'ough  Mohawk  and  Hudson  rivers  was  exposed.  This  was  along  the  present  Lake  Nipissing 
near  North  Bay  and  down  Ottawa  River  to  the  lower  St.  Lawrence.  This  is  called  the  stage 
of  the  Nipissing  Great  Lakes.  With  the  uncovering  of  the  Ottawa  River  outlet  the  waters  of 
the  Lake  Superior  basin  fell  to  a  considerably  lower  level  than  that  occupied  before  and  accord- 
ingly regions  about  the  shores  of  Lake  Superior  which  had  been  submerged  or  had  groups  of 
islands  were  wholly  uncovered.  The  largest  area  of  this  sort  was  the  lowland  east  of  Mar- 
quette, in  the  Upper  Peninsula  of  Michigan  (fig.  67).  Romiuger,  who  described  the  superficial 
deposits  of  this  region,"  was  somewhat  at  a  loss  to  explain  the  mi^iture  of  ground  moraine,  reces- 

o  Kominger,  Carl,  Geol.  Survey  Michigan,  vol.  1  1873,  pp.  15-20. 


448 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


sional  moraines,  assorted  drift,  and  lake  clay  witli  wliicli  tlie  region  is  covered  as  a  result  of  its 
occupation  first  by  ice,  then  by  melting  ice  fronts,  and  later  by  glacial  lakes. 

One  notable  change  was  the  temporary  abandonment  of  the  outlet  from  Lake  Iluion  jjast 
Detroit  to  Lake  Erie.  Lake  Erie  continued  to  drain  into  Lake  Ontario,  which  may  have  been 
an  arm  of  the  sea,  while  Lakes  Superior,  IMichigan,  and  Huron  (the  Nipissing  Great  Lakes) 
drained  independently  to  the  Ottawa.  Another  marked  change  was  the  disconnection  of  the 
Lake  Nipigon  basm  so  that  Lake  Nipigon  at  tliis  time  first  assumed  somewhat  its  present  form 
and  was  independent  of  Lake  Superior.  Isle  Royal,  the  site  of  several  small  islets  at  the  Algon- 
quin stage,  assumed  form  as  one  large  island  of  nearh'  its  present  area.  All  about  the  lake  shore 
the  waters  stood  at  lower  levels.  The  beaches  built  at  the  Nipissing  stage  seem  to  be  the  largest 
that  were  formed  at  any  time  in  the  history  of  the  Lake  Superior  basin.  The.se  beaclics  are  so 
broad  and  the  chfTs  cut  by  the  Nipissing  waves  are  so  high  that  it  has  been  inferred  that  this 
stage  of  the  lake  was  continued  for  a  very  long  time — longer,  in  fact,  to  judge  from  the  strength 


FiGUEE  67.— Part  of  Nipissing  Great  Lakes. 

of  the  shore  lines,  than  the  present  level  of  Lake  Superior  has  been  maintained  as  yet,  though 
postglacial  gorges  are  cut  back  much  farther  at  the  present  level  than  they  were  at  the  Nipissing 

stage. 

EFFECT  OF  TILTING  ON  GLACIAIi  LAKES. 

Up  to  this  point  in  the  histoiy  of  tlie  Lake  Superior  basin  the  lake  waters  fell  every  time  a 
lower  outlet  was  exposed  by  the  northeastward  retreat  of  the  ice  sheet.  For  some  time  before 
this  there  had  been  going  on  a  broad  warping  winch  was  producing  an  uphft  of  the  region  to  the 
north  or  a  sinldng  of  the  region  to  the  south.  The  e^adence  of  this  disturbance  is  found  in  the 
fact  that  the  beaches  of  the  glacial  lakes,  wliich  must  have  been  originally  horizontal  in  jjosition, 
for  the  waters  of  the  lake  were  hoi-izontal,  are  now  inchned  from  north  to  south  at  a  sUglit  angle. 
It  was  not  until  after  the  clo.^e  of  the  Nipissing  stage  that  this  war])ing  of  the  lake  basin  had  any 
very  profound  efiects,  except  to  produce  a  fanhke  splitting  of  glacial-lake  hhore  lines  and  to 


THE  PLEISTOCENE.  449 

cause  temporary  oscillations  in  the  outlets  of  the  Algonquin  and  Nipissing  stages.  During  and 
after  the  Nipissing  stage,  however,  the  tilting  became  sufficient  to  bring  about  a  new  and  rather 
dramatic  change  in  the  history  of  the  glacial  lakes.  It  has  been  stated  that  the  lake  levels 
had  fallen  because  lower  and  lower  outlets  toward  the  northeast  were  exposed  by  the  ice  sheet 
(figs.  G3-0()).  The  normal  result  of  such  a  series  of  changes  would  be  the  establishment  of  a  per- 
manent outlet  of  the  Great  Lakes  along  the  line  of  greatest  depression  between  the  uplands  of  New 
England  and  the  Adirondacks  on  the  one  hand  and  the  Height  of  Land  of  Canada  on  the  other. 
The  Lake  Nipissing  and  Ottawa  River  outlet  was  so  situated;  but  after  the  occupation  of  this 
outlet  for  what  may  have  been  a  longer  time  than  the  present  St.  Lawrence  outlet  has  been 
occupied,  to  judge  from  the  strength  of  the  beaches,  as  already  stated,  the  uphft  of  the  land 
toward  the  north  became  sufficient  to  raise  the  Nipissing-Ottawa  Valley  to  a  liigher  level  than 
another  valley  farther  south,  and  the  latter  valley  became  the  outlet  of  the  Great  Lakes.  The 
three  upper  Great  Lakes  at  this  time,  instead  of  draining  through  Lake  Nipissing  to  Ottawa 
River  or  through  Trent  River  and  Georgian  Bay  to  Lake  Ontario,  were  once  more  turned  south- 
ward and  drained  through  Lake  St.  Clair  past  the  present  site  of  Detroit  into  Lake  Erie,  whence 
the  waters  of  the  four  upper  lakes  once  more  passed  over  Niagara  Falls  to  Lake  Ontario  and 
down  the  St.  Lawrence  by  the  present  route.  The  amount  of  tilting  necessary  to  accomplisli 
this  result  was  not  very  great,  although  that  it  was  greater  than  the  i)revious  tilting  is  proved  by 
the  fact  that  in  places  these  lower  beaches  are  more  liighly  inchned  than  any  above  them.  That 
it  did  not  affect  the  whole  region  is  shown  by  the  horizontality  of  some  of  the  beaches. 

This  tilting  has  continued  up  to  the  present  time  and  is  still  going  on,  as  is  proved  by  several 
kinds  of  evidence.  One  proof  is  found  in  the  fact  that  on  the  south  side  of  Lake  Superior  and 
the  other  Great  Lakes  the  waters  are  being  canted  into  bays  and  river  mouths,  so  that  what 
were  formerly  valleys  are  now  becoming  bays  and  estuaries  (PI.  II,  p.  86),  as  noted  in  northern 
Wisconsin  by  the  land  surveyor  G.  R.  Stuntz"  in  1869.  In  these  southern  rivers  the  lake  water 
extends  backward  far  enough  to  make  river  navigation  possible  for  some  distance,  as  from 
Duluth  17  miles  up  St.  Louis  River  to  Fond  du  Lac;  but  in  all  except  the  largest  rivers  on  the 
north  side  of  the  lake  the  water  cascades  down  in  falls  and  rapids  almost  directly  into  the  basin 
of  the  lake  itself.  The  lower  courses  of  many  rivers  on  the  south  side  of  Lake  Superior  are  so 
broad  that  it  requires  a  double  line  to  represent  them  on  the  map,  whereas  on  the  north  side  of 
the  lake  practically  all  the  rivers  are  so  narrow  that  they  are  represented  by  a  single  line.  Tliis 
canting  of  the  lake  waters  into  the  river  valleys  on  the  south  side  of  the  lake  has  had  a  very 
important  effect  in  connection  with  man's  occupation  of  the  region,  by  producing  good  harbors, 
and  of  such  harbors  that  at  Duluth  and  Superior  is  the  best  (figs.  69,  p.  457,  and  70,  p.  458; 
PI.  V,  A,  p.  112),  having  been  protected  by  the  sub.gequent  building  of  great  sandbars.  To 
the  submergence  of  old  stream  valleys  during  this  tilting  are  due  the  Apostle  Islands,  wliich 
have  been  briefly  described  by  Whittlesey '  and  Irving. "= 

PKESENT  POSITION  OF  RAISED  BEACHES. 

The  effect  of  the  tilting  of  tliis  elevated  shore  line  has  been  to  sid)merge  some  of  the  beaches 
of  the  former  lakes,  so  that  the  Nipissing  shore  line,  for  example,  is  elevated  many  feet  above 
the  level  of  Lake  Superior  on  the  north  shore  of  the  lake,  whereas  on  the  south  shore  it  is  now 
submerged  in  places  b}^  the  lake  waters.  It  has  been  estimated  that  the  shore  line  of  the  Nipis- 
sing stage  in  Lake  Superior  is  25  feet  below  the  present  water  surface  at  Duluth  and  that  this 
shore  line  appears  above  the  present  water  surface  at  Beaver  Baj-,  beyond  which  it  rises  with 
an  average  slope  of  about  7  inches  to  the  mile."* 

Numerous  observations  and  notes  on  these  abandoned  strands  were  made  by  pioneers 
in  the  region.     Some  of  these  by  Sir  William  Logan,  Foster  and  Wliitney,  Bela  Hubbard, 

a  Stuntz,  G.  R.,  Some  recent  geological  changes  in  northeastern  Wisconsin:  Proc.  Am.  Assoc.  Adv.  Sci.,  vol.  18, 1870,  pp.  205-210. 
&  Whittlesey,  Charles,  Geological  sur\-ey  of  Wisconsin,  Iowa,  and  Minnesota,  1852,  pp.  437-438. 
c  Irving,  R.  D.,  Geology  of  Wisconsin,  1873-1879,  vol.  3, 1880,  pp.  72-76. 
d  Taylor,  F.  B.,  Am.  Geologist,  vol.  15, 1895,  p.  307. 

47517°— VOL  52— 11 29   . 


450  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

W.  A.  Burt,  Agassiz,  Desor,  Whittlesey,  and  others  have  already  been  alluded  to.  None  of  these 
furnish  very  specifu;  data  or  contain  more  than  scattered  observations.  A.  C.  Lawson,"  how- 
ever, made  a  very  painstaking  stiuh'  and  instrumental  measurement  of  these  elevated  shore 
lines  on  the  northern  shore  of  Lake  Superior,  and  concluded  that  these  strands  were  horizontal 
and  wore  formed  in  a  great  lake,  held  in  bj^  a  land  barrier  that  was  progressively  lowered  by 
warping.  He  rejected  tlie  idea  of  an  ice  barrier.  Subsequently  F.  B.  Taylor  **  pointed  out 
that  Lawson  and  also  Warren  LTpham,<=  who  supported  Lawson's  conclusion  as  to  the  hori- 
zontality  of  these  shore  lines,  though  recognizing  the  glacial-lake  condition,  had  not  sufficiently 
considered  the  possibility  that  the  sliore  lines  observed  from  point  to  point  along  the  shore  of 
Lake  Superior  were  inclined  instead  of  being  horizontal.  By  field  study  Taylor  demonstrated 
that  the  shore  Imes  Avhich  Lawson  interpreted  as  horizontal  were  indeed  inclined  at  a  small 
angle,  and  pointed  out  conclusively  that  they  were  formed  in  a  glacial  lake  wliose  barrier  was 
an  ice  dam  to  the  east."* 

These  raised  beaches  on  the  north  shore  of  Lake  Superior,  especially  in  the  Michi])icoten 
district,  have  also  been  studied  by  A.  B.  WiIlmott<^  and  by  A.  P.  Coleman,/  who  has  noted 
very  many  more  shore  lines  than  were  measured  by  Lawson.  Near  Lake  Nipigon  Coleman 
has  also  measured  many  new  shore  lines, ^  and  a  number  were  noted  by  C.  R.  Van  Ilise  and 
J.  M.  Clements''  in  a  trip  around  northern  Lake  Superior  in  190L  Observations  on  the  raised 
beaches  in  northern  Lake  Michigan,  Green  Bay,  and  western  Lake  Huron  have  been  made 
by  Taylor,*  Russell,^'  Goldthwait,*  and  others. 

The  writer  took  a  hasty  trij)  around  the  north  shore  of  Lake  Superior  from  Duluth  to 
Saidt  Ste.  Marie  in  1907  and  visited  a  number  of  the  localities  described  by  Lawson.  Although 
feeling  that  Lawson's  observations  in  general  were  most  thorougli  and  accurate,  he  believes 
that  the  conclusion  suggested  by  Ta^'lor  is  fully  warranted  and  that  at  least  the  lower  beaches 
of  this  region  show  a  decided  tilt  to  the  south  and  southwest.  In  evidence  of  the  tilting  and 
the  long  duration  of  the  Nipissing  stage  established  by  Taylor,  he  found  that  near  Duluth  and 
northward  from  that  city  to  Beaver  Bay  the  mouths  of  the  small  postglacial  gorges  contain 
no  bed  rock  but  are  uniformly  either  filled  with  gravel  deposits  or  occupied  by  the  waters  of 
the  lake,  as  at  Lester  Creek,  north  of  Duluth.  Northeast  of  Beaver  Bay  most  of  the  small 
stream  valleys  are  found  to  have  no  gorges  extending  down  to  or  below  the  present  lake  level, 
but  instead  the  streams  flow  over  the  bare  rock  surface  of  the  hillside.  An  especially  good 
illustration  of  this  is  Current  River,  northeast  of  Port  Arthur,  Ontario.  Good  ev-idence  was 
found  that  the  Nipissing  shore  line  dips  under  the  lake  at  Beaver  Ba}',  Minnesota. 

It  has  been  shown  by  G.  K.  Gilbert '  that  the  canting  of  the  lake  basins  is  still  in  progress, 
and  his  estimate  of  the  rate  of  tilting  is  that  the  north  end  of  a  south-southwest  line  100  miles 
long  in  the  Great  Lakes  region  would  in  a  centurj'  be  tilted  0.42  foot  above  the  south  end. 
This  amount  of  tilting,  of  course,  is  small,  but  it  would  be  sufficient  to  divert  the  waters  of 
Lake  Superior  again,  just  as  they  were  once  diverted  from  the  Nipissing  Valley  to  the  St.  Law- 
rence Valley,  turning  them  southward  to  Chicago  River,  where  the  waters  would  once  more 
flow  southward  rather  than  over  Niagara  and  through  the  St.  Lawrence.     More  recent  studies 

o  Sketch  of  the  coastal  topography  of  the  north  side  of  Lake  Superior;  Twentieth  Ann.  Kept.  Geol.  and  N'at.  Hist.  Survey  Minnesota,  1893, 
pp.  230-282. 

Ii  The  Nipissing  Beach  on  the  north  Superior  shore:  Am.  Geologist,  vol.  15, 1895.  pp.  304-314. 

c  Am.  Jour.  Soi.,  3d  ser.,  vol.  49, 1895,  p.  7;  Twenty-second  Ann.  Kept.  Geol.  and  Nat.  Uist.  Survey  Minnesota,  1S94,  pp.  54-66;  Bull.  Geol.  Soc 
America,  vol.  6, 1895,  pp.  21-27. 

i  Taylor,  F.  ]!.,  .\m.  Geologist,  vol.  15, 1895,  pp.  304-314;  vol.  20, 1897,  pp.  111-128. 

e  Rept.  Bur.  Mines  Ontario,  vol.  7, 1898,  p.  193. 

/  Idem,  vol.  8,  pt.  2,  1S99,  pp.  150-158;  vol.  9,  1900,  pp.  175-170;  vol.  11,  1902,  p.  181;  vol.  15,  pt.  1,  lOOC,  pp.  193-199. 

c  Idem,  vol.  10,  pt.  1,1907,  p.  135. 

*  Unpublished  MS. 

■  Taylor,  F.  B.,  The  abandoned  shore  lines  of  Green  Bay:  Am.  Geologist,  vol.  13,  1S94,  pp.  316-:i27;  A  rcconnalsfsance  of  the  abandoned  shore 
lines  ol  the  south  coast  of  Lake  Superior:  Idem,  p.  3fi5;  The  highest  old  shore  line  on  Mackinac  Island:  \m.  Jour.  Sci.,  3d  ser.,  vol.  43, 1892,  pp. 
210-218;  The  Munuscong  Islands:  .\m.  Geologist,  vol.  15,  1895,  pp.  24-33;  The  great  ice  dams  ol  Lakes  Mauniee,  Whittlesey,  and  Warren:  Idem, 
vol.24.  1S99   pp.  (■)-38. 

J  Ru.'iseli,  I.  C,  Ann.  Rept.  Michigan  Geol.  Survey,  for  1904, 1905,  pp.  83-93;  idem  for  1906, 1907,  PI.  III. 

*  Goldthwait,  J.  W..  .\handoned  shore  lines  ol  eastern  Wisconsin:  Bull.  AViscon.sln  Geol.  and  Nat.  lli-st.  Survey  No.  17, 1907,  pp.  43-119;  Jour. 
Geology,  vol.14, 190fi  np.  411-124;  Bull   Illinois  Geol,  Survey  No.  7.  1908,  pp.  M-6S;  lour  Geology,  vol.  ir..  19I1.S,  pp.  4,i9-476. 

'  Modification  ol  the  Great  Lakes  by  earth  movement,  Nat.  Geog.  Mag.,  vol.  8, 1897,  pp.  233-247;  Recent  earth  movement  In  the  Great  Lakes 
region:  Eighteenth  Ann.  Rept.  1'.  S.  Geol.  Survey,  pt.  2, 1898,  pp.  li01-(>47. 


THE  PLEISTOCENE. 


451 


by  J.  W.  Goldthwait"  indicate  that  the  abandoned  shore  Hues  in  the  southern  ))art  of  the  Lake 
Aiichigan  basin  are  horizontal.  The  axis  of  tilting  runs  south  of  Green  Bay.  The  effect  of 
the  presence  of  this  hinge  Ime  will  be  to  postpone  very  much  the  time  before  the  tilting  can 
be  sufficient  to  divert  the  drainage  of  Lake  Superior  and  the  other  Great  Lakes  to  the  Chicago 
outlet. 

A  series  of  observations  as  to  the  fluctuating  level  of  Lake  Sujierior  have  been  made  by 
Capt.  J.  H.  Darling,  of  the  L^nited  States  engineer  office,  at  Duluth,  who  comes  to  the  conclu- 
sion that  so  far  as  evidence  from  two  stations  nearly  on  an  east-west  line,  Duluth  and  Marquette, 
for  eighteen  years  indicates,  there  is  no  adequate  proof  of  a  cliange  in  the  level  of  the  present  water 
surface.  It  seems  possible  to  the  writer,  however,  that  this  fact  of  no  variation  at  two  points, 
one  almost  directly  west  of  the  other,  would  indicate  that  the  axis  of  tilting  runs  nearly  east 
and  west  in  the  Lake  Superior  basin,  as  it  seems  to  run  in  Lake  Michigan. 

One  of  the  great  unsolvetl  problems  of  the  glacial-lake  history  in  the  Superior  and  upper 
Lake  Micliigan  basins  concerns  the  stages  intermediate  between  Lake  Duluth  and  Lake  Cliicago 
or  Lake  Algonquin.  Between  the  time  of  the  St.  Croix  outlet  and  the  Hudson  River  outlet 
Lake  Duluth  must  have  had  an  outlet  to  Lake  Chicago  through  a  series  of  lakes  and  straits, 
including  Portage  Lake  on  Keweenaw  Pomt,  the  possible  marginal  channel  east  of  Manjuette 
in  the  Au  Train  and  Wliitefish  valleys  to  Green  Bay,''  and  perhaps  a  channel  through  Sturgeon 
Bay  in  the  Door  Peninsula  of  Wisconsin.  Nothing  conclusive  can  yet  be  said  as  to  tlie  halts 
of  the  ice  front  or  the  time  of  shifting  from  the  St.  Croix  outlet  to  the  temporary  initial  Lake 
Algonquin  outflow  past  Chicago,  a  stage  which  preceded  the  double  outlets  to  Lake  Iroquois 
and  thence  to  the  Hudson.     Further  observation,  however,  will  settle  these  questions. 

Another  interesting  possibility,  at  present  merely  a  hypothesis,  is  that  which  supposes 
stagnant  ice  in  the  deep  eastern  part  of  the  Superior  basin  with  retreat  southward  from  the 
Height  of  Land,  instead  of  northward  toward  it  as  has  always  been  inferred.  No  evidence 
known  to  the  writer  disproves  this  possibility  and  certam  unusually  high  beaches  in  the  Mar- 
quette and  Michipicoten  districts  suggest  it.  It  is  jiossible  that  this  stagnant  mass  may  have 
become  completely  detached  from  the  retreating  ice  sheet.  At  the  beginnmg  of  this  withdrawal 
marginal  lakes  of  high  level  were  formed  in  the  Micliipicoten  district,  just  as  Lakes  Omini  and 
Kaministikwia  were  formed  earlier  on  the  northwest  side  of  the  lake. 

The  following  table  shows  some  of  the  ju-esent  altitudes  of  the  abandoned  shore  lines,  the 
discrepancies  in  elevation  in  the  same  beach  proving  the  tiltmg  and  indicating  how  the  warping 
varied  from  the  earlier  to  the  later  stages  and  from  one  part  of  the  region  to  another.  Not  all 
the  higher  isolated  beaches  are  listed,  and  some  of  the  correlations  are  tentative. 

Elevations  above  Lake  Superior  {602  feet)  of  some  of  the  abandoned  beaches. 


Glacial  Lake  Duluth  (or 
highest  early  lake  recorded). 

Glacial  Lake  Algonquin. 

Nipissing 
Great 
Lakes. 

Satilt  stage. 

Duluth 

632-535, 510-515, 470-475 

410-415 

314(?) 

203-315.380 

400-4.50 

-25 

0 

30 

60 

90 

105-110 

110-115 

Beaver  Bay 

467-498 
482 

Port  Arthur 

Nipigon 

28 

JaokHsh 

418 
410 

Peninsula  Harbor 

40-45 

Michipicoten 

728,843.470 
315,534,543 

Old  \v  Oman  River 

Root  River 

212-266 
414 
412 

85-148 

10-34 

Sault  Ste.  Marie 

49 
35 
25 

Grand  Marais 

Munising ■ 

Marquette .     .      .          

590 

338 

260,240.236,3.35 

338 

Huron  Mountains 

25 

20 

L' Anse 

590 
718 

Ontonagon  Valley 

Houghton 

410 

25 
40 

Lac  La  Belle 

Pnrnnpine  Mniintain«! 

561 
570 
510 
535 
419 
470-535 

Iron  River 

Maple  Ridge 

Brule-St.  Clair  outlet 

Duluth 

<•  Bull.  Wisconsin  Geol.  and  Nat.  Hist.  Survey  No.  17, 1907,  p.  42;  Jour.  Geology,  vol.  14, 1906,  pp.  411^24,  vol.  16, 1908,  pp.  459-476. 
t  Winchell,  N.  II.,  Am.  Jour.  Sci.,  3d  ser.,  vol.  2, 1871,  p.  19. 


452 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Elevations  above  Lake  Michigan  {580  feel)  of  some  of  the  abandoned  beaches. 


Glacial  Lake 
Algonquin. 


Nlpissing 
Great  Lakes. 


Sault  Sle.  Marie 

Detour 

Oediirviile 

St.  Icnace 

Miiiiiiscon^  Islands. 

Mackinac 

Cooks -Mills 

KnsiKii 

Ganlen  liluH 

Fayette 

Burnt,  liluir 

Escanaha  Hlver 

Gladstone , 

Ford  River 

Pine  Kidf-'e 

Birch  ('reck 

Rock  Island 

WasIiinf:lon  Island. 
Dcatlis  Door  Bluff.. 

p^phraini 

Egg  Harbor 

Graceport 

Sturgeon  Bay 

Wilco.v 

Sawyer 

Clay  Banks 

Little  Suamico 

Dykesville 

Cormier 

Two  Rivers 


412-434 


280 
200 
170 
120 
120 
130 
125 
140 
120 

ino 

110 
50 
99 
95 
79 
62 
51 


40 
61 
40 
45 


30 


30 
24 
20 


GLACIAL  LAKE  DEPOSITS. 

Tlie  deposits  laid  dowii  in  the  glacial  lakes  differ  from  the  deposits  now  being  made  in  the 
Great  Lakes  in  the  rapidity  of  accumulation  and  in  the  character  of  materials  laid  dow-p  in 
water  which  was  fed  by  melting  ice  and  in  which  icebergs  floated.  The  deposits  made  in  these 
glacial  lakes  were  predominantly  clay,  although  sands  and  gravels  were  laid  down  near  the  lake 
shores.  Great  thicknesses  of  these  clays  were  accumulated  at  the  west  end  of  Lake  Superior 
during  the  Nemadji,  Duluth,  and  Algonquin  stages  and  acquired  a  prevailing  red  color  by 
derivation  from  the  Keweenawan  rocks.  These  clays  form  a  distinctly  different  soil  from  that 
found  in  the  region  not  covered  by  marginal  lakes.  Well  boruigs  near  Ashland  and  Sujjerior, 
Wis.,  show  thicknesses  of  100  to  150  feet  or  even  more  of  red  clay,  in  places  with  a  little  blue 
clay,  generally  without  any  stones,  overlying  what  is  reported  as  sand  and  "hard])an."  the 
latter  possibly  glacial  till.  The  total  thickness  of  clay  and  sand  in  one  boring  is  193  feet  and  in 
others  is  over  200  feet.  West  of  Duluth  and  Superior  and  extending  eastward  from  Superior 
on  tlie  south  shore  of  the  jn-esent  lake,  these  thick  lake  clays,  overlying  the  horizontal  Cambrian 
sandstone,  form  a  plain  which  ajipears  horizontal  though  sloping  imperceptibh"  northward." 
This  ])lain  has  been  cut  by  ])ostglacial  streams  into  a  series  of  rather  deep,  steep-sided  gullies, 
which  necessitate  the  buikling  of  a  great  number  of  ])ri(lgos  l)y  the  railroads:  for  oxamjile.  the 
Duluth,  South  Shore  and  Atlantic  between  Ashland  and  Duluth  and  the  Northern  Pacific  and 
Great  Northern  between  Duluth  and  Carlton,  Minii.  The  highways  extending  east  and  west 
across  this  region,  where  the  streams  generally  flow  from  south  to  north,  are  continually  going 
up  and  down  hill  in  crossmg  ridges  and  valleys.  West  of  Duluth  and  south  of  Fond  du  Lac, 
^linn.,  tlicse  gullies  are  of  very  great  depth,  some  as  deep  as  200  feet,  so  that  the  railroads  swing 
far  southwaixl  in  order  to  cross  the  gullies  near  their  heads,  reducing  the  number  and  height  of 
the  bridges  which  must  be  built.  The  bridges  on  the  Great  Northern  Railway  are  in  striking 
contrast  with  those  on  the  Northern  Pacific,  both  in  their  number  and  in  their  height  above  the 
streams,  tlic  latter  railway  crossing  nearer  the  headwaters  of  the  streams.  The  flat  ])lain  of 
these  clays  is  not  es])ecialh'  suited  for  agriculture  and  has  not  been  cleared.  The  clays  were 
covered  with  timber,  but  have  been  devastated  by  fire  and  at  present  constitute  a  rather  deso- 
late countiT  that  is  traversed  in  the  first  hour  of  the  ride  from  Duhitli  to  St.  Paul. 


o  Grant,  U.  S.,  Bull.  Wisconsin  Geoi.  and  N'at.  Hist.  Survey  Xo.  il,  19U1,  p.  ti. 


THE  PLEISTOCENE. 


453 


North  of  Duliith,  as  previously  indicated,  tlie  ice  retreated  southward  toward  the  Lake 
Superior  basin,  and  between  the  Mesabi  range  and  Lake  Superior  the  area  of  flat-lying  lower 
Huronian  rocks  was  the  bed  of  a  great  glacial  lake,  called  Lake  Ui)luuu,  which  gradually  increased 
in  size  as  the  ice  retreated,  and  in  which  great  quantities  of  clay  were  accumulated:  The  inter- 
ference with  drainage  in  this  lake-clay  ]>lain  has  brought  about  the  great  prevalence  of  muskegs 
along  the  Duluth,  Missabe  and  Northern  Railway,  which  pursues  an  almost  mathematically 
straight  course  for  over  25  miles  because  of  the  levelness  of  the  lake-bottom  plain.  Nearly  all 
of  tins  distance  is  through  muskeg  swamps,  mterrupted  here  and  there  by  low  gravel  ridges, 
whicli  are  believed  to  be  portions  of  recessional  moraines  built  at  temporary  halts  of  the  ice 
during  this  southward  retreat  and  later  jtartly  submerged  by  tlie  accumulation  of  lake  clay. 
The  bed  of  glacial  Lake  Agassiz  is  similar  in  nature. 

THE  FOUE  PLEISTOCENE  PROVINCES. 
GROUNDS   FOR  DISTINCTION. 

In  review  of  the  conditions  prevaUmg  in  the  Lake  Superior  region  as  regards  minor  topog- 
raphy and  soil,  it  may  be  stated  that  this  region  includes  four  distinctive  ])rovinces — (1)  the 
Driftless  Area,  (2)  the  area  of  the  older  drift  sheets,  (3)  the  area  overlain  by  tlic  till  and  the 


0 

25 

50            75 

100           125 

150  MILES 

m 

%. 

^ 

-r:;.v:^^''' 

^^ 

Driftless  Old  drift  Last  drift  Lake  deposits 

(tv/th  known  moraines) 

Figure  08,— Sketch  map  showing  Driftless  Area  and  regions  of  older  drift,  last  drift,  and  lake  deposits. 


assorted  glacial  deposits  of  the  last  (late  Wisconsin)  stage  of  glaciation,  and  (4)  the  area  where 
glacial-lake  deposits  predominate.  (See  fig.  68.)  These  provinces  are  bounded  respectively 
by  the  terminus  of  the  outermost  of  the  older  drift  deposits,  by  that  of  the  glacial  lobes  of  the 
Wisconsin  stage,  b}'  the  border  of  the  highest  shore  lines  of  the  great  glacial  lakes  in  the  Lake 
Superior  and  Lake  Michigan  basins,  and  by  the  highest  shore  of  Lake  Agassiz.     Not  all  the 


454  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

glacial  lobes,  and  by  no  means  all  the  glacial  lakes,  were  contemporaneous,  so  the  map  should 
not  1)0  understood  as  roprcscnting  conditions  that  were  ])roducpd  at  any  one  time.  It  merely 
rc])roscnts  four  groups  of  areas  witlun  each  of  which  tlie  average  conditions  are  strikingly 
similar  and  wliich  contrast  vnth  one  another. 

DRIFTLESS  AREA. 

In  the  Driftless  Area  the  minor  topographic  conditions  are  intimatel_v  related  to  the  undcr- 
Ij^ing  rock.  The  drainage  is  mature  (PI.  XXXI,  A).  The  valleys  are  cut  almost  entirely'  by 
streams.  Resistant  rocks  make  prominent  ledges  with  castellated  forms,  and  weak  rocks  arc 
worn  to  insignificant  relief.  Waterfalls  and  rapids  in  the  streams  are  rare.  Lakes  are  absent. 
The  soil  consists  of  the  materials  of  the  underlying  rock  or  of  some  adjacent  material  from  a 
source  uphill  from  its  ])resent  location.  It  usually  grades  downward  with  coarser  and  coarser 
fragments  to  the  undecayed  ledge  from  which  it  has  l)een  derived  by  disintegration.  It  is 
a  typical  local  or  residual  soil. 

AREA   OF   OLDER   DRIFT. 

The  province  of  older  drift  includes  the  regions  adjacent  to  the  Driftless  Area  where  deposits 
were  left  by  one  or  more  of  the  earlier  glacial  advances  before  the  Wisconsin.  The  topography 
and  soil  of  this  proN-ince  are  contrasted  with  that  of  the  Driftless  Area  on  one  hand  and  with 
that  of  the  area  of  Wisconsin  glaciation  on  the  other.  The  preglacial  topography  is  partly 
obscured.  The  valleys  are  due  in  part  to  ineriualities  in  glacial  accumulation  as  well  as  to 
stream  cutting.  The  streams  may  have  rapids  or  waterfalls,  though  these  are  rarer  than  in 
the  region  of  latest  drift.  Lakes  are  rather  rare,  and  many  lakes  and  swamps  have  been  filled 
and  drained.  The  glacial  topography  has  slumped  down  to  a  softened  outline.  The  soil  is 
distinctly  a  transported  soil,  containing  foreign  fragments  quite  different  in  composition  from 
the  un<lerh'ing  bed  rock,  overlying  it  unconft)rmably  as  glacial  soils  always  do,  and  not  grading 
into  it.  This  soil,  however,  contrasts  with  the  soil  of  the  area  of  youngest  drift,  which  is  fresh 
and  unweathered.  Indeed,  the  soils  of  the  areas  of  older  drift  are  leached  of  their  soluble 
constituents  to  some  extent  by  the  action  of  percolating  ground  water,  although  the  degree 
of  solution  is  naturally  less  than  in  the  Driftless  Area.     Tliis   might  be  called  a  modified, 

transported  soil. 

AREA   OF   LAST  DRIFT. 

As  already  described,  the  minor  topography  in  the  province  of  latest  drift  is  of  the  various 
kinds  characteristic  of  a  region  overridden  by  glaciers.  The  province  has  a  mildly  irregular  sur- 
face, covered  by  the  till  or  ground  moraine,  which  in  some  places  completely'  mantles  the  ledges 
(Pis.  XXIX,  A,  p.  432;  XXXI,  B,  p.  436),  in  others  covers  them  thinly  (Pis.  V,  p.  112;  XVI, 
in  pocket),  and  in  still  others  is  almost  al)sent  (Pis.  IV,  B,  p.  90;  XVII,  in  pocket).  It  contains 
drumlins,  terminal  and  recessional  moraine  deposits  (PI.  XXX,  A,  p.  434),  and  the  many  assorted 
glacial  deposits  Uke  the  outwash.  These  minor  topograpliic  forms  and  the  transported  soil, 
which  is  fresh  and  still  retains  its  soluble  constituents,  make  up  the  surface  of  the  great  pro^-ince 
of  the  Wisconsin  drift,  which  includes  the  greater  part  of  the  Lake  Superior  region.  The 
important  feature  about  this  province  is  its  contrast  with  the  adjacent  areas,  where  there  is 
either  no  drift  or  the  older  drift  or  where  even  tliis  youngest  drift  is  overlain  by  glacial-lake 
deposits.  The  contrast  with  the  topography  of  the  older  drift  has  already  been  emphasized 
and  may  be  dismissed  with  the  statement  that  here  the  irregularity  is  greater  and  the  asjiect 
of  the  topography  is  distinctly  fresher.  The  young  streams  have  cut  relatively  insignificant 
courses  in  the  latest  tlrift,  except  along  the  largest  rivers,  and  the  lakes  and  swamps  mostly 
exist  as  at  the  close  of  the  glacial  period,  though  some  are  ])artly  filled. 

AREAS   OF   GLACIAL-LAKE  DEPOSITS. 

The  fourth  Pleistocene  province  includes  not  tlie  areas  covered  by  the  numerous  small 
inland  lakes,  but  the  area  fornierlj-  occupied  by  the  larger  glacial  lakes  which  ovcrsj)read  the 
margins  of  Lake  Superior  and  Lake  Micliigan  and  extended  some  distance  northward  from 
Duluth,  as  well  as  the  bed  of  the  large  glacial  Lake  Agassiz. 


THE  PLEISTOCENE.  455 

In  this  province  the  deposits  consist  chiefly  of  assorted  glacial  drift  of  lacustrine  types, 
showing  a  predominance  of  clay  and  silt,  although  in  the  region  between  Marquette  and  Sault 
Ste.  Marie  sandy  deposits  cover  large  areas.  The  minor  topography  in  this  province  contrasts 
strikingly  with  everytlung  else  in  the  whole  Lake  Superior  region.  In  places  there  is  an  exceed- 
ingly smooth,  monotonous  surface  of  lake  clay  or  sand  covered  with  muskegs  or  forests,  with 
insignificant  stream  valleys,  as  south  of  the  Mesabi  range,  in  Minnesota,  and  in  the  eastern 
part  of  the  northern  peninsula  of  Micliigan;  elsewhere  there  is  a  similar  clay  or  sandy  surface 
which  stands  at  a  high  enough  level  above  the  present  lake  for  streams  to  have  cut  deep,  steep- 
sided  gorges  and  gulhes  in  the  clays,  as  west  and  south  of  Duluth.  The  soils  of  this  lake-bottom 
plain  vary  greatly  in  character,  being  in  some  places  exceedingly  fertile,  as  in  the  valley  of 
Red  River  and  on  the  bed  of  the  extinct  Lake  Agassiz;in  others  sandy,  originally  supporting 
an  excellent  forest,  as  in  the  eastern  part  of  the  northern  penmsula  of  IVIichigan ;  and  in  still 
others  of  the  clayey  character  wliich  is  here  fertile  and  there  sterile,  as  near  Superior,  at  the 
head  of  the  lakes,  and  around  Green  Bay  and  Lake  Winnebago.  Tlic  distriljution  of  these  va- 
rieties of  soil  has  not  yet  been  determined  in  the  greater  part  of  the  Lake  Suijerior  region. 

POSTGLACIAL  MODIFICATIONS. 

Since  the  retreat  of  the  ice  normal  jirocesses  have  begun  to  work  upon  the  region.  These 
are  cluefly  the  atmospheric  agencies  that  accomplish  weathering  and  denudation,  including 
the  chemical  work  of  air,  of  surface  water,  and  of  ground  water;  the  work  of  vegetation  and 
animals;  and  the  erosive  and  constructive  work  of  streams  and  waves.  The  most  notable 
results  thus  far  accomplished  are  the  modification  of  the  glacial  drift  and  the  bed  rock  by 
weathering  and  by  stream  work  and  the  work  of  lakes  in  their  beds  and  on  their  shores. 

MODIFICATIONS  ON  THE  LA.ND. 

The  modification  of  the  glacial  deposits  since  the  retreat  of  the  Wisconsin  ice  sheet  has 
been  exceedingly  shght.  Weathering  has  done  relatively  httle,  not  even  erasing  dehcate 
glacial  striae  except  on  the  more  friable  rocks. 

The  deposits  of  older  drift,  however,  as  described  by  Samuel  Weidman"  and  others,  seem 
to  have  been  much  more  modified  since  their  formation.  The  older  drift  now  contrasts  with 
the  last  drift  in  showdng  a  greater  amount  of  modification  by  stream  action  and  very  much 
less  rehef.  Its  composition  has  been  changed,  the  more  soluble  constituents  of  the  clay  in 
the  till  and  of  certain  of  the  bowlders  having  been  leached  out  by  percolating  water.  Streams 
have  filled  or  drained  the  many  shallow  lakes  wMch  may  have  existed  in  inequalities  in  the 
older  drift  sheets,  and  swampy  areas  are  much  less  prevalent. 

The  Wisconsin  drift  sheet,  which  covers  the  greater  part  of  this  region,  contrasts  strikingly 
with  the  older  drift  in  standing  at  a  lugher  elevation,  in  being  essentially  unmodified  by  streams 
and  in  having  relatively  few  of  its  lakes  and  swamps  filled  or  drained.  Many  of  the  shallower 
lakes,  however,  have  probably  been  converted  into  swamps  by  silting  up  and  by  encroacliing 
vegetation.  Doubtless  many  of  the  muskeg  areas  of  the  Lake  Superior  region  were  previously 
areas  of  shallow  water.  Hall  has  estimated  that  in  northern  Minnesota  the  shallow  glacial 
lakes,  whose  numbers  have  probably  been  greatly  exaggerated,  are  being  extinguished  at  a 
rate  of  about  sixty  a  year.*"  The  outwash  deposits  are  in  places  the  only  ones  that  have  been 
very  much  modified  by  stream  work,  and  the  change  in  these  consists  principaUy  of  the  cutting 
of  terraces,*^  as  in  the  lower  St.  Croix  district  of  western  Wisconsin  and  along  Wisconsin,  Cliip- 
pewa,  Mississippi,  and  other  rivers.  The  greater  part  of  these  terraces  were  probably  developed 
during  and  at  the  end  of  the  glacial  period,  when  the  streams  carried  much  more  water  from  the 
melting  of  the  retreating  ice  sheet.  There  has  been  considerable  postglacial  stream  guhjang, 
especially  in  the  lake  deposits.     The  composition  of  the  glacial  drift  of  Wisconsin  age  has 

o  Bull.  Wisconsin  Geol.  and  Nat.  Hist.  Survey  No.  16, 1907.  pp.  435-488. 

6  Hall,  C.  W.,  The  geology  and  geography  of  Minnesota,  1903,  pp.  178,  181-183. 

c  Wooster,  L.  C,  Geology  of  Wisconsin,  1873-1879,  vol.  4, 1882,  pp.  134-138. 


456 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


remained  essentially  unchanged  in  the  comparatively  brief  time  since  the  retreat  of  the  ice. 
Alluvial  deposits  are  present  and  are  being  formed  constantly,  but  arc  confined  largeh"  to  the 
valleys.  As  alrcad}^  stated,  however,  part  of  what  VVcidman"  discusses  as  alluvial  material 
is  certainly  glacial  outwash. 

MODIFICATIONS  IN  AND  AROUND  THE  GREAT  LAKES. 6 

Since  the  Nipissing  stage,  as  already  stated,  the  waters  of  Lake  Superior  have  been  mark- 
edly fluctuating  in  level,  occupying  lower  and  lower  points  on  the  north  shore  of  Lake  Superior 
and  higher  and  higher  points  on  the  south  shore  of  Lake  Superior  and  tlic  shores  of  the  adjacent 
parts  of  Lakes  Michigan  and  Huron,  as  the  warping  of  the  earth's  surface  in  tliis  region  gradu- 
ally tilted  the  water  southward.  This  tilting  has  caused  a  gradual  postglacial  emergence 
of  the  northern  coast  and  a  gradual  submergence  of  the  southern  coast.  In  northern  Micliigan, 
for  example,  A.  ('.  Lane  has  observed  dead  trees  now  standing  out  in  the  waters  of  Lake  Superior. 


, 

V 

\ 

7\[y^ 

I        Yi       0 

z 

3            4  Miles 

\_/ 

jy 

^ 

LAKE     SUPERIOR 

A 

A.  T 

NIPIS SING     S  TAGE 

\ 

2 

^r^ 

yRapid^i       1 

1 

^"^ 

'  w 

Figure  69. — St.  Louis  River  at  the  stage  when  it  cut  its  valley  and  emptied  directly  into  Lake  Nipissing. 

Although  the  recent  beach  levels  have  not  aU  continued  to  be  occupied  by  the  lake  waters 
at  exactly  the  same  level,  some  rather  distinctive  shore  deposits  of  the  normal  type  have  been 
built  up.  On  the  headlands  chffs  have  been  cut,  and  of  these  chffs  those  on  the  south  shore 
of  Lake  Superior  in  the  Pictured  Rocks  region  "^  are  famous.  Because  of  the  character  of  the 
Lake  Superior  sandstone,  the  attack  of  the  waves  upon  it  has  developed  overhanging  cliffs 

o  Weidmnn,  Samuel,  Bull.  Wisconsin  Geol.  and  Nat.  Hist.  Survey  No.  in,  1907.  pp.  5I4-M7. 

f>  fidouard  Desor  was  the  first  to  dcscrihe  the  Lake  Superior  shore  features  ( Foster,  J.  W.,  and  Whitney,  J.  D.,  Geology  of  the  Lake  Superior 
and  district,  vol.  2,  1851,  pp.  2.W-268),  as  Charles  Whittlesey  (idem,  pp.  270-273)  did  for  Lake  Jtichigan.  In  1880  R.  D.  Irving dcscrilwd  the  coast  in 
the  .Vshland  region  (Geology  of  the  eastern  Lake  Superior  district:  Geology  of  Wisconsin,  1873-1879,  vol.  3, 1880,  pp.  70-72).  I.  C.  Russell  has  described 
some  recent  changes  on  the  north  shores  of  Lakes  Huron  and  Michigan  (.\nn.  Rept.  Michigan  Gcol.  Survey  for  1904, 1905,  pp.  102-105).  .V.  C.  Law- 
son  has  described  the  modern  cliffs,  beaches,  etc.,  of  the  north  shore  of  Lake  Superior  (Twentieth  .\nn.  Rept.  Minnesota  Geol.  and  Nat.  Hist.  Snr\ey, 
1893.  pp.  197-230),  discussing  the  shore  contours  and  the  coaslal  profiles  in  the  various  kinds  of  rocks.  G.  L.  Collie  (Bull.  Geol.  Soc.  .\merica,  vol. 
12, 1901,  pp.  197-211'.)  has  done  some  work  on  the  modern  shore  lines  of  tlie  soulh  const  of  Lake  Superior  in  Wisconsin.  G.  K.  Gilbert  used  many 
Illustrations  from  Lake  Superior  and  northern  Lake  Michigan  in  his  Topographic  Features  of  Lake  Shores  (Fifth  .\nn.  Rept.  W  S.  Geol.  Survey, 
1885,  pp.  75-123). 

c  Foster,  J.  W.,  and  Whitney,  J.  D.,  op.  cit.,  pp.  124-129,  plates. 


THE  PLEISTOCENE. 


457 


and  caves,  as  well  as  isolated  stacks  and,  still  farther  out  in  the  lake,  reefs.  The  attack  of 
the  waves  upon  Cambrian  sandstone,  upon  the  Keweenawan  lavas,  and  upon  the  Algonkian 
and  Archean  rocks  has  produced  different  styles  of  coastal  topogra{)hy,  and  the  cHffs  cut  in 
the  glacial  drift  are  different  from  all  others.  On  the  north  shore  of  Lake  Superior  the  relative 
position  and  resistance  of  certain  dikes  and  sills  have  modified  the  shore  topography,  as  was  long 
ao'o  described  by  Agassiz."  Logan  *  carried  the  idea  of  coast  control  by  dikes  still  further — 
further,  indeed,  than  Irving'^  thought  justified.  The  bold  north  coast  forms  a  striking  scenic 
contrast  to  the  mild  south  shore  of  Lake  Superior,  as  Irving  <*  lias  pointed  out. 

Between  the  headlands  beaches  have  been  formed,  and  these  beaches  are  of  the  usual 
sand  and  gravel  and  bowlder  type,  associated  with  spits,  hooks,  bars,  and  sand  dunes.  In 
places  where  such  beaches  have  been  built  across  the  mouths  of  valleys  or  bays  and  separated 
them  from  the  lake,  ponds  have  been  held  in,  as  on  the  south  shore  of  Lake  Superior  or  the 


4  Miles 


FiGUKE  70.— The  present  St.  Louis  River,  which  has  been  converted  into  an  estuary  by  post-Nipissing  lilting. 

of  sand  spits  which  have  been  buHt. 


The  figure  also  shows  the  two  sets 


east  shore  of  Lake  Micliigan  near  Grand  Traverse  Bay,  where  some  very  large  ponds  of  this 
sort  are  found.  Elevated  examples  of  these  ponds  were  observed  by  C.  R.  Van  Hise  and 
J.  M.  Clements  in  1901  on  the  north  shore  of  Lake  Superior,  along  the  Black  Bay  coast,  form- 
ing a  pecuUar  type  of  lakes  associated  with  the  raised  beaches."  In  the  Micliipicoten  district 
a  bar  of  this  kind  was  thrown  across  the  bay  now  occupied  by  Wawa  Lake  at  the  time  of  one 
of  the  higher  lake  stages,  as  described  by  A.  P.  Coleman.-''  The  modern  and  abandoned  beaches, 
chffs,  caves,  and  skerries  on  Isle  Royal  have  been  described  by  Lane,?  and  the  older  and 
modern  beaches  at  Pigeon  Point,  Minn.,  by  Bay  ley.'' 

lAgassiz,  Louis,  Lake  Superior,  its  physical  character,  vegetation,  and  animals,  1850,  pp.  420-425. 

b  Logan,  W.  E.,  Geology  ol  Canada,  1803,  p.  72. 

c Irving,  R.  D.,  Mon.  V.  S.  Geol.  Survey,  vol.  5, 1883,  pp.  336-337. 

<ildeni,pp.  260-2r,l. 

e  From  unpublished  field  notes. 

/  Rept.  Bur.  Mines  Ontario,  vol.  15,  pt.  1,  190B,  p.  19(1;  Univ.  Toronto  Studies,  Geol.  Series,  1902,  p.  5. 

e  Lane,  A.  C,  Geel.  Survey  Michigan,  vol.  fi,  pt.  1,  1898,  pp.  184-186. 

iBayley,  W.  S.,  Bull.  U.  S.  Geol.  Survey  No.  109,  1893,  F-  15. 


458  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Among  the  striking  shore  deposits  of  Lake  Superior  now  being  formed  are  the  two  great 
bars  or  spits  wliich  extend  across  the  liead  of  Lake  Superior  at  Duluth — Minnesota  Point  and 
Wisconsin  Point.  (Sec  figs.  4,  p.  87,  and  70,  p.  457.)  Their  ends  are  separated  by  a  narrow  flian- 
nel  wliich  formed  the  only  entrance  to  the  Bay  of  Superior  until  the  Government  dredged  the 
canal  near  the  Duluth  shore.  These  bars  have  a  total  length  of  atiout  10  miles  (Minnesota 
Point  6i  miles,  Wisconsin  Point  3^  miles)  and  a  width  varnng  from  a  little  over  an  eighth 
of  a  mile  to  less  than  a  hundred  yards  (PI.  V,  A,  p.  112).  They  have  been  built  up  above  the 
water  by  a  combination  of  two  causes.  The  first  and  more  important  is  the  interference  of 
the  shallowdng  lake  bottom  with  the  passage  of  waves,  causing  waves  to  be  overturned  on  the 
site  of  the  present  Minnesota  and  Wisconsin  points.  The  overturning  stirred  up  the  depo.sits 
at  the  bottom  of  the  lake  and  caused  the  waves  to  heap  up  material  at  tins  locality.  The 
continued  accumulation  of  material  along  tliis  narrow  line  gradually  built  up  a  deposit  tliat 
approached  the  surface  of  the  water  and  was  augumented  b}'  the  deposits  of  the  second  kind, 
namely,  the  materials  derived  from  the  shores  of  the  lake,  which  were  transported  outward 
along  the  submerged  embankment,  under  the  influence  of  the  shore  currents.  The  combina- 
tion of  these  two  agencies  soon  carried  the  spits  a  great  distance  out  frona  the  lake  shores,  and 
they  were  eventually  built  up  above  water  for  a  greater  part  of  the  distance  and  finally  con- 
nected, except  for  a  narrow  outlet. 

Drill  holes  put  down  near  the  Goverimient  canal  at  Duluth  and  at  the  newer  jetties  between 
Wisconsin  Point  and  Minnesota  Point  have  shown  that  the  points  are  built  upon  a  base  of  fine 
lake  clay,  overlain  near  the  shore  by  very  coarse  material,  which  a  short  distance  out  is  replaced 
by  fine  sand.  On  the  Minnesota  side  no  pebbles  are  found  on  the  present  beacli  at  a  greater  dis- 
tance from  the  shore  than  three-quarters  of  a  nule,  showing  that  the  contribution  of  the  coarse 
along-shore  drift  in  the  middle  of  the  point  is  not  very  great  and  that  the  larger  part  of  the  mate- 
rial is  washed  up  by  the  waves,  although  probably  augmented  by  the  material  driftetl  along  the 
beaches.  The  higher  parts  of  these  points,  which  rise  20  or  30  feet  above  the  lake  level,  consist 
of  very  fuie  sand,  built  up  into  sand  dunes  by  the  wind,  and  upon  these  dunes  evergreen  trees 
have  been  able  to  grow. 

About  a  mile  back  from  Minnesota  and  Wisconsin  points  another  pair  of  points  (Rice  Point 
and  Connors  Point)  has  been  built,  separating  St.  Louis  Bay,  where  most  of  the  ore  boats  are 
loaded,  from  Superior  Bay.  These  were  doubtless  formed  as  spits  at  an  earher  date,  in  an 
exactly  similar  manner  to  the  outer  spits,  though  they  have  never  been  connected. 

StUl  farther  up  St.  Louis  River  there  are  projections  from  the  sides  of  the  valley,  like  Grassy 
Point  and  others,  in  some  respects  similar  to  these  points  but  probably  of  an  entirely  different 
origin.  It  seems  probable  that  in  the  post-Nipissing  tilting  of  the  lake  waters  into  the  valleys 
the  St.  Louis  Valley,  which  had  been  rather  deeply  cut  in  the  lake-clay  plam  and  which  had 
developed  to  the  stage  of  a  flood  plaui  with  a  meandering  stream  (fig.  69),  has  been  drowned, 
so  that  portions  of  the  spurs  on  the  valley  sides  now  emerge  from  the  water  and  resemble  bars 
(fig.  70).  Farther  up  the  St.  Louis  the  deposits  in  the  vicinity  of  Spirit  Lake  and  above  to  Fond 
du  Lac  form  a  very  characteristic  tlrowned  flood  plain. 

Smaller  bars,  similar  to  these  at  Duluth,  have  been  formed  in  several  bays  on  the  shores  of 
Lake  Superior,  especially  on  the  south  shore,  where  the  rocks  are  weak  and  easily  suji])ly  mate- 
rial for  the  waves  and  currents  to  move.  The  most  notable  of  these  spits  is  Cliequamegon 
Point,"  near  Ashland,  and  there  are  numerous  smaller  ones,  as  on  Au  Train  Island.  In  Grand 
Traverse  Bay,  Lake  Michigan,  a  great  hook  is  formed  by  the  curving  of  a  similar  sjjit. 

It  is  a  rather  notable  fact  that  almost  none  of  the  streams  flowing  uito  Lake  Superior  have 
been  able  to  build  deltas.  Naturally  the  streams  on  the  south  side  of  the  lake,  lil^e  St.  Louis 
River,  could  not  build  deltas  fast  enough  to  keep  up  with  the  gradual  submergence  of  the  region 
in  connection  witii  the  tilting  of  the  land.  On  the  north  shore,  however,  there  seem  to  be  special 
conditions  which  prevent  the  building  of  deltas  by  several  large  sti'eams.  Nipigon  River  would 
not  build  a  delta  because  it  is  a  relatively  clear  stream,  having  been  strained  of  all  sediment 

o  Collie,  O.  L.,  null.  Geol.  Soc.  .\nierica,  vol.  12,  1901,  pp.  200-207. 


THE  PLEISTOCENE.  459 

before  it  flows  out  of  Lake  Nipigon.  This  is  also  true  of  Michipicoten  Eiver  and  of  a  great  num- 
ber of  smaller  streams  which  at  present  carry  rather  small  amounts  of  sediment  because  they 
flow  through  so  many  lakes.  The  Kaministikwia,  at  Fort  William,  has  the  only  delta  of  any 
notable  size  in  Lake  Superior,  and  tliis  seems  to  have  been  formed  mostly  at  an  earlier  time, 
relatively  little  sediment  being  carried*  by  the  Kaministikwia  at  present. 

Of  the  offshore  deposits  less  is  known  specifically.  As  already  suggested,  these  deposits  are 
accumulating  more  slowly  than  when  melting  glaciers  furnished  both  water  and  sediment  in 
greater  quantities  and  when  stones  dropped  by  floating  icebergs  differentiated  the  silts  from 
tliose  now  going  down.  Coarse  deposits  like  gravel  and  sand  predominate  near  the  beaches  and 
the  river  mouths,  and  rocky  accumulations  are  probably  growing  near  the  cliffs.  In  deep  water 
fine  clay  and  silt  predominate,  as  the  detailed  soundings  of  the  Lake  Sui'vey  charts  show.  The 
several  areas  of  sand,  clay,  etc.,  on  the  lake  bottom  show  appi'opriate  relationsliips  to  the  rocks 
and  the  glacial  deposits  of  the  adjacent  shores,  the  drainage  basins,  the  lake  currents,  etc. 
Deposition  here  contrasts  with  the  postglacial  weathering,  erosion,  transportation,  and  slighter 
deposition  on  the  land. 

SUMMARY  OF  THE  PLEISTOCENE  HISTORY. 

The  Pleistocene  epoch  in  the  Lake  Superior  region  witnessed  four  rather  different  sets  of 
conditions — (1)  in  preglacial  time,  when  the  topography  was  much  as  it  is  now  except  for  cer- 
tain valleys  that  have  since  been  deepened  by  glacial  erosion,  broad  areas  that  have  been  covered 
by  glacial  drift,  and  an  entire  contrast  of  drainage;  (2)  in  the  time  of  advancing  glaciers,  when 
the  land  was  gradually  being  covered  and  eroded  by  an  ice  sheet,  drainage  was  being  modified, 
and  plants  and  animals  were  being  driven  out;  (3)  in  the  time  of  retreating  glaciers,  when  from 
an  extreme  stage  of  glaciation  with  nothing  uncovered  except  the  Driftless  Area  the  present 
topography  was  revealed  by  the  gradual  melting  of  a  largely  stagnant  ice  sheet,  with  the  several 
marginal  lake  stages,  etc.,  and  the  attendant  warping  of  the  earth's  crust;  (4)  in  the  present 
stage  of  modification  of  glacial  deposits,  building  of  stream  and  lake  deposits,  return  of  plants 
and  animals,  and  a  general  attempt  to  restore  the  normal  conditions  that  were  prevalent  before 
the  interruption  by  glaciation. 


CHAPTER  XVII.  THE  IRON  ORES  OF  THE  LAKE  SUPERIOR  REGION. 


By  the  authors  and  W.  J.  Mead. 


HOEIZONS  OF  IRON-BEARING  FORMATIONS. 

The  ages  and  names  of  the  iron-bearing  formations  of  the  Lake  Superior  region  are  as 

follows : 

Brown  ores  associated  with  Paleozoic  and  Pleistocene  deposits  (Spring  Valley,  Wis.). 
Cretaceous  detrital  ores  of  the  western  Mesabi  duitrict  of  Minnesota. 
Clinton  ores  of  the  Silurian  of  Dodge  County,  Wis. 
Algonkian  system: 

Keweenawan  series:  Titaniferous  gabbros  of  Cook  and  Lake  counties,  Minn. 
Huronian  series: 
I  Upper  Huronian  (Animikie  group): 

Biwabik  formation  of  the  Mesabi  district  of  Minnesota. 

Animikie  group  of  the  Animikie  district,  Ontario. 

Ironwood  formation  of  the  Penokee-Gogebic  district,  Michigan  and  Wisconsin. 

Vulcan  formation  of  the  Menominee  and  Calumet  districts,  Michigan. 

Vulcan  iron-bearing  member  of  the  Crystal  Falls,  Iron  River,  and  Florence  districts,  Michigan 

and  Wisconsin. 
Gunflint  formation  of  the  Gunflint  Lake  district,  Canada,  and  VermDion  district,  Minnesota. 
Bijiki  schLst  of  the  Marquette  district,  Michigan. 
Deerwood  iron-bearing  member  of  the  Cuyuna  district,  Minnesota. 
Middle  Huronian: 

Negaunee  formation  of  the  Marquette  district,  Michigan. 
Freedom  dolomite  of  the  Baraboo  district,  Wisconsin. 
Archean  system. 
Keewatin  series: 

Soudan  formation  of  the  Vermilion  district,  Minnesota. 
Helen  formation  of  the  Michipicoten  district,  Ontario. 
Unnamed  formation  of  Atikokan  dLstrict,  Ontario. 
Several  nonproductive  formations  in  Ontario. 

The  ores  of  these  horizons  fall  into  natural  groups  on  the  basis  of  general  characters  and 
origin  as  follows: 

(1)  The  ores  of  the  Lake  Superior  pre-Cambrian  sedimentary  iron-bearing  formations, 
including  practically  all  tiie  ores  produced  from  the  Lake  Superior  region. 

(2)  Titaniferous  magnetites  constituting  magmatic  segregations  in  Keweenawan  gabbros. 
Nonproductive. 

(.3)  Magnetic  ores  representing  ])egmatite  intrusions  in  basic  igneous  rocks.     Doubtfully 
represented  by  Atikokan  and  certain  nonproductive  Vermilion  ores. 

(4)  Residual  or  bog  ores  of  the  Paleozoic  at  Spring  ■\':dloy,  in  northwestern  Wisconsin. 
Slightly  ])roductive. 

(5)  The  Clinton  ores  of  the  Paleozoic  in  Dodge  County,  southeastern  Wisconsin.     Slightly 
productive. 

■KO 


THE  IRON  ORES. 


461 


■^GENERAL  DESCRIPTION   OF   ORES  OF  THE    LAKE   SUPERIOR  PRE-CAMBRIAN 
SEDIMENTARY   IRON-BEARING  FORMATIONS. 

INTRODUCTION. 

The  ores  of  the  pre-Cambrian  sedimentarj-  type  comprise  99  per  cent  of  the  productive 
ores  of  the  region.  They  occur  in  the  Keewatin  series,  the  niichlle  Iluronian,  and  the  upper 
Huronian  (Animikie  group).  Tlie  following  table  shows  the  percentage  of  ore  which  has  been 
rained  from  these  rocks,  by  districts,  from  the  opening  of  mining  in  the  (Ustrict  to  the  close  of 

1909: 

Percentages  of  ores  mined  from  pre-Cambrian  sedimentary  rocks  in  Lake  Superior  region  to  close  of  1909. 


Per  cent 
of  total 
to  close 
of  1909. 

Per  cent 
of  1909 
ship- 
ments. 

Geologic  horizons. 

District. 

Kee-.vatin  series. 

Middle  Huronian. 

Upper  Huronian. 

Per  cent 
of lolal 
to  close 
of  1909. 

Per  cent 
of  1909 
.ship- 
ments. 

I'er  cent 
of  total 
to  close 
of  1909. 

Per  cent 
of  1909 
ship- 
ments. 

Per  cent 
of  tolal 
to  close 
of  1909. 

Per  cent 
of  1909 
ship- 
ments. 

Minnesota: 

43.57 
6.49 

66. 41 
2.61 

43.  .57 

66.40 

6.49 

2.01 

50.06 

69.02 

Michigan: 

11.48 
20.45 
14.71 

7.90 
9.99 
10.  SO 

■ 

1 

11.48 

.54 

14.47 

7.90 

19.91 
.24 

9.21 
.37 

.78 

10.43 

40.64 

28.09 

Wisconsin: 

1.17 

2.07 

.06 

.67 
1.62 

1.17 
2.07 

.67 

1.62 

.06 

3.30 

2.29 

Total            ..  .             

100.00 

100.00 

6.49 

2.01 

20.21 

9.58 

73.30 

87.80 

a  Including  Swanzy  district. 


&  Includes  Iron  Kiver  and  Crystal  Falls  districts. 


A  comparison  of  the  total  production  from  each  of  the  geologic  horizons  with  the  i)ro(luction 
for  1909  shows  that  in  the  past  the  Keewatin  and  middle  Huronian  iron-bearing  formations  were 
more  productive  relatively  than  they  are  now;  and  that  the  upper  Iluronian  is  increasing  its 
proportion.  A  further  increase  in  percentage  of  the  upper  Huronian  ores  is  probably  to  be 
looked  for. 

Notwithstanding  the  fact  that  the  iron-bearing  formations  are  contained  in  three  different 
groups,  separated  by  great  unconformities,  tliey  are  remarkabh^  similar  in  their  lithology,  making 
it  possible  to  discuss  them  essentially  as  a  unit.  These  formations  are  unic|ue  among  most  of  the 
sediments  of  the  globe  with  which  we  are  familiar.  The  early  geologic  conclusions  relating  to 
their  structure  were  based  on  the  assumption  that  formations  so  peculiar  were  developed  at  one 
and  the  same  time,  an  assumption  which  of  course  led  to  inuch  confusion  in  the  interpretation 
of  the  stratigraphy  of  the  region. 

An  attempt  is  made  under  the  following  headings  to  summarize  the  salient  features  of  the 
ores  of  the  region  as  a  whole.  In  earlier  chapters  the  ores  of  the  several  districts  are  separately 
discussed. 

KINDS    OF    ROCKS    IN    THE    IRON-BEARING    FORMATIONS. 

In  the  simplest  terms  the  iron-bearing  formations  of  the  Lake  Superior  region  consist 
essentially  of  interbanded  layers,  in  widely  varyang  proportions,  of  iron  oxide,  silica,  and  com- 
binations of  the   two,  variously  called   jasper  or   jaspilite,  where   anhydrous   and    crystalline 


462  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

(Pis.  XXXII  and  XXXIII),  and  ferruginous  chert"  (PI.  XXXI\',  A,  B),  taconite,  or  ferruginous 
slate  (PI.  XXXIV,  C),  where  softer  and  more  or  le.ss  hydrous.  These  rocks  become  ore  by  local 
emichment,  largely  by  the  leaching  out  of  silica  and  to  a  less  extent  by  the  introduction  of  iron 
oxide.  There  are  accordingly  complete  gradations  between  them  and  the  iron  ores.  Many  of 
the  interme<liate  phases  are  mined  as  lean  siliceous  ores.  In  the  following  descriptions,  therefore, 
the  ores  are  not  in  all  cases  sharply  dilferentiated  from  the  iron-bearing  rocks.  Local  phases 
of  the  iron-bearing  formations  are  amphibolitic  and  magnetitic  cherts  and  slates  (PI.  XXXY), 
cherty  iron  carbonates  (PL  XXXVI),  ferrous  silicate  or  greeiudite  rocks  (PI.  XXXVII),  pyritic 
quartz  rocks,  and  detrital  iron-bearing  rocks  derived  from  older  iron-bearmg  formations.  All 
these  phases  are  found  in  each  district,  but  in  considerably  varying  ])roportions.  One  of  the 
most  significant  variations  with  reference  to  the  origin  of  the  ore  is  in  the  relative  abundance  of 
greenalite  rocks  and  siderite. 

CHEMICAL    COMPOSITION    OF    THE    IRON-BEARING    FORMATIONS. 

The  average  iron  content  of  all  the  original  phases  of  the  iron-bearing  formations  for  the 
region,  not  including  interbedded  slates,  as  shown  by  all  available  analj-ses,  is  24.8  per  cent. 
The  average  iron  content  of  the  ferruginous  cherts  and  jaspers,  from  which  there  has  been  but 
httle  leaching  of  silica,  as  shown  by  all  available  analyses,  is  26.33  per  cent.  The  amphibole- 
magnetite  phases  of  the  formations  show  approximately  the  same  percentage.  The  average 
iron  content  of  the  formations,  as  shown  by  all  available  analyses,  different  phases,  including 
the  ores,  being  taken  in  proportion  to  their  abundance,  is  38  per  cent  for  the  Lake  Superior 
region.  (See  table,  p.  491.)  A  comparison  of  this  figure  with  24.8  per  cent  for  the  original 
siderite  and  greenalite  (see  pp.  167,  527)  and  26.33  per  cent  for  the  ferruginous  cherts  and 
jaspers  from  wliich  silica  has  not  been  removed  (see  pp.  181,  238)  will  show  what  has  been 
accomphshed  in  the  secondary  concentration  of  the  ores.  It  is  possible,  however,  that  the  ores 
have  in  part  been  derived  from  the  richer  phases  of  the  iron-bearing  formations.  So  far  as 
this  is  true,  the  secondary  concentration  accomplished  has  been  less  than  the  comparison  of 
these  figures  might  indicate. 

RATIO    OF    ORE    TO    ROCK    IN    THE    IRON-BEARING    FORMATIONS. 

It  may  again  be  noted  that  the  iron  ores,  though  important  commercially,  form  but  a  very 
small  percentage  of  the  rocks  of  the  iron-bearing  formations.  The  deposits  are  very  large,  but 
are  relatively  insignificant  as  compared  wdth  the  great  adjacent  masses  of  ferruginous  cherts 
and  jaspers  making  up  the  bulk  of  these  formations. 

The  percentages  of  iron  ore  to  rock,  by  weight  (see  p.  492  for  depths),  calculated  from  esti- 
mates of  tonnage  given  on  other  pages,  are  as  follows: 

Proportions  of  ore  to  rock,  by  weight,  in  the  iron-bearing  formations  of  the  Lake  Superior  region. 

Per  cent. 

Marquette  district 0. 110 

Penokee-Gogebic  district 165 

Menominee  and  Crystal  Falls  districts 183 

Mesabi  district 2.  000 

Vermilion  district 0G2 

STRUCTURAL    FEATURES    OF    ORE    BODIES. 

It  will  be  shown  later  that  the  iron  ores  are  the  result  of  subsurface  alterations  of  richer  layers 
of  the  iron-bearing  rocks  and  are  localized  at  places  in  these  layers  where  these  alterations 
have  been  most  effective,  particularly  where  the  ordinary  ground  waters  are  converged  within 

o  Chert,  as  deftned  In  the  text-books,  is  an  amoriihous  and  hydrous  variety  ot  quartz,  but  in  the  field  the  term  has  Ijeen  very  generally  applied 
to  siliceous  bands,  such  as  those  found  in  limestone,  with  little  regard  to  their  microscopic  or  chemical  characteristics.  Some  of  the  so-calleil  cherts 
and  limestones  are  very  fine  grained  or  amoqihous.  The  cherts  of  the  iron-bearing  formations  are  similar  in  every  respect  to  those  of  the  limestones. 
They  show  the  same  irregularity  of  texture,  interlocking  of  quartz  grains,  and  in  places  very  fine  grains.  However,  it  can  not  be  said  that  any  ot 
the  so-called  chert  in  the  Lake  Superior  region  has  been  found  to  be  truly  amorphous  and  hydrous 


PLATE   XXXII. 


463 


PLATE  XXXII. 

Jaspilite. 

A.  Folded  jaspilite  from  Jasper  Bluff,  Ishpeming,  Marquette  district,  Michigan.     The  illustration  beautifully  shows 

the  secondary  infiltration  of  iron  oxide  and  deformation  by  combined  fracture  and  flow.  By  close  obsen'ation 
iron  oxide  of  three  different  ages  may  be  seen.  The  oldest  is  the  dark -gray  hematite.  Intersecting  this  is  the  more 
brilliant  steel-gray  hematite  and  magnetite,  and  cutting  both  of  the  former  are  other  veins  of  brilliant  hematite 
and  magnetite.  The  history  of  the  rock  seems  to  be  briefly  as  follows:  Banded  hematite  and  jas]5er  was  bent  by 
folding,  probably  while  the  rock  was  deep  seated.  During  this  folding  the  hematite  was  mashed.  In  a  later 
stage,  when  the  rock  was  more  rapidly  deformed  near  the  surface,  fracturing  occurred.  This  gave  the  conditions 
for  the  first  infiltration  of  iron  oxide,  and  later,  when  the  rock  was  perhaps  still  nearer  the  surface,  further  defor- 
mation resulted  in  new  fractures.     Finally,  the  crevices  thus  formed  were  filled  with  the  latest  iron  oxide. 

B.  Brecciated  jaspilite  from  Jasper  Bluff,  Ishpeming,  Marquette  district,  Michigan.     The  illustration  gives  e\-idence 

of  the  history  as  shown  by  A.  However,  during  the  final  process  the  layers  of  ja.sper,  which  were  bent  at  the 
earlier  stage,  were  broken  through  and  through,  jiroducing  a  breccia.  The  same  evidences  are  seen  of  three  stages 
of  iron  oxide  as  in  A.  The  less  brilliant  gray  is  the  earliest-mashed  hematite;  the  intermediate  gray  represents  a 
first  infiltration;  after  this  there  was  shattering;  and  finally  the  breccia  was  cemented  by  brilliant  steel-gray 
hematite  and  magnetite. 

464 


z 
■< 

e3 


h- 


3 


-^ 


o 

IT 


a. 


PLATE    XXXIII. 


47517°— VOL  52—11 30  465 


PLATE  XXXIII. 

Jaspilite  and  iiematitic  chert. 

A.  Folded  and  brecciated  jaspilite  of  the  Soudan  formation,  Vermilion  district,  Minnesota.     (After  Clements.) 

B.  Hematitic  chert  from  Negaunee,  Marquette  district,  Michigan.     The  bands  of  chert  are  so  broken  by  movement 

that  they  are  in  some  places  difficult  to  follow.  Many  of  the  fragments  have  roundish  outlines,  due  to  their  partial 
Bolution  and  replacement  by  iron  oxide.  The  material  illustrated  is  frequently  found  very  close  to  the  ore  bodies. 
If  a  portion  of  the  remaining  silica  were  removed  and  iron  oxides  introduced  in  its  place,  it  would  become  iron 
ore.    The  hematite  is  soft  and  the  material  illustrated  is  therefore  called  soft-ore  jasper  by  the  miners. 

466 


U.   S.  GEOLOGICAL  SURVEY 


MONOGRAPH    Lll         PLATE  XXXIII 


(A)       FOLDED  AND  BRECCIATED  JASPILITE  OF  THE   IRON -BEARING  SOUDAN   FORMATION, 
VERMILION   DISTRICT,   MINNESOTA, 

fB)        HEMATITIC  CHERT   FROM    NEGAUNEE,   MARQUETTE    DISTRICT,    MICHIGAN, 


PLATE    XXXIV. 


467 


PLATE  XXXIV. 
Ferruginous  chert  and  slate  of  iron-bearing  Biwabik  formation. 

A.  Gray  ferruginous  chert  (specimen  45027)  from  Chicago  mine,  in  sec.  4,  T.  58  N.,  R.  16  W.     Mesabi  district,  Minne- 

sota. Natural  size.  This  is  one  of  the  characteristic  aspects  of  the  ferruginous  chert.s  of  the  iron  formation.  Under 
the  microscope  iron  oxide  and  chert  can  be  seen  still  marking  the  shapes  of  the  greenalite  granules.  Described 
on  pages  168-170. 

B.  Ferruginous  chert  (specimen  4.5588)  from  Mahoning  mine,  Mesabi  district,  Minnesota.     Natural  size.     The  rock 

shows  interbanding  of  chert  with  iron  oxide.      Described  on  pages  168-170. 

C.  Banded  ferruginous  slate  (specimen  45594)  from  Penobscot  mine,  298  feet  below  ferruginous  chert.     Me.sabi  district, 

Minnesota.     Natural  size.     Described  on  pages  170-171. 

468 


U.  S.  GEOLOGICAL  SURVEY 


MONOGRAPH   Ul         PLATE  XXXIV 


(Ai 


FERRUGINOUS  CHERT  AND   SLATE  OF   IRON-BEARING    BIWABIK   FORMATION 
MESABI   DISTRICT,   MINNESOTA 


PLATE   XXXV. 


469 


PLATE  XXXV. 
Ferruginous  chert  and  schist. 

A.  Amphibole-magnetite  chert  (specimen  48571)  from  Republic,  Mich.     Note  coarsely  crystalline  anhydrous  character 

as  compared  with  ferruginous  cherts  and  jaspilites.     For  discussion  of  origin,  see  pages  545  et  seq. 

B.  Sideritic  magnetite-griinerite  schist  from  sec.  13,  T.  47  N.,  R.  27  W.,  Marquette  district,  Michigan.     The  different 

bands  consist  mainly  of  griinerite,  hematite,  magnetite,  and  quartz,  in  varying  proportions.  The  darker-colored 
bands  contain  much  of  the  iron  oxide.  In  the  lighter  bands  griinerite  is  abundant.  In  all  the  layers  there  is  a 
sufficient  amount  of  residual  siderite  to  show  that  from  this  mineral  and  silica  the  griinerite  formed,  and  from 
the  griinerite,  with  jiartial  or  complete  oxidation,  the  magnetite  and  hematite  developed.  Most  of  the  hematite 
is  of  the  si)ecular  variety,  liut  in  places  l)lood-red  flecks  of  hematite  may  be  seen,  and  parts  of  the  specimeiu  are 
stained  by  limonite.     This  is  doubtless  the  result  of  weathering.     Natural  size. 

470 


U    S.   GEOLOGICAL  SURVEY 


MONOGRAPH   Lll         PLATE  XXXV 


fAJ 


'BJ 


(A)  AMPHIBOLE- MAGNETITE  CHERT  FROM   REPUBLIC,   MICHIGAN. 

(B)  SIDERITIC   MAGNETITE-GRUNERITE   SCHIST   FROM   MARQUETTE   DISTRICT,   MICHIGAN. 


PLATE    XXXVI. 


471 


PLATE  XXXVI. 
Jaspery  filling  in  amygdules  and  cherty  siderite. 

A.  Jaepery  filling  in  amygdules  from  ellipsoidal  basalt  of  the  Crystal  Falls  district,  Michigan.     (Specimen  47554.) 

B.  Cherty  siderite  from  sec.  19,  T.  47  N.,  R.  27  W.,  Marquette  district,  Michigan.     This  is  one  of  the  purest  cherty 

siderites  found  in  the  Marquette  district.  The  gray  material  consists  almost  wholly  of  very  finely  crystalline  and 
opaline  silica  and  of  siderite.  The  liluish-gray  layers  contain  some  silica,  the  greenish  layers  some  siderite.  On 
the  weathered  siuface  the  siderite  is  entirely  decomposed  and  in  place  of  it  is  hematiteand  limonite.  The  begin- 
ning of  the  same  kind  of  alteration  may  be  seen  to  affect  some  of  the  siderite  belts  quite  to  the  center  of  the  speci- 
men. As  examined  in  thin  section  the  secondary  limonite  is  found  to  be  in  pseudomorphous  areas  after  the 
siderite.  Between  the  unaltered  siderite  and  that  which  is  completely  decomposed  there  is  every  gradation, 
different  granules  showing  all  stages  of  the  transformation.     Natural  size. 

C.  Cherty  siderite  from  sec.  13,  T.  47  N.,  R.  4G  W.,  Penokee  district,  Michigan.     (See  Mon.  U.  S.  Geol.  Survey, 

vol.  19,  1892,  PI.  XXI,  fig.  4.)  The  original  cherty  siderite  of  the  Penokee  district  is  represented  perfectly  by 
the  grayish-green  material.  Its  very  close  similarity  to  that  of  the  Marquette  siderite  represented  in  Bis  notice- 
able. The  beginning  of  the  transformation  of  the  siderite  to  limonite  and  hematite  is  beautifully  shown.  The 
transitions  between  the  two  are  clearer  than  in  B.  The  processes  of  change  begin  along  the  bedding  planes  and 
along  intersecting  veins.  These  two  together  make  two  sets  of  nearly  right-angle  planes,  which  doubtless  are 
shearing  planes.  The  veins  are  entirely  filled  with  limonite  and  hematite  and  therefore  are  minute  layers  of 
ore.  The  changes  along  the  bedding  illustrate  the  beginning  of  the  process  which  results  in  the  formation  of 
the  iron-ore  deposits.  It  is  noticealile  that,  as  a  result  of  the  alterations,  the  original  banding  of  the  rock  is 
emphasized,  although  the  emphasizing  bands  are  not  so  regular  as  the  original  sedimentary  laminse.  This 
emphasizing  of  the  original  banding  of  the  iron-bearing  rocks  by  metasomatic  changes  is  a  general  law  for  the 
iron-bearing  formations  of  the  entire  Lake  Superior  region.     Natural  size. 

472 


U.  S.   GEOLOGICAL  SURVEY 


MONOGRAPH    Lll         PLATE  XXXVI 


(J)       JASPERY  FILLING   IN  AMYGDULES   FROM   ELLIPSOIDAL  BASALT  OF  THE 
CRYSTAL  FALLS  DISTRICT,    MICHIGAN. 

( B)  CHERTY  SIDERITE  FROM   MARQUETTE  DISTRICT,   MICHIGAN. 

(C)  CHERTY  SIDERITE  FROM   PENOKEE   DISTRICT,   MICHIGAN. 


PLATE   XXXVII. 


473 


PLATE  XXXVII. 

Geeenalite  kock. 

A.  Greenalite  rock  (specimen  45647)  from  locality  near  Duliith,  Missabe  and  Northern  Railway  track,  1  mile  south 

of  Virginia,  Mesabi  district,  Minnesota.     Granules  of  greenalite,  but  little  altered,  stand  in  a  matrix  of  chert. 
Described  on  pages  165-168. 
A' .  Portion  of  surface  of  specimen  shown  in  A,  slightly  magnified  to  show  greenalite  granules  to  better  advantage. 

B.  Interbanded  greenalite  and  slate  rock  (specimen  45176)  from  100  paces  north  500  paces  west  of  southea-st  corner  of 

sec.  22,  T.  .59  N.,  R.  15  W.,  Mesabi  district,  Minnesota.  Natural  .size.  The  black  portion  of  the  rock  is  slate  and 
the  green  portion  is  made  up  of  greenalite  granides  lying  in  a  matrix  of  chert.  Greenalite  is  characteristically 
associated  with  slaty  layers  in  the  iron-bearing  formation;  indeed  it  is  due  to  their  protection  that  greenalite  has 
been  retained  in  comparatively  unaltered  form.     Described  on  pages  165-168. 

474 


U.  S.  GEOLOGICAL  SURVEY 


MONOGRAPH    Lll  PLATE  XXXVII 


GREENALITE   ROCK   FROM   MESABI   DISTRICT,   MINNESOTA 


THE  IRON  ORES.  475 

the  formation,  owing  to  various  structural  conditions.  Rarely  are  tne  original  layers  of  iron 
formation  rich  enough  to  be  mined  when  they  have  suffered  only  minor  secondary  concentration. 
Because  of  secondary  concentration  the  ores  are  usually  in  the  upper  parts  of  the  formations 
and  always  extend  to  the  surface,  though  they  may  reach  depths  of  2,000  feet  or  more.  It 
may  readily  be  conceived  that  there  are  a  great  variety  of  structural  conditions  which  deter- 
mine the  circulation  of  the  altering  waters  and  therefore  the  localization  and  shapes  of  the  ore 
deposits  witliin  the  formations.  Such  structural  features  are  joints,  faults,  folds,  intersection 
by  igneous  rocks,  impervious  sedimentary  layers  within  or  below  the  iron-bearing  formations, 
and  area  of  exposure. 

The  structural  features  of  the  ores  are  described  principally  in  the  detailed  descriptions 
of  the  ores  of  the  several  districts,  but  some  of  the  more  salient  features  of  the  structural 
relations  are  summarized  below. 

The  development  of  ore  within  the  richer  layers  of  the  iron-bearing  formations  depends 
on  their  accessibility  to  altering  solutions  from  above,  and  the  largest  result  is  given  by  a  wide 
area  of  exposure  of  the  formations,  which  is  in  turn  a  function  of  the  dip.  The  flat-lying 
iron-bearing  formation  of  the  Mesabi  district  exposes  a  greater  surface  to  concentrating  agents 
than  the  steeply  dipping  formation  of  the  Gogebic  district,  of  similar  thickness  and  character, 
with  the  result  that  the  proportion  of  the  formation  altered  to  ore  is  much  greater  in  the  Mesabi 
district.  A  comparison  of  the  actual  areas  of  the  dift'erent  iron-bearing  formations  with  their 
total  shipments  to  date  and  with  their  probable  reserves  shows  a  close  relation  between  area 
and  amount  of  ore  developed. 

Of  more  immediate  and  practical  importance  in  relation  to  the  distribution  of  the  ores 
are  the  structural  conditions,  such  as  impervious  basements  and  fractures,  which  determine 
the  location  of  ores  within  a  given  area  of  the  iron  formation. 

Impervious  basements  for  the  ore  body  may  be  formed  (1)  by  the  intersection  of  the  foot- 
wall  quartzite  with  an  igneous  dike,  as  in  the  Gogebic  district;  (2)  by  irregular  intrusive  masses 
of  basic  igneous  rock,  as  in  the  Marquette  district;  (-3)  by  dolomite,  as  in  the  Menominee 
district;  (4)  by  slate,  as  at  the  lower  horizons  of  the  Negaunee  formation  in  the  Marquette, 
Crystal  Falls,  Iron  River,  and  Florence  districts,  and  at  the  upper  horizon  of  the  Vulcan  forma- 
tion in  the  Menominee  district;  (.5)  by  slate  layers  within  the  iron-bearing  formation,  locally 
developed  in  the  Gogebic  and  Mesabi  districts;  and  (6)  by  granite,  as  in  the  Swanzy  district 
of  Iklichigan  and  very  locally  in  the  Mesabi  district.  Most  of  these  basements  have  the  config- 
uration of  pitching  troughs. 

The  ores  are  likely  to  be  closely  associated  with  fractures  in  the  iron-bearing  formation 
which  give  access  to  altering  solutions,  as  is  particularly  well  illustrated  by  certain  of  the 
deposits  of  the  Mesabi  district  and  by  parts  of  the  deposits  of  the  Gogebic  district  which  pass 
through  faults  in  the  impervious  basement,  and  indeed  is  illustrated  to  a  greater  or  less  extent 
by  practically  all  the  iron  deposits  of  the  region. 

The  relative  importance  of  the  several  structural  features  of  the  ore  deposits  varies  widely 
from  place  to  place.  In  the  Gogebic  district  the  existence  of  impervious  basements  in  the  form 
of  pitching  troughs  seems  to  be  the  essential  structural  feature  of  the  ore  deposits.  Localization 
of  the  ores  within  and  adjacent  to  fissures  in  the  iron-bearing  formation  is  also  apparent.  On 
the  other  hand,  in  the  Mesabi  district  the  conspicuous  feature  is  the  localization  of  the  ores 
by  fractures  in  the  iron-bearing  formation,  the  impervious  basement  being  so  gently  flexed 
as  to  make  it  difficult  to  ascertain  whether  or  not  it  forms  pitching  troughs  that  control  the 
localization  of  the  ore  body. 

SHAPE    AND    SIZE    OF    THE    ORE    BODIES. 

Because  of  the  wide  variety  of  conditions  outlined  under  the  preceding  heading,  the 
shapes  of  the  deposits  of  this  region  are  so  various  that  they  may  collectively  be  designated 
by  the  term  "amoeboid,"  though  there  are  several  groups  of  more  uniform  shape,  as  described 
below.     They  may  be  roughly  tabular  in  a  horizontal  plane,  as  in  the  Mesabi  district,   or 


47(i  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

roughly  tabular  in  steeply  inclined  planes,  or  in  steeply  pitc-hin<r  linear  shoots,  as  in  the 
Menominee  district,  or  they  may  assume  almost  any  conihination  of  these  shapes.  The  mine 
cross  sections  (Pis.  X,  XXVII;  figs.  14,  29,  36,  46,  47,  48)  will  give  the  best  notion  of  the 
sha])es  of  the  ore  bodies. 

The  horizontal  dimensions  known  at  the  surface  range  up  to  a  mile.  Indeed,  in  the  Ilibbing 
area  of  the  Mesabi  district  the  deposits  are  more  or  less  connected  for  a  distance  of  10  miles, 
and  the  horizontal  area  would  range  up  to  2  square  miles.  The  maximum  depth  of  iron  mining 
in  the  Lake  Superior  region  at  the  present  time  is  2,200  feet,  in  the  Gogebic  district. 

It  is  therefore  apparent  that  the  size,  shape,  and  structural  relations  of  the  Lake  Superior 
ores  are  of  widest  variety.  In  the  flat-lpng  formations  of  the  Mesabi  district  the  ore  bodies 
have  wide  lateral  extent  as  compared  with  depth,  have  extremely  irregular  outlines  partly 
controlled  bv  jointing,  abut  irregularly  on  the  bottom  and  sides  against  unaltered  portions  of 
the  iron-bearing  formation,  and  when  the  glacial  overl)urden  is  removed  are  accessible  to  surface 
operations  with  steam  shovels.  Steeply  dipping  formations,  comprising  most  of  the  formations 
of  the  districts  other  than  the  Mesabi,  have  greater  vertical  dimensions  as  compared  with  hori- 
zontal dimensions,  usually  abut  not  only  against  unaltered  parts  of  the  iron-bearing  formation 
but  against  well-defined  impervious  walls  consisting  of  slate,  r|uartzite,  dolomite,  or  bosses  or 
dikes  of  greenstone,  and  must  be  worked  by  underground  mining. 

TOPOGRAPHIC    RELATIONS    OF    THE    ORE    BODIES. 

The  ore  deposits  are  associated  with  hills  or  ranges,  a  fact  that  explains  the  common  use 
of  tlie  term  "range"  in  connection  with  the  ore-producing  districts.  There  are,  however, 
exceptions  to  this  relation  in  the  Cuyuna  district  of  Minnesota  and  perhaps  elsewhere,  as  sho\vn 
in  the  detailed  descriptions.  The  ore  deposits  occur  in  places  on  the  top  of  the  lull,  a.s  in  the 
Vermilion  district;  commonly  in  the  middle  slopes,  as  is  well  illustrated  by  the  Mesabi  district, 
and  on  the  low  ground  adjacent  to  the  hills,  as  in  parts  oi  the  Gogebic,  Marquette,  and  Menom- 
inee districts.  In  general  the  middle  slopes  seem  to  be  favored,  but  there  are  so  many  exceptions 
to  this  that  there  is  no  warrant  for  limiting  prospecting  to  such  localities.  As  the  formation  of 
the  ore  bodies  is  a  function  of  the  rapid  circulation  of  waters  from  above,  it  is  believed  that 
the  common  association  of  the  ore  deposits  with  hUls  mnj  be  due  to  the  fact  that  these  are 
places  where  the  circulating  waters  have  considerable  head.  It  would  not  follow  that  ore  deposits 
should  for  this  reason  be  confined  entireh'  to  the  vicinity  of  liills,  for  circulation,  perhaps  less 
deep,  seems  to  be  effective  also  in  relatively  flat  areas,  as  in  the  Cuyuna  district  of  Minnesota. 
The  efl'ectiveness  of  the  head  at  different  elevations  and  with  different  structural  relations  is 
not  well  known.  It  is  to  be  remembered,  also,  that  the  ore  deposits  have  not  been  concentrated 
entirely  in  relation  to  the  present  topograph)-,  but  that  when  these  deposits  were  formed  the 
topography  was  more  or  less  different,  and  that,  therefore,  ore  deposits  now  found  independent 
of  topographic  elevations  may  still  have  originated  under  control  of  an  elevation  which  has 
since  been  removed.  Notwithstanding  these  various  limit ations,  to  be  considered  in  the  inter- 
pretation of  the  relation  of  ore  deposits  to  topography,  the  present  prevalence  of  by  far  the 
greater  number  of  ore  deposits  on  the  middle  slopes  of  the  ranges  is  extremely  suggestive,  for 
these  are  the  places  where  the  flow  of  meteoric  waters  directly  from  the  surface  should  be  at  a, 
maximum. 

OUTCROPS    OF    THE    ORE    BODIES. 

By  far  the  greater  number  of  the  Lake  Superior  ore  dejiosits  are  softer  than  at  least  one  of 
their  walls.  Thej'  therefore  (jccupy  depressions  which  are  largely  covered  with  glacial  drift  and 
generally  they  do  not  outcrop.  A  few  of  the  ores,  such,  for  instance,  as  the  hard  ores  of  the_  Ver- 
milion and  Marquette  districts,  are  nearly  or  cpiite  as  hard  as  the  wall  rock,  have  resisted  erosion, 
and  here  and  there  project  above  the  mantle  of  drift.  Considering  the  nund)er  of  ore  bodies  in 
the  Lake  Suj)erior  region  and  their  variety-  of  structural  relations,  it  is  surprising  that  so  few  have 
been  found  to  outcro]).  The  lean  siliceous  and  magnetic  parts  of  the  iron-bearing  formations 
have  withstood  erosion  to  such  an  extent  that  they  outcrop  rather  commonly.     These,  together 


THE  IRON  ORES. 


477 


with  magnetic  variations,  have  served  as  guides  to  the  locati<5n  of  the  iron-bearing  formations 
and  have  led  to  the  discovery  of  ores  in  the  covered  areas  by  underground  work. 

In  the  iron-ore  deposits  that  have  their  greatest  tlimensions  on  the  erosion  surface  the  ratio  of 
area  of  iron  ore  to  area  of  iron-bearing  formation  is  greater  than  the  ratio  of  tonnage  of  iron  ore 
to  tonnage  of  iron-bearing  formation.  In  the  Mesaln  district  the  forjiier  runs  up  to  nearly  S  per 
cent  for  the  jiroducing  part  of  the  district ;  in  most  of  the  other  ranges  it  is  far  smaller,  usually 
less  than  1  ])er  cent. 

CHEMICAL    COMPOSITION    OF    THE    ORES. 

The  average  composition  of  the  iron  ore  mined  in  the  I^ake  Superior  region  during  the  years 
1906  and  1909,  as  shown  in  the  table  below,  has  been  calculated  from  the  cargo  analyses  published 
by  the  Lake  Superior  Iron  Ore  Association,  of  Cleveland,  tt)gether  with  analyses  of  ores  of  different 
mine  grades  furnished  by  individual  mining  companies.  The  averages  are  obtained  by  combining 
all  grades  in  proportion  to  their  tonnage,  and  the  talile  represents  more  nearly  the  average  com- 
position of  all  the  ore  mined  in  the  Lake  Superior  region  in  any  one  year  than  anything  before 
attempted.  Analyses  of  iron  ore  used  in  other  ])arts  of  this  report  are  also  taken  from  the  Lake 
Superior  Iron  Ore  Association's  tables  unless  otherwise  stated. 

Analyses  of  iron  ore  may  represent  the  composition  of  a  dried  ore  (dried  at  212°  F.  or  100°  C), 
or  they  may  show  the  composition  of  the  ore  in  its  natural  moist  condition  as  it  comes  from  the 
ground.  The  latter  are  designated  natural  analyses  and  include  the  moisture  or  uncombined 
water  as  one  of  the  constituents  of  the  ore.  The  natural  iron  content  is  the  basis  on  wliich  the 
value  of  ore  is  figured  commercially.  It  may  be  computed  from  the  hon  content  of  the  dried 
ore  and  the  moisture,  as  follows:  Percentage  of  natural  iron  =  percentage  of  iron  in  dried  ore 
X  (100  —  pei'centage  of  moisture).     The  following  average  analyses  represent  the  dried  ore: 


Average  composition  of  total  yearly  production  of  Lake  Superior  iron  ore  for  the  years  1906  and  1909. 

1906. 

1909. 

11.28 

Analysis  of  ore  dried  at  212°  F. : 

59.80 
.0810 
6.83 
1.60 

2.70 

3.92 

58.45 

.091 

Silica                                                                                                                  

7.67 

2. 23 

.71 

.64 

)           .55 

.060 

4.12 

The  range  in  percentages  shown  by  the  analyses  from  which  the  foregoing  averages 
derived  is  as  follows : 


are 


Range  in  percentage  for  each  constituent  of  ores  mined  in  1906  and  1909,  as  shown  by  average  cargo  analyses. 

. 

1900. 

1909. 

0.60    to  17. 40 

Range  in  composition  of  ore  dried  at  212*  F. : 

Iron                                                                                                            

38.15    to  66. 07 
.  0O8  to      .  850 
3.21    to  40. 97 

35. 74    to  C5. 34 

.008  to    1.28 

2. 50    to  40.  77 

.00    to    7.20 

Alumina                                                                

.20    to    3.59 

.  16    to    5. 67 

.00    to    4.96 

.00    to    3.98 

.003  to    1.87 

0.00    to  10.0 

.  40    to  11.  40 

The  sulphur  in  the  Lake  Superior  ores  ranges  from  a  trace  to  1.87  per  cent  and  in  some  of 
the  ores  of  the  Florence,  Iron  River,  and  Crystal  Falls  districts  it  is  present  in  sufficient  cjuantity 
to  affect  the  value  of  the  ore.  Titanium  is  not  present  in  the  Lake  Superior  sedimentary  ores  in 
amounts  sufficient  to  be  harmful.  The  titanium  content  of  the  ores  varies  from  0.1  to  0.2  per 
cent,  TiO,,  but  in  some  of  the  hard  magnetite  ores  of  the  Marquette  district  it  is  found  to  run  as 


478 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


high  as  1 .()  i)cr  cent.  Titanium  is  higher  in  the  paint  rocks  and  interbedded  slates  than  in  the  ores 
themselves. 

The  ])roportions  ami  ranges  of  the  constituents  for  the  individual  districts  are  given  under 
the  discussions  of  the  districts.  Figure  71  shows  the  chemical  compositions  of  all  grades  of  ore 
mined  in  the  region  in  19()(),  in  terms  of  ferricoxide,  silica,  ami  minor  constituents.  This  average 
is  lower  in  iron  than  those  of  previous  years.     (See  pp.  493-494. ) 

The  gratle  of  ore  shipped  and  its  general  uniformity  for  given  districts  and  periods  are  pri- 
marilj'  controlled  by  the  nature  of  the  ores  available,  yet  the  commercial  conditions  to  some  extent 

FERRIC  OXIDE  le 


Mesabi 

Vermilion 

Gogebic 

Marquette 

Menominee 

Canadian 


MINOR 
CONSTITUENTS 
Figure  71.— Triangular  diagram  showing  chemical  composition,  in  terms  of  ferric  oxide,  silica,  and  minor  constituents,  of  all  grades  of  iron 

ore  mined  in  the  Lake  Superior  region  in  1906.    The  ores  of  each  district  arc  indicated  by  distinct  ive  s  j-mbols.    For  descript  ion  of  this  method 

of  platting,  see  p.  182. 

determine  the  grade  shipped.  For  instance,  if  high,  medium,  and  low  grade  ores  are  available, 
a  period  of  financial  depression  may  make  it  possible  to  ship  only  the  highest  grade  ores,  whereiis 
business  prosperity  may  make  it  jiossible  to  mix  considerable  quantities  of  lower  grade  ores 
with  those  of  higher  grade,  thereby  lowering  the  average  grade.  This  control  b\-  comniercial 
conditions  is  further  illustrated  by  the  fact  that  the  acid  Bessemer  steel  process  for  years  deter- 
mined that  an  unusually  high  proportion  of   low-phosi)horus  ores  were  to  be  slui)ped."      The 

a  A  Bessemer  ore  is  one  which  will  with  a  proper  flux  and  coke  make  a  pig  Iron  in  which  the  phosphorus  does  not  exceed  0.1  per  cent.  It  is 
approximately  true  that  a  Bessemer  ore  is  one  in  which  the  content  of  phosphorus  divided  by  the  content  of  iron  gives  a  quotient  not  exceeding 
O.noOT.i.  This  ralio  may  be  chani;eil.  hoHevcr,  by  the  phosphorus  content  of  the  coke  and  limestone  used  with  the  ore  in  tl>e  furnace,  as  it  is 
necessary  to  figure  on  the  phosphorus  in  the  flux  and  fuel  as  well  as  that  in  the  ore  itself. 


THE  IRON  ORES. 


479 


recent  rapid  development  of  the  open-hearth  pr>_.pess  has  allowed  shipment  of  ores  higher  in 
phos])horus.  The  tlevelopment  of  the  basic  open-hearth  process  depends  ultimately  on  the 
availability  of  large  reserves  of  non-Bessemer  ore,  but  in  turn  the  develojmient  of  the  open 
hearth  reacts  upon  and  determines  the  grade  of  ore  shipped  from  any  district  or  for  any  period. 


MINERALOGY    OF    THE    ORES. 

The  iron-ore  minerals  in  general  are  as  follows: 

Magnetite:  Magnetic  oxide  (FejOi),  including  titaniferous  magnetite.     Theoretical    iron    content 

of  the  pure  mineral,  72.4  per  cent;  generally  containing  some  feematite. 
Hematite:  Anhydrous  sesquioxide  (FCnOj),  including  specular  hematite,  red  fossil  ore,  oolitic  ore, 

etc.     Theoretical  iron  content  of  the  pure  mineral,  70  per  cent. 
Brown  ore:  Hydrous  sesquioxide  (FejOj.nH^O),  including  turgite,  limonite,  goethite,  or  a  mixture 

of  these  minerals,  known  locally  as  brown  hematite,  bog  ore,  gossan  ore,  etc.     Theoretical  iron 

content  of  iron  minerals,  59.8  to  66.2  per  cent,  depending  on  degree  of  hydration. 
Carbonate:  Siderite,  iron  carbonate  (PeCOj),  known  locally  as  spathic  ore,  black  band  ore,  etc. 

Theoretical  iron  content  of  the  pure  mineral,  48.2  per  cent. 

The  Lake  Superior  iron  ores  are  (1)  soft,  brown,  red,  slaty,  hydrated  hematites;  (2)  soft 
limonite;  (3)  hard  massive  and  specular  hematites;  (4)  magnetites;  and  (5)  various  gradations 
between  the  other  classes.  The  proportions  for  the  entire  region  of  these  different  classes 
shipped  in  1906,  as  calculated  from  average  cargo  analyses,  are  as  follows: 

Total  production  of  iron  ore  in  Lake  Superior  region,  by  grades,  for  1906. 


Class  of  ore. 


Soft  brown,  red.  slaty,  hydrated  hematite 

Soft  limonite  ores 

Hard  massive  and  specular  hematite 

Magnetite  (less  than  1  per  cent;  included  with  hard  ores) 


35,652,174 
2,741,323 


38,393,497 


Per  cent 
of  total. 


93 

7 


100 


The  approximate  mineral  composition  of  the  average  ore  of  the  entire  region  for  the  years 
1906  and  1909,  calculated  from  the  average  analyses,  is  as  follows: 

Approximate  mineral  composition  of  average  iron  ore  of  Lake  Superior  region  for  1906  and  1909. 


Hematite  1  (more  or  less  hydrated),  with  some  magnetite  (SFejOs.HjO). 

Quartz 

Kaolin 

Chlorite  (and  other  ferromagnesian  siUcates) 

Dolomite 

Apatite  (all  phosphorus  figured  as  apatite) 

Miscellaneous 


100.00 


a  The  iron  minerals  may  be  expressed  in  terms  of  hematite  and  limonite  as  follows:  1906,  hematite  66.60,  limonite  22.00;  1909,  hematite  66.75, 
limonite  19.70.    These  minerals  do  not,  in  fact,  exist  in  these  proportions,  there  being  a  number  of  hydrates  between  hematite  and  limonite. 

The  mineral  compositions  above  given  are  necessarily  only  approximate,  as  ferric  and 
ferrous  iron  are  not  separated  in  the  chemical  analysis,  and  water,  carbon  dioxide,  and  pos- 
sibly a  small  amount  of  organic  matter  are  all  included  under  loss  on  ignition.  The  mineral 
compositions  were  calculated  from  the  average  analyses,  as  follows:  All  phosphorus  was  figured 
as  apatite;  the  remaining  lime  was  combined  with  the  proper  amount  of  magnesia  and  COj 
to  form  dolomite;  the  remaining  magnesia  was  combined  with  the  proper  amounts  of  alumina, 
silica,  and  water  to  form  chlorite;  the  alumina  not -used  for  chlorite  was  taken  with  sufficient 
silica  and  water  to  form  kaolin;  the  remaining  water,  combined  with  the  iron  figured  as  ferric 
oxide,  was  figured  as  hydrated  hematite. 

The  proportions  of  the  different  minerals  for  the  individual  districts  calculated  in  the  same 
way  are  given  in  the  discussion  of  these  districts. 


480  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

In  the  above  table  are  mentioned  the  abundant  minerals  associatetl  with  the  iron,  such 
as  quartz,  kaolin,  and  chlorite.  Many  of  the  minerals  termed  miscellaneous '  in  the  table 
arc  present  in  small  aniounts  at  a  few  places.  Some  of  these  minerals  arc  apatite,  adularia, 
wavellite,  calcite,  dolomite,  siderite,  pyrite,  marcasite,  chalcopyrite,  tourmaline,  masonite, 
ottreUte,  chlorite,  mica,  garnet,  rhodochrosite,  manganite,  pyrolusite,  barite,  gypsum,  martite, 
aphrosidcrite,  analcite,  goethitc,  and  turgite. 

Though  many  of  the  Lake  Superior  ores  are  slightly  magnetic,  there  are  only  two  mines  in 
the  region  which  ship  ores  classed  as  magnetite  ores,  the  Republic  and  Champion,  and  even 
these  ores  are  largely  specular  hematite  with  considerable  quantities  of  magnetite.  There  are 
in  the  region,  however,  great  quantities  of  lean  nontitaniferous  magnetic  iron-bearing  rocks, 
as  at  the  east  end  of  the  Mesabi  range  and  in  the  Gunflint  district,  where  the  Duluth  gabbro 
cuts  and  overlies  the  iron-bearing  formation;  at  both  the  east  and  west  ends  of  the  Gogebic 
range,  where  Keweenawan  intrusive  rocks  cut  the  iron-bearing  formation,  and  in  parts  of  the 
Marquette  district. 

The  magnetite  ores  consist  of  coarse-grained  magnetite-quartz  rock  caiTving  a  considerable 
variety  of  metamorphic  silicates,  including  amphiboles,  pyroxenes,  garnets,  chlorites,  olivines, 
cordierite,  riebeckite,  dumortierite,  etc.  (See  pp.  545  et  seq.)  Locally  pyrite,  pyrrhotite,  and 
iron  carbonate  are  pj-esent.  The  minerals  show  greater  variety  and  more  complex  chemical 
constitution  than  those  of  other  phases  of  the  iron-bearing  formation.  Where  altered  at  the 
surface  the  magnetite  may  be  locally  coated  with  limonite  and  the  silicates  may  have  gone 
over  to  chlorite,  epidote,  and  calcite.  The  yellowish-green  colors  so  develoj)ed  are  extremely 
characteristic  of  the  surface  of  the  exposures. 

PHYSICAL    CHARACTERISTICS    OF    THE    ORES. 

GENERAL   CHARACTER. 

The  ores  range  from  the  massive  and  specular  hematite  and  magnetite  tlu-ough  ores  which 
are  partly  granular  and  earthy  and  partly  in  small  hard  chunl<s  to  ores  which  are  almost  entirely 
soft  and  earthy  (Pis.  XXXVIII  and  XXXIX).  There  is  no  very  sharp  distinction  between 
the  hard  ores  and  the  soft  ores.  The  latter  make  up  the  great  bulk  of  the  annual  shipments; 
of  the  ore  shipped  in  1906  fully  03  per  cent  would  be  classed  locally  as  soft  ores.  The  principal 
hard  ores  come  fi'om  the  ^'ermilion  district  and  fi-om  the  upper  horizons  of  the  Xegaunee  forma- 
tion in  the  Marquette  district.  Most  of  the  soft  ores  contain  small  hard  chunks,  usually 
bounded  by  parallelepiped  phases  due  to  being  broken  up  in  the  bed  by  minute  joints.  Screen- 
ing tests  showmg  the  textures  of  the  typical  ores  for  each  of  the  districts  are  given  in  the  chapters 
on  the  individual  districts.  A  sununary  of  these  screening  tests  for  all  the  Lake  Superior  ores 
is  shown  graphicaDy  in  figure  72.  The  data  of  the  screening  tests  on  the  diflerent  ores  were 
kindly  furnished  by  the  Oliver  Iron  Mining  Company. 

Thei-e  is  a  striking  contrast  in  the  coarse  texture  of  the  magnetite  oi-es  and  the  fine  cherty 
textures  of  the  other  phases  of  the  iron-bearing  formation.  The  cpiartz  grains  in  the  jaspere 
of  the  eastern  part  of  the  Marquette  district  average  from  0.01  to  0.03  millimeter,  whereas  in  the 
western  and  southwestern  portions  of  the  same  district  in  the  amphibole-magnetite  phases  of 
the  iron-bearing  formation  the  quartz  grains  average  about  0.1  to  0.4  millimeter  and  run  as  high 
as  1  millimeter.  The  quartz  grains  of  the  amphibole-magnetite  rocks  may  thus  have  a  million 
times  the  volume  of  those  of  the  jaspers.  The  rjuartz  grains  near  the  gabbro  in  the  eastern  part 
of  the  Mesabi  district  reach  a  diameter  of  3  or  4  millimeters,  but  in  the  central  and  western  por- 
tions of  the  district  they  are  in  general  not  greater  than  0.1  millimeter.  In  a  given  amjihibole- 
magnetite  rock  the  grains  are  fairly  uniform  in  size  and  have  a  tendency  toward  polygonal 
shape  (see  PI.  XIjVII,  A,  p.  548),  whereas  in  the  other  parts  of  the  formation  they  are  most 
irregular  in  size  and  shape  (see  PI.  XIjIV,  p.  534)  and  show  the  characteristic  scalloped 
boundaries  of  cherts. 

The  mineral  density  of  the  ores  ranges  from  3.5  to  5.0  and  averages  about  4.30;  the  pore 
space  ranges  from  less  tl\an  1  per  cent  to  60  per  cent  and  averages  about  35  percent;  and 
the  free  moisture  held  in  this  pore  space  ranges  from  0  to  16  per  cent  and  averages  about  10.42 
per  cent. 


PLATES  XXXVIII-XXXIX. 


PLATE  XXXVIII. 

Characteristic  specimens  of  iron  ores. 

.•1.  Soft  hcmatile  fnim  Mesabi  district,  MiniU'sola. 

B.  Hard  lienialile  I'mm  Ely,  l^Iinnosola. 

C.  Hard  hematite  from  Gosebic  district,  Michif;an. 

Ores  of  these  kinds  furm  93  per  cent  of  the  Lake  Superior  shipiueuts. 


PLATE  XXXIX. 

Characteristic  specimens  of  iron  ores. 

A.  Hard  hematite  from  Marquette  district,  Michigan. 

B.  Specular  hematite  from  Marquette  district,  Michigan. 

C.  Ma.2:netite  from  western  Marquette  dis'rict.  Michisran. 

These  ores  form  7  per  cent  of  the  Lake  Superior  .shipments. 


U    S.   GEOLOGICAL  SURVEY 


MONOGRAPH    Lll         PLATE  XXXVIII 


'A^JJ^SSlA 


CHARACTERISTIC  SPECIMENS  OF  IRON  ORES. 


U.  S.  GEOLOGICAL  SURVEY 


MONOGRAPH   LI  I         PLATE  XXXIX 


CHARACTERISTIC   SPECIMENS  OF   IRON   ORES 


U.  S.  QEOLOQICAL  SURVEY 

6.0  5,5 


CI-  50 


24   26    30    35 


380 


360 


340 


320 


300 


280 


250 


240        220        200 
Pounds  per  cubic  foot 

Cubic  feet   per    long   ton  (2240  pounds) 
\S  17  IS  19         M    .     21     .     ?2 


r4  l'5  l'6  17  I'S  l'9 

Cubic  feet   per   short   ton  (2000  pounds) 


180 


160 


140 


120 


100 


80 


60 


DIAGRAM    SHOWING    RELATION    OF    DENSITY,    POROSITY,   AND    MOISTURE   TO  CUBIC    FEET    PER    TON. 

See  page  432. 


THE  IRON  ORES. 


481 


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'~~~~-^^^:^^  ■- -'^-: 

FiGUKE  72.— Textures  of  Lake  Superior  iron  ores  as  shown  by  screening  tests.  Biweekly  samples,  representing  43  grades  of  ore  and  an 
aggregate  of  22,376,723  long  tons,  were  taken  by  the  Oliver  Iron  Mining  Company  during  1939,  and  tests  were  made  on  the  average  year's 
sample.  The  results  of  mine  tests  are  averaged  for  each  district  in  proportion  to  the  tonnage  mined  to  give  the  figures  shown  on  the  diagram. 


CtTBIC   CONTENTS   OF  ORE. 
RANGE    AND    DETERMINATION. 


The  cubic  content  per  ton  ranges  from  7  cubic  feet  for  the  hard  ores  to  17  cubic  feet  for 
the  soft  ores.  It  depends  on  the  density,  the  jjore  space,  and  the  moisture  and  may  be  cal- 
culated directly  according  to  the  methotl  following. 


47517°— VOL  52—11- 


482  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

The  cubic  content  of  an  ore  is  a  direct  function  of  (a)  true  specific  gravity  of  the  material — 
that  is,  the  specific  gravity  unaffected  by  porosity  or  moisture;  (6)  porosity  of  the  material,  in 
terms  of  percentage  of  volume  occupied  by  pore  space  or  voids;  (c)  percentage  of  moisture  in 
the  material — that  is,  the  percentage  loss  in  weight  on  drying  at  110°  C. 

To  facilitate  the  determination  of  the  cubic  content  of  ores  the  diagram  or  graphic  equation 
shown  in  Plate  XL  was  devised,  expressing  the  relation  between  these  tliree  factors  and  the 
number  of  cubic  feet  per  ton.  Actual  determinations  in  the  ground  are  unsatisfactory  in  that 
they  do  not  show  the  individual  effects  of  the  three  factors  mentioned,  especially  moisture 
content,  which  may  vary  widely  at  different  times  and  jilaces.  By  use  of  the  diagram  the  three 
factors  are  considered  separately  and  their  individual,  relative,  and  net  effects  may  be  obser^'ed. 
The  use  of  the  diagram  is  not  confined  to  iron  ores  but  is  also  applicable  to  other  ore  or  mineral 
substance  in  the  ground. 

USE    OF    THE    DIAGRAM. 

The  operation  of  the  diagram  may  perhaps  be  made  clear  most  easily  by  applying  a  con- 
crete problem  as  an  illustration.  Given  an  ore  with  a  specific  gravity  of  4. ,5,  |)orosity  .30  per 
cent,  and  moisture  7  per  cent.  Select  a  point  on  the  upper  edge  of  the  diagram  indicating  the 
given  specific  gravity  (4.5) ;  from  this  point  move  downward,  as  indicated  by  the  dotted  line, 
to  the  line  representing  the  given  porosity.  (There  are  two  sets  of  inclined  lines  crossing  the 
upper  part  of  the  diagram;  the  less  steeply  inclined  set,  numbered  at  the  left  side  of  the  dia- 
gram, indicates  degree  of  porosity.)  From  this  point  move  upward  to  the  right  along  the  more 
steeply  inclined  lines  to  the  edge  of  the  diagram.  This  point  (3.1.5)  indicates  the  specific  gravity 
as  corrected  for  porosity.  From  this  point  move  directly  downward!  to  the  lower  edge  of  the 
diagram,  where  the  number  of  cubic  feet  per  ton  is  indicated.  This  shows  11.4  cubic  feet  per 
ton  of  dry  material.  The  factor  of  moisture  has  not  yet  been  considered.  Wlien  moisture  is 
present  tlie  material  is  heavier  and  consequently  the  volume  per  ton  smaller.  To  introduce  tliis 
factor  of  moisture,  move  directly  upward  from  the  last  point  (11.4)  to  the  horizontal  line  indi- 
cating the  given  percentage  of  moisture  (7),  and  from  tliis  point  down  the  inclined  Une  to  the 
lower  edge  of  the  diagram,  where  the  number  of  cubic  feet  per  long  ton  is  found  to  be  10.6. 

At  the  lower  edge  of  the  plate  is  a  transformation  table  sho^ving  the  relation  between  cubic 
feet  per  long  ton  (2,240  pounds)  and  cubic  feet  per  short  ton  (2,000  pounds).  For  example, 
10.2  cubic  feet  per  long  ton  is  equivalent  to  9.1  cubic  feet  per  short  ton. 

CONSTRUCTION    OF    THE    DIAGRAM. 

The  following  discussion  of  the  derivation  of  the  diagram  is  given  with  the  idea  that  one  desir- 
ing to  make  use  of  it  would  first  wish  to  be  assured  that  it  rests  on  a  rational  mathematical  basis. 

The  top  and  bottom  Imes  of  the  diagram  proper,  labeled  respectively  "Specific  gravity" 
and  "Cubic  feet  per  ton"  and  connected  by  parallel  vertical  lines,  constitute  a  transformation 
table  by  means  of  which  the  number  of  cubic  feet  per  ton  of  a  material  of  a  given  density  may 
be  at  once  determined  (or  \'ice  versa)  by  moving  vertically  between  the  upper  and  lower  edges 
of  the  diagram.  Immediately  below  the  edge  of  the  diagram  proper  is  a  scale  of  pounds  per 
cubic  foot,  wliich  may  be  used  by  moving  vertically  downward  from  any  point  on  the  ' '  specific 
gravity"  or  "cubic  feet  per  ton"  scales. 

Effect  of  porosity. — The  effect  of  porosity  is  to  decrease  the  density  of  a  substance,  hence 
rock  specific  gravity  is  less  than  mineral  specific  gravity  m  proportion  to  the  degree  of  porosity 
of  the  material  considered.  To  introduce  the  factor  of  porosity  in  the  diagram,  the  upper 
line  was  extended  to  the  right  to  the  point  indicating  a  specific  gra^^ty  of  zero  (not  shoMii  on 
the  diagi-am).  The  Une  at  the  left  edge  of  the  diagram  was  dra\ni  perpendicular  to  the  upper 
edge  and  divided  into  100  equal  divisions,  representing  percentages  of  pore  space.  Each  of  the 
points  of  the  vertical  "porosity"  fine  was  then  connected  with  the  point  mdicating  a  specific 
gravity  of  zero.  Hence  on  moving  vertically  downward  from  any  point  on  the  ■"specific  grav- 
ity" line,  a  succession  of  equally  spaced  lines  are  crossed  indicating  percentages  of  pore  space. 
To  enable  the  diagram  to  show  automatically  the  change  in  specific  gravity  resulting  from  a 
given  porosity  of  a  substance  of  known  mineral  specific  gravity,  a  set  of  parallel  lines  was  drawn, 
properly  connecting  pomts  on  the  "porosity"  and  "specific  gravity"  fines.     These  lines  were 


THE  IRON  ORES.  483 

drawn  parallel  to  the  line  connecting  100  per  cent  porosity  with  zero  specific  gra%dty  and  agree 
with  the  following  formula: 

G,  =  G^{\  -P) 

where  G^  =  rock  specific  gravity,  G^  =  mineral  specific  gravity,  and  P  =  porosity.  The 
diagram  then  automatically  shows  the  relation  between  mineral  specific  gravity,  porosity,  and 
cubic  feet  per  ton.  To  illustrate,  a  certain  ore  with  a  mineral  specific  gravity  of  5.0  has  40 
per  cent  of  pore  space.  Beginning  at  the  point  5.0  on  the  upper  edge  of  the  tliagram,  move 
downward  to  the  line  indicating  a  porosity  of  40  per  cent;  from  this  point  move  along  the 
parallel  inclined  lines  upward  to  the  right,  to  the  edge  of  the  diagram,  where  the  specific  gravity 
as  reduced  by  pore  space  (rock  specific  gravity)  is  found  to  be  .3.0;  immediately  below  this 
point,  on  the  lower  edge  of  the  diagram,  it  is  seen  that  the  ore  runs  11.95  cubic  feet  per  ton  and 
187.25  pounds  per  cubic  foot. 

Effect  ofmmsture. — The  diagram  so  far  takes  no  account  of  moisture  and  hence  is  applicable 
only  to  perfectly  dry  material.  Moisture  when  present  in  an  ore  or  similar  substance  occupies 
the  pore  space.  When  the  pore  space  is  filled  with  moisture  the  material  is  said  to  be  saturated. 
As  the  moisture  occupies  the  natural  openings  in  the  ore,  its  presence  affects  the  weight  of  the 
ore  and  not  its  volume,  hence  its  effect  is  to  increase  the  density  and  ilecrease  the  number  of 
cubic  feet  per  ton.     Moisture  is  expressed  in  percentage  of  total  weight. 

Let  D=  density  as  affected  by  porosity;  then,  as  a  cubic  foot  of  water  weighs  62.5  pounds, 

Cubic  feet  per  ton  =  jp~^^ 

When  moisture  (M)  is  present  the  above  equation  becomes — 

^  ,  .    ,    ,         ^         2,240  (l-M) 
C u Die  reet  per  ton  =      r>^   „„  - — 
.     r  x>X62.5 

The  lower  part  of  the  diagram  is  crossed  by  a  set  of  parallel  horizontal  lines  indicating  per- 
centages of  moisture,  as  showTi  at  the  right-hand  edge  of  the  diagram.  Follo^\'ing  the  above 
equation,  a  set  of  inclined  lines  were  drawn,  properly  connecting  points  on  the  "moisture"  and 
"cubic  feet  per  ton"  lines.  Given  the  numlser  of  cubic  feet  occupied  by  a  ton  of  any  porous 
material  when  dry,  the  effect  of  any  percentage  of  moisture  is  indicated  automatically  by  the 
diagram.  For  example,  a  certain  ore  when  dry  occupies  12  cubic  feet  per  ton;  it  is  desired  to 
know  the  effect  of  10  per  cent  of  moisture.  From  the  point  12  on  the  lower  edge  of  the  diagram 
move  vertically  upward  to  the  horizontal  line  indicating  10  per  cent  moisture;  from  this  point 
move  downward  along  the  inchned  line  to  the  edge  of  the  diagram,  where  it  is  found  that  the 
moist  material  occupies  10.8  cubic  feet  per  ton. 

Moisture  of  saturation. — Up  to  this  point  it  has  been  shown  that,  given  the  mineral  specific 
gravity,  porosity,  and  moisture  content  of  an  ore  or  similar  substance,  the  diagram  automatically 
indicates  the  number  of  cubic  feet  per  ton.  In  many  classes  of  ore  the  factor  of  moisture  is  the 
most  variable  of  the  three  named  above.  The  mineral  specific  gravity  and  porosity  of  an  ore 
determine  the  amount  of  moisture  which  it  can  hold.  This  maximum,  or  moisture  of  saturation, 
may  be  calculated  as  follows: 

6-'„j  =  mineral  specific  gravity. 

D    =  density  of  dry  porous  material. 

P    =  porosity. 

M  =  moisture  of  saturation. 

D   =GM-P). 

D  P 

from  which  P=l  —  rr-  and  M  = 


Substituting  the  value  above  given  for  P — 


M- 


1-^ 

Gm. 


484  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

By  substituting  values  for  D  and  G^  in  the  above  equation  tlie  curves  for  moisture  of  satura- 
tion were  constructed  across  tlie  lower  part  of  the  diagram.  Those,  curves  enable  one  to  determine 
at  once  the  moisture  of  saturation  of  any  material,  given  tlie  mineral  specific  gravity  and  porosity. 
Each  cui^ve  corresponds  to  a  certam  mmeral  specific  gravity,  and  the  moisture  of  saturation 
is  found  by  moving  vertically  from  the  point  indicating  the  number  of  cubic  feet  per  ton  of  the 
dry  material  to  the  proper  curve  for  moisture  of  saturation.  For  example,  an  ore  with  a  mineral 
specific  gravity  of  4.0  and  a  porosity  of  36.0  per  cent  occupies  14  cubic  feet  per  ton  if  dry;  its  mois- 
ture of  saturation  is  found  by  moving  upwanl  from  the  point  14  to  the  curve  (/  =  4.0,  and  reading 
the  indicated  moisture — 12.2  per  cent;  that  is,  12.2  per  cent  of  moisture  would  fill  the  pore 
space  of  tliis  ore. 

Excess  of  moisture  Tmndled  in  mining. — It  frequently  happens  in  mining  tliat  ore  as  hoisted 
to  the  surface  contams  a  larger  percentage  of  moisture  than  it  did  before  it  was  mined;  m  fact, 
it  may  contain  a  percentage  of  moisture  greater  than  the  moisture  of  saturation  of  the  unmined 
ore.  This  may  be  caused  by  the  handling  of  broken  ore  on  undrained  mine  floors.  The  ore 
after  being  broken  doAVTi  has  a  much  larger  percentage  of  voids  than  before  and  hence  a  greater 
ability  to  absorb  and  retain  moisture.  The  diagram  is  useful  in  this  connection  in  showing, 
from  determuiations  of  specific  gravity  and  original  porosity  of  hand  specimens,  the  moisture 
of  saturation  of  the  ore  in  place.  Tliis  figure  compared  with  tlie  percentage  of  moisture  of  ore 
as  it  leaves  the  mine  teUs  at  once  whether  or  not  an  unnecessary  amount  of  water  is  being  hoisted 
with  the  ore,  owing  to  improper  drainage. 

EXPLORATION    FOR    IRON    ORE. 

The  location  of  explorations  within  the  areas  of  the  iron-bearmg  formations  is  determined 
by  outcrops,  by  magnetic  Imes,  by  mining,  and  by  general  geologic  structure.  It  has  been 
possible  to  confine  most  of  the  exploration  to  the  area  of  the  iron-bearing  formations,  but  in 
certain  districts,  notably  the  Cuyuna,  Florence,  Crystal  Falls,  and  Iron  River  districts,  the 
distribution  and  limits  of  the  iron-bearing  formation  are  so  uncertain  that  much  exploratory 
work  has  had  to  be  done  even  to  locate  the  formation.  All  the  facts  bearing  on  the  distribution 
of  the  iron-bearing  formation  discussed  in  this  monograph  are  taken  into  account  in  choosing 
areas  for  exploration.  Some  of  the  larger  mining  companies  employ  their  own  geologists  to 
make  special  reports  on  the  geology  of  given  areas  as  a  preliminary  to  underground  explora- 
tion, and  nearly  all  the  explorers  make  liberal  use  of  all  the  geologic  information  available  in 
localizing  their  work. 

As  the  few  ore  deposits  exposed  at  the  surface  were  found  years  ago,  explorations  are 
now  largely  conducted  by  drilling  and  sinking  test  pits  and  shafts.  The  large  size  of  the 
iron-ore  deposits  makes  it  possible  to  find  and  outline  them  by  drilling  to  an  extent  not 
possible  in  smaller  ore  deposits,  with  the  result  that  the  greater  number  of  ore  bodies,  especially 
in  recent  years,  are  thorouglily  explored  by  drilling  before  mining  begins.  It  has  usually 
been  assumed  that  if  drilling  does  not  locate  an  ore  body  it  is  useless  to  sink  a  shaft  for  tliis 
purpose.  Mming  operations  have  necessarily  disclosed  much  ore  which  hail  not  previously 
been  found  by  drilling,  especially  in  certain  districts  like  the  Menominee  or  the  Gogebic,  where 
the  structural  conditions  are  such  as  to  make  the  location  of  ore  by  drillmg  extremely  dillicult. 
In  the  region  as  a  whole  mining  operations  have  almost  evervwhere  disclosed  greater  reserves 
of  ore  than  the  drilling  had  indicated. 

The  great  dependence  placed  on  drill  work  has  resulted  in  enormous  expenditure  for 
this  purpose.  Accurate  estimates  of  the  amount  of  drilling  done  so  far  in  the  region  can  not 
be  made,  but  a  rough  estimate  compiled  from  tentative  estimates  of  engineers  of  the  several 
districts  is  as  follows: 


THE  IRON  ORES. 

Drilling  done  for  iron  ore  in  the  Lake  Superior  region. 


485 


District. 


Number  of 
drill  holes. 


Average 

deptii  of 

drill  iioies 

(feet).o 


Mesabi 

Vermilion 

Cuyima 

Marquette 

Otber  Michigan  ranges  and  Wisconsin  ranges 


15,000 
1,000 
1,500 
5,000 
4,000 


175 

600 
250 
500 
300 


26,500 


1  Estimates  probably  low. 

This  totals  7,200,000  feet,  or  about  1,363  miles  of  drilling.  At  an  average  cost  of  $3  a 
foot,  which  is  a  low  estimate,  the  total  expenditure  has  been  roughly  $21,600,000. 

It  is  estimated  that  at  the  present  time  there  are  400  drills  in  operation  in  the  region.  In 
the  earlier  days  of  exploration  test  pits  were  relied  upon  to  a  large  extent,  especially  in  areas 
where  the  surface  drift  is  thin  and  the  water  level  below  the  rock  surface.  This  method  of 
exploration,  however,  is  unsatisfactory  because  of  the  great  depth  of  the  drift  at  many  places, 
the  difficulty  of  handling  water,  and  the  difliculty  after  finding  the  ledge  of  penetrating  it  by 
this  method.     In  later  years  the  use  of  test  pits  has  been  largely  superseded  by  drilling. 

Both  diamond  and  churn  drills  are  in  use.  Through  surface  and  soft-ore  formations  the 
churn  drill  is  used.  Much  of  the  Mesabi  district  may  be  so  explored.  The  cost  of  churn 
drilling  has  ranged  from  -fl  to  $3.50  and  averaged  about  $2.50  a  foot,  varying  from  district  to 
district  according  to  accessibility  and  cost  of  transportation  ai)d  other  factors.  The  cost  of 
diamond  drilling  has  ranged  from  $2.25  to  $8  a  foot  and  averages  at  present  about  $3.75,  but 
varies  from  district  to  district.     Test  pits  are  cheap,  averaging  perhaps  $1.25  a  foot. 

The  necessity  for  the  most  careful  study  of  the  structural  geology  in  drilling  is  illustrated 
by  the  frequent  failure  of  drills  to  locate  ore  deposits  even  after  what  seemed  to  be  careful 
drilling  and  the  subsequent  discovery  of  the  deposits  either  by  further  drilling  or  by  mining 
operations.  Indeed,  as  one  comes  to  realize  the  variety  and  complexity  of  underground 
structural  conditions,  he  is  likely  to  become  more  and  more  disinclined  to  submit  a  negative 
report  on  any  property,  no  matter  how  extensively  it  has  been  drilled.  This  difficulty  is 
illustrated  by  the  ore  shoots  in  the  Gogebic  and  Menominee  districts,  many  of  which  have 
been  missed  by  drilling  and  picked  up  in  mming  operations.  Many  of  the  ore  shoots  m  the 
Vulcan  member  of  the  upper  Huronian  slate  of  Michigan  pitch  beneath  the  surface,  following 
the  axes  of  drag  folds,  and  it  is  easy  for  drills  to  pass  one  side  or  the  other,  or,  if  the  drill 
hole  is  inclined,  to  go  above  or  below  them.  On  examination  of  drilling  plats  of  exploration 
areas  it  is  easy  to  see  where  linear  shoots  of  ore  might  pass  through  at  places  not  penetrated 
by  the  drilling.  In  fact,  drilling  in  some  of  these  localities  is  almost  as  uncertain  as  shooting 
a  bu'd  on  the  wing.  There  are  many  ways  of  missing  the  ore.  As  knowletlge  of  structural 
conditions  increases,  however,  adverse  chances  diminish,  with  the  result  that  in  certain  areas 
after  the  local  structural  problems  are  solved,  it  is  possible  to  drill  with  a  high  degree  of  success. 

A  higher  average  of  success  in  drillmg  would  unquestionably  result  if  greater  care  were 
taken  in  the  interpretation  of  drill  records.  The  drill  runner  is  often  allowed  to  report  the 
character  of  the  drillings  and  the  samples  are  not  kept,  with  the  result  that  many  valuable 
inferences  that  might  be  drawn  from  the  lithology,  the  dip  and  strike  of  beilding  and  cleavage, 
and  other  features  are  lost.  Not  infrequently  also  failure  to  plat  drill  records  in  such  a  maimer 
that  they  may  be  considered  in  three  dimensions  may  cause  promising  chances  for  ore  to  be 
overlooked. 

There  has  been  a  considerable  tendency  to  generalize  the  principles  of  ore  occurrence  and 
in  exploration  to  carry  such  principles  fi'om  one  district  to  another.  As  a  matter  of  fact,  although 
some  of  the  basic  principles  are  general  for  the  region,  the  local  variations  of  structure  require 
the  most  careful  study  of  each  area  to  prevent  mistakes  in  interpretation.  When  explorers 
of  the  Gogebic  district,  where  the  ores  lie  in  regular,  impervious,  pitching  basins,  went  to  the 


486  GEOLOGY  OF  THE  LAKE  SUPERIOR  JtEGIOX. 

Mesabi  district,  where  the  rocks  are  of  tlie  same  age,  tiiey  naturally  attempted  to  use  the  same 
methods  in  exploration.  But  here  the  flatter  dip  of  the  formation,  the  shallowness  of  basins, 
the  effect  of  overlying  slates  in  ponding  waters,  and  the  unusually  large  influence  of  joints  in 
localizing  the  concentration  of  ore  made  the  finding  of  ore  largely  a  new  j)rob]em,  which  was 
solved  at  much  expense  and  trouble.  Recognizing  the  danger  of  carrying  the  method  of  explo- 
ration of  one  district  into  another,  certain  explorers  have  gone  to  the  other  extreme  and  have 
attempted  to  disregard  all  guides  derived  from  the  study  of  tlie  structural  geology,  with  results 
even  more  unsatisfactory  than  if  they  had  used  principles  developed  for  other  districts. 

Much  the  greater  part  of  the  exploration  of  the  region  has  been  conducted  without  taking 
the  fullest  advantage  of  all  geologic  knowledge  available,  but  there  has  been  a  rapidly  increasing 
tendency  to  follow  geologic  structure  and  therefore  an  increasing  demand  for  geologic  informa- 
tion, as  shown  by  the  cordial  support  that  the  mining  men  have  given  to  the  efforts  of  the 
United  States  and  State  surveys  in  this  region  and  by  their  considerable  expenditures  for  private 
geologic  surveys.  Certain  of  the  drilling  companies  doing  contract  work  now  have  geologists 
on  their  staff  to  aid  in  the  interpretation  of  records,  notwithstanding  the  fact  that  such  inter- 
pretation is  primarily  in  the  hands  of  their  clients.  The  problems  of  underground  exploration 
are  followed  keenly,  intelligently,  and  energetically  by  a  large  number  of  skilled  men  in  the 
employ  of  mining  companies,  with  the  result  that  advances  are  being  made  ^^^th  a  rapiditj' 
which  is  sometimes  almost  bewildering.  Six  months  may  see  the  development  of  new  facts 
requiring  changes  in  the  interpretation  of  the  drilling  of  a  district.  The  statements  as  to  struc- 
tural conditions  ]>resented  in  another  chapter  of  this  book  may  require  some  modification  b}' 
the  time  the  book  is  given  to  the  public,  because  of  the  amount  of  rapidly  accumulating 
information  in  the  interval  between  the  writing  and  the  printing. 

MAGNETISM   OF  THE    LAKE    SUPERIOR    IRON   ORES   AND  IRON-BEARING 

FORMATIQNS. 

All  ores  of  iron  are  found  to  be  magnetic  when  tested  by  sufficientlj^  delicate  means.  Ordi- 
narily magnetite  is  the  only  iron  mineral  which  causes  conspicuous  disturbance  of  the  magnetic 
needle.  Practically  all  the  Lake  Superior  iron-bearing  formations  contain  at  least  minute 
quantities  of  magnetite,  and  hence  all  exert  an  influence  on  the  magnetic  needle,  but  in  ^\-idely 
varying  degree.  The  iron-bearing  formation  of  the  Vermilion  district  and  other  Keewatin 
areas  is  strongly  magnetic.  The  same  is  true  of  the  formation  in  the  east  end  of  the  Mesabi 
district,  the  Gunflint  district,  the  Cuyuna  district,  and  the  east  and  west  ends  of  the  Gogebic 
district,  and  of  most  of  the  Negaunee  formation  of  the  Marquette  district.  Less  magnetic 
parts  of  the  iron-bearing  fonnations  are  those  producing  principally  hematite  and  limonite, 
as  the  central  and  western  parts  of  the  Mesabi,  the  central  part  of  tha  Gogebic,  and  parts  of 
the  Menominee  and  Crystal  Falls  districts.  The  iron-bearing  member  of  the  Iron  River  district 
of  Michigan  affects  the  magnetic  needle  onl}'  'jcaUy  and  slightly. 

Every  known  iron-bearing  formation  '  i  the  Lake  Superior  region,  Anth  the  exception 
of  that  in  part  of  the  extreme  west  end  o''  the  Mesabi  district,  has  been  outlined  partly  as  a 
result  of  magnetic  surveys.  In  some  of  t/ie  districts,  as,  for  instance,  the  Iron  River  district, 
the  magnetic  variation  is  slight,  but  careful  observations  will  detect  it.  In  addition  several 
magnetic  belts  are  known  in  wliich  exploration  has  not  yet  showTi  the  character  of  the  iron- 
bearing  formation.  On  the  general  map  (PI.  I,  in  pocket)  magnetic  belts  are  not  indicated 
over  all  of  the  iron-bearing  formations.  They  are  showTi  only  in  places  where  the  formation 
is  not  naturally  exjiosed  or  uncovered  by  exploration. 

Strong  magnetic  disturbance  does  not  necessarily  mean  ore,  and,  vice  versa,  ore  does  not 
necessai'ih'  cause  strong  magnetic  disturbance.  Lean  amphibolitic  schists  may  be  highly 
magnetic,  while  rich  hydrated  soft  ore  has  but  little  effect  on  the  needle.  Although  magnetic 
disturbance  is  usually  caused  by  an  iron-bearing  formation,  it  is  also  caused  by  certain  basic 
igneous  rocks,  like  the  ellijjsoidal  basalts  of  tlie  Keewatin  or  gabbro  intrusives.     There  is  Uttle 


THE  IRON  ORES.  487 

difficulty  in  ascertaining  the  cause  of  tlie  attractions,  however,  for  somewhere  along  most  of  the 
magnetic  belts  in  the  Lake  Superior  region  there  are  outcrops  which  indicate  the  nature  of  the 
rock  causing  the  disturbance.  If  the  rock  is  entirely  covered,  it  may  still  be  possible  to  deter- 
mine whether  the  disturbance  means  iron-bearing  formation  or  some  other  rocks.  The  iron- 
bearing  formations  are  sedimentary  deposits  with  certain  linear  characteristics  of  distribution, 
giving  even  lines  or  "belts'"  of  magnetic  attraction,  whereas  the  basic  igneous  rocks  are  likely 
to  cause  a  much  more  irregular  magnetic  field. 

Because  of  the  conditions  above  outlined,  it  is  seldom  practicable  in  the  Lake  Superior 
region  to  draw  from  magnetic  observations  inferences  with  regard  to  the  shapes  of  the  iron- 
ore  deposits  themselves  as  distinguished  fi-om  the  rest  of  the  iron-bearing  formation — such 
inferences  as  have  been  drawn  by  magnetic  surveys  of  deposits  in  eastern  Canada,  Sweden, 
and  elsewhere.  In  those  regions  the  ores  consist  of  magnetite  associated  with  relatively  non- 
magnetic wall  rocks,  and  the  magnetic  disturbances  are  produced  by  the  iron  ore  itself,  not  by 
u-on  ore  and  wall  rock;  hence  it  is  possible  to  draw  satisfactory  inferences  as  to  the  shape  and 
attitude  of  the  iron-ore  deposits.  In  the  Lake  Superior  region  the  magnetic  attractions  are 
useful  in  locating  iron-bearing  formations  and  thus  ultimately  the  iron  ore  by  underground 
exploration,  but  do  not  directly  point  out  the  iron-ore  deposits  themselves.  The  highly-  devel- 
oped Swedish  methods  of  determining  both  the  intensity  and  the  direction  of  the  magnetic  pull 
are  therefore  unnecessarily  detailed  and  slow  for  use  in  the  Lake  Superior  region,  and  when 
attempts  have  been  made  to  locate  ore  deposits  by  them  the  results  have  been  disappointing 

Although  the  iron  ores  may  not  be  discriminated  by  means  of  the  magnetic  disturbances, 
it  is  possible  under  some  conditions  to  draw  useful  inferences  frona  them  as  to  the  dip  or  folding 
of  a  buried  iron-bearing  formation.  A  sharp,  narrow  belt  of  magnetic  attraction  leading  up 
to  a  definite  maximum  usually  means  a  liighly  tilted  formation  presenting  a  narrow  erosion 
edge  at  the  rock  surface,  as  in  the  Gogebic  or  Vermilion  district.  A  wide,  more  irregular, 
and  less  well  defined  belt  of  attraction  is  ordinarily  associated  with  a  flatter  dip,  exposing  a 
greater  area  of  iron-bearing  formation  to  the  erosion  surface.  The  producing  part  of  the  iron- 
bearing  Biwabik  formation  of  the  Mesabi  district  illustrates  tliis.  Unequal  magnetic  gradient 
on  two  sides  of  a  maximum  may  indicate  the  direction  of  dip  of  the  iron-bearing  beds.  The 
outward  dip  of  the  iron-bearing  formation  about  the  Archean  ovals  of  the  Crystal  Falls  district 
is  so  indicated.  Several  roughly  parallel,  more  or  less  discontinuous  magnetic  belts,  here  and 
there  converging  and  joining,  may  indicate  repeated  pitcliing  folds,  as  in  the  Cuyuna  district. 

General  laws  of  interpretation  of  magnetic  attraction  require  much  local  mochfication. 
It  is  usually  riecessary  to  ascertain  for  each  locality  the  magnetic  character  of  the  iron-bearing 
formation,  to  correlate  tliis  with  known  facts  fi'om  outcrops  or  underground  workings,  and 
from  the  knowledge  thus  obtained  to  interpret  the  results  of  the  magnetic  formations  in 
covered  parts  of  the  area  where  the  magnetic  reachngs  alone  are  available.  H.  L.  Smyth,"  in 
connection  with  much  magnetic  field  work  in  the  Lake  Superior  region,  has  developed  mathe- 
matical relations  between  magnetic  fields  and  various  attitudes  of  the  rock  beds  which  may 
serve  as  a  useful  guide  in  detailed  surveys. 

The  instruments  wliich  have  been  used  in  Lake  Superior  magnetic  surveys  are  the  dip 
needle  and  the  dial  compass.  The  dip  needle  determines  the  vertical  component  of  the  mag- 
netic pull,  as  well  as  the  direction  of  the  horizontal  pull;  the  dial  compass  determines  only 
the  direction  of  the  horizontal  pull.  Methods  of  using  and  interpreting  these  instruments  are 
discussed  in  detail  by  Smyth.  The  dial  compass  is  essential  in  most  of  the  work  because  it 
affords  means  of  keeping  accurate  directions  necessary-for  location  and  of  reading  the  horizontal 
component  of  the  magnetic  variation.  It  may  be  used  only  on  sunny  days,  and  thus  mag- 
netic work  in  the  Lake  Superior  region  is  likely  to  be  slow  and  expensive.  The  dip  needle 
may  be  used  at  any  time,  but  in  a  disturbed  field  it  affords  no  means  of  keeping  horizontal 
directions,  and  hence  location.  This  is  an  essential  defect  in  a  country  in  wliich  the  roads 
and  other  works  of  man  afford  little  aid  in  keeping  location. 

In  theory  the  use  of  the  magnetic  needle  is  simple,  but  much  practice  is  required  to  insure 
uniformly  accurate  observations.     The  unskilled  observer  finds  many  pitfalls  in  the  mechanism 

cMon.  U.  S.  Geol.  Survey,  vol.  36.  1899,  pp.  335-373. 


488  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

of  the  instrument,  in  the  manner  of  holding  it,  in  the  effects  of  temperature,  in  electrification 
from  rubbing  the  glass,  etc.  There  is  much  opportunity  for  the  exercise  of  good  judgment  in 
the  determination  of  the  intervals  at  wliich  reachngs  shall  be  taken,  the  direction  and  number 
of  runs,  etc.  These  should  be  varied  for  different  areas,  depending  on  the  structure  found  or 
suspected.  Finally,  the  interpretation  of  the  results  calls  for  consideration  and  careful  bal- 
ancing of  a  great  variety  of^  factors,  capacity  for  wliich  is  acquired  only  by  wide  experience 
and  painstaking  observation. 

MANGANIFEROUS  IRON  ORES. 

All  the  Lake  Superior  iron  ores  contain  minute  quantities  of  manganese,  and  certain  ores 
carry  as  high  as  20  to  25  per  cent.  In  the  Cuyuna  district  of  Minnesota  a  drill  hole  in  the 
iron-bearing  member  averages  13  per  cent  for  the  upper  .35  feet  and  about  2  per  cent  below. 
Another  hole,  in  sec.  28,  T.  47  N.,  R.  29  W.,  has  an  average  of  11.33  per  cent  for  the  upper  30 
feet.     Similar  results  have  been  obtained  from  drilling  in  the  Baraboo  district. 

The  larger  part  of  the  manganiferous  ores  shipped  so  far  have  come  from  the  Gogebic 
district.  Manganiferous  ores  are  often  not  discriminated  from  the  iron  ores  in  figures  of 
slupment,  and  tliis  makes  it  difficult  to  estimate  the  tonnage  of  manganese  iron  ore  and  the 
average  percentage  of  manganese  in  so-called  manganiferous  iron  ores.  E.  C.  Eckel °  estimates 
that  during  1906  the  Lake  Superior  region  produced  about  1,000,000  long  tons  of  low-manganese 
iron  ore  with  an  average  manganese  content  of  about  4  per  cent  and  ranging  as  sliipped  from 
1  to  8  per  cent.  According  to  Burchard,*  the  total  production  of  manganiferous  iron  ore  in 
the  Lake  Superior  region  from  1885  to  1909,  inclusive,  has  been  8,968,449  long  tons,  or  about 
77  per  cent  of  the  total  production  for  the  United  States  during  that  period. 

The  percentage  of  manganese  in  the  manganiferous  ores  of  the  Lake  Superior  region  is 
so  low  that  the  ore  may  not  be  classed  either  as  a  manganese  or  a  liiglily  manganiferous  iron 
ore  hke  those  of  Arkansas  and  Colorado.  It  produces  a  basic  pig.  None  of  the  ore  shipped 
from  the  Lake  Superior  region  has  been  liigh  enough  in  manganese  to  be  available  for  ferro- 
manganese  or  spiegeleisen,  which  require  at  least  15  per  cent  of  manganese. 

Mineralogically  the  manganese  is  mainly  in  the  form  of  pyrolusite  (MnOj).  In  the  Cuyuna 
district  this  has  been  found  at  the  surface  to  be  mixed  with  rhodochrosite  (MnCOj).  The 
psilomelane  so  commonly  associated  with  pyrolusite  in  the  Appalachian  manganese  ores  has 
not  been  especially  looked  for  in  the  Lake  Superior  region  but  is  probably  present. 

The  conspicuous  association  of  manganese  with  the  upper  parts  of  the  iron-ore  deposits 
seems  to  prevail  in  the  Lake  Superior  region,  as  in  deposits  of  manganiferous  iron  ore  in  other 
parts  of  the  United  States. 

IRON-ORE    RESERVES. 

DATA  AVAILABLE  FOB  ESTIMATES. 

Up  to  1910,  335  mines  have  been  in  operation  in  the  Lake  Superior  region,  and  many  thou- 
sands of  test  pits  and  churn  and  diamond  drill  holes  have  been  sunk.  The  mines  and  explora- 
tions, together  with  natural  exposures,  afl'ord  data  for  a  fair  estimate  of  ore  reserves  in  the 
producing  areas.     There  are  considerable  areas  not  yet  explored. 

AVAILABILITY  OF  OKES. 

Evitlently  the  question  of  the  present  and  future  availability  of  the  iron  ores  is  one  of  costs — 
in  mining,  in  transportation,  and  in  the  furnace.     The  costs  are  determined — 

(1 )  By  the  character  of  the  ore  itself,  its  percentage  of  iron  and  deleterious  constituents,  and 
the  nature  of  its  principal  ganguc  material. 

(2)  By  the  cost  of  mining,  whether,  for  instance,  by  open  pit  or  underground  mothoil. 

(3)  By  whether  or  not  the  ore  must  be  concentrated,  as,  for  instance,  the  sandy  taconites 
of  the  western  Mesabi. 

o Mineral  Resources  U.  S.  for  1906,  U.  S.  Geol.  Survey,  1907,  p.  106. 

SBurcbard,  E.  F.,  The  production  of  manganese  ore  In  1909:  Extract  from  Mineral  Resources  U.  S.  for  1909.  U.  S.  Geol.  Survey,  1911,  p.  10. 


THE  IRON  ORES.  489 

(4)  By  the  cost  of  transportation  to  the  furnace.  Between  Vermihon  and  Marquette  ores 
there  is  a  difference  of  about  75  cents  a  ton  in  the  cost  of  transportation  to  the  lower  lakes. 
Viewed  in  another  way,  the  cost  of  transportation  is  the  amount  necessary  to  bring  together 
the  coal,  limestone,  and  iron  and  to  transport  the  finished  product  to  consuming  centers. 
This  introduces  another  set  of  costs  for  ores  smelted  at  the  upper  lakes. 

(5)  By  the  cost  of  reduction  in  the  furnace,  depending  on  the  character  of  the  ore  and  on 
the  success  in  modifying  and  applying  furnace  practice  to  local  conditions.  For  instance,  the 
use  of  by-products  from  coke  in  certain  furnaces  in  the  Lake  Superior  region  makes  approxi- 
mately the  difference  between  profit  and  loss  for  the  combination  of  conditions  there  existing. 

(6)  By  the  nature  of  the  ownersliip.  A  large  corporation  holding  a  variety  of  ores  and 
equipped  to  assemble  the  raw  material  under  the  existing  conditions  can  handle  ore  which 
would  not  be  available  to  a  smaller  company  not  equipped  to  control  the  situation  in  a  large  way. 

In  recent  years  the  average  percentage  of  iron  in  the  ore  shipped  has  varied  between  60 
and  54  per  cent  for  the  ore  in  the  natural  state  (see  pp.  477,  493),  the  grade  on  the  whole  low- 
ering. These  grades  may  be  regarded  as  approximately  the  lowest  average  grades  available  under 
the  conditions  prevaihug  in  those  years.  Low-grade,  high-sihca  ores,  running  as  low  as  40  per  cent 
in  iron,  favorably  located  for  cheap  mining  and  transportation,  have  been  used  to  a  small 
extent  for  mixtures,  as,  for  instance,  ores  in  the  Palmer  area  of  the  Marquette  district  and  in 
the  Menominee  district.  In  most  of  the  region  at  the  present  time  ore  running  50  per  cent 
(natural)  in  metaUic  iron  is  considered  of  about  as  low  grade  as  is  at  present  available,  and 
estimates  are  made  accordingly.  Locally  ores  of  lower  grade  are  included  as  available  ores, 
either  because  of  favorable  conditions  of  niirdng  and  transportation,  because  of  differences  in 
the  policy  of  the  companies  making  the  estimates,  or  because  they  may  be  concentrated  by 
washing,  as  in  the  western  Mesabi. 

The  table  of  production  (see  pp.  49-69)  shows  what  has  been  the  relative  availabihty  of 
ores  of  the  different  districts,  all  factors  considered. 

BE  SERVES  OF   ORE   AT  PRESENT   AVAILABLE. 

ESTIMATES. 

•  The  authors  have  made  no  independent  detailed  estimates  of  Lake  Superior  iron-ore 
reserves  for  this  monograph.  "They  have,  however,  had  access  to  the  detailed  estimates  of 
the  principal  mining  companies  and  to  the  records  of  the  Mnnesota  Tax  Commission  and  are 
from  their  field  study  famihar  with  most  of  the  large  deposits  or  groups  of  deposits.  The 
estimates  here  given  represent  their  judgment  as  to  the  approximate  tonnage  of  ore  now  avail- 
able, based  on  the  above  information.  The  variations  in  the  independent  estimates  of  mining 
companies  and  the  difference  of  opinion  as  to  how  low  a  grade  of  ore  in  any  given  place  is  to 
be  included  in  the  available  ores  give  latitude  for  considerable  variations  in  estimates.  The 
authors  can  claim  no  finality  for  the  figures  published.  They  are  what  seem  to  them  reason- 
able approximations. 

Estimates  of  the  available  pre-Cambrian  iron  ore  of  the  Lake  Superior  region. 

Long  tons. 

Marquette  district 100,  000, 000 

Gogebic  district 60,  000,  000 

Menominee  and  Crystal  Falls  districts 75, 000, 000 

Mesabi  district 1,  600, 000, 000 

Vermilion  district 30,  000,  000 

Cuyuna  district 40, 000, 000 

1,  905, 000, 000 

The  reserve  reported  includes  about  1.30,000,000  tons  of  washable  ores  from  the  western 
Mesabi,  averaging  46  per  cent  of  iron  (dry)  of  non-Bessemer  character.  Of  the  remainder  of 
Mesabi  ores,  approximately  40  per  cent  are  Bessemer. 

There  is  a  further  low-grade  reserve  in  the  CUnton  ores  of  Wisconsin  which  may  be  of  con- 
siderable magnitude.     (See  pp.  566-567.) 


490 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


LIFE    OF   ORE    RESERVES    AT    PRESENT    AVAILABLE. 

Figure  73,  prepared  hy  II.  M.  Roberts,  shows  the  total  production  of  ore  from  the  Lake 
Superior  region  for  30  years  before  1907  and  the  rate  of  increase  of  production.  To  the  close  of 
1910  20.5  per  cent  oi'  the  known  reserves  had  been  consumed.     If  the  above  ostiniates  of 


100 


90 


80 


70 


60 


to 

Id 

Z 
Id 
U 


50 


40 


30 


20 


10 


/ 
/ 
/ 

« 

/ 
/ 

1 

/ 
/ 

/ 
/ 
/ 

/ 

/ 

/ 

' 

i 

/ 

/ 
/ 

/ 

/ 
/ 
/ 

/ 
/ 

/ 

/ 
/ 

/ 

/ 

/ 

/ 
/ 
/ 

/ 

/ 

/ 

1650 


I860 


1870 


1860 


1890 


1900  1910 

YEARS 


I9Z0 


1930 


1940 


1950 


I960 


FlGtntE  73.— Diagram  showing  relation  between  estimated  ore  reserves  of  the  Lake  Superior  region  and  rate  of  production.  The  estimated  reserve, 
1,905,000,000  tons,  plus  the  total  amount  of  ore  miued  to  the  end  of  1910,  is  represented  as  100  per  cent  on  the  vertical  line.  For  each  year 
there  is  shown  the  percentage  of  this  total  which  had  been  removed  to  the  end  of  that  year.  For  example.  15.9  per  cent  of  the  kno«-n  ore  was 
removed  to  the  close  of  1907.  For  the  last  five  years,  1905  to  1910,  the  curve  is  practically  a  straight  line.  If  this  line  is  projected  at  a 
uniform  slope,  it  indicates  complete  exhaustion  of  the  known  reserves  in  1960.  Reasons  are  given  in  the  text,  however,  for  the  belief  that 
the  date  of  exhaustion  will  be  later. 

reserves  at  present  available  are  even  approximately  correct  and  the  rate  of  jiroduction  remains 
the  same  as  that  in  1910,  the  hfe  of  the  ore  deposits  as  now  estimated  will  be  45  years— that 
is,  to  1956.  If  the  rate  of  production  increases  in  the  future  this  time  will  obviously  be 
shorter.  As  some  increase  in  the  rate  of  pi-oduction  seems  Hkely  in  spite  of  the  ])robable 
temporary  recessions  due  to  business  dei)ressions,  if  onty  liigh-grade  ores  are  mined  the  exhaus- 
tion of  the  existing  deposits,  or,  if  not  these,  of  the  amount  of  high-grade  ore  equivalent  to 
that  now  in  sight,  will  probably  occur  earlier  than  this  date.  But  even  this  conclusion  must 
be  modified  by  the  fact  that  In  proportion  as  the  inadequate  supply  of  liigh-grade  ores  becomes 


THE  IRON  ORES. 


491 


depleted  there  wall  be  an  increased  use  of  lower-grade  ores  with  the  high-grade  material,  whose 
life  will  be  thereby  prolonged.  This  factor  is  regarded  as  so  important  as  to  rendoi-  it  probable 
that  the  use  of  the  high-grade  ores  will  be  distributetl  through  a  much  longer  period  than  45 
years,  just  as  there  will  be  first-growth  white  pine  remaining  uncut  long  after  the  date  when 
all  the  white  pine  would  be  gone  at  the  present  rate  of  use.  Also  new  discoveries  of  ore  of  jires- 
ent  commercial  grade  are  made  yearly.  Prior  to  1911  the  discoveries  have  kept  well  ahead  of 
the  sliipments.  The  region  is  now  so  well  known  that  there  is  httle  likelihood  of  discovering 
another  Mesabi  range.  Though  it  is  not  impossible  that  in  the  next  few  years  the  reserves 
may  be  sufficiently  increased  by  discovery  to  keep  pace  with  the  sliipments,  this  is  rather 
unhkely.  Still  less  Ukely  is  it  that  the  increase  of  reserves  will  keep  pace  with  an  acceleration 
of  production.  If,  for  instance,  the  increase  of  production  for  a  year  amounts  to  2,000,000 
tons,  and  it  is  estimated  that  the  present  reserves  will  last  20  years  at  the  lower  rate,  it  will  be 
necessary  in  that  year  of  increase  to  discover  40,000,000  tons  of  ore  in  order  that  the  life  of  the 
reserves  may  not  be  lessened. 

RESERVES  AVAILABLE  FOR  THE  FUTURE. 


ESTIMATES. 

Reserves  available  for  the  future  must  be  considered  as  having  a  present  small  and 
intangible  value,  for  the  reason  that  the  estimates  of  ores  at  present  available  include  all 
ores  wliich  can  be  immediately  mined  or  wliich  will  be  taken  out  in  the  normal  course  of 
development  of  present  mines.  When  we  remember  that  iron  is  one  of  the  most  widely 
disseminated  metals  of  the  earth's  crust  (by  actual  analysis  constituting  4  per  cent  of  all  the 
rocks  of  the  earth),  it  is  apparent  that  only  the  most  arbitrary  limits  can  be  placed  on  future 
reserves.  In  the  following  estimates  of  future  reserves  are  included  rocks  containing  a  per- 
centage of  iron  lower  than  the  percentage  in  the  reserves  at  present  available  but  sufficiently 
liigher  than  that  in  the  common  rocks  of  the  earth's  crust  to  give  them  future  priority  in  use  as 
iron  ores  over  the  average  rocks  of  the  earth's  crust.  It  will  probably  be  many  hundreds  of 
years  before  any  but  an  insignificant  portion  of  these  reserves  available  for  the  future  are 
utihzed.  The  additional  discovery  of  liigh-grade  ores — as,  for  instance,  those  of  the  great  field 
in  Brazil — the  enormous  quantities  of  low-grade  ores  now  available  from  Alabama  and  Cuba, 
the  extension  of  the  known  high-grade  reserves  of  Lake  Superior,  and  the  increased  use  of 
scrap  iron  and  steel  will  postpone  the  use  of  the  bulk  of  the  low-grade  Lake  Superior  reserves 
available  for  the  future.  On  the  other  hand,  the  diminution  in  supply  of  the  reserves  at 
present  available  will  lead  gradually,  and  probably  in  the  not  far  distant  future,  to  the  drawing 
on  minor  amounts  of  these  future  reserves  for  mixtures. 

It  is  to  be  remembered  that  the  available  ores  are  associated  with  iron-bearing  formations, 
which  differ  from  the  ore  mainly  in  having  more  silica  and  which  show  all  gradations  to  the 
ore.  The  character  of  these  formations,  so  far  as  iron  is  concerned,  is  best  shown  by  the  fol- 
lowing table  of  analyses  from  drill  sections  compiled  by  the  Oliver  Iron  Mining  Company: 

Character  of  iron-bearing  formations  in  Lake  Superior  region  (not  including  available  ore). 

Diamond-drill  Averages. 


Range. 

Number  of 
holes. 

Total 
number 
of  feet. 

Number  of 
analyses. 

Average 

percentage 

of  iron. 

Gogebic 

15 
30 
32 
30 
24 

5,890 
4,814 
11,025 
5,287 
5,400 

490 
1,517 
1,726 
1,681 
1,094 

36  65 

Baraboo .                

36.40 

35.12 

Menominee , .                    

37.  93 

Mesabi 

3S.  00 

Other  Sources. 

Marquette 

Trenches 

Levels 

975 

94 
905 

41.53 

Menominee. 

Zfi  40 

492 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


These  analyses  include  both  the  lean  and  the  partly  concentrated  parts  of  the  iron-bearing 
formations,  but  do  not  include  the  available  ore.  If  tlic  partly  concentrated  parts  of  the  forma- 
tion are  left  out  of  consideration,  the  average  would  be  2.5  per  cent  of  iron. 

In  the  following  table  column  4  contains  a  rough  estimate  of  the  tonnage  of  all  iron- 
bearing  formations  outside  of  "available"  ore  to  a  (U-pth  of  1,2.50  feet  for  the  steeply  dipping 
formations  and  to  a  depth  of  400  feet  for  the  Mesahi  district,  where  the  thickness  ranges  from 
a  knife-edge  to  900  feet.  Column  5  contains  a  rough  estimate  of  the  tonnage  of  the  part  of 
the  iron-bearing  formations  which  will  run  above  35  per  cent  in  iron. 

Total  tonnage  of  iron-bearing  formations  to  given  depths  and  tonnage  estimated  to  run  35  per  cent  or  mare  in  iron. 


District. 

(1) 

.\roa. 

(2) 
Depth. 

(3) 
Volume. 

(4) 

Quantity  of  iron 
formation. 

(5) 

Quantity  con- 
tainini;  35  per  cent 
or  more  of  iron. 

Michifran; 

Crystal  Falls  

Sq.  mi. 

7.8 
28.5 
5.6 
5.8 
1.0 

127.0 
15.6 

.7 
5.S 
11.0 

10.0 
6.6 
30.0 

Feet. 
1,250 
1,250 
1,250 
1,250 
1,000 

400 
1,250 

1,250 

1,250 

3.i0 

100 
1.250 
1,250 

Cu.  mi. 
1.85 
&75 
1.30 
1.40 
.20 

9.M 
3.70 

.16 
1.40 
.70 

.19 
1.57 
7.10 

Tons. 

24.100.000.000 
sr.SIXI.UIM.l.lMMJ 
16.  !«KI.  I«K1.I«)0 
IS,  200.  QUI  1.01)0 
2,600,000,000 

125, 000. 000. 000 
4,S,  100, 000, 000 

2,1.50.000,000 
IS,  200. 000.  (KK) 
9,100,000,000 

2.500,000.000 
20.400.000.000 
92,400,000.000 

Tons. 

I,.5or).ono.nno 

10,000.(Ml<l.ll(IO 

Menominee 

3,.50(>.  (HKI.OIIO 

l,2.'i0.l-K10.l»« 

Swanzy                  

260,  iKlO.noO 

Minnesota: 

Mesabi                

30,000,flf)i).OOO 

1,023,000,000 

Wisconsin: 

215,  OM.  000 

Penokee 

1, 250,  (KH  1.000 

910,000.000 

Ontario: 

250.000.000 

2,040.000.000 

North  shore  of  Lake  Superior 

9,240,000,000 

200.000,000 

255.  40 

35  92 

467,4.50,000,000 

67,640,000,000 

We  may  conclude,  therefore,  that  while  the  ores  at  present  available  would  probabl_y  be 
exhausted  within  about  50  j-ears  if  they  alone  were  drawn  from,  the  increasing  use  of  lower- 
grade  ores,  already  begun,  will  lengthen  this  jjeriod  many  times. 


COMPARISON  OF  LAKE  SUPERIOR  RESERVES  WITH  OTHER  RESERVES  OF  THE  UNITED  STATES. 

For  comparison  a  table  showing  ores  available  at  present  and  in  the  future  in  different 
parts  of  the  United  States  is  given  below.  The  figures,  with  the  exception  of  those  for  Lake 
Superior,  are  those  of  the  National  Conservation  Commission." 

Iron-ore  reserves  of  the  United  States  available  at  present  and  in  the  future. 


Commercial  district. 

Ore  at  present 
available. 

Ore  available 
in  the  future. 

Long  tons. 

•29S.000.000 

53S.  440. 000 

1,905.000.000 

315,000,000 

57.  760, 000 

68,950,000 

Long  Ions. 
1 .  095. 000. 000 

2.  Souttieastern 

1  276  5'X)  000 

67, 640,  tmO.  (XX) 

4.  Mi.ssis.sippi  Valley 

570  000  (XX) 

120. 665.  (XX) 

6.  Pacific  slope - 

23,905,000 

3,183,150,000 

70,726,070,000 

1.  Vermont.  Massachusetts,  Coimecticut,  New  York,  New  Jersey.  Pennsylvania.  Maryland,  Ohio. 

2.  VirEinia.  West  VirRinia.  eastern  Kentucky,  North  Carolina,  South  Carolina,  Georgia,  .Vlabama,  eastern  Tcimessec. 

3.  Micnijian.  Minnesota,  Wisconsin. 

4.  Northwest  .\labama,  western  Tennessee,  western  Kentucky.  Iowa,  Missouri.  .Vrkansas.  eastern  Texas. 

5.  Montana,  Idaho,  Wyoining.  Colorado,  Utah,  Nevada,  New  Mexico,  western  Te.xas,  .\rizona. 
C.  Washington,  Oregon,  California. 

It  appears  from  this  table  that  the  Lake  Superior  region  contains  approximately  60  per 
cent  of  the  reserves  at  present  available  and  96  per  cent  of  the  future  reserves,  as  figured 
in  tons.  If  measured  in  units  of  iron,  the  Lake  Superior  reserves  form  a  still  larger  proportion 
of  the  total. 

oBull.  U.  S.  Geol.  Survey  No.  394.  1909,  p.  103. 


THE  IRON  ORES. 


493 


LOWERING    OF    GRADE    NOW    DISCERNIBLE. 

Lower  and  lower  grades  of  ore  are  being  included  in  successive  estimates  of  available  ores. 
A  comparison  of  the  iron-ore  tonnage  of  the  United  States  with  the  production  of  pig  iron  for 
the  last  20  years  shows  a  distinct  increase  in  the  number  of  tons  of  iron  required  to  make  a 
ton  of  pig  iron,  and  thus  a  lowering  of  the  grade  of  iron  ore  mined.  Figure  74,  prepared  by 
H.  M.  Roberts,  compares  the  pig  iron  and  tons  of  ore  used  and  shows  an  average  annual  drop 
in  grade  of  tiie  ores  for  the  last  20  years  of  0.3.5  per  cent  in  iron.     Each  temporary  increase  of 


100 


90 


80 


70 


60 
10 

o 

Sso 

o 

a: 

LJ 


40 


30 


20 


10 


^'^ 

l'' 

''.     / 

V 

\ 

\ 

i 

-I 

1 
1 

r-A       /     . 

~~~1' 

w^ 

V 

V 

A 

V 

■ — 

1886 


1890 


IS94 


1898 


1902 


1906 
YEARS 


1910 


1914 


1916 


1922 


I9E6 


Figure  74.— Diagram  representing  decline  in  grade  of  Lake  Superior  iron  ore  since  1889.  The  light  black  line  represents  the  approximate  average 
percentage  of  metallic  iron  in  the  total  production  for  the  United  States  for  each  year.  The  heavy  black  line  is  the  average  slope  computed 
liy  method  of  least  squares,  from  the  variations  of  the  light  continuous  line.  It  represents  the  average  decline  of  grade  since  1S89,  which  amounts 
to  about  0.35  per  cent  per  year.  The  broken  line  shows  the  percentage  of  the  entire  production  of  the  United  States  which  comes  from  the 
Lake  Superior  region.  As  this  proportion  has  steadily  increased,  it  is  apparent  that  the  drop  in  grade  of  the  iron  ores,  figured  for  the  entire 
United  States,  is  shared  by  the  Lake  Superior  ores. 

production  has  been  followed  by  a  lowering  m  grade,  and  decrease  of  production  has  meant 
raising  of  the  grade  in  about  the  proportion  that  might  be  calculated  from  the  general  drop 
in  grade  with  mcrease  in  production  for  the  last  20  years.  It  is  not  likely  that  the  grade  will 
lower  as  rapidly  in  the  future  as  in  the  past,  for  as  successively  lower  grades  of  ore  are  utilized 
the  amounts  available  are  larger. 

As  the  Lake  Superior  region  produces  nearly  80  per  cent  of  the  iron  ore  of  the  United  States, 
the  conclusion  as  to  lowering  of  grade  drawn  from  the  diagram  may  be  taken  to  apply  conspicu- 
ously to  this  region. 


494  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Tlie  present  marked  tcudeucy  toward  the  use  of  lower-grade  ores  docs  not  necessarily  mean 
that  the  higher-grade  supplies  are  exhausted,  but  simply  that  they  are  being  conserved  for 
the  future.  In  working  a  series  of  deposits  ranging  from  the  highest  to  a  low  grade,  in  strong 
financial  hantis,  it  is  regarded  as  the  best  business  policy  not  to  rob  the  deposits  of  their  liighest 
grade,  as  was  formerly  done,  but  so  to  mix  the  high  and  low  grades  as  to  give  the  maximum 
tonnage  of  an  ore  just  rich  enough  to  be  commercially  available.  The  prospective  short  life 
of  the  highest-grade  ores,  probably  not  more  than  .50  years,  is  undoubtedly  influencing  the 
present  conservative  action.  The  conservation  of  the  higher-grade  supphcs  is  favored  by  the 
marked  concentration  of  control  of  the  industry  in  a  few  hands.  When  the  ore  was  held  by 
many  owners  the  range  of  grade  available  to  each  owner  was  necessarily  limited;  when  it  is  in 
few  hands  the  range  is  greater  and  correspondingly  greater  care  can  be  taken  in  the  proper 
mi.xing  of  grades  in  order  to  yield  a  maximum  amount  of  the  lowest  grade  which  the  market 
will  stand.  In  the  discussion  of  mining  methods  (p.  498)  some  reference  is  made  to  the  care 
taken  in  getting  out  the  proper  grades  from  kny  individual  deposit.  The  same  general  methods 
are  apj)lied  by  the  United  States  Steel  Corporation  in  apportioning  the  desired  ores  among  the 
different  deposits  available. 

EFFECT    OF    INCREASED    USE    OF    LOW-GRADE    ORES. 

If  it  is  established  that  the  high-grade  ores  have  a  limited  life  and  that  the  direction  of  the 
development  of  the  ore  industry  is  now  toward  the  use  of  lower-grade  ores  and  is  likely  to  be  more 
so  in  the  future,  and  that  this  tendency  will  lengthen  greatly  the  life  of  the  ore  deposits,  there 
are  certain  consequences  which  may  be  expected. 

•  1.  The  distribution  of  the  production  of  iron  ore  is  likely  to  be  modified  and  the  relative 
importance  of  iron-mining  centers  will  vary  somewhat.  As  the  grade  falls,  new  "low-grade" 
districts  will  come  into  existence  and  some  old  districts  which  have  had  a  somewhat  precarious 
existence  in  competition  with  higher-grade  districts  will  be  enabled  to  meet  them  on  less  unequal 
terms.  This  will  be  the  effect  not  only  locally  within  the  Lake  Superior  region,  but  also  in  the 
relations  between  the  Lake  Superior  and  other  regions.  Western  iron  ores  not  now  mined 
will  come  into  the  market.  Appalachian  ores,  which  can  even  now,  in  spite  of  their  low  grade, 
compete  with  Lake  Superior  ores  because  of  favorable  conditions  of  transportation  and  prox- 
imity to  smeltmg  materials  and  consuming  centers,  may  in  the  future  attain  an  even  stronger 
position,  for  the  difference  in  composition  of  the  ores  marketed  is  sure  to  become  less,  in  view  of 
the  fact  that  the  change  toward  low  grade  in  the  Lake  Superior  region  is  likely  to  be  much  more 
rapid  than  it  is  on  the  large  low-grade  supplies  of  the  SoutheJist.  The  same  general  arguments 
will  apply  to  the  large  Cuban  reserves. 

This  increased  use  of  lower-grade  southern  Appalachian  ores  is  further  favored  by  the 
distribution  of  the  population  of  the  United  States  and  the  prevailing  freight  rates.  In  a 
personal  communication  Judge  E.  H.  Gary,  chairman  of  the  board  of  directors  of  the  United 
States  Steel  Corporation,  says : 

Under  the  existing  freight  rates  for  the  cruder  forms  of  steel  products,  if  the  freights  from  Birmingham  be  taken 
to  a  series  of  points  extending  approximately  east  and  west,  so  selected  that  the  rate  from  Birmingham  to  each  point 
is  the  same  as  the  rate  from  Chicago  to  that  point  or  the  rate  from  Pittsburg  to  that  point,  and  a  line  be  drawn  connecting 
these  points,  more  than  30  per  cent  of  the  population  of  the  United  States  lives  in  the  territory  south  of  the  line  so 
formed,  and  the  rail  freight  rates  from  Birmingham  to  all  points  in  this  territory  are  lower  than  the  freight  rates  from 
either  Pittsburg  or  Chicago  to  these  points. 

If  a  line  be  located  approximately  north  and  south  by  selecting  the  points  reached  at  equal  freight  rates  from  Chi- 
cago and  Pittsburg,  about  32  per  cent  of  the  population  of  the  United  States  lives  in  the  territory  west  of  this  Pittsburg- 
Chicago  line  and  north  of  the  Birmingham  line,  and  about  38  per  cent  of  the  population  of  the  United  States  lives  east 
of  the  Pittsburg-Chicago  line  and  north  of  the  Birmingham  line. 

The  preeminence  of  the  Lake  Superior  region  is  due  to  the  riclmess  of  its  ores,  wliich  offsets 
relatively  adverse  conditions  of  distance  and  transjiortation.  The  lowering  of  the  grade  of 
ore  will  undoubtedl}'  for  a  time  favor  other  regions  more  than  the  Lake  Superior  region,  but  it 
would  be  rash  to  assume  that  the  preeminence  of  the  Lake  Superior  region  will  be  lost.  The 
lower-grade  su])plies  of  the  Lake  Su))erior  region  will  not  bo  called  into  use  until  long  after 
those  from  other  districts,  and  this  will  make  it  possible  to  maintain  for  a  long  time  a  liigher 
grade  of  output  in  the  Lake  Superior  region  tlian  in  other  ilistricts. 


THE  IRON  ORES. 


495 


2.  As  a  result  of  the  increasing  use  of  low-grade  ores,  tne  distribution  of  blast  furnaces 
and  steel  plants  may  be  changed.  At  present  the  higher  transportation  charges  on  ores  to 
lower  lake  ports  as  compared  with  upper  lake  ports  are  just  about  counterbalanced  by  increased 
cost  of  fuel  and  flux  for  smelting  at  upper  lake  points  as  compared  with  lower  lake  points.  As 
the  grade  of  ore  is  lowered  this  equilibrium  will  be  disturbed. 

3.  As  a  result  of  decrease  in  reserves  of  low-phosphorus  ores,  the  change  from  the  acid 
Bessemer  process  to  the  open-hearth  process  of  steel  making  will  continue.  The  amount  of 
high-grade  Bessemer  ore  now  in  sight  is  scanty.  Attention  should  be  called,  however,  to  the 
fact  that  the  low-grade  ores  which  may  be  drawn  upon  in  the  future  are  not  necessarily  high  in 
phosphorus.  In  fact,  the  ratio  of  phosphorus  to  iron  remains  substantially  the  same  whether 
the  ore  is  lean  or  rich,  the  difference  between  grades  of  ore  being  mainly  in  the  percentage  of 
silica  present.     Lowering  of  grade  may  call  into  use  new  methods  of  smelting  iron. 

4.  The. lowering  of  the  grade  of  the  ore  may  favor  combination  of  capital  in  the  mining 
industry  if  such  combination  will  make  possible  additional  economies  and  the  use  of  a  wider 
range  of  ores. 


COMPARISON    WITH    PRINCIPAL    FOREIGN    ORES. 

The  large  deposits  of  low-grade  limonite  in  Cuba  have  already  been  mentioned.  These 
will  doubtless  be  largely  developed  for  use  of  the  iron  industry  along  the  east  coast  of  the 
United  States.  The  local  ore  supplies  of  England  and  Germany  are  of  low  grade.  Both 
countries  import  high-grade  ores  for  mixture,  partly  hematites  from  Bilbao,  Spain,  and  partly 
magnetites  from  northern  Sweden  and  Lapland.  The  high-grade  Bilbao  deposits  are  nearly 
exhausted.  Sweden  limits  the  exports  of  its  magnetite  ores.  Bessemer  hematites  of  the 
highest  grade  are  known  in  enormous  quantities  within  300  miles  of  the  coast  in  Minas  Geraes, 
Brazil.  vSteps  are  now  being  taken  to  develop  these  deposits.  They  are  likely  to  be  an 
important  factor  in  the  future  in  the  British  and  German  markets,  and  it  is  not  improbable 
that  they  may  be  used  on  the  east  coast  of  the  United  States,  especially  for  mixture  with  the 
Cuban  ores. 

TRANSPORTATION. 

The  transportation  of  tlie  Lake  Superior  ores  is  one  of  the  most  important  factors  determin- 
ing their  availablity.  They 'have  been  able  to  stand  high  transportation  charges  because  of 
their  high  grade. 

MINE  TO  BOAT. 

The  following  table  shows  the  principal  ore-carrying  railways,  distances,  rates,  and  the 
total  tonnage  hauled  to  December,  1 90S : 


Ore-carrying  railroads  of  the  Lake  Superior  region. 

Railroads. 

Ranges  supplying 
traffic. 

Principal  range  shipping 
points. 

Lake  termini  at  which 
ore  docks  are  located. 

Average 
haul. 

Approximate 
average  cost 
per  ton  from 
mine  to  dock. 

Total  iron 

ores  hauled 

to  December, 

1908. 

fVerrailion 

Tower,  Ely 

{■Two  Harbors,  Minn. . . 
Duluth  Minn 

Miles. 
f      70-90 
\           66 
SO 

120 

40 

70 
45 
45 
80 
S3 
03 
12-15 

$0.90-11.00 
.80 
.80 

.80 

.40 

.25 

.25 
.40 

.40 

Tons. 
}      75,153,936 

79,118,051 

'■40,268,854 

Eveleth,Sparta.Biwabik. 
Virginia,  Hibbing,  Cole- 

raine. 
Virginia,  Hibbing,  Nash- 

wauk. 
Hurley ,  Tronwood ,  Besse- 

merj  Wakefield. 
Michigamme,  Negaunee. . 

DuluthjMissabe  and  Northern 

Mesabi 

Great  Northern                        . 

Mesabi    

Sunerior  Wis 

fGogebic 

Ashland  Wis 

Chicago  and  Northwestern 

Marquette 

Escanaba,  Mich 

Marquette,  Mich 

Menominee 

Crystal  Falls 

Iron  River 

Iron  Mountain,  Norw'ay.. 
Crystal  Falls,  Amasa 

6131.219,397 

Duluth,  South  Shoreand*  Atlantic. 

Marquette 

/Marquette 

/Ishpeming,  Negaunee 

28,493,359 

Lake  Superior  and  Ishpeming 

Negaunee,  Ishpeming 

Marquette,  Mich {      ^~-\f. 

17,420.583 

Wisconsin  Central 

Gogebic 

Menominee 

Bessemer,  Hurley,  Iron- 
wood. 

Crystal  Falls,  Iron  Moun- 
tain. 

Ashland  Wis 

50 
40-60 

16.592,713 

Chicago,  Milwaukee  and  St.  Paul. 

Escanaba,  Mich 

a  Since  January  1,  1897. 


ti  Since  ISSO.    Includes  ores  other  than  iron  ores  between  June  1, 1888,  and  July,  1903. 


496 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


Eighty-five  per  cent  of  the  tonnage  has  been  hauled  .'iO  miles  or  more  and  15  per  cent  has 
been  hauled  less  than  50  miles.  The  average  cost  for  hauhng  the  ore  to  the  lake  has  been  60.42 
cents  a  ton. 

Four  of  the  railways  hauling  the  ore  are  controlled  directly  hy  the  companies  owning  or 
mining  the  ore.  The  United  States.  Steel  Corporation  owns  the  Duluth,  Missabe  and  Northern 
and  the  Duluth  and  Iron  Range  railroads;  J.  J.  Hill  controls  the  Great  Northern  Railway;  and 
the  Cleveland-Clill's  Company  the  Lake  Superior  and  Ishpcming  Railway. 


DOCKS. 


The  docks  antl  their  capacities  are  as  follows: 


Record  of  ore  docks  on  the  Great  Lakes. 
[Revised  to  May  1,  1909.    Table  furnished  by  Oliver  Iron  Mining  Co.] 


Kaikoad. 

Location. 

Dock 
No. 

Num- 
ber of 
pock- 
ets. 

Storage 
capacity. 

Height 

from 
water  to 
center 
hinge 
hole. 

Height 

from 
water  to 

deck  of 
dock. 

Width 
of  dofk 
from  out- 
side to 
outside  of 
partition 
posts. 

Length 

of 
spouts. 

Length 
dock. 

Angle 
pockets 

Capacity 

per 
pocket 
to  bot- 
tom of 

stringers. 

Chicago  and  Northwestern . . . 
Do 

Escanaba,  Mich 

do 

I 
3 

4 
5 
6 
1 
2 

1 
2 
3 
4 
5 

be 

2 

3 
4 

1 
2 
3 

4 
5 

1 

1 

1 
2 

1 
cl 

184 
226 
250 
202 
320 
234 
234 

Tons. 
21,143 

28,  792 
34.923 
29,310 
69, 760 
42,120 
42, 120 

Ft.    in. 
28    10 
31      2 
36      6 
28      6 
40 
40 
40 

Ft.    in. 
48    G 

52  8 
59    2 

53  3 
70 
70 
70 

Ft.    in. 
37 
37 
37 
37 

50      2 
50      2 
50      2 

Ft.  in. 

21 

27 

30 

21    8 

30 

30 

30 

Feet. 
1,104 
1,356 
1.500 
1.212 
1,920 
1,404 
1,404 

0            , 

39    30 

45 

43 

40 
45 

45 
45 

Cubic/eet. 
1,918 
1,969 

Do 

do 

2.191 

Do 

do 

2,8.?2 

Do 

...  .do 

4.114 

Do 

Ashland.  Wis 

3,915 

Do 

do 

3,915 

Two  Harbors,  Mum. . . 
do 

1.630 

268, 170 

Duluth  and  Iron  Range 

Do.  . 

202 
208 
170 
108 
168 
148 

40, -100 
41.600 
34, 000 
36,960 
35,450 
43,246 

35      5 

33      5 

40 

37 

39 

40 

39    6 
57    6 
66 
62 

66    9 
73 

49 
49 
49 
49 
49 
53 

27 
27 
27 
29 
30 
32    4 

iil.,38.S 
1,280 
1,054 
1,042 
1,050 
920 

38    42 
38    42 
43    32 
38    42 
43    32 
45 

3,006 
3,006 

Do...: 

do 

3.006 

Do 

do 

3.270 

Do 

do 

3,126 

Do 

do 

4.272 

Duluth,  Minn  . 

1.0B4 

231,656 

Duluth,  Missabe  and  North- 

384 

384 
384 

69,120 

80.640 
119,274 

32 

40  7 

41  9J 

57    6 

67      J 
72    6 

49 

59 
57 

27    9 

27    9 
30     IJ 

2,336 

2,304 
2,304 

45 

45 
45 

2,363 

ern. 
Do 

So 

2.782 

Do 

.  ..do 

3,867 



1.152 

269.034 

374 

.    350 

326 

100,980 
94.500 
88.020 

40 
40 
40 

73 
73 
73 

62      8 
62      8 
62      8 

32    4 
32    4 

32    4 

2.244 
2,100 
1.956 

4S 
45 

45 

4,972 

Do 

do 

4.972 

Do 

.do 

4.972 

Marquette,  Mich 

.     .do 

1,050 
200 
200 
400 

283,500 

Duluth,  South  Shore  and  At- 
lantic. 
Do      . 

28,000 
50,000 

27      9 
40 

47    3 
70  10 

36      8 
51 

21    1 
32    4 

1,200 
1,236 

39    45 
45 

1.839 
3,848 

do     

78,000 

LakeSuperiorand  Ishpemlug. 
WiSGonsui  Central 

200 
314 

36,000 
48,356 

30      9 

40 

54 
66    2 

50 

36 

27    7 
27 

1,232 
1.908 

38    40 

50    45 

2.713 

Ashland,  Wis 

2.435 

Escanaba,  Mich 

do ■ 

Chicago,  Milwaukee  and  St. 
Paifl. 
Do 

240 
240 

50,400 
63,500 

40      2i 

40  Hi 

66    6 
69    2 

52 
54 

120  27 
120  29 
30    4i 

1,500 
1,500 

45 
45 

2,900 
3.150 

Michipicoten,  Ontario. 
Key  Harbor,  Ontario. . 

480 

113,900 

Algoiiia  Central  and  Hudson 

12 
20 

34 

43    4 
61    9 

■  25 

28 

22    6 
30 

311t 
240 

44 

Bay. 

2,000 



o312  feet  single  pockets:  1.070  feet  double  pockets. 

t>  Steel  superstructure  on  concrete. 

c  Pockets  ailed  by  belt  conveyor  from  stock  pile  trestle  30  feet  high. 

The  cost  of  unloading  from  train  to  dock  and  from  dock  to  boat  aggregates  4  cents  a  ton. 
Most  of  the  structures  up  to  the  present  time  have  been  made  of  wood  and  are  so  inilammable 
as  to  require  almost  prohibitory  insurance  rates,  are  easily  choked  in  cold  weather  by  the 
freezing  of  the  water  in  the  ore,  and  are  easih^  lied  up  by  strikes.  Tlic  destruction  or  tying  up 
of  a  dock  is  a  most  serious  setback  to  the  iron-ore  industiy  and  one  which  can  be  less  easily 


(.  S.  GEOLOGICAL  SUfivE 


O^JOGt'sPH   LI'     fl_    1 


A     ORE    DOCKS    AT    TWO    HARBORS,    MINN. 
See  page  497. 


Jl     EXCAVATIONS    AT     STEVENSON,     MINN. 
See  page  497, 


THE  IRON  ORES.  497 

avoided  and  less  quickly  remedied  than  any  other  of  the  misfortunes  affecting  the  industry. 
Steel  is  used  in  new  docks  at  Two  Harbors,  Minn.  (PI.  XLI,  A),  and  this  may  be  the  beginning 
of  a  revolution  in  dock  building.  The  docks  have  undergone  little  stractural  modification  since 
they  were  first  used  in  the  Lake  Superior  region.  There  "is  still  room  for  mechanical  improve- 
ment to  make  the  movement  of  ore  more  certain  and  continuous  between  the  train  and  the  boat. 

BOATS. 

The  ore  is  carried  on  the  Great  Lakes  by  a  fleet  of  vessels  numbering  660  in  1907.  Of  the 
total  tonnage  which  has  gone  dowm  the  Great  Lakes  much  the  largest  percentage  has  gone  to 
Cleveland  and  a  small  percentage  to  Chicago.  The  proportion  going  to  Chicago  is  constantly 
increasing.     The  following  table  shows  tonnage  and  rates  from  upper  Lake  ports  in  1907: 

Quantity  of  ore  shipped  from  upper  Lake  ports  in  1907,  with  rates  per  ton. 


ERRATUM. 

The  rate  stated  on  page  497,  in  the  last  sentence  under  the  heading 
"Dock  to  furnace,"  is  incorrect.  In  1910  the  rate  per  ton  from  Lake  Erie 
docks  to  the  Youngstown  district  was  64  cents,  to  Pittsburgh  $1.04,  and 
to  Philadelphia  $1.53. 


DOCK   TO    FURNACE. 

Still  another  transportation  charge  to  be  added  to  the  ore  is  that  of  unloading  at  the 
Lake  docks  and  short  rail  transportation  to  lower  Lake  furnaces.  From  Conneaut  and 
Ashtabula  to  the  furnaces  the  distance  is  50  miles  and  the  charge  50  cents  a  ton. 

TOTAL    COST   OF    TRANSPORTATION. 

The  average  cost  of  transporting  Lake  Superior  ores  to  the  furnaces  during  1907  was 
$2.14  a  ton.  When  it  is  remembered  that  approximately  three-fourths  of  the  transijortation 
is  done  by  companies  controlling  the  ore  and  that  this  transportation  charge  contains  a  con- 
siderable profit  for  the  mining  companies,  the  real  cost  of  carrying  ore  to  the  furnaces  is  seen 
to  be  considerably  lower. 

Although  the  cost  of  transportation  for  the  ore  has  been  high,  on  the  other  hand  the 
furnaces  have  been  located  fairly  close  to  the  distributing  centers  for  finished  materials,  so 
that  transportation  of  the  finished  material  has  been  correspondingly  less.  As  the  center 
of  population  has  moved  westward,  the  smelting  in  the  vicinity  of  Chicago  has  become  propor- 
tionally more  important  and  the  cost  of  transportation  of  the  ore  proportionally  less. 

METHODS    OF    MINING. 

It  is  the  purpose  here  mere.ly  to  mention  some  of  the  most  elementary  features  of  the 
mining  methods  used  in  the  Lake  Superior  region.  The  ores  in  general  are  taken  from  the 
ground  by  open-pit  and  underground  methods  or  some  combination  of  them.  By  far  the  larger 
number  of  mines  are  underground  mines.  Most  of  the  open-pit  mines  (see  Pis.  XI,  p.  ISO;  XLI, 
B)  are  in  the  Mesabi  district,  where,  in  1908, 63.7  per  cent  of  the  ore  was  so  produced.  The  pro- 
duction of  the  Mesabi  open-pit  mines  is  so  large  that,  notwithstanding  their  small  number  as  com- 
pared with  the  total  number  of  mines  in  the  region,  they  produced,  in  1908,  42  per  cent  of  the 

47517°— VOL  52—11 32  • 


THE  IRON  ORES. 


497 


avoided  and  less  quickly  remedied  than  any  other  of  the  misfortunes  affecting  the  industry. 
Steel  is  used  in  new  docks  at  Two  Harbors,  Minn.  (PI.  XLI,  A),  and  this  may  be  the  beginning 
of  a  revolution  in  dock  building.  The  docks  have  undergone  little  stnictural  modification  since 
they  were  first  used  in  the  Lake  Superior  region.  There  is  still  room  for  mechanical  improve- 
ment to  make  the  movement  of  ore  more  certain  and  continuous  between  the  train  and  the  boat. 

BOATS. 

The  ore  is  carried  on  the  Great  Lakes  by  a  fleet  of  vessels  numbering  660  in  1907.  Of  the 
total  tonnage  which  has  gone  down  the  Great  Lakes  much  the  largest  percentage  has  gone  to 
Cleveland  and  a  small  percentage  to  Chicago.  The  proportion  going  to  Chicago  is  constantly 
increasing.     The  following  table  shows  tonnage  and  rates  from  upper  Lake  ports  in  1907: 

Quantity  of  ore  shipped  from  upper  Lake  ports  in  1907,  with  rates  per  ton. 


Port. 

Shipped  in 
1907. 

Percent- 
age of 
total. 

Rate  per 
ton  to 
lower 
lakes. 

Rate 
times 

peroent- 
age 

carried. 

Escanaba 

Tms. 
5,7(3,988 
3,013,826 
3,437,672 
8,188.906 
7,440,386 
13.445,977 

13.95 
7.30 
8.43 
19.79 
18.00 
32.00 

SO.  60 
.70 
.75 
.75 
.75 
.75 

837 
511 

Marquette 

Ashland 

1,485 

Superior 

Duluth 

2,400 

41,288.755 

99.47 

7.216 

72.16 

The  average  cost  per  ton  of  transporting  all  the  ore  shipped  in  1907  from  the  upper  to  the 
lower  Lake  ports  was  72.16  cents. 

DOCK   TO   FURNACE. 

Still  another  transportation  charge  to  be  added  to  the  ore  is  that  of  unloading  at  the 
Lake  docks  and  short  rail  transportation  to  lower  Lake  furnaces.  From  Conneaut  and 
Ashtabula  to  the  furnaces  the  distance  is  50  miles  and  the  charge  50  cents  a  ton. 

TOTAL   COST   OF   TRANSPORTATION. 

The  average  cost  of  transporting  Lake  Superior  ores  to  the  furnaces  during  1907  was 
$2.14  a  ton.  Wfien  it  is  remembered  that  approximately  three-fourths  of  the  transportation 
is  dcfne  by  companies  controlling  the  ore  and  that  this  transportation  charge  contains  a  con- 
siderable profit  for  th^  mining  companies,  the  real  cost  of  carrying  ore  to  the  furnaces  is  seen 
to  be  considerably  lower. 

Although  the  cost  of  transportation  for  the  ore  has  been  high,  on  the  other  hand  the 
furnaces  have  been  located  fairly  close  to  the  distributing  centers  for  finished  materials,  so 
that  transportation  of  the  finished  material  has  been  correspondingly  less.  As  the  center 
of  population  has  moved  westward,  the  smelting  in  the  vicinity  of  Chicago  has  become  propor- 
tionally more  important  and  the  cost  of  transportation  of  the  ore  proportionally  less. 

METHODS    OF    MINING. 

It  is  the  purpose  here  merefy  to  mention  some  of  the  most  elementary  features  of  the 
mining  methods  used  in  the  Lake  Superior  region.  The  ores  in  general  are  taken  from  the 
ground  by  open-pit  and  underground  methods  or  some  combination  of  them.  By  far  the  larger 
number  of  mines  are  underground  mines.  Most  of  the  open-pit  mines  (see  Pis.  XI,  p.  180;  XLI, 
B)  are  in  the  Mesabi  district,  where,  in  1908, 63.7  per  cent  of  the  ore  was  so  produced.  The  pro- 
duction of  the  Mesabi  open-pit  mines  is  so  large  that,  notwithstanding  their  small  number  as  com- 
pared -with  the  total  number  of  mines  in  the  region,  they  produced,  in  1908,  42  per  cent  of  the 
47517°— VOL  52—11 32  • 


498  GEOLOGY  OF  THE  LAKE  SLTPERIOR  REGION. 

entire  Lake  Superior  shipments.  Stripping  operations  in  the  Mesabi  district,  taking  into 
account  the  removal  of  ore,  are  far  more  extensive  than  the  work  conducted  at  the  Panama 
Canal,  the  total  material  removed  during  1909  in  the  Mesabi  district  being  49,750,000  cubic 
yards  as  compared  with  35,100,000  cubic  yards  at  the  Panama  Canal." 

The  underground  metliods  have  in  common  the  general  use  of  gravity  in  milling  the  ore  to 
lower  levels  fi'om  which  it  may  be  trammed  to  the  shaft  and  then  hoisted  to  the  surface. 
The  ores  are  taken  out  by  square-set  rooms  running  up  from  sublevels,  or  by  top  and  side  slicing 
downward  from  the  upper  parts  of  the  deposits,  or  by  milling  through  untimbered  chutes  to 
levels  below  after  the  surface  material  has  been  taken  from  the  top.  The  cost  of  this  work 
has  ranged  from  40  cents  to  $1.60  a  ton,  or  even  higher.  An  average  figure  would  be  perhaps 
$]  a  ton. 

The  essential  feature  of  open-pit  mining  is  the  removal  of  the  surface  material  and  the 
transfer  of  the  ore  directly  to  railway  cars  wthout  the  intermediate  use  of  the  tram  or  shaft, 
and  without  the  loss  due  to  leaving  pillars.  The  thickness  of  drift  removed  ranges  up  to 
100  feet  or  more.  The  general  method  of  work  is  much  more  scientific  than  would  at  first 
appear,  for  it  is  not  a  matter  of  shoveling  ore  at  random  onto  cars.  The  character  and  physical 
conditions  of  the  deposits  are  determined  by  drilling,  and  the  steam-shovel  cuts  and  tracks  are 
distributed  so  as  to  reach  the  desired  grades  of  ore  by  handling  the  least  possible  amount  of 
waste.  The  possible  grades  which  the  mine  may  produce  are  ascertained,  and  when  a  certain 
grade  is  desired  by  the  market  the  greatest  care  is  taken  to  extract  this  grade  from  the  ore 
body  without  leaving  undesirable  ores  which  must  be  later  moved  at  a  loss.  It  would  be 
obviously  undesirable  to  take  out  a  high-grade  ore  and  leave  a  low-grade  ore  adjacent  which 
could  not  be  sold  because  of  its  low  grade  when  by  mixing  a  high  and  low  grade  it  would  be 
possible  to  get  a  medium  gi'ade  which  could  be  sold.  Extreme  care  is  taken  to  match  the 
different  grades  in  such  a  manner  as  to  leave  them  accessible  at  proper  times.  The  prob- 
lem is  primarily  an  engineering  problem  and  is  worked  out  by  engmeers  from  most  careful 
measurements  and  calculations.  "Wlien  a  request  for  a  certain  grade  of  ore  comes  to  an  open- 
pit  mine,  orders  are  sent  out  to  load  so  many  cars  from  a  certain  cut  and  so  many  cars  from 
another  cut,  or  to  make  a  steam-shovel  cut  in  a  certain  position;  and  it  is  kno'mi  in  advance 
that  the  analysis  of  the  ore  thus  ordered  mil  run  very  close  to  that  required.  The  grading 
of  the  ore  is  becoming  closer  every  year.  In  the  utilization  of  expert  engineering  help  the 
open-pit  mines  are  fully  as  far  advanced  as  any  other  form  of  mining. 

In  connection  with  grading  the  ore  accurate  analytical  chemical  work  on  a  very  large 
scale  is  necessary.  The  work  of  sampling  and  analyzing  the  ores,  both  at  the  mines  and  at 
the  works,  has  been  developed  to  a  remarkable  degree  of  accuracy.  An  illustration  of  tliis  is 
shown  by  the  following  pairs  of  analyses,  representing  the  total  average  of  21,030,909  tons  of 
ore  shipped  by  the  Oliver  Iron  Mining  Company  from  the  Lake  Superior  region  in  1909.  The 
average  from  mine  analyses  was  iron  59.19,  phosphorus  0.068^  moisture  12.22,  silica  6.38;  and 
the  average  of  the  same  ore  as  analyzed  at  the  smelting  plants  was  iron  59.04,  phosphorus 
0.068,  moisture  12.33,  silica  6.66.  This  is  an  exceedingly  close  check  on  perhaps  the  largest 
piece  of  quantitative  chemical  work  recorded. 

The  cost  of  open-pit  work  depends  primarily  on  the  amount  of  overburden  to  be  removed 
and  the  ratio  of  tliis  to  the  size  of  the  ore  body.  The  average  cost  of  loading  on  the  car  may 
be  only  4  or  5  cents  a  ton.  The  average  cost  of  stripping,  however,  to  uncover  a  ton  of  ore 
may  run  from  20  to  30  cents.  It  is  obvious  that  the  figure  would  be  small  where  the  drift  is 
thin  or  where  the  amount  uncovered  is  large  in  proportion  to  the  thickness  of  the  cover,  so 
that  the  cost  of  surface  removal  may  be  charged  against  a  large  number  of  tons.  In  general 
the  cost  of  steam-shovel  mining  has  probably  averaged  less  than  30  cents  a  ton. 

With  this  great  difference  in  cost  in  favor  of  the  open-pit  method  of  mining,  the  question 
may  naturally  be  asked  why  any  of  the  Lake  Superior  ores  are  mined  by  underground  methods. 
For  many  of  the  deposits  the  answer  is  obvious.     Their  larger  dimensions  are  vertical  rather 

aMin.  and  Sci.  Press,  vol.  101, 1910,  p.  769. 


THE  IRON  ORES.  499 

than  horizontal,  requiring  hoisting  apparatus  to  get  them  to  the  surface.  But  even  in  the 
Mesabi  district  37  per  cent  of  the  ores  are  mined  by  underground  methods  and  for  such  mines 
the  reason  is  perhaps  not  so  obvious.  It  may  be  that  the  drift  is  too  thick;  that  the  topog- 
raphy does  not  afford  a  sufficiently  gentle  slope  for  the  approach  of  the  track;  that  adjacent 
land  for  a  proper  approach  is  owned  by  others;  that  the  deposit  may  have  a  considerable  amount 
of  low-grade  material  on  top  which  must  be  moved  before  the  material  of  better  grade  can  be 
obtained.  It  may  be  that  the  company  has  insufficient  financial  resources  to  make  the  large 
initial  expenditure  necessary  for  the  open-pit  method  before  ore  is  mined  or  sold,  or  it  may  be 
that  the  deposit  is  not  sufficiently  large  in  proportion  to  the  expense  of  preparing  it  for  the 
open-cut  method  to  warrant  piHng  up  this  great  advance  charge  against  the  ore  deposit. 

It  may  be  noted  that  the  percentage  of  ore  uncovered  by  open-pit  methods  is  being  rapidly 
increased  and  that  conditions  which  a  few  years  ago  were  regarded  as  insuperable  obstacles  to 
open-pit  handling  are  now  easily  managed.  It  may  be  pointed  out  further  that  tliis  change 
in  methods  has  accompanied  the  combination  of  mining  capital,  strong  concerns  being  able  to 
do  what  the  weaker  concerns  could  not  attempt. 

RATES  OF  ROYALTY  AND  VALUE  OF  ORE  IN  THE  GROUND. 

The  ores  of  the  Lake  Superior  region  are  leased  at  royalties  ranging  from  10  cents  to  .fl..35 
a  ton.  The  average  for  the  region  is  somewhere  between  30  and  50  cents  a  ton.  The  liigher 
figures  appear  in  the  later  leases.  The  Mesabi  range  has  the  highest  general  average  of  royal- 
ties. Here  the  Oliver  Iron  Mining  Company  pays  the  J.  J.  Hill  ore  interests  a  royalty  of  85 
cents  a  ton  on  a  muiimum  of  750,000  tons  for  1907;  this  minimum  to  be  increased  by  750,000 
tons  annually  until  it  reaches  8,250,000  tons  a  year,  after  which  it  remains  constant,  the  royalty 
to  increase  3.4  cents  a  ton  per  year  for  ore  carrying  over  59  per  cent  in  iron. 

The  royalty  rate  practically  measures  the  value  of  the  ore  in  the  ground  to  the  fee  oWTier. 
The  fee  owner  demands  on  an  average  as  high  a  price  as  the  leaseholder  can  afford  to  pay  for 
the  ore.  On  tliis  basis  the  value  of  the  ore  in  the  ground  is  between  10  cents  and  $1  a  ton. 
The  value  is  liigh  in  proportion  as  grade  is  liigh  and  costs  of  mining  and  transportation  are  low. 
The  Minnesota  State  Tax  Commission  has  adopted  an  excellent  classification  of  ore  reserves, 
based  on  compulsory  returns  from  the  mining  companies,  and  has  valued  the  ores  for  purposes 
of  taxation  at  8  to  33  cents  a  ton,  this  valuation  being  40  per  cent  of  what  is  regarded  as  the 
real  value.  The  tax-commission  figures  would  therefore  indicate  that  the  value  of  the  ore  in 
the  ground  is  from  20  to  75  cents  a  ton  in  Minnesota. 

The  present  cash  value  of  a  ton  of  ore  is  obviously  less  than  the  value  which  will  ulti- 
mately be  reafized  from  royalty  after  a  period  of  years.  If  it  be  assumed,  for  instance,  that 
the  ore  must  lie  in  the  ground  15  years  before  the  royalty  is  received,  its  present  cash  value 
would  be  roughly  42  per  cent  of  its  ultimate  royalty  value. 

ORIGIN  OF  THE  ORES  OF  THE  LAKE  SUPERIOR  PRE-CAMBRIAN  SEDIMENTARY 

IRON-BEARING  FORMATIONS. 

OUTLINE    OF    DISCUSSION. 

Under  the  above  heading  are  included  all  the  productive  pre-Cambrian  ore  deposits  of  the 
Lake  Superior  region.     It  is  proposed  to  sliow  in  the  following  discussion — 

That  these  iron  ores  are  altered  parts  of  chemically  deposited  sedimentary  fonnations, 
originally  consisting  mainly  of  cherty  iron  carbonate  and  greenahte. 

That  a  few  of  the  iron-ore  deposits  represent  originally  rich  layers  of  iron  formation,  in 
which  secondary  concentration  has  made  only  minor  changes. 

That  in  by  far  the  greater  number  of  deposits,  mcluding  all  the  larger  deposits,  the  second- 
ary concentration  has  been  the  essential  means  of  enrichhig  iron-formation  layers  to  iron  ores. 

That  the  conditions  of  sedimentation  of  the  iron  formation  may  be  roughly  outlined. 


500  GEOLOGY  OF  THE  LAKE  SUPEKIOK  REGION. 

That  tlic  weatlicring  and  erosion  of  bed-rock  surfaces  of  average  composition  would  be 
iniido()uate  as  a  source  of  tlic  materials  of  tlie  iron-bearini;  sedinipnts,  and  lliat  the  materials 
fortliose  formations  have  been  derived  largel}'  from  basic  igneous  rocks. 

That  some  parts  of  the  sedimentation  accompanied  or  immediately  followed  the  several 
introductions  of  jjre-Cambrian  l)asic  igneous  rocks  into  the  outei-  zone  of  the  earth  and  another 
part  came  under  ordinary  weathering  concUtions  later  than  tlie  extrusions  of  the  parent  basic 
igneous  rocks. 

That  the  chemistry  of  deposition  of  the  iron-bearing  fomiations  under  such  conditions 
may  be  approximated  and  that  original  jjhases  of  the  sedimentary  iron-bearing  formations 
may  be  synthesized  in  the  laboratory. 

That  the  subsequent  oxidation  of  the  iron-bearing  formations,  the  transfer  of  iron  salts, 
and  the  leaching  of  sihca  by  agents  carried  in  the  meteoric  waters  have  secondarily  concen- 
trated the  ores  and  developed  all  but  an  insignificant  portion  of  the  ore  deposits  now  mined. 

That  tliis  second  concentration  has  been  localized  by  a  considerable  variety  of  structural 
and  topographic  conditions. 

That  in  some  places  before  and  in  other  places  after  concentration  the  iron-bearing 
formations  have  been  extensively  modified  by  mechanical  deformation  or  by  igneous  intrusions, 
with  contact  effects  such  as  to  prevent  the  further  concentration  of  ore  deposits. 

That  the  sequence  of  events  developing  the  present  features  of  the  ore  deposits  may  be 
outlmed  for  each  district  and  for  the  region  as  a  whole. 

That  the  development  of  the  ores  in  general  represents  a  partial  metamorphic  cycle. 

THE   IRON    ORES   ARE   CHIEFLY  ALTERED   PARTS    OF    SEDIMENTARY   ROCKS. 

The  iron-bearmg  formations  are  bedded  and  locally  cross-bedded.  The  Iluronian  iron- 
bearing  formations  are  conformable  to  other  sedimentary  formations — quartzite,  conglomerate, 
slate,  and  limestone — and  are  not  diiTerent  from  those  of  the  Keewatin,  which  are  associated 
with  but  little  fragmental  sediment.  They  contain  recognizable  sedimentary  material,  such  as 
iron  carbonate,  greenalite,  shale,  sand,  and  conglomerate.  We  may  anticipate  our  discussion 
of  the  secondary  alterations  of  the  ores  by  stating  that  the  original  constituents  of  the  iron- 
bearing  formations  were  domuiantly  cherty  iron  carbonate  and  iron  silicate  (greenalite),  with 
minor  amounts  of  hematite  and  magnetite  and  with  varyuig  amounts  of  the  constituents  of  the 
mechanical  sediments — mud,  sand,  and  gravel.  In  tracing  the  development  of  the  iron-bearing 
formations  we  must  therefore  inquire  principally  into  the  derivation  of  the  cherty  iron  carbonate 
and  greenalite.  These  two  substances  are  nonclastic,  though  locally  some  clastic  material 
appears  in  them;  as  will  be  shown  later,  they  are  chemical  sediments. 

The  sedimentary  nature  of  the  iron-bearing  formations  scarcely  needs  more  elaborate 
proof.  It  is  so  obvious  in  the  field  that  it  has  been  doubted  by  only  three  geologic  observers. 
Whitney,  Wadsworth,  Winchell,  and  Hille  have  held  these  formations  to  be  of  surface  igneous 
origin  (see  pp.  569-570),  but  as  these  views  are  not  now  regarded  seriously  by  most  men  who 
have  studied  the  subject,  and  as  they  liave  been  abandoned  b\'  Wadsworth,  it  will  be  unnec- 
essary here  to  marshal  evidence  against  them. 

CONDITIONS    OF    SEDIMENTATION. 

IRON-BEAKING   FORMATIONS   MAINLY   CHEMICAL   SEDIMENTS. 

The  iron-bearing  formations  are  regarded  mainly  as  chemical  sedipients  (1)  because  they 
consisted  originall\'  of  iron  carbonate  and  ferrous  silicate  and  possibly  some  iron  oxide,  similar 
to  substances  known  elsewhere  to  be  tieposited  as  chemical  sediments;  (2)  because  they  may 
be  synthesized  in  the  laboratory  by  the  simple  chemical  reagents  which  were  probably  ])resent 
where  the  iron-bearing  rocks  were  formed;  and  (3)  because  they  usually  lack  fragmental  ])ar- 
tides.  To  a  minor  extent  they  are  fragmental  sediments  derived  from  the  erosion  of  earlier 
iron-bearing  ami  other  formations. 


THE  IRON  ORES.  501 

ORDER   OF    DEPOSITION   OF    THE    IRON-BEARING    SEDIMENTS. 

The  greater  mass  of  the  Keewatin  iron-bearing  rocks,  as  exhibited  in  the  Vermihon  dis- 
trict, lies  above  the  Keewatin  basalts  and  porphyries  and  is  infolded  with  them.  Another  part 
is  interbedded  with  the  basaltic  flows.  This  general  association  is  believed  to  hokl  as  a  rule 
for  the  Keewatin  of  the  pre-Cambrian  shield  of  North  America.  The  Keewatin  iron-bearing 
formations  are  in  beds  of  limited  and  irregular  extent  and  thickness.  It  is  concluded  that 
the  dej)osition  of  a  few  feet  of  iron-bearing  sediments  directly  in  shallow  depressions  bottomed 
by  basalt  was  followed  by  the  superposition  of  another  lava  flow,  and  this  in  turn  by  more  iron- 
bearing  sediments,  and  so  on.  Later,  when  the  outflow  of  lava  practically  ceased,  the  main 
mass  of  the  iron-bearing  formation  was  deposited.  Locally  a  little  fragmental  material  went 
down  immediately  upon  the  basalt  basements  before  iron  deposition  began. 

The  deposition  of  the  middle  Huronian,  containing  the  iron-bearing  Negaunee  formation 
of  Michigan,  began  with  a  coarse  conglomerate  and  sandstone  (Ajibik  cpiartzite),  changing 
somewhat  gradually  into  a  mud  (Siamo  slate),  and  this  in  turn  into  a  chemically  deposited  iron- 
bearing  formation  (Negaunee).  In  the  Cascade  or  Palmer  portion  of  the  Marquette  range 
fragmental  quartz  sand  and  ripple  marks  are  conspicuous  in  the  iron-bearing  formation.  South, 
of  the  Marquette  district  the  fragmental  beds  untlerlying  the  Negaunee  formation  are  thin  or 
lacking.  In  certain  districts  the  iron  formation  is  replaced  over  large  areas  by  basic  volcanic 
rocks  (Clarksburg  and  Hemlock  formations  and  perhaps  others  unknown).  In  general,  then, 
during  middle  Huronian  time  local  sedimentation  of  sand  and  clay  was  followed  by  more 
widespread  deposition  of  chemical  iron-bearing  sediments  lacking  fragmental  material  and  by 
simultaneous  igneous  flows. 

The  iron-bearmg  formations  of  the  upper  Huronian  are  the  most  widespread  of  the  pre- 
Cambrian.  Quartz-sand  deposition  (Pokegama  quartzite.  Palms  formation,  and  Goodrich 
quartzite)  was  followed  suddenly  by  the  widespread  deposition  of  chemical  iron-bearing  sedi- 
ments (Biwabik,  Ironwood,  Bijiki,  Vulcan,  etc.),  with  very  msignificant  amounts  of  clastic 
material,  and  this  in  turn  gave  way  somewhat  gradually  to  the  deposition  of  mud  of  probable 
delta  origin  (see  pp.  612-614)  in  masses  so  thick  that  the  thin  iron-bearing  formations  and 
quartzites  previously  deposited  may  be  regarded  as  forming  the  lower  selvage  of  a  mud  forma- 
tion. Thin  slate  layers  and  a  few  quartzite  layers  are  interbedded  with  the  upper  Huronian 
iron-bearmg  formations,  especially  in  their  upper  portions,  and  the  formations  locally  show  a 
tendency  to  be  replaced  along  the  strike  by  slate,  as  in  the  Mesabi,  Gogebic,  and  Menommee 
districts.  In  the  Menominee  district  slate  divides  the  iron-bearing  formation,  and  in  addition 
there  are  considerable  quantities  of  fragmental  quartz  sand,  iron  oxide,  and  ferruginous  slate 
near  the  base  of  the  iron-bearing  formation. 

In  the  Crystal  Falls,  Florence,  Iron  River,  and  Cuyuna  districts  the  ore  is  in  siderite  lenses  in 
the  upper  Huronian  slate,  and  the  basal  fragmental  quartzite  has  been  only  locally  recognized. 
These  occurrences  are  apparently  farther  from  the  base  of  the  formation  than  those  in  the  Mesabi, 
Gogebic,  Felch  Mountain,  and  Menominee  districts,  where  quartz  sand,  iron-bearing  formation, 
and  slate  were  successively  deposited  as  distinct  formations. 

On  the  south  side  of  Lake  Superior,  in  the  western  Marquette,  eastern  Gogebic,  and  north- 
western Menominee  districts  the  deposition  of  the  upper  Huronian  iron-bearing  formations  was 
interrupted  by  the  contemporaneous  extrusion  of  great  masses  of  submarine  ellipsoidal  basalts. 
These  extrusions  may  have  been  more  extensive  than  now  appears,  because  evidence  of  them  may 
be  buried  or  may  have  been  removed  by  erosion. 

ARE   THE   IRON-BEARING   FORMATIONS  TERRESTRIAL   OR    SUBAQUEOUS    SEDIMENTS? 

It  is  beUeved  that  the  iron-bearing  formations  are  subaqueous  for  the  following  reasons: 
1 .  They  were  originally  ferrous  compounds  in  major  part.     Terrestrial  sedimentation  usually 
produces  ferric  oxides — hematite  or  limonite  and  laterite,  except  in  bogs — and  reasons  are 
advanced  elsewhere  to  show  that  only  a  part  of  the  Lake  Superior  iron-bearing  formations  may 
be  so  developed. 


502  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

2.  Tlio  middlo  and  upper  Iluroniau  ir<)ii-l)Paring  formations  arc  parts  of  sedimentary  groups 
containing  quartzites  and  slates  of  probable  subaqueous  origin.  The  slates  are  essentially  delta 
dojiosits. 

3.  All  the  iron-l)('ariag  formations  are  associated  with  basalts  having  conspicuous  ellipsoidal 
structures,  which  can  be  best  explained  as  developed  by  flowing  out  under  water.  They  contrast 
in  this  regard  with  tiio  basic  lavas  of  the  Keweenawan  series. 

4.  Between  the  underlying  basalts,  which  are  probably  subaqueous  extrusions,  and  tlie 
iron-bearing  formations  in. the  Keewatin  series  neither  weathering  nor  erosion  has  taken  place 
except  very  locallj'.     The  two  are  conformable. 

BOG   AND    LAGOON   ORIGIN   OF   PART   OF   THE    IRON-BEARING  ROCKS. 

The  iron-bearing  members  of  the  Crystal  Falls,  Iron  River,  and  Cuyuna  (Ustricts  are  asso- 
ciated with  slates  of  probable  delta  origin,  which  near  the  iron-bearing  rocks  arc  so  uniformly 
black  and  graphitic  and  generally  pyritiferous  that  black  slate  is  usually  regarded  as  a  favorable 
intlication  in  jirospecting  for  ore.  Much  black  slate  in  the  upper  Huronian  is  not  associated 
with  the  iron-bearing  formations,  but  ore  is  almost  never  found  without  the  black  slate.  The 
iron-bearing  rocks  in  such  associations  with  black  slate  are  originally  carbonate.  Smaller 
amounts  of  graphitic  slates  are  found  also  in  connection  Avith  the  Keewatin  iron-bearing  forma- 
tions. The  thicker  iron-bearing  formations  of  the  Mesabi,  Gogebic,  Marquette,  and  Menominee 
districts  are  associated  with  black  slates  to  a  less  degree. 

It  is  suggested  elsewhere  that  some  of  the  slates  most  abundantly  associated  with  the 
iron-bearing  formations  may  represent  delta  deposits,  and  that  the  carbon  content  of  the  iron 
formations  is  probably  to  be  explained  as  organic.  So  far  as  direct  evidence  is  concerned,  the 
organic  origin  of  the  graphite  and  sulphides  in  the  black  slates,  notwithstanding  its  probabilit}', 
should  not  be  regarded  as  proved,  although  there  is  no  reason  to  doubt  such  an  origin.  Similar 
associations  elsewhere,  as  in  the  Carboniferous,  have  been  shown  to  be  truly  of  organic  origin. 
On  the  other  hand,  in  the  Lake  Superior  black  slates,  as  in  all  other  Lake  Superior  pre-Cambrian 
formations,  no  organic  forms  have  been  found. 

These  facts  raise  the  question  whether  the  carbon  of  the  slates  may  not  have  been  effective  in 
the  original  deposition  of  the  iron-bearing  formations,  as  bog  or  lagoon  deposits,  in  the  manner 
of  Carboniferous  and  Cretaceous  carbonates — that  is,  by  the  progressive  burial  of  ferric  oxide 
with  organic  material,  resulting  in  the  reduction  of  the  oxide  and  the  formation  of  iron  carbonate. 
The  way  in  which  reducing  organic  substances  aids  in  dissolving  and  transporting  iron  salts  is 
discussed  on  pages  519-520. 

This  is  probably  the  origin  of  the  discontinuous  carbonate  lenses  in  the  carbonaceous  slates  of 
probable  delta  origin  in  the  upper  Huronian,  but  difficulties  appear  when  we  attempt  to  exj)lain 
in  the  same  waj'  the  main,  tliick,  continuous  masses  of  iron-bearing  formation  of  the  Keewatin, 
middle  Huronian,  and  upper  Huronian. 

HYPOTHESIS  OF  BOG  AND  LAGOON  ORIGIN  NOT  APPLICABLE  TO  THE  MAIN  MASSES  OF  THE 

IRON-BEARING  SEDIMENTS. 

The  main  masses  of  the  iron-bearing  sediments  are  not  closely  associated  with  carbonaceous 
slates;  they  are  not  characteristically  discontinuous  or  lens-shaped,  but  are  extensive  and  tliick; 
they  rest  with  sharp  contacts  on  quartzite,  conglomerate,  or  basalt.  The  Lake  Superior  iron- 
bearing  formations  also  carry  more  chert  than  deposits  of  known  bog  origin  of  the  carbonate  type. 

The  bog  theory  of  origin  involves  the  assumption  that  the  Lake  Superior  region  may  have 
been,  during  each  of  the  iron-depositing  periods,  covered  by  great  bogs  or  lagoons  in  wliich  vege- 
table matter  could  grow  at  or  near  the  surface  of  the  water  over  great  areas,  as  in  lagoons  in 
ai-lvance  of  barriers  thrown  up  b}"  the  sea  encroaclung  over  a  gently  sloping  surface,  or  under 
delta  conditions.  As  a  process  necessarily  confined  to  a  shallow  zone  near  the  surface,  its  con- 
tinuous operatiiju  would  involve  continuous  and  uniform  su])sidence  at  a  rate  connnensurate 
with  tlie  deposition  of  the  iron  salts  in  t)rder  to  j)roduce  the  thicknesses  now  Icuown.     iUthough 


THE  IRON  ORES.  503 

this  theory  is  probably'  applicable  to  some  of  the  thin  lenses  of  small  extent  associated  with  car- 
bonaceous slates,  it  is  not  clear  how  this  process  could  produce  a  thousand  feet  of  iron-bearing 
sediments  sliowing  uniformity  of  lithology  and  bedding  and  having  so  little  extraneous  material 
through  hundreds  of  square  miles. 

HYPOTHESIS   OF   GLAUCONITIC   ORIGIN   NOT   APPLICABLE. 

The  greenaUto  of  the  iron-bearing  formations  of  the  ]\Iesal)i  and  other  districts  is  so  similar  to 
glauconite  as  to  suggest  similarity  in  conditions  of  origin — that  is,  as  filUngs  of  cavities  in  or 
replacements  of  Foraminifera  in  deep-sea  deposits.  Dredgings  have  brought  up  glauconite  from 
deep  and  quiet  waters  but  not  from  places  of  rapid  sedimentation.  No  glauconite  is  laiown 
with  so  little  foreign  material  as  the  greenalite  beds  of  the  iron-bearing  formations.  The  thick- 
ness of  the  deep-sea  glauconite  beds  is  not  known.  In  geologic  sections  the  thickest  known 
deposit  is  35  feet.  The  deposition  of  1 ,000  feet  of  greenalite  beds  in  the  same  manner  as  glaucon- 
ite is  known  to  be  deposited  would  require  a  development  of  Foraminifera  in  the  prc-Cambrian 
not  known  in  any  other  geologic  period. 

IKON-BEARING    SEDIMENTS   NOT    LATERITE    DEPOSITS. 

In  many  parts  of  the  world,  especially  in  tropical  climates,  there  are  bedded  iron  ores  of  the 
laterite  type,  presumed  to  develop  from  the  katamorphism  of  basalt  or  other  basic  igneous  rock 
in  place.  They  are  characteristically  associated  with  bauxite,  clay  (lithomarge  or  bole),  usually 
resting  on  it.  Gradational  tyj^es  between  lateritic  iron  ore  and  igneous  rock  have  been  described. 
The  Lake  Superior  iron  beds  associated  with  basalts  can  not  in  any  considerable  part  be  referred 
to  decomposition  of  the  basalt  in  place  after  the  manner  of  laterite  deposits;  the  almost  complete 
absence  of  clay  associated  with  the  iron  ores  and  the  presence  of  abundant  chert  preclude  this 
explanation.  Although  lateritic  decomposition  of  basalt  surfaces  may  have  been  an  ultimate 
partial  source  of  the  iron  ore,  transportation  and  sorting  have  eliminated  the  clay,  which  would 
be  present  if  the  iron  beds  resulted  from  lateritic  decomposition.  The  principal  impurity  in  the 
Lake  Superior  iron  is  silica.  Tliis  could  not  have  developed  from  decomposition  of  the  basalt 
in  place. 

In  reading  accounts  of  the  origin  of  iron  beds  associated  \vith  basalts  in  different  parts  of  the 
world,"  one  notes  a  tendency  to  ascribe  a  lateritic  origin  to  the  iron  beds,  even  in  places  where 
the  iron  lacks  the  associated  clay  to  be  expected  from  such  a  mode  of  origin.  It  would  seem 
necessarj'  at  least  to  introduce  the  factors  of  sorting  and  transportation  to  explain  these  ores. 
Clay  is  as  stable  as  iron  oxide  under  surface  conditions,  and  so  far  as  quantitative  evidence 
goes,  it  remains  with  the  residual  iron  oxide  in  a  more  or  less  uniform  proportion  throughout 
a  cycle  of  decomposition. 

Finally  the  evidences  of  water  sedimentation  and  physical  separation  of  most  of  the  iron 
formations  and  basalts  are  not  in  accord  with  the  hypothesis  of  lateritic  origin. 

IRON-BEARING    SEDIMENTS    NOT    CHARACTERISTIC    TRANSPORTED    DEPOSITS    OF 

ORDINARY  EROSION  CYCLES. 

The  oxidized  carbonate  lenses  associated  with  the  grapliitic  slates  (see  p.  501)  may  be 
regarded  as  one  of  the  mcidental  results  of  a  normal  erosion  cycle.  The  fragmental  bases  of 
the  Vulcan  formation  in  the  Menominee  district,  of  the  Bijiki  scliist  in  the  Marquette  district, 
and  of  the  Cretaceous  rocks  in  the  Mesabi  distiict  contain  a  great  deal  of  detrital  ferruginous 
chert  and  iron  ore  derived  from  the  breaking  up  of  iron-bearing  rocks  that  he  unconformably 
below,  but  all  these  phases  of  the  iron-bearing  rocks  are  of  minor  importance  as  compared 
with  the  thick  masses  of  iron-bearing  formation  derived  from  the  alteration  of  iron  carbonate 
and  greenalite  rocks. 

"Cole,  G.  A.  J.,  The  red  zone  in  the  basaltic  series  of  the  county  ot  Antrim:  Geol.  Mag.,  decade  5,  vol.  5,  No.  530,  1908,  pp.  341-344. 


504  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

L  It  has  long  been  recognizctl  tlmt  tliere  are  dilliculties  in  tlie  way  of  explaining  the 
thick  and  uniform  masses  of  chemical  sediments  constituting  the  thicker  iron-bearing  forma- 
tions, accompanied  by  so  Uttle  mechanical  sediment,  on  the  assumption  that  the  kon-bearing 
formations  have  been  derived  from  the  weathering  of  average  hind  areas.  If  the  pecuHar 
character  of  chemical  sediments  depends  on  depth  of  water  and  distance  from  the  shore,  then 
the  great  thickness  of  the  formations  involves  uniform  subsidence  over  a  great  area  to  keep  the 
conditions  uniform. 

2.  The  iron-bearing  formations  may  or  may  not  be  associated  with  ordinarj'  clastic  sedi- 
ments. In  the  Keewatin  they  usually  are  not.  The  middle  Huronian  consists,  from  the  base 
up,  of  quartzite,  slate,  and  iron-bearing  formation.  The  upper  Iluronian  where  best  exposed 
consists  of  quartzite,  iron-bearing  formation,  and  slate.  The  association  of  the  Keewatin  iron- 
bearing  formations  with  extrusive  basalts  and  not  with  other  sediments  shows  that  the  iron 
ores  of  the  Keewatin,  at  least,  are  not  the  result  of  dejjosition  in  any  ordinarj^  cycle  of  erosion 
and  deposition,  and  tliis  strongly  suggests  that  the  variety  of  succession  in  the  sedimentary 
iron-bearing  formations  of  the  Iluronian  is  also  not  due  to  ordinaiy  cycles  of  erosion  and  depo- 
sition, and  that  the  deposition  of  the  iron-bearing  formations  probably  was  not  uniformly 
related  to  sea  transgression  or  recession  or  any  other  one  phase  of  a  topographic  cycle. 

The  fact  that  in  many  places  the  sediments  above  and  below  the  Huronian  iron-bearing 
formations  are  different  is  the  only  feature  which  suggests  that  the  deposition  of  iron-bearing 
sediment  is  a  part  of  a  cycle  of  erosion  and  deposition,  though  it  is  conceivable  that  volcanism 
itself  would  cause  this  change,  either  by  efl'ecting  changes  of  levels  of  land  and  water  or  by 
introducing  new  rocks  for  erosion  to  work  upon. 

Until  investigation  has  disclosed  all  the  different  combinations  of  factors  wliich  may  pro- 
duce a  particular  order  of  sedimentation,  it  is  unsafe  to  be  too  positive  in  concluding  that  the 
varied  relations  of  the  iron-bearing  formations  to  the  order  of  sedimentation  indicate  their 
deposition  under  exceptional  conditions.  The  conditions  producing  alternations  of  iron-bearing 
sediment  with  other  sediments  in  varying  succession  may  not  be  necessarily  difi'erent  from 
those  favoring  the  deposition  of  limestone  with  a  variety  of  associations — for  instance,  the 
Paleozoic  Hmestones,  which  in  some  places  overlie  sand  and  in  others  mud  and  are  in  turn  fol- 
lowed by  sand  or  mud.  But  the  lack  of  uniformity  in  the  relations  of  the  iron-bearing  forma- 
tions above  noted  is  taken  to  indicate  a  probability  that  conjunction  of  their  deposition  with  a 
certain  phase  of  a  topographic  cycle  is  not  an  essential  condition  to  their  development. 

3.  Were  the  iron-bearing  formations  derived  from  the  weathering  of  the  older  rocks  against 
which  they  he,  it  would  be  diflicult  to  explain  the  complete  absence  of  weathered  material 
between  certain  bands  of  Keewatin  iron-bearing  formations  and  the  associated  basalts,  or  of 
erosion  irregularities  in  the  underlying  surface. 

4.  The  surface  streams  are  only  locally  carrying  iron  in  quantity  at  the  present  tune.  All 
available  analyses  of  river  waters  show  a  lack  of  iron,  with  the  exception  of  minute  quantities 
in  Ottawa  and  St.  Lawrence  rivers.  Many  of  the  springs  carry  iron,  but  this  is  conspicuously 
deposited  at  the  point  of  escape  and  does  not  join  the  run-off.  These  facts  are  correlated  with 
known  observations  of  the  maimer  of  weathering  of  rocks.  The  ferrous  iron  becomes  oxidized 
and,  next  to  alumina,  is  the  most  stable  of  all  substances  under  surface  conditions.  In  fact, 
so  little  iron  is  lost  by  weathering  that  Merrill,  Watson,  and  others  have  used  both  iron  and 
alumina  as  a  basis  against  wliich  to  measure  the  loss  of  other  constituents. 

5.  If  it  is  regai'dod  as  possible  that  the  iron-bearing  formations  are  derived  from  the  weath- 
ering of  ordinary  land  surfaces,  why  should  the  ii-on-bearing  formations  not  be  reproduced  on 
the  same  scale  in  the  Paleozoic  rocks,  which  were  deposited  on  pre-Cambrian  rocks  similar  to 
those  beneath  the  iron-bearing  formations  ?  The  deposition  of  the  Paleozoic  rocks  was  preceded 
by  perhaps  the  longest  period  of  weathering  of  which  there  is  record  in  the  Lake  Superior  coun- 
try. In  many  parts  of  the  United  States  Paleozoic  and  later  sediments  contain  thin  beds  of 
sedimentary  iron-bearing  formation,  but  these  beds  are  at  their  maximum  insignificant  in  thick- 
ness as  comiiared  with  those  of  the  Lake  Superior  region. 


THE  IKON  ORES.  505 

6.  A  comparison  of  the  composition  of  the  iron-bearing  series  with  the  possible  sources 
from  which  they  might  be  derived  by  ordinary  weathering  further  shows  that  the  iron  is  present 
in  higher  ]:)ercentage  in  the  iron-bearing  formations  than  in  the  rocks  from  which  they  may 
have  been  so  derived. 

The  jaspers  of  the  Keewatin  series  of  the  Vermilion  district  average  between  28  and  38 
per  cent  in  iron,  but  the  associated  basalts  average  0.56  per  cent.  The  jaspers  have  little  other 
sedimentary  material  with  them  to  be  figured  in  this  comparison.  Therefore  the  jaspers  pi'obably 
derived  their  iron  from  some  other  source  than  the  weathering  of  the  adjacent  basalts,  or  the 
complementary  fragmental  detritus  was  washed  away. 

The  middle  Huronian,  containing  the  iron-bearing  Negaunee  formation,  has  an  average 
iron  content  of  11.72  per  cent,  as  indicated  by  the  available  figures  of  composition  of  the  three 
formations  of  the  middle  Huronian  and  their  relative  tliickness.  Because  of  the  unconformity 
at  the  top  there  is  a  question  as  to  what  factor  should  be  added  for  materials  that  have  been 
eroded,  but  there  is  no  evidence  that  any  large  amount  of  material  has  been  taken  away,  and  as 
part  of  the  material  which  has  been  removed  belonged  to  the  iron-bearing  formation,  this 
factor  can  not  be  assumed  to  cause  much  change  in  the  figures  given. 

The  composition  of  the  rocks  of  the  ancient  land  area  from  which  the  middle  Huronian  may 
have  been  derived  by  weathering  is  not  definitely  known,  but  it  may  be  supposed  to  be  not  far 
from  the  average  given  by  Clarke"  for  igneous  and  crystalhne  rocks,  in  which  the  iron  content 
is  4.46  per  cent.  Were  the  shore  made  up  of  basic  rocks  such  as  the  Kitchi  schist  or  Mona  schist 
the  iron  content  would  be  about  9  per  cent.  It  is  thus  apparent  that,  whether  we  regard  average 
igneous  rocks  or  basic  rocks  as  representing  the  original  land  from  which  the  middle  Huronian 
may  have  been  derived  by  weathering,  the  sediments  contain  a  considerable  excess  of  ii'on  not 
accounted  for. 

The  iroii  content  of  the  upper  Huronian  of  the  Mesabi  and  Gogebic  districts  ranges  from 
6  to  9  per  cent,  depending  on  the  thickness  of  slate  which  is  chosen  for  the  calculation.  The 
smallest  percentage  is  liigher  than  that  of  the  average  igneous  rock  that  may  be  supposed  to 
represent  the  land  area  from  which  these  sediments  were  derived.  The  highest  is  about  equal 
to  the  percentage  of  iron  in  the  greenstones. 

In  general,  then,  if  it  is  assumed  that  all  of  the  iron  of  the  ancient  land  areas  was  trans- 
ferred and  contributed  to  the  iron-bearing  formations  that  were  being  deposited  in  neighboring 
submerged  areas  (which,  as  above  shown,  it  was  not),  this  would  not  be  enough  to  account  for 
the  iron  in  the  iron-bearing  rocks  when  the  associated  sediments  are  taken  into  account  and 
allowance  made  for  complementary  secUments  deposited  elsewhei'e. 

The  major  part  of  the  iron  of  the  iron-bearing  formations  was  originall}'  deposited  as  a 
chemical  secUment  from  solution.  In  view  of  the  fact  that  in  weatheiing  only  a  small  propor- 
tion of  the  iron  present  is  observed  to  be  carried  off  in  solution,  the  rest  remaining  as  insoluble 
ferric  oxide,  it  becomes  even  more  apparent  that  the  iron-bearing  formations  were  not  derived 
by  chemical  solution  and  deposition  of  the  materials  of  average  land  areas.  A  similar  conclu- 
sion is  to  be  drawn  from  the  silica  content  in  the  iron-bearing  formations.  ■ 

Sihca  of  course  is  derived  abundantly  from  the  weathering  of  rocks  in  cold  solutions  and  is 
precipitated  principally  in  the  form  of  chert  in  limestones.  The  part  mechanicall}'  carried  is 
deposited  as  c^uartz  sand,  differing  in  texture  from  the  chert.  The  latter  mode  of  derivation 
is  practically  excluded  for  the  iron-bearing  formations  of  the  Lake  Superior  region  because  they 
contain  only  small  amounts  of  fragmental  quartz  at  a  few  localities  and  horizons.  If  we 
attempt  to  ascribe  the  cherts  of  the  iron-bearing  formations  to  weathering,  we  ma}'  look  only 
to  the  sihca  carried  in  solution.  To  have  produced  the  thick  iron-bearing  formations  contain- 
ing an  average  of  about  70  per  cent  by  volume  of  chert,  the. solution  of  silica  must  have  pro- 
ceeded on  an  enormous  scale,  probably  too  large  to  be  explained  by  ordinary  weathering. 
That  some  chert  was  so  derived,  just  as  some  iron  and  some  fragmental  quartz  were  so  derived, 
is  altogether  likely,  and  it  would  be  difficult  to  prove  the  contrary.  The  percentage  of  chert 
in  the  iron- bearing  groups  described  on  page  461  ranges  upward  from  63  per  cent  in  weight, 

o  Clarke,  F.  W.,  The  data  of  geochemistry:  Bull.  U.  S.  Geol.  Surrey  No.  330, 1908,  p.  26. 


506  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

wliile  Clarke's  average  of  igneous  and  crystalline  rocks,  which  jpoight  represent  the  composition 
of  an  average  surface  under  weathering,  is  a  little  less  than  62  per  cent  in  silica  and  the  basic 
wreenstones  contain  less  than  50  per  cent  in  silica.  Hence,  even  if  all  the  siUca  had  been  leached 
(together  witli  the  iron)  from  these  rocks  (which  never  happens),  it  would  not  j-icld  a  percent- 
age of  silica  as  large  as  that  known  in  the  iron-bearing  groups.  Organic  agencies  might 
locahze  precipitation  of  silica  in  certain  areas,  but  not  enough  to  account  for  existing  pro- 
portions over  tlu^  entire  region. 

The  calcium-magnesium  content  furnishes  still  another  argument.  In  the  average  crj^stal- 
line  or  igneous  rocks  or  in  the  basic  igneous  rocks  or  in  sediments  derived  from  the  igneous 
rocks,  calcium  preponderates  over  magnesium,  but  in  the  iron-bearing  formations  the  average 
proportion  of  magnesium  to  calcium  is  over  5  to  1 . 

It  appears  m  general  that  the  composition  of  the  pre-Cambrian  sedimentary  groups  con- 
taining the  iron-bearing  formations  (Uffers  from  that  of  the  average  crystalline  rocks  wliich 
formed  the  shores  at  those  periods  in  having  a  liigher  content  of  iron  and  sUica  and  in  having 
a  tlifTcrent  calcium-magnesium  ratio.  It  might  be  that  the  extensions  of  these  iron-beaiing 
sedimentary  groups  outside  of  the  Lake  Supeiior  region  would  be  of  such  different  composition 
as  to  brmg  the  average  more  nearly  down  to  what  would  be  expected  from  derivatives  of  the 
crystallme  rocks.  Yet  it  is  beUeved  that  the  excess  of  certain  constituents  in  the  Lake  Superior 
sedimentary  groups  that  carry  the  iron-bearing  formations  over  those  wliich  seem  to  have  been 
probably  available  from  ordinary  weathering  is  not  counterbalanced  by  corresponding  defi- 
ciencies elsewhere,  for  the  reason  that  the  sections  on  which  these  figures  are  based  are  taken 
through  a  wide  area  in  the  Lake  Superior  region,  and  for  the  further  reason  that  this  peculiar 
composition  is  repeated  over  this  wide  area  in  the  rocks  of  three  successive  geologic  epochs. 
If  the  occurrence  of  iron-bearing  formations  in  the  Lake  Superior  region  is  simply  a  matter  of 
areal  segregation,and  concentration  of  the  normal  products  of  weathermg,  it  is  verv  remarkable 
that  this  areal  concentration  should  always  have  resulted  in  bringing  these  peculiar  iron- 
bearing  phases  in  the  same  region.  We  conclude,  therefore,  that  the  excess  of  iron  and  silica 
and  the  reversal  of  the  calcium-magnesium  ratio  in  the  sedimentary  groups  carrying  the  iron- 
bearing  formations,  as  compared  with  the  average  crystalline  rocks  from  wliich  they  might 
have  been  derived  by  erosion,  is  probably  to  be  regarded  as  evidence  that  some  unusual  source 
of  material  was  available. 

7.  It  appears,  then,  from  the  foregoing  paragraphs  that  there  are  objections  to  regarding 
the  iron-bearing  formations  entirely  as  sediments  produced  by  weathering  of  the  rocks  that 
were  most  abundant  in  the  adjacent  lands.  It  is  not  meant  to  imply  that  ordinary  erosion 
and  katamorpliic  processes  which  are  known  to  segregate  iron-bearing  sediments  were  set 
aside  in  this  region.  Indeed,  as  already  indicated,  there  is  definite  evidence  that  some  of  the 
kon-beaiing  sediments  were  so  produced.  But  it  seems  that  these  processes  are  not  adequate 
to  explain  the  facts.  In  character  and  size  the  iron-bearing  formations  are  unique  as  chemical 
sediments  and  differ  from  other  chemical  sediments  derived  by  normal  weathering  processes. 
Some  unuSual  and  additional  factor  seems  to  be  required  to  explam  them.  Such  a  factor  is 
discussed  under  the  following  headings. 

ASSOCIATION    OF    IRON-BEARING     SEDIMENTS    WITH    CONTEMPORANEOUS    ERUPTIVE 

ROCKS. 

• 

All  the  Lake  Superior  iron-bearing  formations  are  more  or  less  closely  related  in  time  and 
place  to  basalt  Hows,  usually  rich  in  iron  at  j)resent  and  giving  evidence  of  having  exudetl 
iron  salts  at  the  time  of  their  consolidation.  The  iron-bearkig  formations  of  the  Keewatin 
series  have  such  relations  to  the  associated  ellipsoidal  basalts  as  to  point  to  their  do]iositionin 
the  short  periods  separating  the  successive  flows  of  basalt  or  inimediatel}-  followmg  (ho  prui- 
cipal  extnisions.  Detailed  evidence  of  this  has  been  noted  in  a  number  of  places  and  especially 
in  the  Vermilion  distiict.  (See  pp.  126-127.)  The  Negaunee  fornialion  of  the  middle  Iluronian 
is  associated  with  abundant  contemporaneous  igneous  activity,  iiroducing  ellipsoidal  basalts  of 
submarine  origin  and  other  extrusive  rocks  similar  to  those  in  the  Keewatin  series  in  many 


THE  IRON  ORES.  507 

places  in  the  Marquette  district,  especially  at  the  west  end  (the  volcanic  Clarksburg  forma- 
tion), and  in  the  Crystal  Falls  and  adjacent  districts  (the  Hemlock  formation).  The  iron- 
bearing  formations  of  the  upper  Huronian  (Animikie  grou]^)  are  associated  with  igneous  activity 
similar  to  that  of  the  preceding  periods  in  the  Marquette  district  (the  Clarksburg  formation), 
in  the  Gogebic  district  (the  volcanic  rocks  of  the  east  and  west  ends  of  the  district) ,  and  in  the 
Menominee,  Florence,  and  Iron  River  districts.  The  iron-beaiing  formation  of  the  Animikie 
group  on  the  north  shore  of  Lake  Superior  is  not  associated  with  basic  greenstones  of  known 
contemporaneous  development,  but  as  shown  on  pages  213-214  there  is  little  doubt  of  its  direct 
contmuity  with  the  rocks  of  the  Cuyuna  district  and  the  upper  Huronian  of  the  south  shore, 
wliich  are  associated  with  basic  volcanic  rocks. 

Especially  remarkable  are  the  evidences  of  the  close  association  of  iron-bearing  sediments 
and  basaltic  flows  in  the  upper  Huronian  of  Michigan.  Here  ellipsoidal  basalt,  basalt  tuffs, 
and  ashes  are  so  intermuigled  with  the  iron-beariiig  formation  and  stained  by  secondary  alter- 
ation that  there  is  difficulty  in  discriminating  them.  Recent  work  has  shown  the  existence  of 
more  of  the  igneous  rocks  than  had  before  been  suspected.  Drill  holes  in  tlie  Iron  River  and 
Amasa  areas  of  Michigan  pass  through  igneous  beds  from  2  to  50  feet  thick  in  the  midst  of  the 
iron-bearing  formation.  In  these  places  the  eye  can  scarcely  detect  the  break  between  the 
grayish  and  greenish  carbonate  slates  of  the  iron-bearmg  sediments  and  the  fine-grained  greenish 
basalts  and  tuffs.  Under  the  microscope  the  surface  of  contact  is  seen  to  be  an  extremely  irregular 
one,  the  carbonate  apparently  irregularly  replacmg  part  of  the  greenstone.  This  replacement 
has  not  been  accompanied  by  any  oxidation.  It  is  found  in  drill  holes  hundreds  of  feet  beneath 
the  surface,  apparently  in  an  association  determined  at  the  time  of  the  deposition  of  the  iron- 
bearing  formation.  In  the  Keewatin  of  the  Vermilion  district  of  Minnesota  similar  close  asso- 
ciation may  be  observed  between  the  jaspers  and  the  basalts.     (See  PI.  XLVIII,  p.  564.) 

The  significance  of  the  apparent  gradation  of  carbonate  of  iron  and  siUca  and  their  altera- 
tion products  into  the  greenstone  is  not  yet  fuUy  apparent.  It  can  scarcely  be  doubted  that 
tliis  relation  was  developed  at  the  time  of  the  deposition  of  the  iron-bearing  formation,  prob- 
ably soon  after  the  extrusion  of  the  igneoiis  rocks.  It  is  suspected  that  these  phases  represent 
a  transition  between  reactions  associated  with  the  hot  igneous  masses  and  the  normal  precipi- 
tation of  a  sedimentary  formation.  Attempt  has  been  made  in  the  laboratory  to  reproduce 
these  remarkably  close  relations  by  some  combination  of  igneous  and  sedimentaiy  processes, 
but  thus  far  without  successful  results. 

Probably  of  significance  in  connection  with  the  derivation  of  the  iron-bearing  formations 
is  the  fact  that  in  many  places  acidic  intrusive  and  extrusive  rocks  of  the  porphyry  type  closely 
foUo^v  extrusive  basalts  and  are  locally  even  more  closely  associated  with  the  iron-bearing  for- 
mations than  the  basalts  themselves.  This  relation  is  well  illustrated  in  the  Vermilion  district, 
where,  in  a  series  of  mterbedded  basalt  flows,  jaspers,  and  amygdaloidal  porphyries,  the  igneous 
rock  immediately  next  to  the  jaspers  is  commonly  porphyry  as  weU  as  basalt.  (See  fig.  13, 
p.  123.)  Similar  conditions  appear  in  the  Woman  River  district  of  Ontario"  and  elsewhere. 
It  is  suspected  that  this  relation  is  more  general  than  is  yet  known.     (See  p.  513.) 

The  amount  of  igneous  material  extruded  is  not  measured  by  the  areas  of  upper  Huronian 
volcanic  rocks  now  exposed,  for  extensive  extrusive  rocks  were  undoubtedly  present  in  parts 
of  the  formation  that  have  been  removed  by  erosion  and  exist  in  parts  not  yet  uncovered.  It 
is  suggested  in  the  chapter  on  the  Keweenawan  (Chapter  XV)  that  the  present  shore  of  the  Lake 
Superior  basin  was  the  locus  of  the  extrusion  of  the  Keweenawan  igneous  rocks.  If  the  basin 
began  to  form  in  Animikie  time,  as  is  thought  possible  (see  pp.  622-623),  a  siiaular  suggestion, 
for  similar  reasons,  might  be  made  for  the  Animikie  group,  in  which  case  the  north  shore  Ani- 
mikie may  really  not  be  so  distant  from  igneous  rocks  as  now  appears.  The  iron-bearing 
formation  of  the  Animikie  group  of  the  north  shore  is  thus  associated  in  time  with  igneous 
extrusions,  but  may  be  somewhat  distant  in  place. 

a  Allen,  R.  C,  Iron  formation  of  Woman  Elver  area;  Eighteenth  Ann.  Rept.  Ontario  Bur.  Mines,  pt.  1, 1909,  pp.  254-262. 


508  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

The  deposition  of  the  lower  Huronian  was  not  accompanied  by  basic  flows,  and  it  does  not 
contain  a  well-developed  iron-bearing  formation.  The  Paleozoic  of  the  Lake  Superior  region 
lack.s  basic  igneous  rocks  and  also  lacks  iron-bearing  formations  like  those  of  the  pre-Cambrian. 

ASSOCIATION    OF    IRON-BEARING    SEDIMENTS    AND    ERUPTIVE    ROCKS    OUTSIDE    OF  THE 

LAKE   SUPERIOR  REGION. 

The  derivation  of  the  iron-bearing  formations  from  the  associated  igneous  rocks  is  sug- 
gested by  the  close  association  of  these  rocks  not  only  in  the  Lake  Superior  country  but  in 
other  ])arts  of  the  world. 

Practically  all  the  numerous  iron-beaiing  sediments  extending  through  the  Height  of 
Land  country  of  Canada,  as  far  east  as  the  Quebec  boundary,  are  interbedded  with  basalt  flows. 
Most  of  these  belts,  in  the  writers'  judgment,  belong  in  the  Keewatin. 

On  the  east  coast  of  Hudson  Bay  there  are  younger  Algonkian  rocks  containing  an  iron- 
beaiing  formation,  interbedded  wdth  fragmental  sediments  and  elhpsoidal  basalts.  As  Low  " 
had  called  attention  to  the  similarity  of  these  iron-bearing  sediments  to  those  of  the  upper 
Huronian  or  Animikie  of  the  Lake  Superior  region,  the  junior  author  visited  them  in  1909  and 
found  a  veiy  close  similarity,  even  to  the  possession  of  carbonate  and  greenalite  phases.  Freedom 
from  vegetation  and  precipitous  shores  afford  fine  exposures  for  study.  Fragmental  sediments 
of  the  type-  now  bemg  formed  along  the  shores  are  interbedded  with  extiiisions  of  elhpsoidal 
basalt  which  give  evidence  by  their  textures  and  associations  of  having  been  extruded  along 
tidal  flats,  and  by  their  high  content  of  jasper  and  magnetite  of  having  been  rich  in  iron 
salts  at  the  time  of  their  extrusion.  Immediately  following  the  basalt  comes  the  iron-bearing 
formation,  closely  associated  with  volcanic  muds.  It  requires  no  preconceived  hypothesis  to 
lead  the  observer  to  the  view  that  the  extrusion  of  the  igneous  rocks  was  the  variant  in  the 
normal  conditions  of  sedimentation  necessary  to  produce  the  iron-bearing  formations.  The 
story  is  so  clear  that  it  is  possible  to  outhne  the  probable  conditions  of  sedimentation  in  some 
detail." 

Geikie  "^  remarks  concerning  lower  Carboniferous  basalts  of  the  Fife  coast: 
These  lavas  are  thin  sheets,  often  not  more  than  15  or  20  feet  in  thickness,  and  they,  as  well  as  the  associated  tuffs 
are  intercalated  among  shallow-water  deposits,  such  as  cyprid  shales  and  limestones,  coal  seams  with  fire  clays,  thin 
sandstones,  and  ironstones.     Some  of  the  basalts  have  caught  up  portions  of  the  mud  on  the  sea  bottom,  but  in  others 
the  muddy,  sandy,  or  ashy  sediment  of  the  next  deposit  has  fallen  into  the  interspaces  between  the  pillows. 

He  also  says'*  concerning  the  basaltic  lavas  of  County  Tyrone,  Ireland: 

These  greenish  lavas  are  occasionally  interleaved  with  gray  flinty  mudstones,  cherts,  and  red  jaspers,  which  are 
more  particularly  developed  immediately  above.  In  lithological  character,  and  in  their  relation  to  the  diabases,  tliese 
siliceous  bands  bear  the  closest  resemblance  to  those  of  Arenig  age  in  Scotland,  but  no  recognizable  Kadiolaria 
have  yet  been  detected  in  them. 

Describing  the  Carboniferous  volcanoes  of  the  Isle  of  Man,  Geikie  e  says : 

Pauses  in  the  succession  of  eruptions  are  marked  by  the  intercalation  of  seams  of  limestone  or  groups  of  limestone, 
shale,  and  black  impure  chert.  Such  interstratifications  are  sometimes  curiously  local  and  interrupted.  They  may 
be  observed  to  die  out  rapidly,  thereby  allowing  the  tuff  above  and  below  tliem  to  unite  into  one  continuous 
mass.  They  seem  to  have  been  accumulated  in  hollows  of  the  tuff  during  somewhat  prolonged  inter\-als  of  volcanic 
quiescence,  and  to  have  been  suddenly  brought  to  an  end  by  a  renewal  of  the  eruptions.  There  are  some  four  or  five 
such  intercalated  groups  of  calcareous  strata  in  the  thick  series  of  tuffs,  and  we  may  regard  them  as  marking  the  chief 
pauses  in  the  continuity  or  energy  of  the  volcanic  explosions. 

Again,  Geilde  ^  states  that  in  the  Carboniferous  volcanoes  of  Devonshire — 

Bands  of  black  chert  and  cherty  shale  are  interpolated  among  the  tuffs,  which  also  contain  here  and  there  nodular 
lumps  of  similar  black  impure  earthy  chert — an  interesting  association  like  that  alluded  to  as  occurring  in  the  l"ar- 
lK)niferous  volcanic  series  of  the  Isle  of  Man,  and  like  the  occurrence  of  the  radiohirian  cherts  with  the  Lower  Silurian 
volcanic  series. 

o  Low,  .V.  p..  Report  on  an  e>qiloration  of  the  east  coast  of  Hudson  Bay  from  Cape  Wolstcnholme  to  the  south  end  of  James  Bay:  .\nn.  Kept. 
Geol.  Survey  Canada,  vol.  13,  new  stT.,  pi.  L),  l'ju;i,  pp.  45—10. 

<>  I.cith,  C.  K.,  .\n  .Mgonkitin  liusin  in  Hudson  Hay — a  comparison  with  the  Lake  Superior  basin:  Econ.  Geology,  vol.  5, 1910,  pp.  227-246. 

c  .\bstracts  I'roc.  Oeoi.  Soc.  London,  session  19()T-S,  London,  1908,  p.  42. 

d  Geikie,  j\jchil)ald,  Ancient  volcanoes  of  Great  Britain,  vol.  1,  London,  1897,  pp.  240-241. 

<•  Idem,  vol.  2, 1897,  p.  24. 

/  Idem,  vol.  2, 1897,  p.  36. 


THE  IRON  OKES.  509 

The  following  section  in  Tertiary  volcanoes  of  the  Antrim  Plateau  of  Ireland  is  described 
by  the  same  author:" 

Upper  basalt,  compact  and  often  columnar  sheets. 

Brown  laminated  tuff  and  volcanic  clays. 

Laminated  brown  impure  earthy  lignite,  2  feet  3  inches. 

Brown  and  red  variegated  clays,  tuffs,  and  sandy  layers,  with  irregular  seams  of  coarse  conglomerate 
composed  of  rounded  and  sul>angular  fi'agments  of  rhyolite  and  ba.salt,  3  feet  4  inches. 

Brown,  red,  and  yellowish  laminated  tuffs,  mudstones,  and  bole,  with  occasional  layers  of  fine  con- 
glomerate (rhyolitic  and  basaltic),  pisolitic  iron-ore  band,  and  plant  beds,  8  feet  10  inches. 

Lower  basalt,  amygdaloidal. 

The  pale  and  colored  clays  that  occur  in  this  marked  sedimentary  intercalation  have  doubtless  been  produced 
by  the  decomposition  of  the  volcanic  rocks  and  the  washing  of  their  fine  detritus  by  water.  Possibly  this  decay  may 
have  been  in  part  the  result  of  solfataric  action.     *     *     * 

*  *  *  The  original  area  over  which  the  iron  ore  and  its  accompanying  tuffs  and  clays  were  laid  down  can  hardly 
have  been  less  than  1,000  square  miles.  This  extensive  tract  was  evidently  the  site  of  a  lake  during  the  volcanic 
period,  formed  by  a  sulisidence  of  the  floor  of  the  lower  basalts.  The  salts  of  iron  contained  in  solution  in  the  water, 
whether  derived  horn  the  decay  of  the  surrounding  lavas  or  from  the  discharges  of  chalybeate  springs,  were  precipitated 
as  peroxide  in  pisolitic  form,  as  similar  ores  are  now  being  formed  on  lake  bottoms  in  Sweden.  For  a  long  interval 
quiet  sedimentation  went  on  in  this  lake,  the  only  sign  of  volcanic  energy  during  that  time  being  the  dust  and  stones 
that  were  thrown  out  and  fell  over  the  water  basin  or  were  washed  into  it  by  rains  from  the  cones  of  the  lava  slopes 
around. 

Concerning  the  Tertiary  volcanoes  of  the  plateau  of  Small  Isles,  Geikie*  writes: 

It  is  a  noteworthy  fact  that  the  sedimentary  intercalations  among  the  Canna  basalts  generally  end  upward  in 
carbonaceous  shales  or  coaly  layers.  The  strong  currents  and  overflows  of  water,  which  rolled  and  spread  out  the  coarse 
materials  of  the  conglomerates,  gave  way  to  quieter  conditions  that  allowed  silt  and  mud  to  gather  over  the  water 
bottom,  while  leaves  and  other  fragments  of  vegetation,  blown  or  washed  into  these  quiet  reaches,  were  the  last  of 
the  suspended  materials  to  sink  to  the  bottom. 

The  Arenig  eruptions  in  the  Silurian  of  North  Wales  contain  interesting  sediments,  described 
by  Geikie  '^  as  follows: 

Many  of  the  tuffs  that  are  interstratified  with  black  slates  (?  Lingula  flags)  at  the  foot  of  the  long  northern  slope  of 
Cader  Idris  consist  mainly  of  black-slate  fragments  like  the  slate  underneath,  with  a  variable  proportion  of  gray  volcanic 
dust.     *    *    * 

One  of  the  most  interesting  deposits  of  these  interludes  of  quiescence  is  that  of  the  pisolitic  ironstone  and  its 
accompanying  strata  on  the  north  front  of  Cader  Idris.  A  coarse  pumiceous  conglomerate  with  large  slaglike  blocks 
of  andesite  and  other  rocks,  seen  near  Llyn-y-Gadr,  passes  upward  into  a  fine  bluish  grit  and  shale,  among  which  lies 
the  bed  of  pisolitic  (or  rather  oolitic)  ironstone  which  is  so  widely  diffused  over  North  Wales.  The  finely  oolitic 
structure  of  this  band  is  obviously  original,  but  the  substance  was  probalily  deposited  as  carbonate  of  lime  under  quiet 
conditions  of  precipitation.  The  presence  of  numerous  small  Lingidie  in  the  rock  shows  that  molluscan  life  flourished 
on  the  spot  at  the  time.  The  iron  exists  in  the  ore  mainly  as  magnetite,  the  original  calcite  or  aragonite  having  been 
first  replaced  by  carbonate  of  iron,  which  was  subsequently  broken  up  so  as  to  leave  a  residue  of  minute  cubes  of 
magnetite. 

Radiolarian  cherts  are  characteristically  associated  with  sandstones  and  basalts,  partly 
ellipsoidal,  at  Point  Bonita,''  Angel  Island,^  and  at  many  other  points  in  the  Coast  Ranges  of 
California.     In  describing  the  eruptive  rocks  of  Point  Bonita,  Ransome  saj's:  ^ 

Spheroidal  basalt,  apparently  similar  to  that  described,  has  been  noted  by  the  writer  at  Tiburon,  Marin  County 
at  Port  Harford,  San  Luis  Obispo  County;  and  on  the  summit  of  the  north  peak  of  Mount  Diablo.  It  is  noteworthy 
that  in  these  widely  separated  occurrences  the  rock  is  always  associated  with  the  red  jaspers,  and  with  what  is  apparently 
the  San  Francisco  sandstone. 

These  cherts  were  called  "phthanites"  by  Becker  ff  and  regarded  as  due  to  secondary  silici- 
fication.     Lawson  ''  and  Ransome,^'  on  the  other  hand,  regard  them  as  original  siliceous  deposits 

a  Geikie,  Arcliibald ,  Ancient  volcanoes  of  Great  Britain,  vol.  2, 1897,  pp.  204-205. 

t  Idem,  vol.  2, 1897,  p.  223. 

c  Idem,  vol.  1, 1897,  pp.  180-lSl. 

dRansome,  F.  L.,  The  eruptive  rocks  of  Point  Bonita:  Bull.  Dept.  Geology,  Univ.  California,  vol.  1,  1893,  pp.  71-114. 

e  Ransome,  F.  L.,  The  geology  of  Angel  Island;  Bull.  Dept.  Geology  Univ.  California,  vol.  1, 1S94,  pp.  193-240. 

/  Ransome,  F.  L.,  The  eruptive  roclcs  of  Point  Bonita:  Bull.  Dept.  Geology  Univ.  California,  vol.  1. 1893.  pp.  109-110. 

e  Becker,  G.  F.,  Geology  of  the  quicksilver  deposits  of  the  Pacific  coast:  Mon.  U.  S.  Geol.  Survey,  vol.  13,  1SS8,  pp.  10.5-108. 

*  Lawson.  A.  C,  Sketch  of  the  geology  of  the  San  Francisco  peninsula:  Fifteenth  Ann.  Kept.  U.  S.  Geol.  Survey,  1895,  pp.  420-426. 

<  Ransome,  F.  L.,  The  geology  of  Angel  Island:  Bull.  Dept.  Geology  Univ.  California,  vol.  1, 1894,  p.  200. 


510  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

which  are  changed  into  red  jaspers  and  glaucophanic  jaspers  here  and  there  at  igneous  contacts. 
These  cherts  locally  pass  into  iron  ore  and  are  characteristically  associated  with  njanganese 
beds."  The  cherts  are  characterized  by  mbiute  oval  spots  found  in  part  to  represent  radio- 
larian  remains,  but  in  part  of  unknown  origin.     Lawson''  discusses  their  origin  as  follows: 

It  thus  seems  to  the  writer  that  the  bulk  of  the  silica  can  not  be  proved  to  be  the  extremely  altered  d(5bris  of 
Radiolaria.  The  direct  petrographical  suggestion  is  that  they  are  chemical  precipitates.  If  now  we  accept  this  hj^poth- 
esis,  it  becomes  apparent  that  there  are  three  possible  sources  for  the  silica  so  precipitated,  \'iz,  (1)  siliceous  springs 
■in  the  bottom  of  the  ocean,  similar  to  those  well  known  in  volcanic  regions;  (2)  radiolarian  and  other  siliceous  remains, 
which  may  have  become  entirely  dissolved  in  sea  water;  and  (3)  volcanic  ejectamonta,  which  may  have  become 
similarly  dissolved.  The  last  is  the  least  probable,  because  we  are  not  actually  familiar  with  such  a  reaction  as  the 
solution  of  volcanic  glass  by  sea  water.  Our  ignorance  is,  however,  no  proof  that  such  solution  may  not  take  place 
under  special  conditions.    *    *    * 

The  hypothesis  of  the  derivation  of  the  silica  from  siliceous  springs  and  its  precipitation  in  the  bed  of  the  ocean 
in  local  accumulations,  in  which  radiolarian  remains  became  embedded  as  they  dropped  to  the  bottom,  seems,  there- 
fore, the  most  adequate  to  explain  the  facts,  and  there  is  nothing  adverse  to  it  so  far  as  the  writer  is  aware.  The  abun- 
dance of  the  Radiolaria  may  be  due  to  the  favorable  conditions  involved  in  the  excessive  amount  of  silica  locally  present 
in  the  sea,  or  simply  to  the  favorable  conditions  for  preservation  afforded  by  this  kind  of  rock.  If  the  springs  were 
strong,  the  currents  engendered  might  in  some  places  have  been  sufficient  to  deflect  sediment-laden  counter-currents, 
and  this  may  serve  to  explain  the  general  absence  of  clastic  material  in  the  chert. 

The  Pilot  Knob  deposits  of  Missouri  are  interbedded  with  porphyry  flows,  tuffs,  and  ashes, 
suggesting  close  genetic  relation  between  igneous  rocks  and  sediments. 

Illustrations  could  be  multiplied,  but  enough  have  been  cited  to  show  that  basalts,  espe- 
cially the  ellipsoidal  phases,  are  characteristically  interbedded  with  more  or  less  graphitic 
slates,  clays,  cherts,  jaspers,  volcanic  tuffs,  iron  ores,  and  in  places  sandstone.  Practically  all 
the  features  of  the  association  of  basalt  with  sediments  described  for  the  above-mentioned 
districts  are  to  be  seen  in  the  Lake  Superior  region.  The  explanations  of  these  associations 
in  other  regions  therefore  become  significant  in  the  study  of  the  origm  of  the  Lake  Superior  ores. 

In  general  there  seems  to  be  little  doubt  that  some  genetic  relationship  e-xists  among 
surface  basalts,  carbonaceous  slates,  cherts,  and  jaspers,  to  which  attention  has  been  called  by 
several  writers  worldng  from  different  standpomts."^  They  agree  that  most  of  the  carbonaceous 
materials  are  organic,  that  the  deposition  is  largely  subacjueous,  and  that  some  of  the  associated 
iron  is  deposited  partly  through  the  agency  of  weathering  assisted  by  organic  means.  Lawson 
suggests  that  the  cherts  and  jaspers  may  be  the  result  of  inorganic  chemical  deposition  by  hot 
solutions.  In  the  Lake  Superior  region  the  iron-bearing  formations  are  much  thicker  and 
they  have  certain  phases,  notably  the  greenalite  or  ferrous  silicate  phase,  which  are  not  common 
elsewhere,  all  these  features  seeming  to  favor  the  hj-pothesis  that  the  iron  formations  are  in  part 
related  to  the  more  or  less  direct  contribution  of  the  iron-bearing  materials  by  hot  concentrated 
solutions  from  the  igneous  roclis. 

SIGNIFICANCE  OF  ELLIPSOIDAL  STKUCTURE  OF  EBtTPTIVE  ROCKS  IN  RELATION  TO  ORIGIN 

OF  THE  ORES. 

The  basalts  associated  with  the  iron-bearing  formations  have  so  commonl}'  the  peculiar 
ellipsoidal  or  pillow  structure  that  one  is  led  to  assume  that  conditions  favorable  to  the  develop- 
ment of  the  ellipsoidal  structure  may  be  also  favorable  to  the  deposition  of  the  iron  ore  in  this 
district.  Clements  **  has  described  the  structure  in  some  detail  for  the  Crj'stal  Falls  district, 
and  from  comparison  with  occurrences  elsewhere  concludes  it  to  have  been  probably  a  submarine 
extrusive,  similar  to  the  aa  lavas  of  Hawaii  described  by  Button.''     Dah^  f  reaches  the  same 

a  Lawson,  A.  C,  op.  cit.,  pp.  423-424. 

b  Idem,  pp.  425-42C. 

c  Wo  have  received  too  late  for  discussion  a  paper  on  British  pillow  lavaa  and  the  rocks  associated  with  them,  by  nenrj-  Dewey  and  J.  S. 
Fleet  (Gcol.  Mag.,  vol.  8.  Dec.  5, 1911,  pp.  202-209,  241-24S),  emphasizing  the  genetic  association  of  cherts  and  ellipsoidal  iHisalts.  .Vlbilization  of 
thefeldspars  of  the  basalts  is  regarded  as  evidence  of  pneuraatolytic  emanations,  containhig  soda  and  silica  in  solution  and  possibly  other  sub- 
stances. The  cherts  are  deposited  by  those  emanations.  This  independent  conclusion  is  remarkably  in  accord  with  the  inferences  drawn  in  this 
monograph. 

<l ricments,  J.  M.,The  Crystal  Falls  iron-bearing  district  of  Michigan:  Mon.  U.  S.  Geol.  Survey,  vol.  30, 1899,  pp.  112-124. 

tDutton,  C.  E.,  Hawaiian  volcanoes:  Fourth  .\nn   Rcpt.  U.  S.  Oeol.  Survey,  1884,  pp.  95-90. 

/  Daly.  R.  .\.,  Variolitic  pillow  lava  from  Newfoundland:  .\in.  Geologist,  vol.  32, 1903,  p.  77. 


THE  IRON  ORES.  511 

conclusion  for  the  variolitic  pillow  lavas  of  Newfoundland.  Later,  from  a  personal  studj-  of 
Hawaiian  volcanoes,  Dal}^"  regards  the  ellipsoidal  and  aa  lavas  as  different,  though  he  is  not 
disposed  to  question  the  subaqueous  origin  of  elli])soidal  lavas.  Geikie*  repeatedly  cites  the 
probable  subaqueous  origin  of  the  ellipsoidal  structure,  based  on  his  observations  in  Great 
Britain  and  Ireland.  Clement  Reid  "  has  recently  concluded  that  the  pillow  lavas  near  Port 
Isaac  in  t'ornwall  are  of  submarme  origin,  and  in  the  discussion  of  Reid's  paper  Geikie ''  remarked 
that  all  the  examples  of  pillow  lavas  with  which  he  was  acquainted  were  undoubtedly  true  lavas 
and  belonged  to  submarine  eruptions.  Some  of  them,  however,  must  have  been  poured  out  in 
shallow  water,  as  is  particularly  ob.servable  in  the  case  of  the  lower  Carboniferous  basalts  of  the 
Fife  coast.     (See  quotation  on  p.  508.) 

Femier^  concludes  that  ellipsoidal  and  other  structures  in  the  traps  of  the  Newark  group 
are  evidence  of  flowage  of  the  traps  into  lakes.     He  says: 

When  we  came  to  examine  the  lava  itself  we  saw  that  it  carried  in  its  own  mass  plain  evidences  of  the  structural 
changes  which  were  produced  by  the  presence  of  the  lakes  and  of  the  water-bearing  strata  beneath.  Whereas  beyond 
the  borders  of  the  lakes  the  lava  was  of  a  close,  firm  texture  and  showed  a  condition  of  quiet  and  tranquillity  during 
the  process  of  cooling  and  hardening,  over  the  area  of  lake  bottom  there  was  evidenceof  violent  agitation  having  affected 
it  during  the  initial  flows,  and  rapid  cooling  and  the  production  of  much  glaesy  material  during  succeeding  flows,  fol- 
lowed still  later  by  the  crystallizing  effects  wrought  by  heated  waters  and  the  production  of  secondary  minerals. 

By  others  the  ellipsoidal  structure  has  been  regarded  as  the  result  of  rapid  coolmg  or  rapid 
flow  developmg  large  blocks  that  have  rolled  one  over  another,  a  process  which  may  have  been 
subaerial  or  subaciueous,  or  both.  This  is  the  explanation  offered  by  Cole  and  Gregory.^  Ran- 
some  ^  concludes  for  the  ellipsoidal  structure  in  the  basalt  of  Point  Bonita,  California,  that  one 
sluggish  outwelliiig  of  lava  was  piled  upon  another  to  form  the  whole  mass  of  the  flow,  the 
blocks  or  ellipsoids  being  incidental  to  the  cooling  and  movement.  He  makes  no  reference  to 
submarine  or  subaqueous  origin.  Russell ''  observes  that  the  ellipsoidal  structure  found  locally 
in  the  Snake  River  basalts  is  developed  by  the  flowage  of  the  basalts  into  lake  basms,  but  con- 
cludes *'  that  whether  the  lava  develops  the  ropy  or  pillow  or  block  structure  is  determined  by — 

the  ratio  between  rate  of  cooling  and  the  rate  of  motion.  But  this  ratio  is  not  the  same  for  different  lavas,  ^\^len  a 
lava  sheet  cools  without  motion,  neither  a  characteristic  pahoehoenor  anaa  surface  is  produced.  Many  of  the  older 
sheets  of  Snake  River  lava  illustrate  this;  they  are  simply  plane  surfaces,  composed  of  either  vesicular  or  compact 
granular  basalt. 

The  explanation  of  the  origin  of  aa  adopted  above  was  not  accepted  by  Dana,  J  who  suggests  that  the  breaking  of 
a  lava  crust  may  be  due  to  moisture  derived  from  the  rocks  over  which  lava  flows  and  leading  to  quicker  cooling  in 
certain  areas  than  in  others.  Such  an  occurrence,  however,  even  if  proved  to  exert  an  influence,  seemingly  introduces 
a  variation  into  a  more  general  process  without  supplanting  the  controlling  conditions. 

Dr.  Tempest  Anderson  and  Dr.  Flett  *  describe  such  structure  developing  subaerially  at 
Mount  Pelee,  and  Anderson  '  describes  it  also  developing  subaerially  in  Iceland. 

The  evidence  seems  to  be  that  the  ellipsoidal  structure  is  both  subaqueous  and  subaerial 
in  its  development,  that  it  is  produced  by  the  rolling  of  blocks  developetl  during  the  flow  of  the 
lava  as  a  result  of  cooling,  and  that  its  development  is  therefore  determined  bj^  the  speed  of 
flow  and  the  rate  of  cooling,  which  in  turn  may  be  aflfected  by  entrance  into  water.  Where 
associated  with  sediments,  the  structure  seems  to  be  with  little  doubt  subaqueous  in  origin, 
as  concluded  by  Geikie.  In  the  Lake  Superior  region  the  interbedding  of  ellipsoidal  basalts 
with  sediments  of  subaqueous  origin,  according  well  with  the  associations  of  basalt  flows 
and  sedimentary  rocks  that  are  observed  elsewhere,  seems  to  be  adequate  evidence  that  the 

o  Verbal  communication. 

(>  Geikie,  .\rchibald,  Ancient  volcanoes  oJ  Great  Britain,  London,  1897.  i 

cReid,  Clement,  and  Dewey,  Henry,  The  origin  of  the  pillow  lava  near  Port  Isaac  in  Cornwall;  .\bstracts  Proc.  Geol.  Soc.  London,  session 
1907-8,  London,  1908,  p.  42. 
rfldem. 

eFenner,  C.  N.,  Featuresof  trap  extrusions  in  New  Jersey:  Jour.  Geology,  vol.  ll'i,  1908,  p.  320. 

/Cole,  G.  A.  J.,  and  Gregory,  J.  W.,  On  the  variolitic  rocks  of  Mont  Gen(>vre:  Quart.  Jour.  Geol.  Poc,  vol.  40, 1890,  p.  310. 
ffRansome,  F.  L.,  The  eruptive  rocks  of  Point  Bonita,  California:  Bull.  Dept.  Geology,  Univ.  California,  vol.  1, 1S93,  p,  112. 
^  Russell,  I.  C,  Geology  and  water  resources  of  the  Snake  River  plains  of  Idaho:  Bull.  U.  S.  Geoi.  Survey,  No.  199, 1902,  pp.  82  e(  seq. 
ildem,  p.  98. 

;  Dana,  J.  D.,  Characteristics  of  volcanoes,  New  York,  1890,  pp.  242-244. 
i- Cited  in  .Vbstracts  Proc.  Geol.  Soc.  London,  session  1907-8,  London,  190S,  p.  42. 
Udem,  p.  44. 


512  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

ellipsoidal  structure  of  the  Lake  vSuperior  basalts  is  largely  of  subaqueous  origin.  It  should 
not  be  a.ssumed,  however,  that  all  the  ellipsoidal  basalts  of  the  Lake  Superior  region  are  neces- 
sarily subaqueous.  The  region  is  a  large  one,  the  conditions  arc  varied,  the  ellipsoidal  struc- 
tures are  locally  associated  with  structures  ordinarily  regarded  as  of  subaerial  origin,  ellipsoidal 
structure  is  known  elsewhere  to  develop  subaerially,  hence  it  is  rather  likely  that  a  part  of 
the  structures  in  the  Lake  Superior  region  are  of  subaerial  origin.  There  is  little  prospect  that 
evidence  will  be  forthcoming  to  determine  exactly  the  quantitative  importance  of  the  subaerial 
deposit  as  compared  with  the  subaqueous  deposit;  indeed,  there  seems  to  be  little  need  of  such 
determination  wlien  it  is  recognized  tliat  both  are  present.  Qualitativel}'  the  evidence  favors 
the  subaqueous  origin  of  the  major  part  of  the  ellipsoidal  basalts. 

ERUPTIVE    ROCKS    ASSOCIATED    WITH    IRON-BEARING    SEDIMENTS    OF    LAKE    SUPERIOR 

REGION   CARRY   ABUNDANT   IRON. 

Abundant  sulpliides  and  associated  magnetite  are  disseminated  in  quartz  veins  and  irregular 
quartz  masses  through  the  ellipsoidal  greenstones  of  the  Lake  Superior  region  antl  of  much  of 
the  pre-Cambrian  shield  of  Canada.  The  abundance  of  these  sulphides  through  all  parts  of 
these  greenstones  has  been  noted  by  many  observers.  They  are  exceptionally  conspicuous  in 
the  Canadian  part  of  the  region,  where  erosion  has  cut  down  into  the  fresh  rocks  and  exposed 
sulpiride  veins  that  have  not  had  time  to  be  deeply  oxidized  at  the  surface  since  the  glacial  epoch. 
That  certain  of  the  sulpliides  and  the  associated  magnetites  of  the  basic  igneous  rocks  crystal- 
lized soon  after  the  crystallization  of  the  igneous  roclcs,  and  are  not  later  secondary  replacements 
of  such  rocks,  is  shown  by  evidence  of  several  kinds,  as  follows: 

1.  They  are  minutely  disseminated  tlirough  the  greenstone  and  grade  into  pegmatitic  veins. 

2.  The  sulphides  and  the  greenstones  of  this  t^i^e  are  colimital,  and  the  sulphides  are  not 
found  so  abundantly  in  any  other  rocks,  a  fact  wliich  would  be  difficult  to  explain  were  the 
sulphides  the  result  of  later  introduction  by  percolating  meteoric  waters  or  by  later  extrusions. 

3.  The  matrix  of  the  ellipsoidal  basalt  flows  is  in  places  so  higlily  charged  with  magnetite 
as  to  disturb  the  magnetic  needle  greatly,  and  the  amount  of  magnetite  is  much  less  at  the 
ellipsoids.  Illustrations  of  this  are  found  in  the  Hemlock  formation  in  the  vicinity  of  the 
Armenia  and  Mansfield  mines,  in  the  Crj'stal  Falls  district  of  Michigan,  and  in  the  Keewatin 
basalts  associated  with  jaspers  southwest  of  Elyj  in  the  Vermilion  district  of  Minnesota.  The 
matrix  being  the  last  part  of  these  masses  to  crystallize,  the  magnetite  is  obviously  introduced 
late  in  the  extrusion  of  the  mass.     Sulphide  of  iron  is  present  in  the  same  relations. 

4.  Many  of  the  amygdules  in  the  basalts  are  wholly  or  partly  filled  \\dth  magnetite  or 
jasper,  or  both.  Near  the  Gibson  mine,  south  of  Amasa,  in  the  Crystal  Falls  district  of  ilichi- 
gan,  red  jasper  fillings  in  amygdaloids  are  verj'  conspicuous.  The  amygdule  fillings  in  general 
are  characteristic  of  hot  solutions  such  as  would  accompany  the  extrusion  of  the  mass  and  not 
of  cold  meteoric  solutions.     (See  PI.  XXXYI,  A.) 

5.  Plate  XLVIII  (p.  564)  shows  a  Keewatin  basalt  with  gradation  phase  through  siliceous 
basalt  into  banded  sihceous  iron-bearing  formation.  In  the  area  from  wluch  these  specimens 
were  taken,  as  well  as  in  other  parts  of  the  Iveewatin,  it  is  practically  impossible  to  draw  a  line 
between  unaltered  basalt  and  the  iron-bearing  formation.  Tliis  gradation  seems  to  be  one 
developed  on  the  original  sohdification  of  the  mass.  The  fi-eshness  of  the  basalt,  the  lack  of 
katamorphism  along  the  contact  with  the  quartz,  and  the  extremely  vague  surfaces  and  general 
lack  of  vein  structures  are  not  characteristic  of  later  introductions  of  the  quartz  after  weathering. 

6.  Some  parts  of  the  magnetic  iron-bearing  formations  are  so  related  to  the  associated 
basalts  as  to  suggest  that  the  iron  represents  pegmatitic  vein  material  wliich  developed  directly 
from  the  igneous  rock.     Such  instances  are  cited  for  the  Atikokan  and  Vermilion  districts. 

Evidence  is  everywhere  to  be  found  that  these  various  iron  salts  associated  with  the  surface 
extrusive  rocks  represent  remnants  of  outpourings  of  concentrateil  iron  solutions  after  the  main 
mass  of  the  basalt  had  crystallized.  Deep-seated  equivalents  of  the  basaltic  extrusive  rocks 
are  believed  to  bo  the  gabbros  which  carry  large  masses  of  titaniferous  magnetite  representing 
iron  salts  that  diil  not  have  an  opportunity  to  escape  at  the  surface. 


THE  IRON  ORES.  513 

The  fact  that  in  some  places  the  iron-bearing  formation  seems  to  be  related  to  late  acidic 
phases  of  extrusions,  us  has  been  noted  on  page  507,  suggests  the  extrusion  of  the  iron  and  the 
acidic  phases  as  extreme  differentiation  products  from  the  magma.  The  association  of  extremes 
of  this  type  is  not  uncommon. 

GENETIC    RELATIONS    OF    UPPER    HURONIAN    SLATE    TO    ASSOCIATED   ERtTPTIVE    ROCKS. 

The  iron-bearing  formations  of  the  upper  Huronian  are  so  closely  associated  with  slate 
that  evidence  bearing  on  the  origin  of  the  slate  throws  light  on  the  origin  of  the  associated 
iron-bearing  formations.  In  figure  76  (p.  612),  prepared  by  S.  II.  Davis,  the  miner-ilogical  com- 
position of  the  upper  Iluronian  slate,  calculated  from  chemical  composition,  is  compared  graphi- 
cally with  that  of  a  variety  of  other  claj^s  and  soils.  It  appears  from  this  comparison  that  the 
slate  as  a  whole  gives  evidence  hj  its  composition  of  bemg  less  leached  of  its  bases  than  average 
slates  or  residual  clays  and  that  it  has  been  derived  from  basic  rocks.  It  may  be  due  partly 
to  weathering  of  the  greenstones,  to  direct  contribution  of  volcanic  ash  and  muds,  and  possibl}' 
even  to  direct  reaction  with  sea  water.      (See  pp.  610-614.) 

MAIN    MASS    OF    IRON-BEARING    SEDIMENTS    PROBABLY    DERIVED    FROM    ASSOCIATED 

ERUPTIVE   ROCKS. 

The  close  association  of  iron-bearing  sediments  with  contemporaneous  basic  eruptive  rocks 
in  the  Lake  Superior  region  and  in  other  parts  of  the  world,  the  riclmess  of  these  eruptive 
rocks  m  iron  salts,  and  the  probable  derivation  of  the  upper  Huronian  slates  associated  with 
the  iron-bearing  formations  from  the  eruptions  make  it  a  plausible  hypothesis  that  these  iron- 
rich  eruptive  rocks  were  the  principal  source  of  the  iron  in  the  iron-bearing  sediments.  As 
to  the  manner  in  which  the  iron  was  transferred  from  the  eruptive  rocks  to  the  place  of  sedi- 
mentation, there  are  several  possible  hypotheses.  (1)  It  may  have  been  transferred  in  hot 
solutions  migrating  from  the  eruptive  material  during  its  solidification,  carrying  iron  salts 
from  the  interior  of  the  magma  which  had  never  been  crystallized;  (2)  so  far  as  the  lavas  were 
subaerially  extruded,  iron  may  have  been  transferred  by  the  action  of  meteoric  waters  working 
upon  the  crystallized  iron  minerals  in  the  magma,  either  hot  or  cold;  (.3)  the  iron  may  have 
been  transferred  by  direct  reaction  of  the  hot  magma  with  sea  water,  in  which  the  iron-bearing 
sediments  were  deposited. 

DIRECT    CONTRIBUTION    OF    IRON    SALTS    IN    HOT    SOLUTIONS    FROM    THE    MAGMA. 

That  the  igneous  rocks  contributed  some  of  their  iron  solutions  directly  to  the  water  in 
which  the  iron-bearing  sediments  were  being  deposited  is  suggested  by  the  fact  that  basic 
extrusive  rocks  have  a  widely  developed  ellipsoidal  structure,  which  has  been  ascribed  by  many 
observers  to  submarine  extrusion.  (See  pp.  510-512.)  If  these  lavas  are  submarine,  then 
any  iron  salts  extruded  must  have  been  contributed  directly  to  the  ocean.  It  will  be  shown 
in  the  following  pages  that  if  the  salts  were  so  contributed  simple  and  probable  chemical 
reactions  would  develop  the  original  greenalite  or  iron  silicate  phases  of  the  iron-bearing 
formations.  Such  phases  largely  lack  the  carbonaceous  slates  so  closely  associated  with  the 
carbonates.  It  was  found  in  the  laboratory  that  the  precipitation  of  the  greenalite  phase  of 
the  iron-bearing  formations  required  heat  in  the  presence  of  carbon  dioxide  and  the  probable 
presence  of  salt  water,  in  both  contrasting  with  the  precipitation  of  iron  carbonate,  wliich  goes 
on  in  cold  solution,  favored  by  the  presence  of  reducmg  organic  agencies.  Direct  contribution 
would  favor  the  deposition  of  the  iron  salts  in  a  ferrous  condition  in  the  absence  of  reducing 
carbonaceous  material  and  would  avoid  the  oxidation  and  precipitation  which  they  woiUd 
undergo  if  partly  carried  subaerially. 

Further,  the  fact  that  iron-bearing  formation  seems  to  be  lacking  in  association  with 
certain  similar  greenstones  in  the  Lake  Superior  region  and  Canada  may  be  evidence  that  the 
iron-bearing  formations  derive  tlieir  materials  by  direct  magmatic  contributions.  Such  con- 
47517°— vol.  52—11 33 


514  GEOLOGY  OF  THE  LAIvE  SUPERIOR  REGION. 

tributions  are  known  to  be  local  and  variable  in  composition,  and  this  may  explain  the  localized 
distribution  of  the  iron-bearing  formations.  If  derived  entirely  by  weathering  of  basic  igneous 
rocks,  iron-bearing  formations  should  be  more  abundant  in  association  with  igneous  rocks 
outside  of  the  Lake  Superior  region. 

The  percentages  of  both  iron  and  silica  in  the  iron-bearing  formations  seem  to  be  too  high 
for  direct  derivation  from  crystallized  basalt  by  weathering.  Tiioy  soeni  to  accf)r(l  better  with 
the  hyi)othesis  that  the  iron  and  silica,  especially  tlie  silica,  were  precipitated  from  concentrated 
solutions  coming  directly  from  the  magma.  The  local  presence  of  acidic  igneous  rocks  between 
the  lavas  and  the  basalts  and  tlie  fact  that  the  acidic  rocks  are  slightly  later  than  tlie  basalts 
suggest  that  the  development  of  the  iron-bearing  formation  came  at  a  time  when  acidic  phases 
of  the  extrusion  were  coming  out.  The  iron  salts  and  the  acidic  phases  then  might  represent 
the  extreme  differentiation  products  of  a  primary  magma  of  which  the  basalt  was  the  first 
extrusion. 

Favoring  the  hypothesis  of  direct  contribution  of  the  iron  salts  from  the  lava  to  the  sea 
water  into  which  it  was  poured  is  the  lack  in  many  places  of  any  fragmental  material  between 
the  ia'on-bearing  formation  and  the  contemporaneous  lava  on  which  it  rests,  the  mutual  con- 
formity at  these  places,  and  the  absence  of  any  erosion  channels  in  tlie  greenstones.  In  the 
Vermilion  district  of  Minnesota  bands  of  iron-bearing  formation  have  been  traced  for  consider- 
able distances  resting  directly  upon  the  amygdaloidal  upper  surface  of  a  lava  How,  showing  no 
evidence  of  intervening  erosion  and  having  a  contact  like  a  knife  edge. 

The  subacpieous  extrusion  of  igneous  rocks  would  mean  the  sudden  destruction  of  any 
organic  material  m  the  near-by  sea,  to  judge  from  results  observed  near  present-day  extrusions. 
It  has  been  shown  that  after  an  eruption  the  sea  floor  has  been  covered  to  a  depth  of  several 
feet  off  Hawaii  by  dead  fish  and  other  organic  material.  It  is  entirely  jjossible  that  this  may 
explain  the  origin  of  some  of  the  carbonaceous  materials  so  closely  associated  with  the  iron- 
bearing  formations,  especially  in  the  Keewatin,  where  seams  of  rich  graphitic  slate  are  locally 
associated  with  the  iron-bearing  formation  and  the  basalt.  It  is  possible  also  that  this  material 
might  be  a  source  for  the  carbon  dioxide  necessary  for  the  formation  of  the  iron  carbonates. 
Quantitatively  it  is  probably  inadequate  to  explain  either  the  amount  of  carbon  dioxide  neces- 
sary for  the  formation  of  the  iron  carbonates  or  the  amounts  of  carbon  to  be  seen  in  the 
associated  slates.  It  is  mentioned  merely  as  a  possible  source  of  a  part  of  these  substances. 
Its  importance  can  not  be  quantitatively  demonstrated. 

So  far  as  the  parent  igneous  rocks  were  extruded  subaerially,  the  escaping  iron  solutions 
would  be  mingled  with  meteoric  waters,  perhaps  deriving  additional  iron  salts  from  the  breaking 
up  of  crystallized  minerals  described  under  the  next  heading. 

CONTRIBUTION   OF  IRON   SALTS   FROM    CRYSTALLIZED   IGNEOUS   ROCKS   IN    METEORIC   WATERS. 

Some  of  the  basaltic  extrusive  rocks  have  textures  indicative  of  subacrial  crystallization. 
Atmospheric  agencies,  therefore,  have  been  applied  during  the  transfer  of  the  iron  solutions 
to  the  ocean.  Weathering  agents  would  effectively  attack  sulphi<les  at  or  above  the  surface 
of  the  water,  especially  when  aided  by  organic  material  and  residual  heat.  Umler  ordinary 
weathering  these  sulphides  oxidize  and  form  soluble  iron  sulphate,  which  becomes  available 
for  the  sedimentation  of  the  iron-l)earing  formations.  The  same  reaction  iilierates  free  sul- 
phuric acid,  which  may  attack  the  iron  in  the  adjacent  rocks.  Still  further,  it  has  been 
found  that  acidic  gaseous  emanations  from  igneous  rocks  attack  readily  tiie  adjacent  rocks, 
leaching  from  them  their  iron,  partly  depositing  it  in  place  as  hydrated  oxide  and  partly 
carrying  it  away  in  solution  as  a  sulphate.  A  highly  instructive  cpiaiuitative  study  of  the 
Hawaiian  basalts  by  Maxwell  "  shows  the  effectiveness  t>f  acidic  solutions  of  tliis  kind  in 
decomi)osing  the  rocks  antl  segregating  the  iron.  The  marked  softening  and  disintegration  of 
the  rocks  nuij^  furnish  a  source  for  the  unusually  large  amount  of  basic  mud  associated  with 

o  Maxwell,  Walter,  Lavas  and  soils  ot  the  Hawaiian  Islands!  Rept.  Exper.  Sta.  Uaivaiian  Sugar  Planters'  Assoc,  Div.  Agr.  and  Chem.,  Special 
Bull.  A,  Honolulu,  1905,  pp.  S-22. 


THE  IRON  ORES.  515 

the  iron-bearing  formation.  It  is  entirely  conceivable  that  some  of  the  thin  bands  of  the 
iron-bearing  formation  interbedded  with  basic  flows,  with  little  other  sedimenta,ry  material, 
may  be  essentially  residual  iron  oxide  or  laterite  deposits  developed  in  this  way.  This  seems 
especially  likely  where  the  iron-bearing  formation  is  high  in  alumina,  as,  for  instance,  in  some 
of  the  hornblendic  Keewatin  belts  or  in  the  iron  ranges  near  Lake  Nipigon,  where  E.  S. 
Moore"  has  found  dumortierite.  However,  the  generally  low  percentage  of  alumina  in  the 
iron-bearing  formations  seems  to  show  that  for  the  most  part  they  may  not  be  regarded  as 
metamorphosed  residual  products  of  rock  alteration. 

Vegetation  is  known  to  develop  on  basic  extrusive  lavas  with  great  rapidity,  as  indicated 
by  the  cultivation  of  the  slopes  of  Vesuvius  and  Hawaiian  volcanoes  m  an  incretUljly  short  time 
after  eruptions,  and  hence  organic  agencies  may  have  aided  in  the  transfer.  The  chemistry  of 
the  transfer  of  iron  salts  through  these  agencies  is  discussed  elsewhere  (pp.  .519- .520).  Favor- 
ing the  view  that  weathering  is  a  factor  in  the  process  is  the  fact  that  parts  of  the  original 
rocks  of  the  iron-bearing  formation  are  made  up  of  iron  carbonate  associated  with  black  car- 
bonaceous slates,  such  as  may  have  developed  in  delta  deposits.  (See  p.  502.)  There  is  no 
more  reason  to  doubt  the  organic  origin  of  the  carbon  in  these  slates  than  that  of  the  carbon 
in  the  carbonaceous  slates,  iron-bearing  formation,  and  basalts  in  County  Antrim,  Ireland, 
and  elsewhere,  except  that  definite  organic  forms  are  lacking. 

The  iron-bearing  formations  grade  locally  into  phases  rich  in  calcium  and  magnesium 
carbonates,  as  at  Guniiint  Lake  and  in  the  east  end  of  the  Gogebic  cUstrict.  It  is  usually 
assumed  that  calcium  antl  magnesium  carbonates  are  ordinary  products  of  weathering  and 
sedimentary  deposition. 

It  may  be  asked  why  weathering  did  not  also  deposit  iron  abundantly  in  the  Paleozoic 
sea  when  it  advanced  later  on  these  same  rocks.  To  some  slight  extent  iron  was  so  deposited 
at  the  Chnton  horizon.  The  answer  is  believed  to  lie  partly  in  the  essential  contemporaneity 
of  the  basic  extrusive  rocks  with  the  associated  iron-bearing  formations,  indicating  that  the 
process  of  derivation  of  the  iron  salts  and  deposition  went  on  soon  after  the  extrusion  of  the 
igneous  rocks,  very  rapidly  at  first  owing  to  juvenile  contributions  and  to  leaching  during  the 
residual  heat,  but  slowly  later  when  the  rocks  were  colder  and  the  easily  accessible  sulpliides 
had  been  reached.  Still  later,  when  the  Paleozoic  sea  came  over  the  area,  while  it  derived  some 
iron  fi-om  these  rocks,  it  was  unable  to  do  the  work  on  the  same  scale  as  was  accomplished 
immediately  after  their  extrusion.  Since  glacial  time  alteration  of  pyrites  in  the  pre-Cambrian 
sliield  has  penetrated  only  a  fraction  of  an  inch  or  at  most  a  few  inches  below  the  striated 
glacial  surfaces,  indicating  a  relatively  slow  alteration  of  these  substances  under  ordinary 
weathering — probably  too  slow  to  account  for  the  heavy  and  rapid  chemical  deposition  of 
iron-bearing  formation  without  admixture  of  fragmental  material. 

Powdered  Keewatin  rocks  containing  abundant  iron  sulphide  have  been  treated  with 
oxygenated  waters  and  kept  agitated  for  a  period  of  six  weeks.  A  slight  amount  of  sulphuric 
acid  was  also  introduced  to  accelerate  the  alteration.  At  the  end  of  this  time  barely  enough 
iron  had  gone  into  solution  to  be  detected  by  the  most  refined  methods. 

The  slate  that  is  so  abundantly  present  with  the  ujiper  Huronian  iron-bearing  formations 
gives  evidence  in  its  composition  of  derivation  from  the  greenstone.  (See  p.  612.)  It  is  in 
part  doubtless  derived  by  weathering  of  the  type  here  described.  In  part  also  the  slate  repre- 
sents volcanic  dust  and  mud  directly  deposited  from  the  volcanic  extrusions,  and  in  part  it 
may  result  from  reaction  between  the  hot  lavas  and  sea  waters  described  below. 

CONTRIBUTION    OF    IRON    SALTS    BY    REACTION    OF    HOT    IGNEOUS    ROCKS    WITH    SEA    WATER. 

When  basaltic  magmas  are  extruded  into  the  ocean  there  is  reaction  with  the  salt  water. 
The  behavior  of  basic  lavas  when  extruded  into  salt  water  has  not  been  carefully  observed. 
There  seems  to  be  a  tendency  in  Hawaii  and  Iceland  for  rapid  powdering  and  disintegration  at 
these  contacts.     What    the  chemical  results  are  is  not    apparent.     When  pottery  is  sprayed 

»  Geology  of  Onaman  iron  range  area:  Ann.  Rept.  Ontario  Bur.  Mines,  vol.  18,  pt.  1, 1909,  pp.  212-215. 


516  GEOLOGY  OF  THE  LAKE  SITPERIOR  REGION. 

with  salt  water  wliiie  hot,  a  glaze  of  sodium  siUcatc  (water  glass)  is  formed,  which  is  more  or 
less  soluble.  In  connection  with  the  present  study  fresh  basalts  were  heated  in  a  muffle  furnace 
to  a  temperature  of  1,200°  C,  a  temperature  sufficient  to  fuse  the  exterior,  and  then  i)lunged  into 
salt  water  of  the  composition  of  sea  water,  the  result  being  a  violent  reaction,  producing  princi- 
pally soilium  silicate  (see  p.  525)  but  also  bringing  a  small  amount  of  iron  into  solution. 
From  the  available  evidence  it  seems  likely  that  such  a  process  may  account  for  part  of  the 
sodium  silicate  wliich,  by  reaction  with  ferrous  salts,  produces  the  greenalite  with  excess  of 
silica.  (See  pp.  521-523.)  The  experiment  does  not  seem  to  suggest  an  adecjuate  source  for 
the  iron  in  this  reaction.  There  was  also  during  tliis  reaction  a  tendency  toward  disintegra- 
tion. Tliis  may  indicate  ar  partial  source  for  some  of  the  muds  so  closely  associated  with  the 
iron-bearing  formations. 

CONCLUSION  AS  TO  DERIVATION  OF  MATERIALS  FOR  THE  IRON-BEARING  FORMATIONS. 

Ordinary  processes  of  weathering,  transportation,  and  deposition  of  iron  salts  from  terranes 
of  average  composition  were  as  effective  in  the  pre-Cambrian  of  the  Lake  Superior  region  as  in 
other  times  and  ])laces,  but  these  processes  account  for  only  thin  and  relatively  unimportant 
phases  of  the  iron-bearipg  rocks;  for  instance,  the  lenses  of  iron  carbonates  associated  \\ith 
graphitic  slates  of  the  upper  Huronian,  probably  deposited  in  lagoons  and  bogs  of  a  delta. 
For  the  derivation  of  the  unique  thick  and  extensive  iron-bearing  formations  of  the  Lake  Supe- 
rior region  it  is  necessary  to  appeal  to  some  further  agency.  This  is  bebeved  to  be  furnished 
by  the  large  masses  of  contemporaneous  basic  igneous  rocks.  The  association  of  sedimentary 
iron-bearing  formations  and  basic  igneous  rocks  is  known  in  mam^  localities  outside  of  the 
Lake  Superior  region.  The  iron  salts  have  been  transferred  from  the  igneous  rocks  to  the 
sedimentary  iron-bearing  formations  partly  b}'  weathering  when  the  igneous  rocks  were  hot 
or  cold,  but  the  evidence  suggests  also  that  they  were  transferred  jjartly  by  direct  contril)ution 
of  magmatic  waters  from  the  igneous  rocks  and  perhaps  in  small  part  by  direct  reaction  of  the 
sea  waters  upon  the  hot  lavas. 

VARIATIONS   OF   IRON-BEARING   FORMATIONS   WITH   DIFFERENT   ERUPTIVE    ROCKS    AND 

DIFFERENT   CONDITIONS   OF   DEPOSITION. 

The  basalts  contributmg  the  iron  being  both  subaerial  and  subaqueous  in  their  extrusion,  it  is 
to  be  expected  that  the  contribution  of  iron  to  the  bodv  of  water  in  which  the  iron-bearing  forma- 
tions were  being  deposited  was  both  direct  and  indirect.  Evidence  is  not  available  which  wUl 
clearly  discriminate  iron-bearing  formations  contributed  to  the  ocean  in  these  two  ways.  In 
general  the  parts  of  the  iron-bearing  formations  originally  consisting  of  carbonate  seem  to  be 
related  to  the  indirect  contribution  from  the  igneous  rocks  through  the  agencies  of  weathering, 
and  the  parts  of  the  iron-bearing  formations  originally  consistmg  of  greenalite  or  iron  silicate 
seem  to  have  been  contributed  in  the  main  directly  to  the  waters  without  intervening  atmos- 
pheric or  organic  agencies.  The  locally  close  association  of  these  two  types  of  the  original 
iron-bearuig  rocks  indicates  the  close  association  of  direct  and  iiidirect  methods  of  contribu- 
tion of  iron-bearing  materials.  The  fact  that  the  upper  Huronian  iron-beaiing  formation  in 
the  ilesabi  district  was  largely  greenalite,  while  the  upper  Huronian  iron-bearing  formation  of 
the  Gogebic  district  was  lai'gely  carbonate,  might  therefore  signifj'^  simply  that  in  one  district 
the  salts  had  been  derived  primarily  from  subaerial  weathering  and  in  the  other  from  sul>- 
acjucous  contribution,  but  in  each  district  partly  in  both  ways  and  in  both  districts  essentialh' 
from  the  same  I'ocks.  It  is  noted  elsewhere  that  in  many  places  where  the  greenahtc  anil 
carbonate  occur  together  the  greenalite  occupies  the  lower  horizon.  Tliis  might  be  explained 
.not  only  l)y  conditions  of  sid)aerial  contribution  succeedmg  subaciucous  contribution,  but, 
as  explauied  elsewhere,  by  the  more  rapid  settling  of  the  greenalite  when  i)recij)itated  simul- 
taneously with  the  carbonate. 

The  iron-1)oaring  lavas  extnided  at  three  widely  separated  jierioils  could  scarcely  be  expected 
to  produce  iron-bearing  formations  of  exactly  the  same  character,  even  were  the  conditions  of 


THE  IRON  ORES.  517 

deposition  the  s;ime,  for  in  so  far  as  the  ores  were  directly  contributed  by  magmatic  soUitioiiSj 
they  were  subject  to  extreme  variations  in  composition. 

The  conditions  of  deposition  of  iron  salts  were  also  different  during  these  three  periods  of 
volcanism.  The  Keewatin  lavas  were  extruded  in  larger  quantities  than  at  any  later  time  and 
the  associated  iron-bearing  formations  constituted  only  discontinuous  beds  between  the  hot 
extrusives,  but  in  the  middle  and  upper  Huronian  the  extrusions  were  much  less  abundant  and 
sedimentation  proceeded  on  a  larger  scale  and  less  directly  under  the  influence  of  igneous  rocks. 
Although  some  of  the  differences  between  these  three  formations  are  explained  by  later  alteration, 
it  is  believed  that  the  highly  amphibolitic  and  magnetitic  character  of  the  Keewatm  was 
partly  determined  at  the  time  of,  or  soon  after,  its  deposition,  in  contrast  with  the  prevailing 
deposition  of  ferrous  carbonate  and  ferrous  silicate  at  the  later  periods.  In  the  discussion  of  the 
secondary  concentration  of  the  ores  it  will  "be  shown  that  the  ores  of  the  Keewatin  have  under- 
gone far  less  secondary  concentration  than  the  later  ores.  This  is  certainly  in  part  due  to 
anamorphic  changes  before  the  katamorphic  agents  had  an  opportunity  to  work,  but  possibly 
in  part  also  to  original  differences  in  texture  and  composition,  possibly  because  the  Keewatin 
as  a  whole  seems  to  contain  a  lower  percentage  of  iron  than  the  succeeding  formations,  and 
partly  because  of  the  small  area  of  the  formations  exposed  to  concentrating  agencies.  (See 
pp.  474-475.)  The  Keewatin  series  had  produced  only  6.5  per  cent  of  the  total  shipments  to  the 
close  of  1909.  The  Keewatin  seems  to  occupy  the  same  subordinate  position  in  Canada,  and 
as  the  area  of  Keewatin  in  Canada  is  relatively  greater  than  that  of  later  iron-bearing  forma- 
tions, the  chances  of  finding  ore-  there  are  relatively  smaller  than  in  other  parts  of  the  Lake 
Superior  region. 

It  would  be  expected  also  that  the  iron  salts  closely  associated  with  the  igneous  rocks  would 
be  less  regular  in  their  thickness  and  more  generally  separated  into  different  belts  by  intercalated 
igneous  rocks  than  those  at  a  distance  from  the  areas  of  extrusion.  The  latter  seem  to  be  illus- 
trated by  the  Animikie  ores,  which  attain  their  maximum  development  on  the  north  shore  of 
Lake  Superior,  tlie  nearest  Icnown  extrusive  rocks  being  west  of  the  lake  or  possibly  under  the 
lake.  The  remarkably  uniform  character  of  the  iron-bearing  formation  and  the  rest  of  the 
Animilde  group,  distinguishing  it  from  all  other  pre-Cambrian  iron-bearing  formations,  may 
well  be  due  to  its  distance  from  the  contemporaneous  volcanic  activity,  for,  in  view  of  the  con- 
nection of  the  ores  with  igneous  rocks  above  outlined,  it  would  seem  to  be  more  than  a  coinci- 
dence that  the  most  uniform  and  widespread  of  the  iron-bearing  formations  should  be  the 
farthest  removed  from  volcanic  activitj'.  Variation  in  the  iron-bearing  formations  with  varying 
distance  from  the  igneous  rocks  is  more  definitely  shown  by  the  iron-bearing  formation  of  the 
Gogebic  district,  which  at  the  east  end  of  the  range,  where  associated  with  extrusive  rocks, 
is  extremely  varied  in  its  composition  and  is  broken  into  different  belts  by  other  sediments 
and  by  igneous  beds.  The  material  of  this  portion  of  the  formation  may  also  originally  have 
been  deposited  in  small  part  as  magnetite  or  hematite  rather  than  sideriteor  greenalite.  The 
irregularity  diminishes  toward  the  west,  though  still  existing  at  Sunday  Lake.  For  many  miles 
west  of  Sunday  Lake  the  iron-bearing  formation  was  deposited  as  a  continuous  thick  formation 
with  less  amounts  of  other  sediments.  These  differences  may  be  j^artly  due  to  varying  condi- 
tions of  temperature  and  materials  present,  as  discussed  on  page  526,  and  are  undoubtedly 
due  in  part  to  the  fact  that  near  the  exti-usions  there  were  sudden  and  violent  oscillations  in 
level,  requiring  frequent  alternations  of  sediments,  while  farther  away  these  oscillations  were 
less  marked  and  the  movement  was  a  comparatively  uniform  one  of  sinking,  perhaps  due  to 
the  general  extrusion  of  the  lavas  from  the  region. 

Moreover,  shore  conditions  of  deposition  may  well  have  been  different  from  those  offshore. 
It  has  been  noted  that  the  upper  Huronian  iron-bearing  formations  in  the  Mesabi,  Gogebic, 
ilenominee,  and  Felch  ^fountain  districts  are  clearly  defined  formations  originally  contaming 
greenalite  and  carbonate  between  quartz  sand  below  and  shale  above,  and  that  in  these  districts 
they  come  relatively  close  to  the  older  rocks,  suggesting  a  possible  shoi-e  comlition.  In  the 
Cuyuna,  Florence,  and  Iron  River  districts  the  iron-bearing  members,  originally  sideritic,  are 


518  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

in  numerous  layers  and  lenses  in  the  slates.  These  are  probably  higher  in  the  series  and  may 
also  represent  offshore  conditions. 

It  may  be  argued  that  similar  basic  igneous  rocks  elsewhere  extruded  near  or  under  the  sea 
are  not  accompanied  by  deposition  of  iron-bearing  formation  on  such  a  scale.  That  iron-bearing 
rocks  are  present  on  a  smaller  scale  in  such  association  elsewhere  is  shown  on  pages  508-510.  It 
should  be  remembered  that  only  veiy  exceptionally  do  igneous  rocks  of  any  sort  carry  ores  with 
them.  There  are  many  areas  of  Tertiary  eruptive  rocks  and  but  few  Goldfield  camps.  So  far  as 
the  Lake  Superior  iron-bearing  formations  derive  their  materials  from  direct  magmatic  contrilni- 
tion  of  igneous  rocks,  they  are  likely  to  be  localized  by  reason  of  these  exceptional  contributions. 
Tliis  may  explain  why  all  of  the  similar  pre-Cambrian  basalts  in  Canada  or  elsewhere  in  the 
Lake  Superior  region  are  not  associated  with  iron  ores,  though  the  geologic  conditions  are 
apparently  the  same.  It  follows  from  the  foregoing  statements  that  the  ores  are  not  derived 
from  basic  igneous  rocks  in  general  but  from  certain  ones. 

It  may  be  further  argued  that  wliile  the  iron-bearing  formations  of  the  Keewatin  may 
have  readily  been  derived  from  the  relatively  abundant  associated  greenstones,  the  iron-bearing 
formations  of  the  Huronian  are  so  extensive  as  compared  with  the  contemporaneous  volcanic 
rocks  that  they  could  scarcely  have  been  derived  from  those  rocks.  Such  an  argument  would  be 
without  definite  basis,  however,  because  there  is  no  known  quantitative  relation  between  volume 
of  igneous  rock  and  volume  of  materials  derived  from  it  as  igneous  after-effects.  The  iron  ores 
of  the  Iron  Springs  district  of  Utah  show  a  wide  range  in  abundance  as  compared  with  the 
parent  igneous  rocks.  The  contemporaneous  volcanic  activity  in  the  midille  Huronian  was 
extensive,  being  represented  by  the  Hemlock,  part  of  the  Clarksburg,  and  other  volcanic 
formations.  That  in  the  upper  Huronian  was  less  in  amount,  but  is  represented  by  most  of 
the  Clarksburg,  in  the  eastern  part  of  the  Gogebic  district,  and  some  of  the  greenstones  of  the 
Menominee  district;  moreover,  it  may  well  be  that  the  present  Lake  Superior  basin  was  the 
locus  of  much  more  abundant  upper  Huronian  flows,  for  reasons  wliich  are  mentioned  on 
pages  507-508. 

CHEMISTRY  OF  ORIGINAL  DEPOSITION  OP  THE  IRON-BEARING 

FORMATIONS. 

NATTJKE   OF  THE    PROBLEM. 

The  experiments  specifically  described  in  the  following  paragraphs,  if  not  otherwise  cred- 
ited, have  been  made  in  the  geological  and  chemical  departments  of  the  Universit}^  of  Wisconsin, 
principally  by  M.  E.  Diemer,  in  cooperation  with  W.  J.  Mead,  R.  D.  Hall,  and  others,  to  meet 
conditions  specified  by  the  authors. 

The  problem  is  to  explain  the  original  deposition  in  tliick  formations  of  greenalite  ([FeMg] 
SiOj.nHsO),  siderite  (FeCOj),  chert  (SiOj),  and  perhaps  some  hematite,  magnetite,  and  limonite, 
in  intercalated  layers  of  varying  proportions,  under  conditions,  if  our  preceding  conclusions  are 
valid,  ranging  from  ordinary  cycles  of  weathering,  transportation,  and  deposition  to  direct  con- 
tribution of  iron  solutions  from  the  hot  igneous  extrusives  to  the  water  in  which  the  sediments 
were  deposited. 

Obviously  a  wide  range  of  chemical  processes  has  been  involved  in  the  development  of  the 
iron  ores.  It  is  unlikely  that  all  are  known.  It  is  the  aim  of  the  following  paragraphs  to 
indicate  as  definitely  as  possible  certain  processes  wMch  seem  likely  to  have  been  important, 
without  impHcation  that  these  are  necessarily  the  only  ones  contributing  toward  the  observed 
results. 

The  iron  may  have  been  carried  as  a  ferrous  salt  of  silicic,  carl)onic,  sul]>huric,  hj-drochloric, 
or  other  acids  present,  or  as  FeO  in  presence  of  IIjO  at  liigh  temperatures  it  may  have  been  in 
excess  of  the  available  acid  radicles.  It  appears  now  as  an  origmal  constituent  of  basic  extru- 
sive rocks  in  the  form  of  sulpludos,  magnetite,  hematite,  chlorite,  and  the  pyroxenes  ami  amplii- 
boles.  The  absence  of  greenalite  and  ferrous  carbonate  as  such  among  these  ^original  constitu- 
ents, and  also  the  absence  in  the  ferrous  silicate  and  carbonate  of  alkalies,  which  are  associated 


THE  IRON  ORES.  519 

with  the  iron  as  original  constituents  in  the  igneous  rocks,  seem  to  prechide  the  direct  contri- 
bution of  the  iron  as  ferrous  sihcate  or  ferrous  carbonate  from  tlie  igneous  rocks  and  to  require 
certain  sifting  ami  simplifying  reactions  by  outside  agencies  to  explain  the  composition  of  the 
original  iron-formation  rocks.  It  will  be  assumed  in  the  following  discussion  that  the  iron 
is  carried  as  a  ferrous  salt.  From  the  abuntlance  of  iron  sulphides  in  the  original  igneous  rocks 
and  in  their  pegmatitic  after-effects  it  will  be  assumed  further  that  the  acid  radicle  is  sulphuric. 
This  is  done  also  for  convenience  in  experimenting.  It  is  not  meant  to  exclude  other  possible 
combinations  of  tlie  iron  above  mentioned.  Carbonic  acid  was  doubtless  present.  Other  combi- 
nations than  these  would  serve  fully  as  well  in  the  essential  steps  of  tlie  process  below  outlined. 

FORMATION   OF   IRON   CARBONATE    AND    LIMONITE. 

The  close  association  of  man}-  of  the  thinner  carbonate  ))ands  of  the  upper  Huronian 
with  black  carbonaceous  and  pyritiferous  slates,  an  association  similar  to  those  found  in  the  "Coal 
Measures"  and  elsewhere,  suggests  that  the  iron  carbonate  may  be  the  result  of  reduction  of  ferric 
hydrate  by  organic  material  buried  with  it  in  deltas,  bogs,  or  other  similar  places.  Hydrogen 
sulphide  characteristic  of  these  conditions  would  react  upon  part  of  the  carbonate  of  iron,  pro- 
ducing the  iron  sulphide,  thereby  giving  both  iron  carbonate  and  ii'on  sulphide  in  association 
with  carbonaceous  rocks. 

Van  Hise  "  says : 

As  to  the  form  in  which  the  iron  salts  enter  the  seas,  we  can  judge  only  by  analogy,  but  if  the  present  be  a  guide  to 
the  past,  the  iron  was  chiefly  as  a  carbonate  and  to  a  subordinate  extent  as  a  sulphate,  although  it  might  have  been  in 
part  in  the  form  of  other  salts.  When  the  iron  salts  reach  the  lagoon,  they  are  precipitated  under  favorable  condi- 
tions as  ferric  hydrate  or  possibly  in  part  as  basic  ferric  sulphate.  Supposing  the  iron  salt  to  be  carbonate,  it  would  be 
precipitated  according  to  the  following  reaction: 

4FeC03+3H20+20=2Fe,03.3HoO+4C02. 

Where  this  process  goes  on,  on  an  extensive  scale,  limonite  bodies  are  built  up. 

It  was  formerly  supposed  that  this  reaction  took  place  as  a  result  of  the  work  of  oxygen  and  moisture  alone,  and  this 
is  true  to  some  extent.  But  recent  observation  has  shown  that  where  in  lagoons  iron  carbonate  is  abundant  the  oxidation 
is  largely  performed  through  the  agency  of  a  class  of  bacteria  called  the  iron  bacteria.  It  has  been  found  thatthese  bac- 
teria are  unable  to  exist  without  the  presence  of  iron  carbonate  or  manganese  carbonate,  but  the  iron  carbonate  is  the 
chief  compound  used.  This  material  they  absorb  into  their  cells.  There  the  iron  carbonate  is  oxidized  and  the  limonite 
is  precipitated.     Says  Lafar: 

"The  decomposing  power  of  these  organisms  is  very  great,  the  amount  of  ferrous  oxide  oxidized  b>'  the  cells  being  a 
high  multiple  of  their  own  weight.  This  high  chemical  energy  on  the  one  hand,  and  the  inexacting  demands  in  the 
shape  of  food  on  the  other,  secure  to  these  bacteria  an  important  part  in  the  economy  of  nature,  the  enormous  deposits  of 
ferruginous  ocher  and  bog  iron  ore,  and  probably  certain  manganese  ores  as  well,  being  the  result  of  the  activity  of  the 
iron  bacteria.  "6 

Evidence  is  furnished  of  the  precipitation  of  the  limonite  of  bog  iron-ore  deposits  in  this  manner  by  the  discovery 
in  some  of  them  of  large  numbers  of  the  sheaths  of  the  iron  bacteria.  <^  Further  evidence  of  the  importance  and 
activity  of  these  bacteria  is  furnished  by  their  partly  or  completely  closing  water  pipes  of  cities  where  the  water  con- 
tains a  considerable  amount  of  iron  carbonate.  ^ 

The  iron  part  of  the  salts  carried  down  to  the  sea  as  a  sulphate  would  be  likely  to  be  thrown  down  as  basic  ferric 
sulphate,''  according  to  the  following  reaction: 

12FeS04+60-|-(x-F9)H,0=Fe,(SOj3.5Fe,,03.xHoO-|-9H.,S04. 

The  material  thrown  down  as  a  hydrated  ferric  oxide  and  basic  ferric  sulphate  is  mingled  with  more  or  less  of  organic 
material,  and  a  deposit  of  considerable  thickness  may  thus  be  built  up.  This  depcj.sit  is  below  the  level  of  ground  water 
and  is  therefore  in  the  zone  of  incom])lete  oxidation,  or  is  under  the  conditions  of  the  belt  of  cementation.  The  oxygen 
required  for  the  partial  oxidation  of  the  organic  material  is  derived  in  part  from  the  ferric  oxide,  and  the  iron  is  reduced 
to  the  ferrous  form;  but  probably  this  reaction  does  not  take  place  on  an  important  scale  at  the  surface.  The  reducing 
agent  may  be  regarded  as  carbon,  carbon  monoxide,  or  some  of  the  hydrocarbons,  such  as  methane.  The  result  is  the 
same  in  any  case.  The  oxygen  and  the  carbon  produce  carbon  dioxide,  and  thus  the  conditions  are  reproduced  for  the 
production  of  iron  carbonate.     A  representative  reaction  may  have  been  as  follows: 

2Fej03-3H,0+3C02-fC=4FeC03-l-3H20. 

u  Van  Hise,  C.  R.,  k  treatise  on  metamorphism:  Mon.  V.  S.  Geol.  Survey,  vol.  AT,  1904,  pp.  825-827. 
ii  Lafar,  F.,  Teclinical  mycology,  vol.  1,  Lippinrolt  <t  Co  ,  1S98,  p.  361. 

c  Fischer,  .V.,  The  structure  and  functionsof  bacteria,  trans,  by  A.  Coppen  Jones,  Clarendon  Press,  Oxford,  1900,  p.  09. 
d  Lafar,  F.,  op.  cit.,  p.  3r.l. 

«  Pickering,  S.  P.  U.,  Ontheconstitutionofmolecularcompounds;  the  molecular  weight  of  basic  ferric  sulphate:  Jour.  Chem.  Soc.  London,  vol. 
43, 1883,  p.  182. 


520  GEOLOGY  OF  THE  I^VKE  SUPERIOR  REGION. 

Beck  summarizes  the  conditions  of  solution,  transjiortation,  and  deposition  of  iron  under 
weatherinfi;  processes,  especially  witii  reference  to  organic  agencies.  In  his  discussion  of  the  origin 
of  lake  and  bog  ores,  he  sa^-s:" 

What  was  the  nature  of  the  solutions?    The  following  are  the  chief  solvents: 

1.  Sulphuric  acid  formed  by  the  decomposition  of  iron-bearinf;  sulphides. 

2.  Carbonic  acid  supplied  by  the  air  and  by  decaying  organisms,  and  to  some  extent  by  the  living  animals.  This 
enables  it  to  attaclv  various  silicates. 

3.  Organic  acids  also  play  a  part.  These  are,  moreover,  transformed  into  carbonic  acid  by  oxidation,  when  v^e- 
table  masses  decompose.  In  the  presence  of  decaying  vegotaVjle  matter  deprived  of  an  adequate  oxygen  supply,  iron 
sesquioxide  is  reduced  to  ferrous  oxide,  which  forms  .soluble  double  salts,  with  humus  acids  and  ammonium. 

The  precipitation  of  iron  from  these  dilute  solutions  may  take  place  in  various  ways. 

In  solutions  of  iron  sulphate  the  mere  addition  of  ammonium  humate,  which  is  always  present  in  the  brown  waters 
of  peaty  areas,  effects  a  precipitation  of  iron  oxide  and  later  on  of  ferric  hydrate. 

From  carbonated  solutions  the  iron  is  precipitated  as  ferric  hydrate  by  the  escape  of  carbonic  acid  into  the  air,  or  by 
its  absorption  by  plant  cells.  The  deposition  of  iron  carbonate  is  only  possible  when  the  air  is  excluded  or  in  the  pres- 
ence of  organic  matter,  which  seems  to  harmonize  with  the  known  facts  concerning  spherosiderite  and  black-band  ores. 

From  humates  and  other  organic  compounds  the  ferric  hydrate  is  precipitated  by  the  oxidation  of  the  humus  acids 
and  their  decompo-sition  into  carbonic  acid  and  water.  Here,  too,  the  plant  cell  accelerates  this  process  by  furnishing 
oxygen.  Lastly,  by  the  mingling  of  iron  humates  anS  sulphates,  the  sulphuric  acid,  which  kept  the  iron  sesquioxide 
in  solution,  unites  with  ammonium,  and  iron  is  precipitated  as  hydroxide  or  as  ferric  humate. 

In  this  action,  the  life  processes  of  plants  take  a  part,  entirely  independent  of  any  products  of  plant  decay.  Accord- 
ing to  Ehrenberg,  the  algae,  especially  the  so-called  iron  alg;e,  Galionella  Jerruginea  Ehrenb.,  are  active  ore  precipitants, 
coating  their  cell  walls  with  ferric  hydrate  and  opaline  silica.  This  alga  is  abundant  on  the  sea  bottoms.  According 
to  the  recent  works  of  Molisch  and  Winogradsky,  these  and  most  other  supposed  algse  are  ciliated  bacteria  of  different 
kinds,  especially  Lcptothrix  ochracea.b 

The  silica  of  these  ores  may  originally  have  been  held  in  solution  as  alkaline  silicates,  which  are  supposed  to  be 
decomposed  by  carbonic  acid.  This  silica  is  precipitated  simultaneously  with  the  ferric  hydrate.  The  phosphoric 
acid  was  certainly  present  as  ammonium  phosphate  and  is  precipitated  at  first  as  iron  phosphate  and  as  calcium  phos- 
phate in  calcareous  ores.    *    *    * 

We  saw  that  in  the  case  of  lake  ores  the  deposition  took  place  quite  slowly.  This  process  is  more  rapid  where  the 
drainage  from  the  gossan  of  a  lai^e  pj-rite  deposit  is  carried  into  a  lake  basin,  or  into  the  sea,  or  where  mining  operations 
produce  an  inflow  of  great  quantities  of  iron-bearing  mine  waters.  Thus  the  bottom  of  Lake  Tisken,  near  P'alun,  is 
covered  with  a  layer  of  ocher  mud  several  meters  thick  that  has  been  furnished  by  the  neighboring  pyrite  stock. 
The  bed  of  the  Rio  Tinto  carries  ocher  mud  and  diatoms  derived  from  the  waters  of  the  copper  mines  as  far  as  Palos  in 
Huelva  Bay.  That  this  was  the  case  even  before  mining  began  at  that  locality  is  proved  by  the  deposit  of  iron  ore  on 
the  Mesa  de  los  Pinos  and  the  Cerro  de  las  V'acas.  These  limonite  deposits  were  formed  in  a  bog  which  was  afterwards 
dissected  by  the  river.  The  ironstones  contain  plant  remains  of  the  same  character  as  the  present  flora.  Slabs  of  tliis 
ore  were  used  by  the  Romans  for  tombstones.'^ 

Iron  carbonate  is  Iviiown  to  be  directly  ]ircciijitated  when  a  ferrous  salt  comes  into  contact 
with  calcium  carbonate,  as,  for  instance,  when  ferrous  solutions  from  intrusives  penetrate 
a  limestone.  The  presence  of  any  calcium  carbonate  in  the  waters  or  sediments  at  the  time 
of  the  deposition  of  the  Lake  Superior  iron  formations  may  have  reacted  with  any  ferrous 
salts  present  to  produce  carbonate,  but  we  have  no  direct  evidence  of  this. 

The  above-noted  processes  do  not  seem  adecjuate  to  account  for  all  the  iron  carbonates 
of  the  Lake  Superior  region,  for  some  of  them,  as,  for  instance,  in  tlic  (n)gebic  district,  are  in 
much  thicker  masses  than  have  been  found  elsewhere  associateil  with  carbonaceous  seams, 
are  comparatively  free  from  carbon  and  sulphides,  and,  moreover,  show  remarkablj'  close 
association  witli  certain  iron  silicates  called  greenalite,  to  which  they  are  partly  secondary 
ami  which  are  thought  to  develop  in  another  wa}-.  Laboratorj-  reactions  between  iron  silicates, 
iron  carbonates,  and  carbon  dioxide,  discussed  under  another  heading  (p.  526),  suggest  other 
processes  of  iron  carbonate  deposition. 

NATURE   OF   CARBONATE    PRECIPITATE. 

The  precipitate  of  ferrous  carbonate  is  apple-green  in  color,  is  flocculent,  settles  slowly, 
and  shows  a  distinct  tendenc}'  in  settling  to  segregate  into  bands  separated  by  greater  or  less 

oBcck,  Richard, Tlienatureof  ore  depositsCtr.  by  W.H.  Weedi,  vol.  1,1905,  pp.lOl-lOi.    See  also  Van  Jlise,  C.  R.,  A  treatise  on  nletamo^ 
phlsm;  Men.  U.  S.  Geol.  Survey,  vol.  47, 1904,  p.  550. 

6  Weed,  W.  II.,  GeoloKical  work  of  plants:  Am.  Geologist,  June,  1S94.    Walther,  liinleitung  indieGeoiogie,  Jena,lS93-94,p.65S. 
e  Louis,  H.,  Ore  deposits,  2d  ed.,  1896,  p.  41. 


THE  IRON  ORES.  521 

amounts  of  free  silica.  No  tendency  is  observed  in  this  substance  toward  the  development 
of  globular  forms,  antl  in  this  connection  it  is  suggested,  in  view  of  Lehniann's  inferences  cited 
on  page  525,  that  tlie  iron  carbonate  lias  a  strong  tendency  to  crystallize. 

PRECIPITATION   OF   GREENALITE. 

PROCESSES. 

Evidence  has  been  presented  elsewhere  (pp.  166-16S)  to  show  that '  greenalite 
(Fe(Mg)Si03.nH20)  is  different  from  glauconite  and  probably  from  other  green  silicate 
granules  which  have  been  described.     It  may  be  reproduced  in  the  laboratory. 

In  all  the  reactions  and  experiments  in  which  silicic  acid  was  used,  it  was  in  aqueous 
solution  aloi.g  with  sodium  chloride.  This  was  for  two  reasons:  (1)  The  silicic  acid  was 
prepared  by  neutralizing  sodium  silicate  of  the  composition  Na^O.-SSiOj  with  hydrochloric 
acid.     Thus: 

Na^O.SSiOj  +  2HC1  =  2NaCl  +  SSiO^.H^O. 

(2)  The  methods  of  experimentation  chosen  approximated  the  natural  conditions  under  which 
the  greenalite  was  deposited,  and,  to  our  belief,  this  was  in  the  presence  of  sea  water.  On 
starting  with  a  soluble  ferrous  salt,  for  convenience  ferrous  sulphate,  the  following  reactions 
are  found  to  be  significant  with  reference  to  the  origin  of  greenalite: 

1.  A  solution  of  ferrous  salt  when  boiled  with  silicic  acid  (prepared  as  above  stated)  pro- 
duces (in  the  absence  of  air)  no  precipitate,  showing  that  silicic  acid  and  a  ferrous  salt  do  not 
react  to  form  greenalite. 

2.  Ferrous  sulphate  reacts  directly  with  solutions  of  silicates  of  the  alkalies,  producing 
a  granular  precipitate  corresponding  in  composition  to  the  water  glass  used  in  the  precipitation. 
Thus: 

FeSO,  +  Na^O.SSiOj  =  Fe0..3SiO,  +  Na^SO,. 

It  is  shown  on  page  522  that  this  precipitate  is  composed  of  ferrous  silicate  (FeSiOj)  and 
free  silica. 

If  a  soluble  magnesium  salt  is  present  with  the  ferrous  salt  in  the  above  reaction,  it  will 
be  precipitated  as  MgSiOg  (or  the  silicate  corresponding  to  the  composition  of  the  water  glass 
used),  explaining  the  presence  of  some  MgO  in  the  greenalite. 

3.  That  the  precipitate  FeO.-SSiO,  consists  of  ferrous  silicate  (FeSiOj)  and  free  silica  is 
shown  by  the  following  experiments: 

When  the  precipitate  (FeO.SSiOj)  formed  under  the  given  conditions  is  dissolved  in  strong 
NaOH  and  reprecipitated  by  neutralization  of  the  large  excess  of  alkali  by  hydrochloric  acid, 
the  composition  of  tlie  resulting  precipitate  is  FeSiOj  (by  analysis),  and  the  remaining  silica 
of  the  FeO.SSiO,  is  held  in  solution  as  colloidal  silicic  acid. 

Furthermore,  when  FeO.SSiOj  is  boiled  with  water  silica  is  taken  into  solution,  while  the 
iron  remains  in  the  precipitate,  and  the  ratio  lFeO:3Si02  becomes  gradually  less  and  approaches 
Fe0:Si02.  This  process  can  not  be  carried  to  the  extent  of  FeO:SiO,,  however,  as  the  iron 
of  the  compound  oxidizes,  and  when  it  oxidizes  the  combined  silica  becomes  soluble,  so  that 
no  distinction  can  be  made  between  the  silica  of  the  compound  and  the  uncombined  excess 
silica.     When  greenalite  (FeO.SiO,)  alone  is  boiled  with  water  no  silica  goes  into  solution. 

The  composition  of  the  greenalite  is  shown  further  by  the  fact  that  when  boiled  with 
water  tlu-ough  which  carbon  dioxide  is  being  passed,  iron  and  silica  go  mto  solution  in  the 
proportions  1:1. 

4.  When  the  proportions  of  silica  and  alkali  are  varied  in  the  water  glass,  there  is  variation 
in  the  total  amount  of  silica  precipitated. 

5.  Wlien  the  ferrous  salt  is  in  excess,  in  the  precipitate  the  proportion  of  the  iron  to 
total  silica  precipitate  is  relative  to  that  of  the  sodium  silicate,  but  when  the  water  glass  is 
in  excess,  the  proportion  of  iron  and  silica  is  variable,  depending  on  the  tem])erature  at  whicli 
it  is  formed.  This  is  shown  by  the  precipitates  resultmg  from  mixing  solutions  of  ferrous 
sulphate  and  water  glass. 


522  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Tlic  ])recipitatcs  arc  mixturos  of  ferrous  silicate  iiiid  fi-ee  silica: 

(a)  The  precipitate  from  cold  solutions  witii  ferrous  suit  in  excess  may  be  expressed  as 
FeO.SSiO,. 

(b)  The  precipitate  from  cold  solutions  with  water  glass  in  excess  mav  lie  expressed  as 
FeO.oSiO;. 

(f)  The  ))recipitate  from  hot  solutions  with  ferrous  salt  in  excess  may  be  expressed  as 
FeO.SSiO^. 

(d)  Tlie  precipitate  from  liot  solutions  with  water  f!;lass  in  excess  maj'  be  expressed  as 
Fe0.1t)SiO,. 

6.  RegartUess  of  the  various  proportions  of  iron  to  total  silica  obtained  under  the  conditions 
stated  in  jtaragraj^hs  4  and  5,  the  iron  silicate  formed  has  the  character  of  greenalite  and  the 
variation  in  composition  is  entirely  in  the  amount  of  free  silica  precipitated. 

7.  The  precipitation  of  ferrous  silicate  requires  neutral  or  slightly  alkaline  conditions. 
The  substance  is  soluble  in  acids  and  strong  alkalies.  When  water  glass  is  added  to  a  ferrous 
solution  which  is  acid  with  hydrochloric  or  sulphuric  acid,  there  is  no  precipitation,  but  when  ' 
this  is  neutralized  with  alkali,  a  ferrous  silicate  precipitate  results.  LTnder  stronglj- alkaline 
conditions  it  will  not  precipitate,  being  held  in  more  or  less  of  a  colloidal  solution,  which  has  a 
greenish  niuddj'  appearance. 

Thus  the  materials  necessary  to  make  greenalite  might  be  carried  for  some  distances  in  acid 
or  alkaline  solutions  before  precipitation.  Hydrochloric  acid  is  formed  simultaneously  with 
the  sodium  silicate.  This  would  act  as  a  solvent.  If  the  solution  were  alkaline,  deposition 
would  come  by  neutralization  with  an  acid,  such  as  carbon  dioxide. 

8.  The  iron  of  the  ferrous  silicate  precipitated  in  the  absence  of  oxygen  is  entirely  in  the 
ferrous  condition.  The  freshly  precipitated  ferrous  silicate  was  thorougldy  wasiied,  dissolved 
in  sulphuric  acid,  and  the  ferrous  iron  titrated.  This  gave  the  ferrous  iron  of  the  salt — 0.154 
per  cent.     Then  the  total  iron  was  calculated  as  FeO,  the  result  being  0.159  per  cent. 

9.  When  oxygen  is  available,  variable  percentages  of  ferric  oxide  develop  in  the  silicate. 

10.  Greenalite  may  also  be  produced  by  using  other  ferrous  salts  as  cliloride,  according  to 

the  reaction — 

FeClo  +  Na^SiOg  =  FeSiO^  +  2XaCl. 

11.  As  first  precipitated  the  greenalite  and  silica  are  hydrous.  If  they  are  allowed  to 
stand  and  dry,  out  of  contact  with  the  air,  the  percentage  of  water  becomes  progressively  less. 
Presumably  this  loss  may  go  on  for  an  indefinite  time  and  to  an  indefinite  extent.  Analyses 
of  greenalite  rocks  of  the  Mesabi  district  show  considerable  variation  in  the  amount  of  combined 
water. 

12.  Greenalite  may  be  formed  by  the  reaction  of  alkaline  silicata  and  iron  bicarbonate." 

NATURE    OF    GREEN.A.I.ITE    PKECIPIT.XTE. 

When  formed  by  any  of  the  processes  above  mentioned  the  greenalite  and  associated  silica 
first  constitute  a  green,  flocculent  precipitate.  As  this  precipitate  settles  a  granular  structure 
practically  identical  with  that  observed  in  the  Mesabi  slides  is  ile\eloped.  The  optical  properties 
also  are  the  same.  The  precij^itate  has  been  pressed  and  dried,  a  slide  cut  from  it,  and  a  photo- 
micrograph taken  (PI.  XLII,  B).  A  comparison  of  this  plate  with  one  taken  of  the  greenalite 
rock  (PI.  XLII,  A)  shows  identity  of  textures  which  csin  not  be  mistaken,  in  spite  of  the 
imperfections  of  the  granules  developed  artificially  in  cutting  the  slides.  As  the  precipitate 
settles,  there  is  also  to  be  observed  a  distinct  tendency  toward  banding. 

The  only  feature  in  which  the  artificial  greenalite  granules  diil'er  from  those  of  the  Mesabi 
district  is  in  lacking  the  small  ])ercentage  of  magnesia  found  in  Jlesabi  granules.  No  attempt 
was  made  to  introduce  magnesia  artificially,  but  there  would  seem  to  be  no  inherent  chemical 
difficulty  in  the  association  of  the  magnesium  with  the  iron,  an  association  characteristic  of 
silicate  rocks. 


oBischof,  Gustav,  Elements  of  chemical  and  physical  geology  (tr,  by  B.  11.  Paul),  vol.  2,  London,  1S55,  p.  71. 


PLATE   XLII. 


523 


PLATE  XLII. 
Photomicrographs  of  natural  and  artificial  greenalite  granules,  cherty  sedeeite, 

AND  concretionary  FERRUGINOUS  CHERT. 

A.  Greenalite  granules  (specimen  45705,  slide  16395)  from  Cincinnati  mine.     Without  analyzer,  X  40.     The  granules 

are  for  the  most  part  unaltered,  and  are  dark  green,  light  green,  or  yellow.  Some  of  them  show  alterations  to 
iron  oxide  and  to  dark-green  chloritic  material.  \Miere  altered  they  become  dark  brown,  black,  or  dark  green. 
The  matrix  is  entirely  chert.  Evidence  of  crushing  is  to  be  observed  in  minute  cracks  ramifjang  through  the 
elide.  Note  the  remarkable  similarity  in  shapes  of  these  granules  to  those  of  the  green  granules  in  Clinton  ores, 
illustrated  in  Plate  XLV  (p.  536). 

B.  Greenalite  granule  in  matrix  of  silica  artificially  produced  in  the  laboratory.     Without  analyzer.     See  description 

(pp.  522-523). 

C.  Photomicrograph  of  cherty  siderite  altering  to  ferruginous  chert  (specimen  6138,  slide  1173),  from  north  shore  of 

Gunflint  Lake,  T.  65  N.,  R.  3  W.,  Minnesota:  Animikie  group.  In  ordinary  light.  X  25.  The  figure  illustrates 
the  formation  of  iron  oxides,  pseudomorphous  after  siderite.  A  background  of  chert  contains  nimierous  small 
roundish  and  rhombohedral  areas  of  siderite  and  ii'on  oxide.  Between  the  little-altered  and  wholly-altered 
siderite  a  complete  gradation  is  seen. 

D.  Concretionary  ferruginous  chert,  developed  from  alteration  of  cherty  siderite  (specimen  9048.  slide  28S6),  from 

the  SE.  i  sec.  27,  T.  46  N.,  R.  2  E.,  Wisconsin.  In  ordinary  light,  X  25.  In  a  cherty  background  are  beautiful 
concretions,  which  are  composed  of  concentric  rings  of  iron  oxide  and  chert.  One  concretion  particularly  is  very 
fine, 'showing  many  closely  packed  concentric  rings.     Silica  is  seen  breaking  across  these  rings  in  a  few  places. 

524 


U.  S.  GEOLOGICAL  SURVEY 


MONOGRAPH   Lll     PL.  XLII 


PHOTOMICROGRAPHS. 


D. 


THE  IRON  ORES.  525 

The  reason  for  the  greenalite  taking  the  globuhir  form  is  probably  found  in  the  surface 
tension  between  the  precipitate  and  the  hquid,  which  tends  to  make  the  smallest  possible 
surface  of  contact  between  them,  just  as  mercur}-  in  contact  with  the  air  will  tend  to  take  a 
globular  form  in  response  to  surface  tension.  Such  forms  are  commonly  observetl  in  precipi- 
tates. Lehmann  °  has  investigated  them  extensively  and  finds  them  to  develop  characteristi- 
cally in  preci])itates  which  lack  strong  crystallizing  tendency.  He  also  finds  such  forms  in 
other  precipitates  to  be  intermediate  steps  toward  crystal  form,  and  presents  interesting  photo- 
micrographs of  incipient  development  of  these  forms  from  these  intermediate  globular  stages.  He 
has  used  the  expressive  term  "  liquid  crystals  "  for  these  intermediate  globular  stages.  Correlat- 
ing the  development  of  liquid  crystals  as  intermediate  steps  in  the  formation  of  crystals,  where  the 
substances  are  of  strong  crystallizing  power,  with  the  fact  that  similar  forms  and  not  cr3'stals 
are  likely  to  be  the  permanent  form  taken  by  substances  of  low  crystallizing  ]:)Ower,  he  is  led 
to  suggest  that  the  permanent  retention  of  globular  forms  is  a  consequence  of  low  crystallizing 
power,  the  substance  in  its  attem]5t  to  organize  itself  having  reached  the  stage  of  a  liquid  crystal 
but  not  having  gone  further. 

Hydrated  iron  oxide  also  tends  to  take  on  globular  forms  when  precipitated. 

SOURCE    OF    AT.KALINE    SILICATES    NECESSARY    TO    PRODUCE    GREENALITE. 

The  above-described  reactions  indicate  that  it  is  necessary  to  account  for  the  ])resence  of 
alkaline  silicate  rather  than  free  silicic  acid  to  produce  the  desired  results.  Soluljle  alkaline 
silicates  are  laiown  to  be  one  of  the  common  results  of  rock  decay,  but  a  comparison  of  the 
amount  of  silica  available  from  the  basic  rock  by  weathering  with  that  concentrated  in  the 
iron  formations  shows  such  an  excess  in  the  iron  formations,  as  well  as  absence  of  alkalies,  as 
to  lead  us  to  search  for  another  possible  source  for  the  alkaline  silicates.  Sodium  silicate  is 
furnished  by  the  reaction  of  sea  water  upon  hot  silicate  magmas  of  extrusive  basalts  or  por- 
phyries or  upon  the  siliceous  solutions  coming  from  these  extrusives  as  igneous  after-efi'ects  or 
forming  a  part  of  them.  Abundant  vein  quartz  inclosing  iron  sulphides  in  the  basalts  and 
porphyries  is  taken  to  represent  remnants  of  siliceous  solutions  which  did  not  escape.  These 
reactions  were  suggested  by  the  common  practice  in  pottery  making  of  producing  a  water-glass 
glaze  by  spraying  salt  water  against  the  hot  silicates.  Under  ordinary  conditions  hydrochloric 
acid  is  much  stronger  than  silicic  acid,  decomposing  many  of  the  silicates,  but  when  heated 
hydrochloric  acid  is  volatile  and  silicic  acid  is  not,  hence  silicic  acid  may  then  displace 
hydrochloric  acid  from  its  salts  and  produce  alkaline  silicates. 

By  neutralization  of  water  glass  with  hj'drochloric  acid  a  solution  of  silicic  acid  is  obtained, 
along  with  sodium  chloride  formed  in  the  reaction.  To  obtain  the  neutral  point — that  is,  the 
point  where  the  sodium  silicate  is  just  decomposed — methyl  orange  may  be  used  as  an  indicator. 
This  solution  is  boiled  for  sonje  time,  and  the  indicator  shows  that  the  solution  has  become 
alkaline,  showing  that  alkaline  silicate  has  formed.  If  again  neutralized  by  hydrochloric  acid 
and  strongly  boiled,  the  alkalinity  returns.  A  solution  of  sodium  chloride  when  boiled  without 
the  addition  of  the  silicic  acid  shows  no  such  alkalinity. 

A  solution  of  silicic  acid  and  sodium  chloride,  if  evaporated  to  diyness  and  heated,  but  not 
to  fusion,  has  considerable  alkalinity  when  redissolved  in  water,  showing  that  alkaline  silicate 
has  formed. 

From  these  experiments  it  appears  that  sodium  chloride  can  be  slightly  decomposed  by 
silicic  acid  or  silica  in  boiling  solution,  forming  sodium  silicate,  but  when  heated  to  a  higher 
temperature  sodium*  chloride  decomposes  more  readily. 

In  addition  to  conducting  the  reactions  with  free  silicic  acid,  salt  water  was  spra^^ed  upon 
a  hot  Keewatin  basalt,  with  the  result  that  a  water-glass  glaze  was  produced  by  reactions 
similar  to  those  above  described. 


o  Lehmann,  O.,  Fliissige  Kristalle  sowie  Plastizitat  von  Kristallen  im  Allgemeinen,  molekulare  Umlagerungen  und  Aggregatzustandsander- 
ungen,  Leipzig,  1904. 


526  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

REACTIONS    BETWEEN    GREENALITE    AND    IKON    CARBONATE,    OR    CARBON    DIOXIDE. 

1.  A  source  of  tlic  injii  carboiiiitc  appears  in  tlic  I'cactiou  of  carbon  dioxide  upon  the  ferrous 
silicate  (f^reenaiite),  either  cold  or  hot,  as  follows: 

FeSiOj  +  CO2  =  FeCO,  +  SiO^. 

L'.  Solid  FeSiO.,  and  free  silica  boiled  with  water  through  which  carbon  dioxide  is  passed 
shows  iron  and  sihca  in  solution  in  the  ratio  FeOiSiO,: : 0.0320 -.0.0.302,  indicating  that  the 
greenalite,  and  free  silica  to  only  a  slight  extent,  are  taken  into  solution. 

3.  If  carbon  dioxide  is  passed  through  water  in  the  cold  containing  solid  gi-eenalite  (FeSiOj) 
for  twenty  liours,  iron  and  silica  are  taken  into  solution  in  about  the  proportions  1  to  1.  Less, 
however,  is  dissolved  than  when  tlie  solution  is  hot. 

4.  If  precipitated  ferrous  carbonate  and  precipitated  ferrous  silicate  of  the  composition 
FeO.SSiOj,  instead  of  ferrous  silicate  and  carljon  dioxide,  are  boiled  in  water,  carbon  dioxide  is 
given  olT  and  greenalite  remains  according  to  the  foUowhig  reaction: 

2FeC03  +  FeO.SSiO^  =  SFeSiO,  +  2C0,. 

In  the  cold  solution  l)otli  remain.     This  reaction  is  similar  to  5,  below,  as  it  is  probable  that 
part  of  the  silica  is  in  the  form  of  silicic  acid  and  not  combined  with  the  iron. 

5.  A  solution  of  silicic  acid  was  boiled  with  precipitated  ferrous  carbonate.  The  composition 
of  the  precipitate  from  several  determinations  was  variable  but  in  each  case  showed  decomjx)- 
sition  of  the  ferrous  carbonate  by  the  silicic  acid,  producing  greenalite.  This  decomposition 
continues  until  the  sdicic  acid  is  entirely  precipitated  as  ferrous  silicate. 

6.  Alkaline  silicate  and  iron  bicarbonate  react  to  form  iron  silicate." 

These  results  show  that  carbon  dioxide  will  break  up  precipitated  ferrous  silicate,  either 
cold  or  hot,  producing  iron  carbonate,  which  is  probably  m  solution  as  the  soluble  bicarbonate. 
The  precipitation  of  the  iron  carbonate  would  follow  from  the  loss  of  carbon  dioxide. 

The  experiments  show  further  that  when  carbonate  is  actually  thrown  out  it  may  be  reacted 
upon  by  silicic  acid  or  alkaline  silicates  when  heated,  drivmg  oif  carbon  dioxide,  indicating  that 
the  silicate  is  the  more  stable  under  such  conditions.  In  the  cold  no  reaction  takes  place;  the 
silicate  and  carbon  dioxide  may  exist  side  by  side.  In  short,  there  is  a  constant  tendency  for 
the  development  of  a  bicarbonate  and  the  precipitation  of  a  carbonate  of  iron  in  the  presence  of 
carbon  dioxide,  but  the  precipitate  is  stable  only  when  cold. 

It  appears,  therefore,  that  the  probable  chemical  result  of  the  extrusion  of  the  igneous 
rock  into  salt  water  carrying  ferrous  salts,  with  or  without  free  silicic  acid,  is  the  formation  of 
ferrous  silicate  with  the  simultaneous  precipitation  of  free  sdica  in  proportions  varying  with 
conditions;  that  of  the  soluble  salts  of  the  bases  which  may  have  been  simultaneouslv  delivered 
the  salt  of  magnesia  would  form  an  insoluble  compound  with  the  alkaline  silicate  and  be  pre- 
cipitated with  the  iron;  that  such  iron  silicate  is  the  first  and  nio.st  stable  salt  to  form  under 
conditions  of  heat;  that  so  far  as  carbon  dioxide  is  present  it  will  tend  to  dccom])ose  the  silicate, 
taking  the  iron  into  solution  as  iron  bicarbonate,  which,  however,  will  remain  as  iron  carbonate 
after  precipitation  only  when  cold,  being  decomposed  when  hot  by  reaction  with  silicic  acid,  or 
alkaline  silicates.  The  first  precipitate  after  the  extrusion  of  the  lava  wouUl  tlierefore  tend 
to  be  greenalite,  unless  the  solution  is  acid  or  strongly  alkaline,  preventing  i)recipitation 
until  neutralized.  It  is  likely  to  be  acid  from  the  presence  of  hydrochloric  acid  formed  as  a 
by-i)roduct  of  the  reactions  above  described.  Removal  of  this  acid  by  heat  would  lead  to  pre- 
cijjitation  of  greenalite.  Tiie  presence  of  a  large  amount  of  carbon  dioxide,  also,  would  ])revent 
the  jjrecipitation  of  greenalite,  holding  the  sid)stance  in  solution  until  loss'of  carbon  dioxide 
from  the  bicarbonate  allowed  the  ])recipitation  of  the  carbonate.  The  deposition  of  green- 
alite therefore  depends  on  the  absence  of  carbon  dioxithi  or  other  acid;  the  deposition  of 
carbonate  depends  on  the  ai)unclant  jiresence  of  carbon  dioxiile.  In  the  last  analysis  tiie  law 
of  mass  action  determines  wliich  of  the  two  shall  form.  Iron  carbonate  replacing  greenalite  is 
often  observed  in  the  Mesabi  district. 

"  Bischof,  Custav.  Elements  of  choiiiical  anJ  physical  geology  (tr.  by  B.  IT.  Paul),  vol.  2, 1855,  p.  70. 


THE  IRON  ORES.  527 

SOUUCE    OF   CARBON   DIOXIDE    FOB   REACTIONS   WITH   GREENALITE. 

The  reactions  above  describeil  require  a  source  for  tlie  carbon  dioxiile.  One  source  may 
be  the  carbonaceous  shites  so  abumhmtly  associated  with  the  carbonates.  Their  distilhition 
during  the  period  of  deposition  of  the  carbonates  woukl  furmsli  carbon  dioxide  -for  these  reac- 
tions. Another  source  may  be  the  igneous  rock  from  vvliich  the  greenahte  solutions  are  held 
to  have  come.  To  quote  Chamberhn  and  Salisbury,"^  "The  chita  now  at  command  seem  to 
indicate  tliat  carbon  choxide  increases  greatly  in  relative  abundance  as  volcanic  action  dies  away. 
Great  quantities  of  this  gas  are  often  given  forth  long  after  all  signs  of  active  volcanism  have 
disappeared." 

DEPOSITION    OF   HEMATITE,  MAGNETITE,  ANI^    SILICA    DIRECTLY  FROM   HOT    SOLUTIONS. 

Certain  facts  have  been  described  for  the  Keewatin  iron  formations  indicating  that  the 
present  hematitic  and  magnetic  jaspers  may  not  be  the  result  of  alteration  of  earlier  ferrous 
compounds,  but  are  original  precipitates  in  the  present  form.  The  same  kinds  of  solutions  from 
these  igneous  rocks  being  postulated  as  seem  to  be  required  to  produce  the  greenahte  and 
carbonate,  the  iron  oxides  could  be  produced  by  the  following  reactions: 

6FeS0,  +.30  =  2Fe,(SO,)3  +FeA- 
9FeS0,  +40  =  3Fe.(SO,)3  +Fe30,. 

Magnetite  may  be  formed  in  high  temperatures  by  the  reaction  of  ferrous  iron  and  water, 
according  to  the  following  reaction: 

3FeO  +  H^O  =  FegO,  +  H^. 

Tills  reaction  is  reversible.  As  it  tloes  not  require  change  in  volume,  probably  pressure  does 
not  control  it.  As  the  development  requires  evolution  of  heat,  the  formation  of  magnetite  is 
favored  by  lowering  of  temperature.  Travers  ^  and,  later,  R.  T.  Chamberlin*^  have  showm 
that  the  free  hydrogen  in  rocks  may  be  developed  in  tiiis  manner  by  artificial  heating. 

Tliis  carries  us  a  step  farther  back  toward  the  direct  pegmatitic  after-effects  and  magmatic 
segregations  producing  the  magnetites  discussed  on  pages  561-562. 

DEPOSITION   OF   IRON    SULPHIDE. 

Iron  sulphides  are  exceptionally  abundant  in  connection  with  the  carbonaceous  slates  and 
iron  carbonates,  presvnnably  deposited  in  bogs  or  deltas.  Hydrogen  sulphide  characteristic  of 
these  conditions  would  react  upon  iron  carbonate  and  produce  iron  sulpiride. 

Hydrocarbon  distillates  from  muds  or  shales,  such  as  are  commonly  given  off  in  marshes, 
would  accompUsh  direct  reduction  of  soluble  iron  sulphates  to  iron  sulphides. 

The  existence  of  iron  sulphides  as  magmatic  segregations  and  deep-seated  contact  minerals 
points  to  another  mode  of  origin  of  iron  sulphide.  The  ferrous  sulphate  and  silicic  acid  solu- 
tions coming  from  extrusives  being  again  postulated,  the  reduction  of  ferrous  sulphate  directly 
to  the  sulphide  could  be  brought  about  in  the  presence  of  hydrogen  sulphide  or  hydrogen,  both 
of  which  might  be  emanations  from  the  same  mass. 

The  reaction  of  ferrous  sulphate  and  magnetite  would  produce  iron  sulphide,  according 
to  the  following  reaction: 

FeSO,  +  sFcjO^  =  FeS  + 1 2Fe203. 

CORRELATION  OF  LABORATORY  AND  FIELD  OBSERVATIONS. 

It  appears,  therefore,  that  the  principal  original  iron-bearing  constituents  of  the  iron  for- 
mations, greenahte  and  carbonate,  as  well  as  the  suborilinate  ones,  may  be  produced  in  the 
laboratory  with  comparatively  simple  reactions  under  conditions  ranging  from  those  similar 

o  Chamberlin,  T.  C,  and  Salisbury,  R.  D.,  Text-book  of  geology,  vol.  1. 1904,  p.  590. 

i>  Travers,  II.  ^ .'.,  Proc.  Roy.  Soc.,  vol.  04, 1S98,  pp.  130-1-12. 

cChamberlin,  R.  T.,  The  gases  in  rocks:  Pub.  Carnegie  lust.  So.  lod,  1908. 


528  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

to  weathering,  transportation,  and  deposition  at  ordinary  temperatures,  aided  Ijv  organic 
reducing  materials,  to  conditions  of  direct  contribution  of  iron-bearing  salts  from  the  hot  igne- 
ous rocks  to  the  locus  of  deposition.  Carbonate  or  greenalite  might  develop  under  either  set 
of  conditions,  but  on  the  whole  tlie. former  set  seems  to  be  more  favorable  chemically  to  the 
development  of  iron  carbonate  associated  with  the  carbonaceous  slates  and  the  latter  set 
more  favorable  to  the  development  of  greenalite.  It  also  appears  that  iron  carbonate  may 
develop  from  reactions  of  greenalite  with  carbon  dioxide,  and  this  is  regarded  as  an  adequate 
though  not  necessary  means  of  precipitation  of  iron  carbonate  knowTi  to  be  more  or  less  free 
from  carbonaceous  material  and  in  close  association  with  greenalite.  Iron  carbonate  secondary 
to  greenalite  is  commonly  observed.  It  maj^  be  noted  that  when  carbon  dioxide  reacts  upon 
greenalite,  carbon  dioxide  is  introduced  and  nothing  is  taken  awaj'.  The  percentage  of  silica 
in  the  cherty  carbonate  is  therefore  less  than  the  percentage  of  free  silica  in  the  original  cherty 
greenalite.  The  exact  difference  in  percentage  will  dej)end  on  the  proportion  of  greenaUte  to 
free  sihca  chosen  for  the  reaction.  An  average  of  all  the  iron  carbonate  analyses  available  from 
the  Lake  Superior  iron  formations  gives  iron  24. .56  per  cent,  silica  4L15  percent.  An  aver- 
age of  all  the  greenalite-chert  analyses  gives  iron  2.5.05  per  cent,  silica  55.80  per  cent.  These 
figures  are  derived  from  a  sufficiently  large  number  of  samples  to  make  them  fair  averages. 
Their  validity  is  strengthened  also  by  their  accordance  with  the  comjiosition  of  the  alteration 
products,  the  ferruginous  cherts  and  jaspers,  the  average  composition  of  which  has  been  closel}' 
ascertained.  The  lower  relative  silica  content  of  the  iron  carbonates  is  thus  suggestive  though 
not  decisive  evidence  of  the  derivation  of  some  of  the  carbonates  from  the  greenalite  rocks. 
A  condition  also  pointing  to  reactions  between  iron  silicate  and  carbon  dioxide  to  produce 
iron  carbonate  is  the  conspicuous  absence  in  the  greenalite  and  in  some  of  the  iron  carbonate  of 
the  bases  which  form  soluble  compounds  with  the  silicates,  especially  calcium  and  the  alkalies, 
and  the  presence  in  the  greenalite  and  iron  carbonate  of  magnesia,  a  substance  wliich  forms  an 
insoluble  compound  with  the  alkaline  silicates.  The  average  content  of  these  minor  con- 
stituents in  the  greenalite  and  carbonate  is  as  follows:     . 

Average  magnesium,  calcium,  sodium,  and  potassium  content  in  greenalite  and  cherty  carbonate. 


Greenalite 
rock. 


Cherty 
carbonate. 


Magnesium 

Calcium 

Sodium  and  potassium. 


Per  cent. 

4.20 

.08 

None. 


Percent. 
S.20 
.86 
None. 


The  above  argument  will  not  apply  to  the  exceptional  iron  carbonates  which  show  gradations 
into  limestones  and  ferrodolomites,  as,  for  instance,  at  Gunflint  Lake  and  at  the  east  end  of  the 
Gogebic  district. 

Wlaether  u-on  carbonate  develops  by  reaction  of  greenahte  upon  carbon  dioxide  or  under 
the  ordinary  surface  weathering  conditions  in  the  presence  of  organic  material,  when  we  look 
into  the  probable  sequence  of  events  following  the  extrusion  of  the  original  iron-bearing  igneous 
rocks  and  leading  up  to  the  deposition  of  the  iron  formations,  we  note  that  in  either  case  the 
probable  tendency  would  be  to  develop  greenahte  first  and  then  carbonate.  Also  so  far  as  the 
two  arc  precipitated  at  the  same  time,  the  higher  density  of  the  greenalite  would  make  it  settle 
first,  the  carbonate  following  later,  as  shown  by  laboratoiy  c.xperuuent.  ^^^len  the  ingretlients 
of  the  upper  Iluronian  (quartz  sand,  mud,  greenalite,  and  iron  carbonate)  are  shaken  up 
together  m  a  vessel  of  water  and  allowed  to  settle,  a  clean  layer  of  sand  is  fonned  at  the  bottom, 
showmg  a  most  distinct  contact  with  the  la3'er  next  above.  Then  follows  greenalite  with  some 
carbonate  and  mud,  then  carbonate  and  mud  with  some  greenalite,  and  finally  mud  with  some 
carbonate. 

Thus,  whatever  emphasis  is  put  upon  the  different  ways  of  producmg  iron  carbonate,  it 
seems  probable  that  in  any  iron-bearing  formation  greenalite  materials  would  be  more  abundant 
near  the  bottom  of  the  formation,  or  near  shore,  and  the  carbonate  higher  up,  or  offshore. 


THE  IRON  ORES.  529 

The  distribution  of  the  greenalite  and  carbonate  rocks  in  the  upper  Huronian  is  remarkably 
in  accord  with  inferences  drawn  from  the  chemistry  of  tiieir  deposition.  GreenaHte  is  as  yet 
known  only  at  the  lowest  horizons  of  the  upper  Huronian  and  is  exposed  in  tlie  Mesabi,  Felch 
Mountain,  and  Menominee  districts  and  to  a  slight  extent  in  the  Gogebic  district.  In  the  upper 
part  of  the  iron  formation  of  the  Mesabi  district  iron  carbonate  becomes  relatively  more  abun- 
dant, and  just  beneath  the  overlying  Virginia  slate  forms  a  layer  up  to  20  feet  in  thickness.  la 
higher  parts  of  the  upper  Huronian  associated  with  the  slate  in  the  Cuyuna,  Crystal  Falls,  Iron 
River,  and  Florence  districts  the  iron  formation  consists  dominantly  of  iron  carbonate. 

The  presence  of  the  carbonate  near  the  base  of  the  series  in  the  Gogebic  district  would 
imply  under  the  above  principles  a  proportionally  greater  abuntlance  of  carbon  dioxide  there 
than  in  the  Mesabi  district,  for  unknown  reasons. 

SECONDARY    CONCENTRATION    OF    THE    ORES. 

GENERAL    STATEMENTS. 

The  secondary  alteration  of  the  iron  formations  to  ore  has  been  accomplished  by  both 
chemical  and  mechanical  processes,  under  conditions  of  weathering,  with  modifications  due  to 
folding,  deep  burial,  and  proximity  to  igneous  intrusions. 

All  the  ores  are  partly  the  result  of  secondary  concentration,  but  some  have  suffered  more 
and  some  less  concentration.  Layers  of  iron  formation  origmally  rich  in  iron  hav  become 
iron  ores  by  less  concentration  than  liave  layers  of  iron  formation  originally  poor  in  iron.  In 
a  few  places  in  the  region,  as  in  the  east  end  of  the  Gogebic  district  and  in  parts  of  the  Mesabi 
district,  there  is  evidence  that  certain  layers  of  iron  formation  were  originally  nearly  rich 
enough  in  iron  to  be  mined  as  iron  ores,  after  only  a  slight  amount  of  secondary  alteration.  In 
such  places  the  shape  and  dimensions  of  the  original  layers  determine  essentially  the  shape  and 
dimensions  of  the  iron  ore  deposits.  Wliere  secondary  concentration  has  been  largely  effective 
ill  producing  the  iron  ore,  as  it  has  in  most  of  the  larger  deposits  of  the  region,  the  shape  and 
distribution  of  the  ore  bodies  are  determined  by  the  structural  conditions  which  localize  the 
secondary  concentration,  rather  than  by  the  ]>rimary  bedtling  of  the  iron  formation. 

The  essential  secondary  changes  in  the  development  of  the  ores  have  been  effected  by 
weathering.  The  ores  once  formed,  alterations  effected  by  dynamic  action,  igneous  intrusion, 
or  redeposition  as  fragmental  sediments  may  be  regarded  as  for  the  most  part  subsequent  and 
modif3'mg  factors,  tendmg  to  change  somewhat  the  character  of  the  ores  and  ore  deposits, 
but  adding  little  to  their  size  or  richness.  Dynamic  and  igneous  metamorphism  actmg  before 
the  concentration  of  the  ores  tends  to  inhibit  ore  concentration  by  making  the  iron  forma- 
tion refractory  to  weathering  agencies.  In  the  following  treatment  emphasis  will  be  placed 
accordingly. 

CHEMICAL  AND   MINERALOGICAL  CHANGES   INVOLVED   IN   CONCENTRATION  OF  THE   ORE 

UNDER    SURFACE    CONDITIONS. 

OUTLINE    OF    ALTERATIONS. 

It  requires  only  the  most  general  field  observation  to  bring  out  the  fact  that  the  iron  forma- 
tions are  being  and  have  been  rapidly  altered  by  percolating  waters  carrying  oxygen,  carbon 
dioxide,  and  other  constituents  from  the  surface  and  that  the  present  characteristics  of  the 
formations  are  considerably  different  from  those  they  had  when  they  first  became  consolidated. 
Now  they  consist  mainly  of  ferruginous  chert  and  jasper,  uith  subordinate  quantities  of  iron 
ore,  paint  rock,  greenalite,  iron  carbonate,  amphibole-magnetite  rock,  etc.  Formerly  they 
were  more  largely  cherty  iron  carbonate  or  greenalite.  Fortunately  the  alterations  have  not 
everywhere  gone  far  enough  to  obhterate  all  the  original  phases  of  the  iron  formations.  Grada- 
tions may  be  observed  between  original  cherty  iron  carbonate  or  greenalite  phases  of  the  forma- 
tions and  the  dominant  alteration  products,  ferruginous  cherts  and  jaspers  and  iron  ores.  The 
47517°— VOL  52—11 34 


530  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

former  are  found  in  protected  places  beneath  slate  or  other  impervious  cappings;  the  latter 
occur  in  portions  of  the  formations  exposed  to  percolating  oxidizing  waters.  The  former  are 
ferrous  compounds,  unstable  under  conditions  of  surface  weathering;  tiu'  latter  are  the  stable 
oxides,  end  products  of  weathering.  The  ferruginous  cherts,  jaspers,  and  iron  ores  furthermore 
retain  textures  characteristic  of  carbonate  and  greenahte,  thereby  betraying  their  derivation 
from  these  substances.  This  is  especially  noticeable  in  the  ores  and  cherts  derived  from  green- 
ahte, the  pecuhar  granular  shapes  of  the  greenahte  being  conspicuous  in  its  derivatives.  The 
red,  brown,  and  j'ellow  colors  of  the  altered  phases  of  the  formations,  the  ores  and  ferruginous 
cherts,  contrast  strongly  with  the  gray  and  green  of  the  original  cherty  carbonate  and  greenahte, 
making  the  alterations  conspicuous  to  the  eye,  especially  along  fissures  in  the  original  rocks. 

The  secondary  alterations  of  iron  carbonate  and  greenahte  rocks  to  iron  ore  involve  (1) 
oxidation  and  hydration  of  the  iron  minerals  in  place,  (2)  leaclaing  of  sihca,  and  (.3)  introduc- 
tion of  secondary  iron  oxide  and  iron  carbonate  from  other  parts  of  the  formations.  These 
changes  may  start  simultaneously,  but  the  first  is  usually  far  advanced  or  complete  before  the 
other  two  are  conspicuous.  The  early  products  of  alteration  therefore  are  ferruginous  cherts — • 
that  is,  rocks  in  wliich  the  iron  is  oxidized  and  hydrated  and  the  sihca  not  removed.  The 
later  removal  of  sihca  is  necessary  to  produce  the  ore.  The  secondary  introduction  of  iron 
oxide  and  iron  carbonate  in  cavities  left  by  the  leaching  of  sihca  is  of  httle  importance  in  the 
alteration  of  the  greenahte  rocks  to  ore.  In  the  alteration  of  the  carbonates  to  ore  it  is  fre- 
quently a  conspicuous  feature.  The  alteration  of  the  original  rocks  of  the  iron  formations  to 
ore  may  therefore  be  treated  under  two  main  heads — (1)  oxidation  and  hydration  of  greenahte 
and  siderite,  producing  ferruginous  chert;  (2)  alteration  of  ferruginous  chert  to  ore  by  leaching 
of  sihca,  -with  or  without  secondary  introduction  of  iron. 

OXIDATION  AND  HYDRATION  OF  THE  GREENALITE  AND  SIDERITE  PRODUCING  FERRUGINOUS  CHERT. 

The  oxidation  of  the  cherty  iron  carbonates  and  greenahtes  to  hematite  or  hmonite  pro- 
duces ferruginous  cherts  of  varying  richness.  (See  Pis.  XLII,  C,  D;  XLIII-XLY.)  During 
these  changes  tlie  iron  minerals  for  the  most  part  are  altered  in  place,  but  iron  ma}'  also  be 
transported  and  redeposited.  Evidence  of  this  is  abundant  in  the  stalactitic  and  botryoidal 
ores  lining  cavities  or  incrusting  secondary  quartz  crystals  and  numerous  veins  of  ore  cutting 
across  the  beddmg  of  the  formation.  It  will  be  sho\\^^  in  the  follo^v•ing  discussion,  however,  that 
the  principal  enrichment  of  the  ore  takes  place  in  connection  with  the  removed  silica,  although 
in  several  districts  the  introduction  of  iron  is  very  important.  The  oxidation  and  hydration  of 
the  original  iron  imnerals  are  expressed  in  the  following  reactions: 

4FeC03  (siderite)  +  nHjO  +  20  =  2Fe203.nH20  +  400^. 
4Fe(Mg)Si03.nH20  (greenahte)  +  20  =  2Fe203.nH,0  +  4SiOj. 

The  alteration  of  the  iron  minerals  is  facUitated  by  smaU  amounts  of  acids  carried  by  per- 
colating waters.  Carbonate  of  iron  is  soluble  ^nth  difficulty  in  pure  water  and  not  easily  soluble 
with  an  excess  of  carbon  dioxide.  On  the  otlier  hand,  it  is  easil}'  soluble  in  either  of  the  stronger 
acids,  sulphuric  or  hydrocliloric.  Sulphuric  acid  results  from  the  decomposition  of  the  iron 
sulphide  in  the  original  carbonates  and  in  the  adjacent  pyritiferous  greenstones  and  slates. 
The  reaction  may  be^ 

FeS^  +  H,0  +  70  =  FeSO,  +  H,SO,  .  , 

This  is  aided  in  turn  by  carbon  dioxide  in  the  water.  Thus  the  iron  sulphide  is  oxidized  to 
ferrous  sulphate^  ^\^th  the  simvdtaneous  production  of  sulphuric  acid,  wliich  attacks  the  iron 
carbonates  and  changes  them  to  soluble  ferrous  sulphate.  In  the  Micliipicoten  ihstrict,  where 
glacial  erosion, has  cut  deep,  sulphides  are  found  abundantlv  with  the  carbonates.  Sulphate 
of  iron  is  present  in  veins  in  the  ores  of  the  Iron  River  chstrict.  Baj-ley  "  found  the  white 
efBorescence  characteristic  of  Menominee  ores  to  be  essentially  sochum  sulphate  with  tlie  for- 
mula of  Glauber  salt,  Na^SO,  +  lOHjO,  which  he  regards  as  the  result  of  decomposition  of  pyrite 

oBayley,  \V.  S.,  The  Menominee  iron-bearing  district  ot  Michigan:  Mon.  I'.  S.  Geol.  Survey,  vol.  K\  1904,  pp.  390-391. 


PLATE    XLIII. 


531 


PLATE  XLIII. 
Photomicrographs  of  greenalite  granules. 

A.  Greenalite  rock  (specimen  45178,  elide  15652)  from  100  paces  north  500  paces  west  of  the  southeast  corner  of  sec.  22, 

T.  59  N.,  R.  15  W.,  Mesabi  district,  Minnesota.  Without  analyzer,  X  50.  The  slide  is  selected  to  show  both 
the  fresh  and  the  slightly  altered  granules.  Note  the  peculiar  greenish-yellow  color  of  the  granules,  their 
irregular  shape,  and  their  curving  tails,  some  of  which  seem  to  connect  with  adjacent  granules.  The  homogeneous 
greenish  yellow  colors  represent  the  unaltered  parts.  The  bright-green  and  dark-green  colors  represent  grunerite 
which  has  been  developed  from  the  alteration  of  the  greenalite.  The  dark  green  is  perhaps  in  small  part  iron 
oxide.     Described  on  pages  165-168. 

B.  The  same  with  analyzer,  X  50.     The  unaltered  portions  of  the  granules  are  nearly  or  quite  dark  under  crossed  nicols. 

^\^lere  the  granules  have  altered  to  griinerite  the  polarization  colors  appear.  The  matrix  consists  of  fine-.grained 
chert  in  which  the  individual  particles  are  very  irregular  in  shape  and  size.     Described  on  pages  165-168. 

532 


U.  S.  GEOLOGICAL  SURVEY 


MONOGRAPH    Lll         PLATE  XLIII 


PHOTOMICROGRAPHS  OF  GRUNALITE  GRANULES. 


PLATE    XLIV. 


533 


PLATE  XLIV. 

Photomicrographs  of  ferruginous  chert  showing  later  stages  of  the  alteration  of 

greenalite  granules. 

A.  Ferruginouschertwith  granules(8pecimen  45063,  slide  15563)  from  nearcenterof  sec.  22,  T.  60  N.,  R.  13\V.     With- 

out analyzer,  X  50.  The  granules  are  outlined  and  in  part  replaced  by  iron  oxide.  The  matrix  is  chert.  The 
complex  nature  of  one  of  the  granules  is  to  be  noted.  Apparently  one  complete  small  granule  is  entirely  inclosed 
in  another  large  one.     Described  on  pages  168-170. 

B.  Griineritic  ferruginous  chert  (specimen  45603,  slide  15974)  from  Clark  mine.     With  analyzer,  X  50.     The  rock 

consists  of  chert  and  iron  oxide  and  griinerite.  The  iron  oxide  is  a  yellowish-brown  hydrated  variety,  which 
is  with  difficulty  distinguished  from  the  griinerite.  The  granules  have  been  entirely  obliterated.  Described 
on  pages  168-170. 

634 


U.  S.  GEOLOGICAL  SURVEY 


MONOGRAPH   Lll     PL.   XL!V 


PHOTOMICROGRAPHS. 


PLATE   XLV. 


535 


PLATE  XLV. 
Photomicrographs  of  granules  and  concretionary  structures  in  Clinton  iron  ores. 

A.  Granules  in  Clinton  iron  ore,  from  lower  bed,  Sand  Mountain,  New  England  City,  Ga.     Loaned  by  C.  H.  Smyth,  jr. 

Without  analyzer,  X  40.  Granules  of  black  and  dark-brown  hydrated  hematite  stand  in  a  matrix  of  calcite. 
The  latter  areas  within  the  granules  are  also  calcite.  Traces  of  organic  shells  in  these  slides  are  abundant.  The 
granule  a  little  to  the  right  of  the  center  shows  this  especially  well.  There  can  be  no  doubt  as  to  the  fact  that  the 
granules  are  for  the  most  part  replacements  and  accretions  about  shells  and  particles  of  shells.  It  is  apparent 
also  that  there  is  a  marked  tendency  for  the  granules  to  take  on  rounded  and  oval  forms  regardless  of  the  shape 
of  the  original  particles  of  shell.  Note  the  remarkable  similarity  of  these  granules  in  shape  to  the  greenalite 
granules  illustrated  in  Plate  XLII,  A,  B. 

B.  Green  oolites  in  Clinton  ore,  from  Clinton,  N.  Y.     Loaned  by  C.  H.  Smyth,  jr.     With  analyzer,  X  40.     Concen- 

tric layers  of  chloritic  and  siliceous  substance,  of  various  shades  of  green  and  yellow,  surround  angular,  subangular, 
and  rounded  grains  of  quartz .  The  concentric  greenish  and  yellowish  bands  under  crossed  nicols  show  black  crosses 
characteristic  of  concretionary  structures.  The  matrix  is  mainlycalcite.  but  there  are  present  also  small  particles 
of  quartz. 

536 


U.  S.  GEOLOGICAL  SURVEY 


MONOGRAPH   Lll     PL.   XLV 


PHOTOMICROGRAPHS. 


THE  IRON  ORES.  537 

and  muscovite.  Iron  sulpliides  and  chalcopyrite  are  also  common  as  vein  fillings.  Sul])hates 
are  found  in  mine  waters.  (See  pp.  54.3-544.)  Humus  acids  are  also  well  known  to  aid  in 
the  solution  of  the  iron. 

Precipitation  of  the  iron  from  ferrous  solutions  would  be  caused  (1)  by  direct  oxidation 
and  precipitation  as  limonite;  or  (2)  by  reaction  with  alkaline  carbonate,  producing  iron  car- 
bonate, which  in  this  form  in  the  presence  of  oxygen  alters  almost  immediately  to  hydrated 
iron  oxide;  or  (3)  by  loss  of  carbon  dioxide.  A  small  amount  of  secondary  iron  carbonate, 
where  iron  is  carried  in  solution  as  bicarbonate,  observed  locally  in  each  of  the  districts,  is 
incidental  to  the  mam  process  of  oxidation  producmg  ferruginous  cherts. 

The  oxidation  of  the  iron  in  the  carbonate  and  greenalite  goes  on  much  more  easily  and 
rapidly  than  the  removal  of  the  silica  and  may  afl'ect  most  or  all  of  the  carbonate  or  greenaUte, 
producing  ferruginous  cherts,  before  the  removal  of  the  silica  has  gone  fai'  enough  to  be  appre- 
ciable. ,  An  epitome  of  the  storey  for  the  formation  is  presented  by  almost  any  hand  specimen  of 
iron  carbonate  or  greenalite.  The  ferruginous  cherts  are,  therefore,  intermediate  phases  between 
the  original  greenalite  or  siderite  and  the  ore,  and  the  principal  removal  of  the  silica  is  subse- 
quent to  the  formation  of  the  ferruginous  cherts.  Given  sufficient  time  and  the  other  necessary 
favorable  conditions  and  any  part  of  them  may  become  ore.  In  districts  where  greenalite 
is  the  dominant  origmal  iron  compound,  so  far  as  can  be  determined,  the  layers  of  chert  in 
the  ferrugmous  cherts  prior  to  their  alteration  to  ore  are  not  veiy  different  m  number,  iron 
content,  and  degree  of  hj'dration  from  those  in  the  greenalite  rocks,  indicating  but  little 
transfer  of  iron,  though  localh^  the  segregation  of  silica  and  iron  oxide  into  bands  is  more 
accentuated.  In  districts  where  carbonate  is  an  important  original  iron  salt,  the  rearrange- 
ment, transportation,  and  introduction  of  iron  salts  are  quantitatively  important.  This  is 
probal)ly  due  to  the  structural  conditions  described  on  page  538.  Slight  rearrangements  of 
the  iron  ore  are  to  be  seen  in  the  concretions  composed  of  alternate  concentric  laj'ers  of  chert 
and  iron  oxide  developed  during  the  alteration.  These  develop  both  from  the  iron  carbonate 
and  from  the  greenalite. 

Not  uncommonly  oxidizetl  greenahte  cherts  are  found  alongside  of  unoxicUzed  iron  car- 
bonate cherts.  At  first  thought  this  would  seem  to  indicate  the  readier  oxidation  of  the 
greenalite  than  the  carbonate,  but  it  is  not  certam  that  this  is  the  case,  for  it  is  sometimes 
found  that  the  carbonate  in  these  relations  is  secondary,  and  another  possibility  is  that  the 
greenalite  was  oxidized  at  the  time  of  its  precipitation  rather  than  secondarily. 

ALTEKATION   OF   FERRUGINOUS    CHERT    TO   ORE    BY   THE    LEACHING   OF   SILICA,    WITH   OR    WITHOUT 

SECONDARY    INTRODUCTION    OF    IRON. 

PROCESSES   INVOLVED. 

Ore  may  be  formed  (1)  by  taking  awa}'  silica  from  the  ferrugmous  cherts,  leavmg  tlie 
iron  oxide;  (2)  by  taking  out  silica  and  introducing  iron  in  its  place;  or  (3)  by  adding  iron 
to  an  extent  sufficient  to  make  the  percentage  of  sdica  a  small  one.  In  the  last  case  there 
would  necessarily  be  a  large  increase  in  volume.  Quantitative  tests  show  that  (1)  is  of  greatest 
importance,  that  (2)  is  effective  only  in  some  of  the  ores  derived  from  carbonates,  and  that 
(3)  is  practically  negligible. 

Measurements  of  pore  space  of  the  ores  derived  from  the  alteration  of  ferruginous  cherts 
of  greenalitic  origin  brmg  out  the  facts  that  pore  space  approximates  the  volume  of  silica 
which  has  been  removed  (see  pp.  184-185),  when  there  has  been  little  slump;  in  other  words,  the 
filling  of  the  pore  space  in  the  ores  by  sUica  would  nearly  reproduce  the  composition  of  the  fer- 
ruginous cherts.  It  wHl  be  shown  also  that  the  leaching  of  silica  from  the  ferruginous  cherts 
derivetl  from  greenalite  alterations  does  not  materially  affect  the  character  of  the  iron  oxides, 
especially  their  degree  of  hydration,  and  that  therefore  the  nature  of  the  ore  of  the  deposit 
is  primarily  determined  by  the  changes  which  the  greenalite  undergoes  when  it  alters  to  the 
oxide  bands  of  the  ferruginous  cherts. 

Measurements  of  pore  space  in  ores  derived  from  ferruginous  cherts,  which  in  turn  have 
been  derived  from  the  alteration  of  iron  carbonate,  show  that  the  pore  space  is  less  than  the 


538  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

volume  of  the  silica  wliieh  has  beeu  removed.  (See  p.  24L)  This  is  due  i)artly  to  shimp, 
but  mamly  to  the  fact  that  secondary  iron  oxide  partly  fills  the  openings. 

The  ran<^o  in  wliich  there  is  conspicuous  absence  of  evidence  that  iron  has  been  transported 
to  any  considerable  extent  is  the  Mesabi,  where  the  flat  dip  exposes  a  larye  portion  of  tlie  for- 
mation directly  to  oxidizing;  waters,  and  oxidation  works  down  more  or  less  uniformly  from  the 
surface,  leaving;  few  imoxidizcd  portions  to  contribute  soluble  iron  salts  to  be  earned  down 
and  mixed  witli  deeper  oxidizing  solutions  following  cluuincls  from  the  surface.  In  the  other 
districts,  where  the  evidence  of  the  carrying  of  iron  is  ])Iiiin,  the  formations  are  so  tilted  that 
the  underground  courses  of  oxidizing  waters  from  the  surface  pitch  deeply,  lea^-ing  unoxi- 
dized  iron  formation  above  as  a  source  for  soluble  iron  salts,  which  may  be  taken  into  solution 
and  carried  down,  and,  by  reaction  wdth  oxidizing  waters,  precipitate  the  iron  oxide.  This 
deej)  circulation  of  oxidizing  waters  afforded  by  steeply  tilted  formations  permits  the  leaching 
of  silica  at  tlepth,  thus  providing  openings  in  wliich  the  iron  carried  m  solution  from  the  upper 
unoxidized  portions  of  the  formation  may  be  deposited.  It  is  m  tliis  essential  that  the  ilesabi 
conditions  differ  from  those  of  the  other  ranges. 

Silica  dissolved  from  the  iron  formations  has  been  in  small  part  redeposited  in  veins,  both 
in  ore  and  rock  and  in  the  crystallized  quartz  linings  of  uiany  cavities  in  the  ore,  and  in  part 
has  joined  the  run-off.  The  process  is  going  on  to-day,  for  mine  and  svirface  waters  carry' 
siUca  (see  pp.  540-544),  an<l  quartz  linings  of  cavities  may  be  seen  to  have  developed  since  mining 
explorations  began.  It  has  been  suggested  that  the  abundant  chert  in  the  ferruginous  cherts 
themselves  might  represent  materials  previously  leached  from  other  parts  of  the  formations 
and  redeposited.  As  the  cherts  are  very  dense  (see  p.  545),  there  would  be  no  room  for  the 
addition  of  secondary  silica  except  that  made  by  the  volume  change  in  the  alteration  of  iron 
minerals  or  by  the  previous  leaching  of  silica.  Undoubtedly  cavities  of  both  sorts  have  been 
filled  to  a  certain  extent  by  silica.  But  the  process  of  the  average  increase  of  silica  would 
involve  a  reversal  of  the  one  which  is  actually  observed  to  occur — that  is,  the  leaching  of  silica 
from  tlie  ferruginous  cherts,  producing  the  ores.  We  are  forced  to  the  conclusion  that  while 
as  in  any  metamorphic  process  in  the  belt  of  weathering  silica  is  removed  and  silica  is 
deposited,  the  former  change  is  predominant.  A  parallel  may  be  cited  in  the  development  of 
caves  in  limestone  by  solution  and  deposition,  the  process  of  solution  predominating. 

CONDITIONS  FAVORABLE  TO  LEACHING  OF  SILICA. 

The  loss  of  silica  from  the  ferruginous  cherts  on  a  large  scale  requires  exceptionally  favorable 
conditions.  These  conditions  seem  to  be  (1)  the  ready  access  of  dissolving  solutions  to  large 
surfaces  of  the  chert  and  (2)  the  alkaline  character  of  the  dissolving  solutions. 

The  fine  and  irregular  grain  of  the  cjuartz  in  the  ferruginous  cherts  affords  large  surfaces 
of  contact  with  the  water,  thereby  favoring  solution.  It  is  noted  that  where  the  cherts  have 
been  coarsely  recrj'stallized  under  the  influence  of  intrusives  there  is  much  less  tendency  for  the 
silica  to  go  into  solution.  Much  of  the  silica  in  the  cherts  is  cherty  or  opahne  and  thus  easily 
soluble. 

The  conditions  favorable  to  rapid  and  abundant  flow  of  water  are  due  largely  to  structural 
causes,  wliich  are  discussed  on  pages  474-475.  A  large  amount  of  water  is  needed  to  effect  the 
removal  of  silica.  Merrill  "  estimates  that  the  removal  of  a  unit  of  silica  requires  10,000  times  its 
weight  in  water.  The  removal  of  a  large  amount  of  silica  from  the  iron  format  ions  wliich  has  been 
necessary  to  produce  the  ore  deposits  has  therefore  required  a  large  amount  of  water  for  each 
unit  removed^n  other  words,  free  and  vigorous  flow.  Thus  is  explained  the  concentration 
of  the  ores  along  zones  of  easy  flow  Mhcre  water  is  abundantly  concentrated. 

SOLXTTION  OF  SILICA  FAVORED  BY  ALKALINE  CHARACTER  OF  WATERS. 

The  solution  of  silica  is  favored  by  the  alkaline  character  of  the  waters.  Alkaline  car- 
bonates react  upon  quartz,  forming  soluble  alkaline  silicates  with  release  of  carl)ou  dioxide. 
Sodium  carbonate  may  not  stand   in  a  glass  bottle  without  dissolving  it.     Well  waters  in  the 

o  Merrill.  G.  P.,  Rocks,  rock  weathering,  and  soils,  New  York,  1897,  p.  238. 


THE  IRON  ORES.  539 

vicinity  of  Ironwood,  Mich.,  obviously  the  same  waters  that  are  entering  the  formations,  are 
throughout  alkahne  in  their  reactions.  Many  solution  cavities  left  by  the  leacliing  of  silica 
are  lined  by  ailularia  crystals  (potassium  feldspars),  as  in  the  cavities  left  by  the  leacliing  of 
quartz  pebbles  from  the  ore  at  the  base  of  the  upper  Iluronian  of  the  Marquette  district. 

All  the  ore  deposits  of  the  Lake  Superior  region  are  close  enough  to  igneous  roclcs  to  have 
been  altered  by  waters  which  have  probably  derived  an  alkaline  content  from  the  leaching  of 
the  igneous  rocks.  In  the  Mesabi  district  all  the  waters  entering  the  iron  formation  have  pre- 
viously come  down  across  the  Giants  Range  granite  and  in  a  few  places  have  jnet  granite  dikes 
within  the  formation  and  thoroughly  leached  them  of  their  bases.  In  the  Goge])ic  tlistrict  the 
dikes  (see  analyses,  p.  240)  closely  associated  with  the  ores  have  been  so  thorouglily  leached  oi 
their  bases  that  the  residual  clayey  material  is  known  as  paint  rock  or  soaps  tone.  A  glance  at 
the  maji  of  the  Marquette  district  (PI.  XVII,  in  pocket)  will  show  the  abundance  of  basic 
intrusive  rocks  in  the  iron-bearing  areas.  These  again  near  the  contact  with  the  ores  have  been 
altered  to  soap  rock  and-  paint  rock,  thereby  delivering  their  bases  to  the  solutions  which  have 
developed  the  ore.  In  the  Crystal  Falls  district  the  relation  is  not  less  obvious.  The  ores  are 
throughout  not  far  from  the  basic  eruptive  roclvs.  In  the  Cuyuna  district  intrusive  rocks 
are  everywhere  associated  with  the  ores.  In  the  Menominee  district  the  relation  is  not  so 
obvious,  although  the  igneous  rocks  appearing  on  both  sides  of  the  Menominee  trough  may 
well  have  afl'ected  the  character  of  the  water  in  the  ores.  Probably,  however,  tlie  waters  have 
been  rendered  effective  principally  by  solution  of  the  dolomite  associated  with  or  immediately 
underlying  nearly  all  the  deposits.  It  is  noted  in  nearly  all  the  districts  that  where  the  ore 
comes  into  contact  with  slate  the  slate  has  been  altered  to  paint  rock.  A  comparison  of  the 
composition  of  paint  rock  and  the  unaltered  slate  shows  that  alkalies  have  been  taken  out  in 
the  development  of  the  paint  rock.  Here  again,  then,  is  a  factor  favoring  the  alkaline  character 
of  the  waters. 

TRANSFER   OF  IRON  IN  SOLUTION. 

So  far  as  iron  is  carried  in  solution  it  is  probably  in  the  early  stages  of  the  alteration  of  any 
particular  part  of  a  formation,  when  there  are  still  ferrous  compounds  to  work  upon.  AVlien 
nothing  but  ferric  iron  remains,  this  is  insoluble  and  the  princi})al  further  alteration  is  the 
removal  of  silica.  If  the  iron  finds  lodgment  in  the  formation  before  the  silica  is  taken  out  it 
can  be  only  on  a  small  scale,  for  the  voids  are  not  large  enough  to  contain  much  ore.  When  iron 
is  introduced  after  all  the  silica  is  taken  out,  its  introduction  may  not  materially  change  the 
percentage  of  iron  in  the  ore.     It  will  merely  reduce  the  pore  space. 

SECONDARY  CONCENTRATION  OF  THE  ORES  CHARACTERISTIC  OF  WEATHERING. 

Quartz  is  ordinarily  regarded  as  practically  insoluble  in  surface  waters.  It  might  be 
argued  that  the  conditions  above  cited  are  not  peculiar  to  iron  formations  alone  but  may  be 
found  elsewhere,  and  the  question  is  raised  whether  elsewhere  quartz  is  largely  taken  into  solu- 
tion. We  believe  that  quartz  is  taken  into  solution  under  ordinarj^  conditions  of  weathering 
to  a  larger  extent  than  is  general^  recognized,  and  that  it  is  apparently  stable  because  it  is 
usually  associated  with  more  soluble  constituents,  thereby  contrasting  with  the  iron  formations, 
where  the  quartz  is  associated  with  less  soluble  constituents  against  which  the  loss  of  quartz 
may  be  measured.  A  series  of  three  analyses  of  fresh  granite,  partly  altered  granite,  and  much 
weathered  granite  from  Georgia  published  by  Watson,"  when  recalculated  in  terms  of  minerals, 
shows  that  in  the  early  stages  of  the  alteration  the  quartz  is  but  little  affected,  but  that  in  the 
last  stage  there  is  unquestionable  evidence  of  considerable  leaching  of  free  quartz. 

In  general  comparison  of  analyses  of  fresh  and  weathered  igneous  and  other  rocks  shows 
that  iron  and  alumina  are  the  two  most  stable  constituents  and  that  in  weathering  under  oxidizing 
conditions  silica  is  lost  more  readily  than  the  iron.  The  iron  formation,  consisting  principally  of 
iron  minerals  and  silica  and  lacking  alumina,  would  therefore  be  expected  to  retam  its  iron 

«  Watson,  T.  L.,  Granites  and  gneisses  of  Georgia:  Bull.  Geol.  Survey  Georgia  No.  9A,  1902,  p.  302. 


540  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

under  weaLliering  to  a  greater  extent  than  the  sihca,  and  in  so  doing  has  followed  the  general 
laws  of  katamorphism.  The  absence  of  evidence  of  transfer  of  iron  during  secondary  concentra- 
tion on  a  large  scale  is  in  strong  contrast  with  its  transportation  in  large  amounts  in  the  primary 
concentration.  The  secondary  local  transfers  of  iron  in  the  fcn-ous  condition  before  it  is  oxidized 
to  the  stable  form  are  characteristic  of  both  tlic  iron  formation  and  igneous  rocks  and  do  not  dis- 
prove the  general  ]iiinciple  above  stated. 

In  general  the  same  processes  of  weathering  that  have  j)r()duced  residual  clay  from  igneous 
rocks  are  the  ones  which  have  secondarily  concentrated  the  iron  ores.  Most  igneous  rock  con- 
tains so  little  iron  and  so  much  alumina  and  silica  that  secondary  concentiation  fails  to  ])roduce 
an  iron  ore  directly  from  it;  it  produces  an  iron-stained  clay.  Exce])tionalIy,  however,  as  from 
the  serpentine  rocks  of  Cuba,  which  have  a  low  content  of  alumina,  secondary  concentration 
has  produced  a  mixture  of  iion  ore  and  clay,  and  the  clay,  by  extreme  weathering  at  the  surface, 
has  broken  down  further,  by  loss  of  sUica,  to  bauxite.     The  result  is  a  lateritic  iron  ore." 

MECHANICAL   CONCENTRATION   AND   EROSION   OF   IRON   ORES. 

The  loosening  of  silica  grains  by  solution  locally  makes  it  possible  for  them  to  be  carried 
mechanically  by  the  meteoric  wateis.  This  jjiocess  becomes  one  of  some  importance  when  the 
openings  have  been  made  sufficiently  large  by  solution.  Where  the  mine  waters  are  dammed, 
there  is  very  commonly  a  considerable  sediment  of  fine-grained  chert  sand.  This  process 
probably  also  exjilains  the  occurrence  of  finely  granular  chert  sand  in  seams  and  crevices  in 
certain  Mesabi  ore  deposits.  The  process  is  probably  more  conspicuous  now  tlian  it  was  before 
mining  ojjenings  gave  a  chance  for  the  accumulations  of  these  silica  sands.  It  is  diflicult  to  see 
where,  under  original  conditions,  these  sands  could  have  been  deposited.  The}'  are  found  filling 
openings  underground  only  to  a  very  small  extent,  and  it  is  unhkely  tliat  they  would  follow 
the  underground  waters  into  the  run-ofl'. 

So  far  as  pore  space  has  been  lessened  by  mechanical  slump  anywhere  througli  tlie  iron 
formation,  this  amounts  to  a  decrease  in  volume  antl  increases  the  amount  of  iron  in  a  given 
volume  of  iron  formation.  It  is  shown  in  the  chapters  relating  to  the  difl'erent  districts  that 
this  j)rocess  has  gone  on  to  a  consideral)le  extent. 

Locally,  as  at  the  base  of  the  Vulcan  formation  in  the  Menominee  district,  at  the  base  of 
the  Goodrich  (juartzite  in  the  Marcpiette  district,  and  in  the  Cretaceous  of  the  Mesabi  district, 
there  is  fragmental  detritus  deriveil  l)y  the  processes  of  disintegration,  transportation,  and 
sedimentation  from  earlier-formetl  iron-bearing  formations.  T\liere  this  includes  sorting,  it 
amounts  to  mechanical  concentration  (PI.  XL VI). 

Artificial  concentration  of  ore  by  removal  of  the  chert  through  washing  is  now  being  practiced 
on  tiie  western  Mesabi,  where  the  chemical  processes  have  gone  just  far  enough  to  loosen  tlie 
chert.  There  is  an  enormous  mass  of  material  available.  In  fact,  this  is  the  partly  altered 
iron-beaiing  formation  itself,  rather  than  concentrations  within  the  fornuition.  A  consiilerable 
amount  of  iron  is  lost  during  the  process,  because  the  iron  and  chert  are  attached  to  each  other. 
Much  of  the  silica  in  the  ferruginous  cherts  is  so  very  fine  grained  and  so  intimately  associ- 
ated with  the  iron  that  it  could  probably  never.be  se|)arated  by  crushing  anil  washing.  That 
separation  is  possible  on  tiie  western  Mesabi  is  due  to  the  banded  nature  of  the  ferruginous 
chert  and  the  fact  that  the  finer-grained  portions  have  been  removed  by  solution,  leaving  the 
larger  j)ieces  of  chert  loose. 

GENERAL   CHARACTER   OF   MINE    WATERS. 

Tlie  mine  waters  of  all  but  tlie  deepest  jiaits  of  some  of  liie  mines  are  characterized  by 
considerable  contents  of  carbonates  of  the  alkalies  ami  alkaline  earths,  together  with  silica. 
Iron  is  usually  present  only  in  traces  or  entireh'  lacking. 

The  shallower  mine  waters  are  represented  bj-  the  following  analyses: 

oLelth.C.  K.,an(l  Mead,  W.  J.,  Origin  ol  the  Iron  ores  of  central  and  northeastern  Cuba:  Bull.  .\in.  Inst.  Min.  Eng.  No.51, 1911,  pp.  217-229. 


PLATE    XL VI. 


541 


PLATE  XLVI. 

Ore  and  jasper  conglomerate  and  ferruginous  chert. 

Ore  and  jasper  conglomerate  from  Saginaw  range,  Marquette  district,  Michigan.  This  is  a  t\pifal  basal  conglom- 
erate of  the  Goodrich  quartzite  of  the  upper  Huronian.  The  detritus  consists  almost  wholly  of  various  materials 
denved  from  the  Negaunee  formation,  including  jasper,  chert,  and  ore.  There  is  present,  however,  some  quartz 
derived  from  the  Archean.  A  close  examination  of  the  illustration  shows  that  secondary  hematite  and  ma-netite 
have  largely  formed  in  the  spaces  between  the  grainsabout  many  of  the  jasper  fragments,  and,  indeed  have'partly 
replaced  the  jasper  fragments  themselves.  This  is  beautifully  shown  at  the  lower  left-hand  comer  of  the  fi-iu-e 
In  the  places  where  the  basal  conglomerate  is  fine  grained  these  replacements  by  iron  oxide  may  be  almost  com- 
plete, m  which  case  an  iron-ore  deposit  is  formed.  Of  such  an  origin  is  the  iron  ore  of  the  Volunteer  and  some 
other  mines. 

Ferruginous  chert  from  point  south  of  Jackson  mine,  Marquette  district,  Michigan  (sec.  1,  T.  47  N.,  R  27  W  ) 
The  iron  oxide  and  chert  were  largely  concentrated  into  bands  before  the  last  folding.  At  the  time  of  the  folding 
radial  cracks  were  formed,  especially  in  the  chert  layers,  owing  to  the  position  of  the  rock  on  the  rrown  of  an 
anticline.  Along  these  cracks  the  silica  has  to  some  extent  been  leached  out  and  iron  oxide  introduced  One 
light-colored  area  of  chert  appears  to  be  a  secondary  infiltration,  but  it  was  apparently  present  before  the  last- 
folding,  as  it  is  fractured  in  the  same  way  as  the  other  layers.     Natural  size. 


U.  S.  GEOLOGICAL  SURVEY 


MONOGRAPH   Lll         PLATE  XLVI 


(A)    ORE   AND  JASPER   CONGLOMERATE   FROM   MARQUETTE   DISTRICT,   MICHIGAN 
fBj    FERRUGINOUS   CHERT 


THE  IRON  ORES. 


543 


Analysis  of  water  from  a  drift  between  the  Hull  and  RuM  mines  west  of  Ribbing ,  Mesabi  district/^ 

[Parts  per  million.] 

CO2 71 

SiOj 22.  35 

SO, 2.2 

PO4 Trace? 

Fe Not  a  trace. 

Ca 10.1 

K .97 

Analysis  of  water  from  Newport  mine,  Gogebic  district. 
[Parts  per  millioD.    Analyst,  R.  D.  Hall,  University  of  Wisconsin.] 

SiO, 8. 43 

Fe^Oj 1.  23 

AI2O3 6 

CaO 21.  3 

MgO 16.1 

NajO 5.  4 

K,0 1.83 

CI 6.0 

SO3 .■ 10.8 

CO2  (combined)  as  carbonate 30.  5 

CO2  (free)  as  bicarbonate 18.  0 

PA None. 

Drying  at  100° 108.  3 

Ignited ' 68.  6 

Some  of  the  deep  mine  waters  have  been  found  to  be  highly  concentrated  solutions  of 
calcium  and  sodium  chlorides,  according  to  the  following  analyses.  Such  waters  are  extremely 
corrosive  in  boilers  and  pumps. 

Analyses  of  Michigan  iron  mine  waters." 


Vulcan. 

Ishpem- 
ing. 

Iron- 
wood. 

Hurley. 

Bessemer. 

Cham- 
pion. 

Republic 

Total  solids  (soluble) 

0.340 
.2.'i0 
.052 

0.344 

0.232 

0.142 
.0876 

1.493 

0.309 

7.15 

Organic 

(.020) 

.055 

.051 
.061 
.060 
(?) 
.038 

Tr. 
.000 

.  ori4 
Tr. 
.043 

.163 
.019 
.062 
.006 
.028 

.013 

.010 
.018 

.171 

(.007) 
.062 
.011 
.030 

.004 

.011 
.002 

.046 
.034 
.022 
(.047) 
.005 
/    With 
t       ?Na 
.010 
.001 

.070 
.016 
.025 
.003 
.009 

}      (?) 
.012 
.001 

(.037) 
.638 
.220 
.081 
.073 

Tr. 

.010 
.020 

(.050) 
.060 
.040 

(.039) 
.008 

Chlorine  (CI) 

3.061 
.817 

25  36 

7  202 

Sodium  (Na) .          ..   .. 

7  ^^9 

.500 

Pnta.<y:inm  (K). 

Silica  (Si02) 

Alumina  (AI2O3). 

.003 

Not  det. 
0 
.37 

700 

Tr. 

Sulphate  ion  (SCO 

.013 

.011 

.040 

.005 

.058 

.040 

1  045 

.263 

.99 
0 
.70 

.332 

3.2 
.315 
.68 

.309 

8.9 
1.8 
(1.47) 

.205 

.65 
1.38 
1.18 

.141 

1.56 
.187 
.31 

1.137 

.345 
.127 
.092 

Ratios: 

Ca :  CI2 

.66 
.65 
.66 

.27 

Na:Clj 

''84 

SO,  :C1 

.12 

a  Furnished  by  A.  V.  Lane,  State  geologist  of  Michigan.  Mnrch,  1909. 

Highly  mineralized  waters  have  also  been  found  on  the  eighth  level  of  the  Great  Western 
mine,  in  the  Crystal  Falls  district  of  Michigan.  No  analyses  are  available,  but  tests  by  the 
mine  chemists  showed  the  presence  of  calcium  chloride  and  magnesium  sulphates. 

The  upper  carbonate  waters  are  abundant,  rapidly  flowing,  dilute,  and  more  or  less  direct 
from  the  surface,  carrying  gases  of  the  atmosphere.  They  are  the  w  aters  which  accomplish  the 
major  part  of  the  secondary  concentration  of  ore,  as  showTi  by  the  limitation  of  the  ore  deposits 
to  places  of  rapid  circulation  of  these  waters  and  further  by  the  known  chemical  eflfects  of 
waters  of  this  type  upon  the  original  iron-bearing  formation. 


o  Mon.  U.  S.  Geol.  Survey  vol.  43, 1903,  p.  264. 


544  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

The  deep  chloride  waters  are  relatively  minute  in  quantity  and  liigldy  coneentratecl.  Their 
distribution  is  very  irregular.  Small  reservoirs  may  be  tapped  and  exliausted  alongside  of 
flows  of  fresher  water.  Waters  of  very  similar  characteristics  are  found  in  the  deep  copper 
mines  of  the  Lake  Superior  region,  in  the  deep  levels  of  the  Silver  Islet  silver  mine,"  on  the 
north  shore  of  Lake  Su|>erior,  in  deep  wells  in  the  Paleozoic  of  the  upper  Mississippi  Valley,  in 
the  granites  of  the  Piedmont  area  of  Georgia,  and  elsewhere.  Their  characteristics  seem  not 
to  be  related  to  certain  kinds  of  rocks  or  ore  deposits,  but  to  depth  and  stagnant  conditions. 
Chlorine  is  present  in  minute  quantities  in  original  igneous  rocks  and  in  nearl}'  all  surface  waters. 
Its  salts  tend  to  remain  in  solution,  while  the  salts  of  other  acids  are  more  largely  precipitated. 
With  a  given  amount  of  water,  there  seems  likely  to  be,  therefore,  a  progressive  relative  accumu- 
lation of  chlorine  salts.  Such  is  the  case  in  salt  waters  at  the  earth's  surface,  where  a  large 
factor  in  the  accumulation  is  the  lack  of  sufficient  circulation  to  carry  off  and  dilute  the  salt 
waters  that  are  developing  by  evaporation.  In  deep  underground  Maters  there  is  essentially 
the  same  condition  of  stagnancy,  and  therefore  we  suggest  jirogressive  accumulation  of  soluble 
chlorine  salts.  In  the  shallower  mine  waters  the  rapid  circulation  and  accession  of  fresh  waters 
from  the  surface  prevent  such  accumulation  of  salt. 

The  proportion  of  sodium  chloride  to  calcium  cliloride  in  deep  mine  waters  in  the  Lake 
Superior  region  becomes  relatively  less  with  increase  in  depth,  indicating  that  the  increasing 
content  of  chlorine  is  able  to  hold  not  only  all  sodium  present  but  larger  amounts  of  calcium. 
The  materials  in  solution  under  any  conditions  must  be  regarded  as  representing  the  residual 
solutions  from  which  all  possible  insoluble  minerals  have  already  crystaUized  out.  All  the 
Lake  Superior  mines,  both  iron  and  copper,  are  associated  with  basic  rocks  in  which  calcium 
greatly  predominates  over  the  sodium,  so  that  whenever  the  sodium  is  taken  care  of  by  the 
clilorine  present  there  should  always  be  a  considerable  excess  of  calcium  available. 

Lane,  who  has  given  special  attention  to  deep  mine  w* aters  and  who  has  brought  together  the 
analyses  above  quoted,  otl'ers  quite  "another  explanation  for  the  characteristics  of  these  deep 
waters.  He  beUeves  them  to  be  connate  or  fossil  sea  waters,  included  in  the  rocks,  both  igneouf 
and  sedimentary^  during  submarine  deposition.  The  fact  that  they  differ  from  present  sea 
water  in  having  so  large  a  proportion  of  calcium  chloride  he  ascribes  to  a  possible  change  in 
composition  of  the  sea  water  during  geologic  time  in  the  direction  of  increasing  the  proportion 
of  sodium  chloride  as  compared  with  calcium  chloride  to  the  present  known  proportion  of  sea 
water.  We  do  not  follow  him  in  this  conclusion  because  of  the  fact,  already  cited,  that  these 
pecuhar  salt  waters  seem  to  be  characteristic  not  only  of  marine  sediments  but  of  sediments  of 
subaerial  origin,  of  surface  eruptives,  and  of  plutonic  igneous  rocks.  They  are  related  to  depth 
and  stagnancy  rather  than  to  kind  of  rock  or  geologic  horizon.  There  seems  to  be  no  adequate 
reason  for  regarding  these  waters  as  fossil  sea  waters,  for  all  the  essential  kinds  of  conditions 
which  produce  the  salt  water  of  the  ocean  are  present. 

LOCALIZATION  OF  THE  ORES  CONTROLLED  BY  SPECIAL  STRUCTURAL  AND  TOPOGRAPHIC 

FEATURES. 

From  the  foregoing  discussion  it  appears  that  the  iron  ores  constitute  concentrations  in  the 
exposed  parts  of  the  iron-bearing  formations  accomplished  on  the  average  mainly  b}^  the  removal 
of  associated  silica,  leaving  the  iron  oxidized  and  in  larger  percentage,  but  to  an  important 
extent  accomplished  also  by  solution,  transportation,  and  redeposition  of  the  iron  when  it  was 
still  in  its  soluble  ferrous  condition.  The  agents  of  alteration  are  surface  waters  carrying  oxygen 
and  carbon  dioxide  from  the  atmosphere.  The  accessibihty  to  the  iron-bearing  formations  of 
these  agents  therefore  determines  the  location,  shape,  and  size  of  the  deposits.  The  structural 
conditions  favoring  such  accessibihtj'  have  been  summarizeil  in  the  earlier  part  of  this  chapter 
(see  pp.  474-475),  and  are  discussed  in  some  detail  in  connectit)n  with  the  ores  of  the  individual 
districts.  They  may  be  merely  mentioned  here.  The  most  favorable  condition  is  afforded 
by  wide  area  of  exposure  of  the  formation,  which  in  turn  is  a  function  of  the  dip.     Fractures, 

» iDgall,  E.  D.,  Report  on  mines  and  mining  on  Lake  Superior:  Ann.  Rept.  Oeol.  Survey  Canada  for  18$7-SS,  vol.  3,  pt.  2, 1889,  p.  2SB. 


THE  IRON  ORES.  545 

impervious  basements,  and  varying  porosity  also  serve  to  concentrate  the  circulation.  Ores 
are  not  found,  however,  in  some  places  where  area,  fractures,  and  impervious  basements  seem 
to  be  favorable  for  ore  concentration.  This  is  beheved  to  be  due  in  some  part  to  the  denseness  of 
the  cherts  in  these  places,  preventing  access  of  water.  Wherever  the  rocks  are  dense  the  silica 
is  not  removed.  The  amphibole-magnetite  cherts,  the  unaltered  greenalite  and  siderite  rocks, 
and  the  quartzites  associated  with  the  iron-bearing  formations  all  have  very  Uttle  pore  space,  as 
shown  by  a  considerable  number  of  determinations.  Silica  is  not  removed  directly  from  these 
rocks.  On  the  other  hand,  the  ferruginous  cherts,  resulting  from  the  alteration  of  cherty  iron 
carbonates  and  greenalites,  contain  pore  space  averaging  about  5  per  cent,  developed  by  the 
lessening  of  the  volume  of  the  iron  minerals  during  their  alteration  from  the  ferrous  to  the 
ferric  form.  Tliis  pore  space  is  so  distributed  as  to  give  the  water  access  to  all  parts  of  the 
rock  mass.  The  size  of  grain  is  so  small  that  for  each  grain  there  is  a  large  surface  in  proportion 
to  volume.  But  even  the  ferruginous  cherts  are  locally  so  dense  that  they  do  not  allow  ready 
access  of  water.  Several  possible  reasons  may  be  suggested  for  this  unusual  density.  (1)  The 
ferruginous  cherts  at  these  places  may  not  have  been  derived  by  alteration  from  cherty  car- 
bonates or  greenahtes  but  may  have  been  deposited  directly  in  their  present  form  as  chemical 
sediments  mth  small  pore  space.  It  has  been  shown  that  this  could  easily  go  on  with  the 
deposition  of  greenahte  and  carbonate.  This  explanation  would  seem  to  be  especially  Ukely 
to  hold  for  certain  of  the  amphibohtic  cherts  of  the  Keewatin,  wliich  are  intimately  associated 
with  basalt  flows  both  above  and  below  and  wliich  it  is  entirely  conceivable  might  have  been 
originally  deposited  in  a  condition  different  from  those  of  the  cherty  carbonates  and  greenalites 
of  the  later  iron-bearing  formations.  (2)  Metamorpliism  of  the  cherts  under  pressure  after  pore 
space  had  been  developed  by  oxidation  of  the  iron  minerals  may  have  closed  the  openings  before 
the  siUca  had  been  taken  out.  Cherts  wliich  have  been  much  folded  and  contorted  at  so  great 
depth  as  to  be  deformed  without  fractures  are  almost  invariably  dense.  The  Keewatin  iron- 
bearing  formations  are  the  oldest  and  have  naturally  suffered  more  from  such  metamorpliism 
than  the  later  formations,  and  this  may  be  a  factor  in  the  barrenness  of  the  Keewatin.  On  the 
other  hand,  larger  areas  of  the  upper  Huronian  are  comparatively  Uttle  deformed  and  pore 
spaces  formed  by  the  oxidation  of  the  iron  minerals  have  remained  substantially  open  since 
upper  Huronian  time.  (3)  The  openings  may  have  been  closed  by  infiltrated  sihca  and  iron. 
In  the  Marquette  jasper,  secondary  materials  completely  heal  the  rock.  The  relative  importance 
of  these  conditions  affecting  pore  space  varies  from  place  to  place  and  between  the  different 
iron-bearing  formations,  and  this  variation  is  beheved  to  account  in  large  measure  for  the  marked 
differences  in  enrichment  of  different  formations  and  different  parts  of  formations. 

Undoubtedly  the  processes  of  secondary  concentration  above  described  tend  to  affect  to 
a  greater  or  less  degree  all  the  exposed  surface  of  the  iron-bearing  formations.  It  is  not  unlikely 
that  in  long  periods  of  slow  denudation  ores  may  have  actually  covered  all  of  this  surface. 
It  is  equalljr  obvious,  however,  that  the  covering  had  various  depths,  depemlmg  on  a  consider- 
able variety  of  structural  conditions.  The  glacial  denudation  has  scraped  off  ore  which  may 
once  have  developed  at  the  surface,  and  little  has  developed  since.  There  remam  only 
the  lower  parts  of  the  deposits  left  by  denudation.  A  discussion  of  the  structural  conditions 
governing  the  ore  deposits  is  therefore  really  a  discussion  of  the  conditions  determining  their 
lower  limit  and  configuration.  The  structural  and  topographic  conditions  of  each  of  the  dis- 
tricts are  summarized  in  other  chapters. 

QUANTITATIVE    STUDY   OF    SECONDARY   CONCENTRATION. 

The  nature  of  the  secondary  concentration  of  Lake  Superior  iron  ores  has  been  in  the 
past  inferred  almost  entirely  from  qualitative  evidence.  The  extensive  commercial  develop- 
ment of  the  ores  of  this  region  during  recent  years  now  makes  available  data  for  quantitative 
study  of  the  origin  and  concentration  of  the  ores.  Although  there  is  a  great  similarity  in  the 
secondary  concentration  of  all  the  iron  ores  of  the  Lake  Superior  region,  certain  local  difler- 

47517°— VOL  52—11 35 


546  GEOLOGY  OF  THE  LAKE  SUPERIOR   REGION. 

ences  require  that  each  of  the  several  districts  be  discussed  independently.     This  is  done  in  the 
chapters  on  the  several  districts. 

The  average  change  in  secondary  concentration,  based  on  all  available  analyses  (seep.  181), 
is  graphically  expressed  in  figures  20  (p.  189)  and  31  (p.  245). 

ALTERATIONS    OF   IRON-BEARING    FORMATIONS    BY    IGNEOUS    INTRUSIONS. 

ORES   AFFECTED. 

The  changes  described  in  the  foregoing  sections  have  completed  the  development  of  the 
ore  deposits  of  the  Mesabi,  Gogebic,  Menominee,  part  of  the  Marquette,  Crystal  Falls,  Iron 
River,  Florence,  and  Cuyuna  districts,  wliich  yield  roughly  93  per  cent  of  the  total  ore  mined 
annually  in  the  region.  Other  ores,  such  as  the  hard  ores  of  the  Marquette  and  Vermihon 
districts  and  the  magnetic  rocks  of  the  Mesabi  and  Gogebic,  have  suffered  certain  additional 
vicissitudes  of  anamorphic  alterations  by  igneous  intrusion,  thus  becoming  the  hard,  dense, 
recrystalhzed,  more  or  less  magnetic,  dehydrated,  and  silicated  ores  described  below.  (See 
Pis.  XXXV,  p.  470,  and  XLVII.)  The  development  of  some  of  these  characteristics  may  have 
been  synchronous  with  the  deposition  of  the  iron-bearing  formation  under  the  influence  of 
contemporaneous  igneous  extrusives,  discussed  on  page  527,  but  whatever  the  probabiMty 
of  this  there  is  no  doubt  that  characteristics  of  this  kind  have  been  developed  mamly  bv  later 
intrusives. 

The  intrusion  of  small  masses  of  igneous  material,  as  the  dikes  in  the  Gogebic  district  and 
certain  of  the  bosses  in  the  Marquette  district,  has  apparently  but  slightly'  metamorphosed 
the  iron-bearmg  formation,  ^\^lere  great  masses  of  igneous  material  have  come  into  contact 
with  the  iron-bearing  formation,  however,  marked  results  have  followed,  as  near  the  Duluth 
gabbro,  the  gabbro  of  the  western  Gogebic  district,  and  the  intrusives  of  the  western  Marquette 
district. 

POSSIBLE    CONTRIBUTIONS   FROM   IGNEOUS   ROCKS. 

The  characteristic  features  of  the  amphibole-magnetite  rocks  of  the  iron-bearing  forma- 
tions described  above  become  more  accentuated  in  approach  to  the  igneous  rocks,  leaving  no 
doubt  that  they  are  the  metamorphic  result  of  the  intrusion  of  the  gabbro.  The  facts  available 
indicate  to  some  extent  also  the  processes  through  which  this  result  is  accomplished.  The 
question  first  to  be  answered  is  whether  or  not  the  iron-bearing  formation  owes  its  character- 
istics near  the  contact  to  direct  contribution  from  the  hot  intrusives  or  to  the  recrystallization 
of  substances  already  in  the  iron-bearing  formation.  The  essential  similarity  of  composition 
of  the  amphibole-magnetite  rocks  with  that  of  the  ferruginous  cherts  (see  p.  204)  argues  against 
large  introduction  of  materials  from  the  gabbro.  Had  such  materials  been  introduced  on  a 
large  scale  they  would  probably  have  considerably  changed  the  proportions  of  the  elements 
present,  for  otherwise  it  would  be  necessary  to  assume  that  the  materials  contributed  from  the 
gabbro  had  been  in  the  same  proportion  as  those  originally  present  in  the  iron-bearing  forma- 
tion. The  magnetite  in  the  gabbro  is  titanic,  while  that  in  the  adjacent  iron  formation  is  not. 
The  higher  sulphur  content  in  tlie  amphibole-magnetite  rocks  may  mdicate  direct  contribution 
of  sulphur,  though  this  may  also  be  original  in  the  iron-bearmg  formation.  (See  pp.  550,  552.) 
Wliether  or  not  there  was  some  small  introduction  of  materials  from  the  gabbro,  the  bulk 
analyses  of  the  amphibole-magnetite  rocks  are  so  similar  to  those  of  the  other  phases  of  the 
iron-bearing  formation  as  not  to  require  the  assumption  of  delivery  of  hot  solutions  from  the 
gabbro  to  the  iron  formation. 

Furthermore,  there  is  no  regular  variation  in  the  composition  of  metamorphic  phases  of 
the  iron-bearing  formation  through  the  several  hundred  feet  from  the  contact  for  which  these 
phases  are  known  in  many  places  to  extend.  Finally,  the  very  fact  that  the  metamorphic 
phases  of  the  iron  formation  extend  so  far  and  so  uniformly  from  the  gabbro  contact  argue 
against  their  development  by  accession  of  materials  from  the  gabbro. 

It  is  conchuled,  therefore,  that  the  princijial  efl'ect  of  the  intrusion  of  the  gabbro  into  the 
iron-bearing  formation  was  that  of  recrystallization  of  substances  already  present  and  not  by 
contribution  of  solutions. 


PLATE    XL VII. 


547 


PLATE  XLVII. 

Photomicrographs  of  ferruginous  and  amphibolitic  chert  of  iron-bearing  Biwabik 
formation  near  contact  with  duluth  gabbro. 

A.  Actinolitic,  griineritic,  and  magnetitic   chert  (specimen  45141,  slide  15621)  from  southeast  of  center  of  sec.  17, 

T.  60  N . ,  K.  12  W. ,  Mesabi  district,  Minnesota.  Without  analyzer,  X  50.  This  rock  is  close  to  the  contact  with 
the  Duluth  gabbro  and  shows  the  tj-pical  alterations  characteristic  of  the  contact.  The  chert  is  in  much  larger 
particles  than  in  the  western  portion  of  the  range  away  from  the  contact.  The  particles  fit  in  somewhat  regular 
polygonal  blocks.  The  iron  oxide  is  magnetite  instead  of  hydrated  hematite,  and  actinolite  and  griinerite  are 
present.  The  amphiboles  are  in  small  quantity  in  the  slide  shown,  but  the  short  actinolite  needles  may  be  seen 
inclosed  in  the  quartz.     (See  PI.  XXXV.) 

B.  Actinolitic  slate  (specimen  9555,  slide  3190)  from  Penokee  Gap,  NW.  \  sec.  11,  T.  44  N.,  R.  3  W.,  Wisconsin.     In 

polarized  light,  X  165.  The  section  is  a  typical  actinolitic  slate.  The  quartz  is  completely  crystallized.  The 
magnetite  has  mostly  well-defined  crystal  outlines  and  is  manifestly  the  first  mineral  to  crystallize,  being  scat- 
tered uniformly  through  the  section  without  any  regard  to  the  actinolite  and  quartz  and  therefore  included  by 
both  of  them.  The  actinolite  is  in  its  characteristic  blades  and  sheaf-like  forms,  hav-ing  a  radial  arrangement  of 
its  fibers.  It  is  as  plainly  the  second  mineral  to  crystallize,  as  needles  of  actinolite  everjTvhere  penetrate  the 
quartz,  but  never  the  magnetite.  The  quartz  constitutes  a  background  for  the  magnetite  and  actinolite  and 
includes  them  in  such  a  manner  as  to  make  the.  conclusion  certain  that  it  must  in  the  main  have  crystallized 
subsequently  to  the  formation  of  the  magnetite  and  actinolite.     (See  PI.  XXXV.) 

548 


U.  S.  GEOLOGICAL  SURVEY 


MONOGRAPH   LH         PLATE  XLVII 


PHOTOMICROGRAPHS  OF  FERRUGINOUS  AND  AMPHIBOLITIC  CHERT  OF  IRON-BEARING 
FORMATION   NEAR  CONTACT  WITH   DULUTH   GABBRO. 


THE  IRON  ORES. 


549 


TEMPERATURE   UNDER   WHICH   CONTACT   ALTERATIONS   WERE    EFFECTED. 

The  significant  discovery  by  Wright  and  Day,"  of  the  geophysical  hiboratory  of  tlie  Carnegie 
Institution  of  Wasliington,  that  quartz  crystalhzed  below  575°  dift'ers  in  its  properties  from 
quartz  crystallized  above  tliis  temperature  affords  a  satisfactory  means  of  determining  tlie 
temperatures  at  which  the  quartz  of  the  iron-bearing  formation  has  crystallized.  Doctor  Wright 
has  kindly  determined  for  us  the  properties  of  the  quartzes  in  specimens  from  different  parts 
of  the  Lake  Superior  iron-bearing  formations,  some  of  them  clearly  developed  under  katamorphic 
conditions,  some  of  them  near  the  contact  with  the  gabbro.     His  observations  are  as  follows: 

Properties  of  quartz  crystals  from  iron-bearing  formations. 


Speci- 
men No. 

Number 
of  sec- 
tions cut. 

Average 
diameter 
(mm.). 

Circular  polarization. 

Twinning,  etch  flgures.u 

R. 

L. 

H.-fL. 

Character 
of  inter- 
growth. 

Number 

not 
twinned. 

Number 
twinned. 

Character  of  twinning. 

A 
B 

29955 
29450 

5 
6 
4 
6 

7 
5 

1.5 
2.0 

4 

1 

6 

Regular 

do 

2 

3 

fi 
4 
4 

Regular  large  patches. 

Do. 
Regular. 
Often  irregular  and  small. 

3 
3 

1 
3 

2 

n  Etched  1}  hours  in  cold  commercial  hydrofluoric  acid. 

.\.  Crystalline  quartz  in  ore  from  Vermilion  district. 

B.  Crystalline  quartz  in  ore  from  Mesabi  district. 

29955.  Coarsely  recrystallized  iron-bearing  formation,  300  feet  from  gabbro  contact,  northwest  of  Paulson  mine  camps,  Gnnflint  district,  north- 
eastern Minnesota. 

29450.  Coarsely  recrystallized  iron-bearing  formation  in  actual  contact  with  Duluth  gabbro  at  east  end  of  Fay  Lake,  Gunflint  district,  north- 
eastern Minnesota. 

The  quartz  of  Nos.  A  and  B  occurs  in  clear  crystals  and  free  from  fracture.s.  The  usual  -f-  and  —  unit  rhomlio- 
hedrons  are  present;  also  the  prism  faces.  On  A  crystals  there  is  also  present  the  rhombohedron  (1121)  and  a  trigonal 
trapezohedron  form;  this  in  itself  is  proof  that  the  A  quartz  was  formed  below  575°. 

The  aljove  observations  show  conclusively  that  the  A,  B,  and  29955  quartzes  [distant  from  gabbro  contacts]  have 
not  been  heated  above  575° ;  that  they  were  formed  below  that  temperature.  Specimen  29450  [at  gabbro  contact]  is  less 
regular  in  its  behavior  and  resembles  in  that  respect  the  quartz  of  .some  pegmatites.  It  is  not  as  shattered  as  granite 
quartzes  usually  are  and  yet  is  not  so  regular  as  the  definitely  lower  temperature  quartzes.  I  concluded  that  in  the 
pegmatites  such  quartz  was  formed  probably  near  the  inversion  temperature  575°,  because  pegmatite  dense  quartz 
is  definitely  the  low  a  form  while  some  pegmatite  quartz  is  definitely  high  6  quartz.  This  was  proved  on  one  and 
the  same  dike. 

It  seems  to  me  probable,  therefore,  that  the  temperature  of  formation  of  the  quartz  band  in  specimen  29450  was 
not  far  from  575°. 

It  is  obvious  from  these  results  that  the  iron-bearing  formation  as  a  whole  has  not  been 
fused,  for  its  fusion  temperature  is  certainly  higher  than  575°.  This  conclusion,  together  with 
the  one  above  referring  to  the  lack  of  transfer  of  material  from  the  gabbro  to  the  iron-bearing 
formation,  emphasizes  strongly  the  probabihty  that  the  metamorphism  of  the  iron  formation 
near  the  gabbro  was  primarily  the  result  of  recrystallization  below  fusion  temperature,  with 
the  aid  of  heat  from  the  gabbro. 


CHARACTER  OF  IRON-BEARING  FORMATIONS  AT  THE  TIME   OF  INTRUSIONS  OF  IGNEOUS 

ROCKS. 

Wliat  were  the  constituents  originally  present  in  the  iron-bearing  formations  at  the  time 
of  the  intrusion?  Were  they  the  ferruginous  cherts  earlier  developed  from  the  alteration  of 
cherty  carbonates  or  gi'eenalite  rocks,  or  were  they  the  chcrty  carbonates  and  greenalite  rocks 
themselves?  If  prior  to  the  intrusion  of  the  igneous  rock  the  iron  existed  as  ferric  hydrate, 
then  the  change  to  magnetite  involved  deoxidation.  This,  according  to  Moissan,''  will  occur 
at  .300°  in  30  minutes  in  a  hydrogen  atmosphere.  The  presence  of  an  actively  reducmg  agent 
of  tliis  type  along  igneous  contacts,  wliile  perhaps  locally  probable,  can  not  be  proved  on  any 

a  Wright,  F.  E.,  and  Larsen,  E.  S.,  Quartz  as  a  geologic  thermometer:  Am.  Jour.  Sci.,  4th  ser.,  vol.  27,  1909,  pp.  421-427. 
6  Moissan,  H.,  Compt.  Rend.,  vol.  84,  p.  129C. 


550  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

large  scale.  If  prior  to  the  intrusion  of  the  igneous  rock  the  iron  was  in  the  ferrous  condition, 
either  as  greenahte  or  carbonate,  then  moderate  heat  was  sufficient  to  produce  magnetite  by 
robbing  the  associated  water  of  part  of  its  oxygen.  (See  p.  526.)  This  alteration  is  thought 
in  general  to  be  a  more  common  one  than  the  reduction  of  iron  to  magnetite  from  the  ferric 
state.  In  the  Lake  Superior  region  there  is  field  evidence  also  that  the  development  of  the 
amphibole-magnetite  rocks  has  been  more  largely  accomplished  by  partial  oxidation  of  the 
ferrous  iron  than  by  the  reduction  of  ferric  oxide.  In  places  in  the  Lake  Superior  region,  where 
there  is  good  field  evidence  that  the  iron-bearing  formation  had  been  exposed  and  altered  to 
ferruginous  cherts  before  the  introduction  of  igneous  rocks — as,  for  instance,  in  the  eastern  part 
of  the  Marquette  district  or  at  Sunday  Lake  in  the  Gogebic  district — it  is  found  that  the  contact 
effect  of  the  intrusives  has  been  to  produce  the  bright-red  banded  specular  jaspers  or  black 
magnetitic  jaspers  rather  than  ampliibole-magnetite  rocks.  In  the  Marquette  district  it  was 
long  ago  noted  that  the  lower  parts  of  the  Negaunee  formation  in  contact  with  intrusives  devel- 
oped amphibole  and  magnetite,  while  the  upper  parts  developed  the  banded  specidar  jaspers. 
The  cement  in  these  rocks  is  usually  magnetite.  Smyth"  argued  that  tliis  present  ditference 
in  the  character  of  the  rocks  at  upper  and  lower  horizons,  especially  for  the  Republic  trough, 
is  so  uniform  as  to  indicate  an  original  difference  in  the  beds  at  these  horizons.  The  magnesia 
content  of  the  ampliibole-magnetite  rocks  for  the  most  part  seems  to  be  like  that  of  the  original 
greenalites  and  carbonates  rather  than  that  of  their  altered  derivatives,  ferruginous  cherts. 
In  the  alteration  of  carbonates  or  greenalites  to  cherts  magnesia  is  lost.  (See  p.  528.)  Had 
the  ampliibole-magnetite  rocks  developed  from  the  ferruginous  cherts,  it  would  be  necessary 
to  assume  that  magnesia  had  been  introduced  in  just  the  percentage  of  the  original  siderite 
and  greenalite  rocks. 

Sulphur  is  also  more  abundant  in  the  original  phases  of  the  iron-bearing  fonnation  than 
in  its  katamorphosed  products,  though  no  figures  are  available  to  show  what  the  average  sul- 
phur content  is,  because  analyses  have  ordinarily  been  made  of  the  greenalite  and  siderite 
where  free  from  sulphur.  Contact  or  deep-seated  mctamorphism  would  not  remove  this  sul- 
phur, and  this  is  thought  to  be  the  probable  explanation  of  the  high  sulphur  in  the  amphibole- 
magnetite  rocks.  The  alternative  explanation  is  that  sulphur  had  been  introduced  directly 
from  the  igneous  rocks. 

CHEMISTRY   OF   ALTERATIONS. 

The  chemistry  of  the  alterations  from  original  ferrous  compounds,  greenalite  and  siderite, 
to  ampliibole-magnetite  rocks  presents  less  difficulty  than  that  of  the  alteration  of  ferruginous 
cherts,  or  ferric  compounds,  to  the  amphibole-magnetite  rocks.  The  former  alteration  requires 
partial  oxidation  of  a  ferrous  compound ;  the  latter  requires  reduction  of  a  ferric  compound, 
which  is  thought  to  be  much  less  common. 

On  the  assumption  that  the  ampliibole-magnetite  rocks  had  developed  directly  from  the 
cherty  iron  carbonates  and  greenalites,  the  changes  would  be  substantially  as  follows:  *" 

Where  the  carbonate  is  nearly  pure  siderite,  griinerite  is  produced,  according  to  the  following  reaction: 

reC03+Si02=FeSi03+COj, 

with  a  decrease  of  volume  of  32  per  cent,  provided  the  silica  be  a  solid  and  the  carbon  dioxide  escape.  Where  the 
original  material  was  hydrous  ferrous  silicate,  greenalite,  simple  dehydration  only  is  necessary  to  form  the  griinerite. 
Where  the  iron-bearing  carbonate  bears  calcium  and  magnesium  in  considerable  quantity,  instead  of  griinerite 
being  produced  sahlite  or  actinolite  may  be  formed.  Supposing  the  carbonate  to  be  normal  ankerite,  sahlite  is  pro- 
duced, according  to  the  following  reaction: 

CaFeCACaMgCjOe-|-4Si02=Ca2MgFeSi40,j-l-4C02, 

with  a  decrease  in  volume  of  37  per  cent,  provided  the  silica  lie  solid  and  the  carbon  dioxide  escape. 
From  ankerite  actinolite  may  be  produced,  according  to  the  following  reaction: 

3(CaFeC20„.CaMgC20e)-|-8Si02=Ca2Mg3Fe3Si024-t-SCOo+4CaC03, 

with  a  decrease  in  volume  of  23  per  cent,  provided  the  silica  be  a  solid,  the  CaCOs  formed  remain  as  a  solid,  and  the 
carl)ou  dioxide  escape. 

o  Mon.  IT.  S.  Oeol.  Survey,  vol.  28, 1897,  p.  530. 

»  Vau  Hlse,  C.  R.,  A  treatise  ou  metaraorphism:  Mon.  U.  S.  Geol.  Survey,  vol.  47, 1904,  pp.  S34-S37. 


THE  IRON  ORES.  551 

If  a  more  ferriferous  and  less  calcareous  iron-bearing  carbonate  be  taken,  it  would  not  be  necessary  to  suppose  any 
calcium  carbonate  to  have  separated. 

The  iron-bearing  carbonates  may  be  very  impure,  just  as  limestones  may  be  impure;  and  in  this  case  there  may 
develop  various  other  minerals.  In  proportion  as  impurities  are  mingled  with  the  carbonates,  other  amphiboles  and 
the  pjToxenes,  micas,  garnets,  and  other  heavy  minerals  such  as  olivine  may  abundantly  develop;  and  thus  there  may 
be  produced  a  great  variety  of  rocks,  such  as  garnetiterous  magnetite  rocks,  micaceous  griinerite  rocks,  etc.  As  the 
impurities  become  abundant  and  the  silicates  other  than  griinerite,  sahlite,  and  actinolite  more  prominent,  the  altera- 
tions become  nearly  those  of  the  fragmental  rocks.     Between  the  two  there  are,  of  course,  all  gradations. 

But  as  a  matter  of  fact,  the  two  silicates  which  most  extensively  form  by  the  alterations  of  the  iron-bearing  carbon- 
ates in  the  zone  of  anamorphism  are  actinolite  and  griinerite.  Where  these  reactions  are  complete  we  may  have,  in 
place  of  the  iron-bearing  carbonate,  actinolite  rocks,  griinerite  rocks,  and  all  gradations  between  them. 

Where  the  iron-bearing  formation  is  originally  greenalite,  the  alteration  to  the  amphiboles 
would  be  simply  one  of  dehydration. 

The  development  of  magnetite  directly  from  the  iron  carbonates  is  possible  by  the  following 
reactions; 

2FeC03  +  FeSj  -f  2H2O  =Fe30,  +  2H2S  +  2C02,° 
3FeC03  +  H,0  =Fe30,  +  3C0.  +  H, 
3FeC03  =Fe30,  +  CO  +  2C02,° 
BFeCOj  -t-  O  =Fe30,  +  BCO^." 

Carbon  dioxide  is  driven  off  at  temperatures  probably  as  low  as  400°.     At  these  and  higher 
temperatures  the  ferrous  iron  remaining  will  rob  the  water  of  its  oxygen,  forming  magnetite. 

Siderite  at  red  heat  passes  into  a  magnetic  oxide  with  the  formation  of  both  carbonic  acid  and  carbonic  oxide. 
According  to  Dobereiner  this  reaction  takes  place  as  follows  :o 

5FeC03=3FeO.Fe203+4C02+CO. 

Glasson.b  however,  says  that  4FeO.Fe203  results,  at  first  giving  two  parts  of  CO.,  and  one  of  CO,  but  that  later  the 
proportion  changes  to  five  parts  of  COj  and  one  of  CO."^ 

Van  Hise  "  says,  again : 

Observation  in  the  field  show.s  beyond  question  that  the  change  fi'om  iron  carbonate  to  magnetite  takes  place  on 
an  extensive  scale.     Wliich  of  the  above  reactions  is  tlae  more  important  may  be  an  open  question. 

The  alteration  of  greenalite  to  magnetite  is  possible  by  the  following  reaction: 

3FeSi03nH20  +  O  =Fe30,  +  SSiO,  -f  nH^O. 

■^Tiich  of  the  above  rocks  develops  at  a  given  place  depends  not  only  upon  the  original  composition  of  the  rocks, 
but  upon  the  nature  of  the  alteration.  For  instance,  where  in  the  original  rock  silica  is  subordinate  and  nearly  pure 
siderite  abundant,  a  quartzose  magnetite  may  develop,  as  at  various  places  in  the  Lake  Superior  region.  WTiere  the 
conditions  are  such  that  the  silicates  form,  the  development  of  the  actinolite  or  gi-iinerite  uses  up  both  the  iron  carbonate 
and  the  silica,  and  an  actinolite  rock  or  a  gi'iinerite  rock  may  be  produced.  Wliere  silica  was  originally  an  abundant 
constituent  both  magnetite  and  the  silicates  are  likely  to  develop.  Thus  we  have  various  proportions  of  all  the  min- 
erals, producing  the  magnetite-quartz  rocks,  the  actinolite-magnetite-quartz  rocks,  the  griinerite-magnetite-quartz 
rocks,  the  actinolite-quartz  rocks,  and  the  griinerite-quartz  rocks. <2 

BANDING   OF   AMPHIBOLE-MAGNETITE    ROCKS.  <■ 

Usually  a  given  formation,  or  member,  does  not  show  a  perfectly  homogeneous  arrangement  of  the  mineral  particles. 
The  original  sedimentary  rock  is  banded,  and  the  different  bands  have  different  compositions.  Naturally  the  trans- 
formation of  these  bands  produces  different  combinations  of  minerals.  Moreover,  during  the  recrystallization  there 
is  a  tendency  for  minerals  of  the  same  kind  to  segregate.  Hence,  in  any  of  the  above  cases,  where  as  a  whole  a  certain 
Bet  of  minerals  are  dominant  within  a  rock,  a  single  mineral,  or  two  combined,  may  be  largely  segregated  in  bands;  and 
in  the  alternate  bands  the  other  minerals  be  largely  segregated.  Thus  a  banded  rock,  consisting  mainly  of  magnetite 
and  quartz,  may  have  a  banded  appearance  as  the  result  either  of  the  segregation  of  the  quartz  and  magnetite  in  sepa- 
rate bands  or,  more  commonly,  the  segregation  of  more  quartz  and  less  magnetite  in  one  band  and  less  quartz  and  more 
magnetite  in  another  band.     In  a  similar  manner  alternate  bands  may  be  made  up  of  actinolite  or  griinerite  with  quartz 

o  Van  Hise,  C.  R.,  op.  cit,.,  p.  838. 

b  Cited  by  Gmelin- Kraut,  Anorganisehe  Chemie,  vol.  3,  p.  319. 

c  Chambcrlin,  R.  T.,  The  gases  iu  rocks:  Pub.  Carnegie  Inst.  No.  106, 1908,  p.  61. 

d  Van  Hise,  C.  R.,  op.  eit.,  p.  8.39. 

'Idem,  pp.  839-840. 


552  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

in  variiius  proportions,  and  of  actinolite  or  griinerite  with  magnetite  in  various  proportionp.  In  still  other  instances 
the  bunding  may  be  due  to  the  combining  of  actinolite  or  griinerite,  magnetite,  and  quartz  in  various  proportions. 
In  general,  therefore,  the  alterations  of  the  rock  do  not  destroy  the  original  sedimentary  banding,  but,  on  the  contrary, 
emphasize  it.  The  staking  banded  appearance  of  actinolitic  and  griineritic  rocks  is  one  of  their  most  characteristic 
features. 

BECRYSTALLIZATION  OF   atTAKTZ. 

The  recrystallization  of  quartz  under  these  anamorphic  reactions  has  multiplied  the  size 
of  the  prain  maiw  times,  as  mentioned  in  the  discussion  of  the  individual  districts.  The  rccrj-s- 
taUization  of  quartz  has  largety  followed  the  development  of  magnetite,  for  magnetite  with 
crystal  outlines  is  often  observed  to  be  completely  inclosed  in  large  clear  quartz  crystals  with 
no  strain  effects. 

HIGH   STTLPHXTB   CONTENT   OF   AMPHIBOLE-MAGNETITE   ROCKS. 

The  amphibole-magnetite  rocks  usually  carrj^  a  higher  percentage  of  iron  sulphide  than 
other  phases  of  the  iron-bearing  formations.  If  iron  sulphide  plays  the  important  part  assigned 
to  it  in  the  early  portion  of  this  discussion  (see  pp.  518-519),  iron  sulphides  may  be  supposed 
to  have  been  locally  deposited  throughout  the  iron-bearing  formations  with  the  carbonates  and 
greenalites.  These  would  be  the  first  substances  to  be  altered  by  the  surface  waters  and,  going 
quickly  into  solution,  would  greatly  accelerate  the  concentration  of  the  ore,  but  during  the 
alteration  of  iron  carbonate  or  greenalite  to  amphibole-magnetite  rocks  there  is  no  opportunity 
for  oxidizing  solutions  to  get  at  the  sulphides  and  hence  the}"  remain.  The  refractoriness  of  the 
amphibole-magnetite  rocks  also  prevents  subsequent  oxitlization.  In  the  Gunflint  Lake  dis- 
trict of  Minnesota  the  sulphide  is  in  the  form  of  pyrrhotite,  which,  according  to  Moissan "  and 
Allen,  *  is  developed  through  the  application  of  heat  to  pyrite. 

An  alternative  explanation  of  the  high  sulphur  is  that  it  was  secondarily  contributed  by 
the  hot  intrusives.     For  this  there  is  no  direct  evidence. 

SECONDARY  IRON  CARBONATE  LOCALLY  DEVELOPED  AT  IGNEOUS  CONTACTS. 

In  a  few  localities,  as  at  Gunflint  Lake,  Minnesota,  in  the  Animikie  district,  and  at  Sunday 
Lake,  in  the  Gogebic  district,  coarsel}^  crystallized  iron  carbonate  is  found  close  to  the  igneous 
rock,  this  material  doubtless  being  produced  by  recrystallization  of  the  original  finer  carbonate. 

CONTACT  ALTERATIONS  NOT  FAVORABLE  TO  CONCENTBATION  OF  ORE  DEPOSITS. 

The  anamorphic  changes  above  described  do  not  fav'or  the  transfer  and  segregation  of  con- 
stituents of  the  u'on-bearLng  formations.  They  tend  rather  to  combine  them.  Localh*  there 
is  evidence  that  iron  is  carried  in  solution  under  these  conditions,  in  the  fact  that  cements  in 
fractures  are  largely  magnetite  and  the  iron  is  usually  in  coarser  bands.  If  the  intrusions  come 
before  the  original  iron-bearing  formation  has  become  porous  tlu'ough  the  loss  of  its  silica, 
the  rocks  do  not  have  the  openings  for  the  transfer  of  solutions.  Even  had  openings  existed 
in  some  places,  the  deep-seated  pressures  exerted  by  great  batholiths,  like  the  Dulutli  gabbro, 
have  been  sulficient  to  make  tlie  rock  undergo  rock  flowage,  thereb}-  closmg  openmgs.  If 
other  conditions  were  favorable  there  would  still  be  the  lack  of  abundant  surface  waters  to 
leach  the  silica.  So  far  as  the  iron-bearing  formation  ]ia<l  been  previously  altered  and  con- 
centrated to  ore  under  weathering  conditions,  the  intrusions  of  the  igneous  rocks  woidil  have 
the  effect  of  dehj^drating  and  recrystallizing  the  ores,  but  not  of  further  concentrating  them. 

There  are  but  two  highly  magnetic  deposits  in  the  Lake  Sui)erior  country  which  have  been 
mined  as  ore.  In  the  Republic  district  of  Michigan  magnetitic  specular  hematite  is  interlayered 
with  bright-red  and  black  jaspers  in  which  the  iron  oxide  is  hematite  and  magnetite.  Near 
the  base  of  the  formation  amphiboles  arc  abundant  and  the  formation  is  lean  in  iron.  The 
upper  part  of  the  formation  seems  to  be  essentially  the  result  of  anamorphism  of  a  previously 

a  Moissan,  n.,  Traitd  de  clilmie  minCralc.  vol.  4,  Metamorphism,  p.  565. 

*  Allen,  E.  T.,  Sulphides  of  iron :  Summary  in  Ann.  Kept.  Geophys.  Lab.  Carnegie  Inst.,  1910,  pp.  104-105  (reprint). 


THE  IRON  ORES.  553 

formed  iron  oxide  and  jasper  zone  in  which  there  lias  been  some  concentration  of  iron  ore.  The 
lower  portion  of  the  formation  is  regarded  as  the  result  of  the  anamorphism  of  an  original 
carbonate  formation  not  exposed  to  weathering  prior  to  the  introduction  of  the  igneous  rocks. 
In  both  cases  conditions  of  rock  flowage  incident  to  the  folding  and  intrusion  have  aided  the 
direct  contact  effects.  The  upper  part  of  the  formation  has  suffered  most  from  the  readjust- 
ment along  the  surface  of  the  contact  between  the  unconformable  middle  and  upper  Huronian. 
The  probable  sequence  of  events  is  discussed  on  pages  277-279. 

At  Champion,  Mich.,  in  the  Marquette  district,  the  development  of  the  magnetite-ore 
deposit  is  explained  in  much  the  same  way.  These  ores  have  been  found  to  contain  a  larger 
percentage  of  titanium  than  is  usual  in  the  Lake  Superior  ores  of  the  sedimentary  type,  some 
samples  of  the  Champion  ore  containing  as  much  as  1.66  per  cent  of  TiOa.  It  is  possible  that 
this  may  represent  a  direct  contribution  from  the  intrusive. 

The  titaniferous  magnetite  deposits  in  the  Lake  Superior  region  are  not  results  of  the  con- 
tact alteration  of  an  iron-bearing  formation,  but  are  rather  magmatic  segregations  in  the  igneous 
rocks.  Also  certain  of  the  black  magnetite  rocks  of  the  iron-bearing  formations  closely  asso- 
ciated with  surface  extrusive  rocks  may  have  been  the  result  of  direct  contribution  from  the 
igneous  rocks  and  not  contact  metamorphism,  as  already  indicated.     (See  p.  527.) 

The  alterations  above  described  are  essentially  constructive  or  anamorphic  in  their  nature, 
tendmg  to  produce  more  complex  mineral  substances,  and  do  not  accomplish  simplification 
and  segregation  sufhcientty  to  develop  ore  deposits,  in  these  respects  contrasting  markedly 
with  weathering  alterations  which  the  formation  undergoes  at  the  surface  away  from  the 
influence  of  igneous  rocks. 

Therefore,  so  far  as  the  amphibole-magnetite  rocks  contain  ores,  these  ores  are  probably 
originally  rich  iron  layers  in  the  iron  formation  which  may  have  been  partly  concentrated 
durmg  katamorphism  precedmg  anamorphism.  The  anamorphic  processes  have  not  aided 
their  concentration. 

SURFACE    ALTERATIONS   OF   AMPHIBOLE-MAGNETITE    ROCKS. 

After  the  griineritic  or  actinolitic  rocks  have  developed  in  the  zone  of  anamorphism,  in 
consequence  of  denudation  they  may  pass  into  the  zone  of  katamorphism,  or  even  into  the  belt 
of  weathering.  Then  will  begin  the  processes  of  oxidation,  hydration,  and  carbonation,  as  a 
result  of  which  the  magnetite  is  slightly  changed  to  hematite  or  limonite  and  the  amphibole  or 
other  silicates  may  decompose  into  clilorite,  epidote,  and  calcite.  However,  as  magnetite 
and  the  iron-bearing  ampliiboles  are  very  refractory,  tliis  process  is  exceedingly  slow  and  usually 
has  affected  only  comparatively  thin  layers  of  materials  adjacent  to  the  surface  or  adjacent  to 
openings  in  the  rock.  Indeed,  the  reactions  of  the  belt  of  weathering  and  the  upper  part  of  the 
belt  of  cementation,  wliich  may  produce  lai-ge  iron-ore  bodies  where  they  have  the  original 
iron-bearing  carbonates  or  the  hydrous  ferrous  silicates  to  work  upon,  have  nowhere  in  the 
Lake  Superior  region  formed  large  ore  bodies  where  they  are  working  upon  the  griineritic  and 
actinolitic  rocks. 

The  iron  content  of  the  ampliibole-magnetite  rocks  is  not  materially  different  from  that 
of  the  ferruginous  cherts,  allowance  being  made  for  a  slight  difference  in  degree  of  oxidation 
and  hydration  of  iron.  The  leaching  of  silica  from  tliis  rock  would  produce -an  ore  as  rich 
as  that  derived  from  the  alteration  of  cherts,  but  as  a  matter  of  fact  the  silica  is  usually  not 
leached  from  these  rocks  and  ore  deposits  derived  from  them  are  small  and  rare.  The  external 
conditions  for  their  alteration  are  essentially  the  same  as  those  for  the  alteration  of  the  cherts, 
topograpliically,  structurally,  and  chemically,  and  so  the  failure  of  the  waters  to  leach  the  silica 
from  them  and  concentrate  the  iron  must  be  ascribed  to  the  condition  of  the  rock.  Microscopic 
examination  shows  that  the  quartz  is  much  more  coarsely  crystallized  than  in  the  ferruginous 
cherts.  The  grains  will  average  a  thousand  times  the  mass  of  those  of  the  cherts.  (Compare 
Pis.  XLIV  and  XLVII.)  It  has  undergone  marked  recrystallization,  winch  has  completely 
obliterated  the  minute  particles  or  any  pore  space  that  the  cherts  may  have  had,  and  also 


554  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

crystallized  any  amorphous  chert  originally  present.  The  pore  space  is  less  than  1  per  cent, 
as  compared  with  about  5  per  cent  in  the  ferruginous  cherts.  The  result  is  that  the  waters 
have  fewer  openings  into  which  to  penetrate  and  far  less  surface  of  quartz  upon  which  to  work. 
This  fact  alone  is  believed  to  be  sufficient  to  account  for  the  lack  of  leaching  of  silica.  However, 
it  may  be  also  pointed  out  that  some  of  the  silica  has  combined  with  the  iron  in  resistant 
ampliiboles  which  do  not  yield  readily  to  the  surface  waters.  The  rocks  are  hard,  dense,  and 
crvstalline,  being  obviously  much  more  diflicult  for  the  waters  to  attack  either  mechanically 
or  chemically  than  the  ferruginous  'cherts.  They  usually  stand  at  a  liigher  elevation,  other 
structural  conditions  being  approximately  the  same,  indicating  their  resistance  to  erosion. 
In  the  Mesabi  district  the  elevation  of  the  upper  Iluronian  iron-bearing  formation  where  it  is 
altered  to  ampliibole-magnetite  rock  at  the  east  end  of  the  district  is  fulh'  200  feet  higher  on 
an  average  than  that  farther  west,  where  the  rock  is  altered  to  ferruginous  chert. 

It  follows  from  the  foregoing  that  anamorpliic  processes  of  the  original  iron  carbonates 
and  greenalites  producing  amphibole-magnetite  rocks  are  not  only  unfavorable  to  the  direct 
development  of  ores,  but  they  put  the  formation  in  condition  to  resist  the  action  of  ordinary 
katamorphic  concentrating  agencies. 

SXJMMABY  OF   ALTERATIONS   OF   IRON-BEARING   FORMATIONS   BY  IGNEOUS  INTRUSIONS. 

As  a  result  of  igneous  intrusions,  iron-bearing  formations  become  recrystallized  and  coarser 
in  grain. 

The  average  chemical  composition  is  not  essentially  changed  except  bv  dehydration,  and 
perhaps  locally  by  introduction  of  sulphur  or  other  constituents,  but  the  mineral  composition 
is  greatly  changed. 

The  density  has  been  increased  and  the  pore  space  lessened. 

The  alterations  of  the  carbonate  and  greenalite  rocks  have  produced  ampliibole-magnetite 
rocks.  The  alterations  of  the  ferruginous  cherts  and  soft  ores  have  produced  banded  red 
jaspers  and  hard  ores. 

The  changes  under  the  influence  of  intrusions  are  those  of  anamorphism  unfavorable  to  the 
development  of  ore  deposits,  but  originally  rich  iron  la3"ers  may  remain  as  ores.  The  anamorpliic 
products,  once  formed  and  exposed  at  the  surface,  are  found  to  be  too  refractory  to  undergo 
alterations  to  ores. 

ALTERATION    OF    IRON-BEARING    FORMATIONS    BY    ROCK    FLOWAGE. 

Mechanical  deformation  has  accomplished  different  changes  in  the  iron-bearing  formations, 
depending  on  whether  it  is  effected  by  fracture  or  by  flowage  and  whether  the  iron-bearing 
formation  Was  in  its  original  carbonate  or  greenalite  form  at  the  time  of  the  deformation,  or 
had  been  altered  to  ferruginous  chert  and  ore  or  to  actinolitic  and  griineritic  rocks.  Fracturing 
has  opened  up  avenues  for  water  circulation,  as  discussed  on  pages  474-475.  Here  is  consid- 
ered the  effect  of  rock  flowage  only. 

The  iron  carbonates  and  greenalites  were  not  considerably  altered  by  rock  flowage,  for 
subsequent  to  the  folding  they  unilerwent  the  normal  alterations  to  ores  and  ferruginous  cherts. 
Tliis  is  especially  well  illustrated  by  the  folded  upper  Huronian  iron-bearing  formation,  in 
which  original  carbonates  are  still  found  where  the  surface  alterations  have  not  reached  them. 

The  alterations  of  the  ferruginous  cherts  and  ores  under  mechanical  pressure  have  been 
very  conspicuous  in  the  Keewatin  ores  of  the  Vermilion  district,  in  parts  of  the  Negaunee 
formation  nearest  the  contact  with  the  upper  Iluronian  in  the  Marquette  district,  and  else- 
where. (See  PI.  XXXIX,  B,  p.  480.)  The  Vermilion  ores  have  been  rendered  hard,  crystal- 
line, dehydrated,  locally  somewhat  schistose,  more  or  less  magnetic,  locally  brecciated,  and 
cemente(l  by  vein  quartz  and  later  by  iron  oxide  (hematite  and  magnetite).  As  the  ores  stand 
in  the  Ely  trough  they  contain  much  pore  space  because  of  their  coarsely  brecciated  condition. 
The  ferruginous  cherts  of  the  Vermilion  district  have  simultaneously  been  recrystallized  and 
cemented,  and  the  iron  minerals  have  gone  through  the  same  series  of  physical  and  chemical 


THE  IRON  ORES.  555 

changes  as  in  the  ores.  The  net  result  is  the  production  of  a  rock  having  a  composition  similar 
to  that  of  ferruginous  chert,  with  a  large  proportion  of  magnetite  and  with  a  small  amount  of 
pore  space. 

In  the  Marquette  district  the  post-Iiuronian  folding  developed  a  marked  shear  zone  at 
the  contact  of  the  Negaunee  formation  with  the  overlying  detrital  ferruginous  base  of  the  upper 
Huronian,  with  the  result  that  the  ore  was  dehydrated  and  rendered  crystalline,  developing 
coarsel}'  crystalline  specular  hematite  or  micaceous  hematite  and  porphyritic  magnetite,  accom- 
panied by  a  marked  elimination  of  pore  space.  The  extent  of  the  mashing  is  best  indicated  by 
the  quartz  pebbles  in  the  detrital  base  of  the  upper  Huronian,  some  of  which  are  much  flattened. 

The  effect  on  the  ferruginous  cherts  or  jaspers  has  been  to  make  the  iron  bands  brightly 
specular. 

Aside  from  these  effects  noted  near  tlie  contacts  of  tlie  upper  and  middle  Huronian,  the 
later  folding  has  not  essentially  changed  the  characters  of  the  iron-bearing  formation.  Smyth  " 
discusses  it  thus: 

It  has  been  said  that  the  griinerite,  quartz,  and  iron  oxides  of  the  iron-liearing  member  have  a  verj'  distinct  banded 
arrangement  and  yet  are  not  original  minerals,  and  that  this  Ijanding  is  parallel  to  the  upper  and  lower  boundaries 
of  the  formation.  It  is  probaljle  that  a  set  of  parallel  structural  planes  has  controlled  the  segregation  of  the  present 
constituent  minerals  during  the  changes  through  which  the  rock  has  passed,  and  that  these  planes  must  have  been 
original  bedding  planes.  As  the  parallel  Ijanding  is  confined  to  this  one  direction,  it  is  certain  that  during  its  devel- 
opment no  other  system  of  parallel  planes  existed  in  the  rock.  The  last  severe  folding,  which  has  determined  the 
larger  structural  features  of  the  Marquette  district,  has  also  affected  the  rocks  in  a  more  intimate  way.  In  certain 
localities  strong  minor,  even  minute  crenulations  have  been  produced,  and  also  parallel  cleavage,  which  sometimes 
traverses  the  lianding  of  the  rock  at  right  angles,  The  little  folds  are  often  broken  and  faulted  and  the  siliceous  banda 
reduced  to  fragments.  Along  the  parallel  cleavage  planes  movement  has  often  taken  place,  as  is  shown  by  the  dis- 
placement of  a  particular  Ijand  on  the  two  sides.  Along  this  secondary  cleavage,  which  dates  from  the  period  of  gen- 
eral folding  after  upper  Marquette  time,  no  great  development  of  new  minerals,  except  the  iron  oxides,  has  taken 
place,  while  the  displacement  which  the  minute  faulting  has  caused  in  the  banding  conclusively  proves  that  this 
structure  was  present  before  the  folding. 

Allen''  finds  similar  conditions  in  the  Woman  River  district  of  Ontario,  where  riebeckite 
and  magnetite  are  cut  by  later  cleavage. 

The  effects  of  mechanical  deformation  in  the  zone  of  flowage  may  be  summarized  as  follows : 

As  a  result  of  mechanical  deformation  the  ores  have  become  dehydrated,  crystalline,  in 
some  places  specular  and  schistose,  lacking  pore  space,  locally  brecciated,  and  irr  part  rece- 
mented  by  quartz  and  iron  oxide. 

The  ferruginous  cherts  have  become  recrystallized  and  deliydrated,  in  some  places  sliglitly 
deoxidized,  tending  to  produce  the  banded  red'  and  black  jaspers.  These  alterations  of  the 
cherts  are  not  certainly  discriminated  from  those  due  to  the  intrusion  of  igneous  rocks. 

Deformation  by  flowage  does  not  aid  concentration  by  surface  waters,  but  on  the  other 
hand  it  does  not  so  affect  the  original  carbonates  and  greenalites  that  surface  waters  may  not 
later  alter  them  to  ores. 

.      CAUSE  OF  VARYING  DEGREE  OF  HYDRATION  OF  LAKE  SUPERIOR  ORES. 

The  Lake  Superior  iron  ores  include  both  hydrous  and  anlij^drous  varieties — magnetite, 
hematite,  limonite,  and  several  intermediate  hydrates.  The  iron  ores  of  tlie  region  as  a  whole 
are  low  hydrates  of  iron,  containing  an  average  of  about  2  per  cent  combined  water.  The  most 
hydrous  of  the  pre-Cambrian  ores  are  those  of  the  Alesabi  range,  which  average  an  amount  of 
combined  water  equivalent  to  a  ferric  hydrate  having  4.5  per  cent.  Locally  ores  containing 
almost  as  much  water  as  limonite  are  fouitd,  but  this  is  exceptional.  Some  of  the  ores  are 
crj'stalhne  hematite  and  magnetite. 

Are  the  differences  in  hydration  of  the  different  beds  due  to  differences  in  original  char- 
acter, or  to  differences  in  secondary  alterations?     These  questions  are  answered  only  in  part. 

a  Smyth,  H.  L.,  The  Republic  trough:  Mon.  U.  S.  Geol.  Survey,  vol.  2S,  1S97,  pp.  531-.'>32. 

t>  Allen,  R.  C,  Iron  formation  of  Woman  River:  Eighteenth  Ann.  Rept.  Ontario  Bm-.  Mines,  pt.  1,  1909,  pp.  254-262. 


556  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Guy  II.  Cox  lias  assembled  tlie  vjirious  experimental  data  on  the  su})ject  and  supplemented 
them  b_y  laboratory  experiments  of  his  own. 

From  meteoric  solutions  under  ordinary  temperatures  at  the  surface  the  precipitates  of 
iron  are  ferric  hydrates  containing  29  per  cent  of  water,  which  rapidly  changes  in  contact  with 
water  into  limonite.  containing  14.44  per  cent  of  water. 

The  presence  of  alumina,  lime,  and  magnesia  to  combine  with  the  iron  may  prevent  dehy- 
dration."    If  left  for  several  years,  the  ore  becomes  dehydrated  and  crystalline.* 

Increase  in  temperature  and  pressure  on  the  solutions  at  tlie  time  of  precipitation  will 
lower  the  hj'dration  of  the  precipitated  salt.  At  a  temperature  of  500°  magnetite  may  be 
precipitated  directly  from  solution.  Slight  variations  in  the  degree  of  hydration  in  a  precipi- 
tate are  determined  by  tlie  form  in  which  the  iron  is  held  in  solution,  by  the  precipitating 
agents,  and  by  the  strength  of  the  solutions,  though  so  far  as  experimental  data  go  the  range 
of  variation  due  to  these  causes  is  small. 

Secondary  alterations  have  little  eflFect  on  anliydrous  ores,  but  liydrous  ores  may  easily 
lose  part  of  their  water  by  moderate  increase  in  temperature  and  by  pressure  such,  for  instance, 
as  that  involved  in  freezing,  where  the  water  is  allowed  to  escape.  It  appears  also  that  in  an 
ore  containing  various  hydrates,  solution  will  dissolve  the  highest  hydrates,  leaving  the  residue 
in  a  lower  state  of  hydration,  but  that  the  redeposition  of  tlie  dissolved  part  as  a  higher  hydrate 
may  result  in  net  increase  of  hydration  for  the  residue  and  dissolved  parts  combined. 

It  appears,  therefore,  that  conditions  of  high  temperature  and  pressure,  either  during  the 
original  deposition  of  the  iron  salts  or  during  their  secondary  alterations,  favor  the  development 
of  anliydrous  salts,  thereby  explaining  the  occurrence  of  crystalline  hematite  and  magnetite 
in  the  iron-bearing  formations  near  igneous  contacts  or  where  djmamically  metamorphosed. 
It  is  shown  elsewhere  that  [magnetite,  perhaps  even  hematite,  may  have  been  precipitated 
directly  from  the  hot  solutions  coming  from  some  of  the  basic  igneous  rocks,  or  that  the  iron 
salts  may  first  have  been  deposited  as  greenalite  and  iron  carbonate  which  subsequently  altered 
under  conditions  of  high  temperature  and  pressure  to  magnetite  and  hematite,  or  that  the 
iron  salts  were  first  deposited  as  greenalite  and  hematite,  subsequently  altered  to  limonite,  and 
then  dehj'drated  by  the  high  temperature  and  pressure  of  anamorphic  conditions  to  hematite 
and  magnetite.  In  all  these  cases  the  heat  from  some  adjacent  igneous  rock  or  the  pressure 
developed  from  rock  flowage  seems,  from  field  evidence,  to  be  an  essential  factor. 

However,  hematite  and  various  hytlrates  are  found  minutely  interliedded  in  parts  of  the 
iron-bearing  formations  where  there  is  no  evidence  of  the  effect  of  unusual  heat  or  pressure. 
A  hand  specimen  may  show  several  layers  of  iron  oxides  with  varymg  degrees  of  hydration. 
These  differences  persist  in  the  ferruginous  cherts  and  jaspers  and  in  the  ores  into  which  the 
ferruginous  cherts  and  jaspers  grade.  Moreover,  they  seem  to  be  independent  of  distance  from 
rock  surface  and  of  dip  of  beds.  In  steeply  inclined  beds  layers  with  different  degrees  of  hydra- 
tion may  be  found  to  continue  from  the  surface  to  great  depth  with  no  relative  change  in 
hydration. 

These  remarkable  and  persistent  variations  in  hydration  in  closely  associated  layers  ma}' 
have  been  due  to — 

1.  Differences  in  the  original  substances  in  different  layers,  whether  carbonate  or  greena- 
lite. The  iron-bearing  formations  were  originally  anhydrous  iron  carbonate  and  hytlrous 
silicate,  both  of  which  have  altered  when  weathered  to  hydrous  oxides.  It  has  not  been  ascer- 
tained that  there  is  any  specific  difference  in  degree  of  hydration  of  the  alteration  products  of 
the  greenalite  and  carbonate,  though  on  the  whole  the  beds  in  the  Mesabi  district,  containing 
the  most  greenalite,  are  the  most  hydrous. 

2.  Difference  in  time  of  alteration  of  the  greenahte  and  carbonate,  vnth  accompanying  slight 
variations  of  temperature  and  pressure.     The  hydration  of  different  layers  has  taken  place  at 

o  Spring,  W.,  Neues  Jahrb.,  vol.  1,  ISDO,  pp.  47-ti2  (cited  by  Moore,  E.  8.,  Eighteenth  Ann.  Rept.  Ontario  Bur.  Mines,  pt.  1, 1909,  p.  194). 
t>  Wittsteln.  G.  C,  Vierteljahresschrltt  fiir  Pharmacie,  vol.  1, 1852,  p.  275  (cited  by  Moore,  E.  S.,  Eighteenth  Ann.  Rcpt.  Ontario  Bur.  Mines, 
pt.  1, 1909,  p.  194). 


THE  IRON  ORES.  557 

different   times  when   the   temperature  conditions   anil   jiressure  conditions  may   have   been 
sHghtly  diflferent,  although  of  these  differences  we  have  no  knowletlge. 

3.  Selective  secondary  alterations  of  the  iiydrates  formed  by  the  first  alteration  of  the  green- 
alite  and  carbonate.  Freezing  (seasonal  and  glacial)  and  moderate  depth  of  cover  may  tend 
to  dehydrate  the  ores  and  probably  have  contributed  to  the  low  average  degree  of  hydration  of 
the  bedded  hematites.  So  far  as  experimental  evidence  goes,  these  ores  would  have  their 
highest  degree  of  hydration  at  the  time  of  precipitation,  and  all  influences  acting  upon  tliem 
subsequent^,  even  moderate  seasonal  variations  in  temperature  and  moderate  depth  of  burial, 
would  tend  toward  lowering  the  degree  of  hydration. 

It  might  be  expected  that  the  result  of  seasonal  variations  in  temperature  and  the  pres- 
sure of  overlying  rocks  would  result  in  a  uniform  variation  in  hyth'ation  from  the  surface  down- 
ward. No  evidence  of  this  sort  has  been  found  in  the  ore  bodies.  It  should  be  noted,  however, 
that  the  effect  of  freezing  would  be  toward  dehydration  at  the  surface  and  the  effect  of  pres- 
sure would  be  toward  dehydration  with  depth.  Instead  of  uniform  change  iia  hydration  one 
way  or  another  from  surface  to  depth,  the  most  conspicuous  change  in  hydration  is  between 
closely  interbedded  layers  of  the  iron-bearing  formations. 

I  The  selective  effect  of  solution  and  redeposition  might  have  influence;  for  instance,  waters 
percolating  rapidly  along  a  certain  bed  or  fissure  might  dissolve  the  more  hydrated  ores,  carry 
them  off,  and  redeposit  them,  leaving  the  residue  with  a  lower  degree  of  hydration.  Slight 
original  variations  in  hydration  would  thereby  be  emphasized.  Other  unknown  causes  may  be 
operative. 

According  to  Stremme,"  hydration  is  favored  by  salt  content  and  carbon  dioxide  content 
of  the  altering  solutions.  The  salt  and  acid  content  apparently  influence  the  degree  of  hydra- 
tion of  the  u'on  oxide  by  lowering  the  vapor  pressure  of  the  solution.  Each  ii-on  hydrate  is 
supposed  to  have  its  own  vapor  pressure,  which  is  the  minimum  pressure  of  water  vapor  with 
which  the  hydrate  can  remain  in  equilibrium  at  any  given  temperature. 

We  may  conclude  in  general  that  the  hydrous  ores  of  the  Lake  Superior  region  have  devel- 
oped under  ordinary  concUtions  of  temperatiu'e  and  pressure  near  the  surface,  that  the  anhy- 
drous ores  exhibit  the  effects  of  heat  and  pressure,  and  that  the  differences  in  hydration  of  closely 
intermingled  layers  of  the  iron-bearmg  formations  have  requu'ed  some  influence  of  a  selective 
sort,  the  nature  of  wliich  may  be  suggested  but  not  proved. 

SEQUENCE    OF    ORE    CONCENTRATION. 

We  have  touched  upon  each  of  the  factors  going  to  determine  the  present  character  and 
structural  relations  of  the  ores.  To  complete  the  picture  we  have  now  to  dwell  upon  the  chron- 
ologic development  of  the  ores. 

The  beginning  of  the  processes  of  secondary  concentration  must  be  placed  for  the  Archean 
ores  in  early  Huronian  time  and  for  the  middle  Huronian  ores  in  the  time  between  the  middle 
and  upper  Huronian.  Iron-formation  fragments  in  the  basal  conglomerates  of  these  divisions 
tell  to  some  extent  what  had  previously  happened  to  the  iron-bearing  formations  of  the  ohler 
land.  At  the  base  of  the  upper  Huronian  rich  ferruginous  detritus  was  formed  at  the  beginning 
of  upper  Huronian  time.  In  certain  places  the  iron-bearing  formation  within  the  upper  Huro- 
nian was  exposed  by  erosion  before  Keweenawan  time  and  went  through  a  set  of  changes  in  the 
time  interval  between  the  Huronian  aiid  Keweenawan  similar  to  those  that  affected  the  lower 
Huronian  iron-bearing  formation  in  inter-Huronian  time.  This  is  shown  by  the  detritus  of  the 
Keweenawan  basal  conglomerate  and  by  the  development  of  red  jaspers  and  hard  ores  from 
the  soft  varieties  near  the  contact  of  Keweenawan  and  upper  Huronian  in  eastern  .Gogebic  dis- 
trict. In  those  districts  in  which  great  masses  of  Keweenawan  rocks  were  laid  down  upon  the 
Huronian  rocks  before  the  iron-bearing  formation  had  been  exposed  to  weathering,  the  concen- 
tration of  the  ore  could  not  have  begun  until  the  Keweenawan  was  cut  through  in  the  erosion 

a  Strenime,  H.,  Zur  Kenntnis  der  wasserhaltigen  und  wasserfreien  Eiseno.xydbilduDgen  in  den  Sedimentgesteinen:  Zeitschr.  prakt.  Geologic 
vol.  IS,  No.  1, 1910,  pp.  18-23  (reviewed  in  Econ.  Geology,  vol.  5, 1910,  p.  499). 


558  GEOLOGY  OF  THE  LAIvE  SUPERIOR  REGION. 

period  precedincr  Cambrian  time,  and  it  is  rather  probable  that  this  hmitation  also  applies  to 
other  districts.  Clearly  the  process  in  each  district  began  when,  as  a  result  of  the  great  oro- 
geiiic  movements  and  the  attenilant  denudation,  the  iron-hearing  formation  was  exposed  to  the 
weathering  forces.  In  most  of  the  districts  this  occurred  in  the  great  time  gap  represented  by 
the  unconformity  between  the  Keweenawan  and  the  Cambrian.  At  this  time  were  concen- 
trated most  of  tlie  great  ore  deposits  of  the  upper  lluronian  of  the  region  and  the  ores  at  the 
middle  and  lower  horizons  of  the  Negaunee  formation  of  the  micklle  Huronian. 

Wherever  the  Cambrian  remains  in  or  near  the  iron  districts  it  contains  iron-ore  frag- 
ments, jaspers,  and  clierts  in  its  basal  conglomerate.  In  the  Menominee  district  these  are  rich 
enough  to  be  mined.  The  process  of  ore  concentration  was  therefore  well  advanced  before 
Cambrian  time. 

In  the  Alesabi  district  remnants  of  Cretaceous  beds  overlie  some  of  the  ore  deposits,  par- 
ticularly in  the  western  parts  of  the  range.  At  the  basal  horizons  of  these  beds  are  detrital 
iron  ores  derived  from  the  Biwabik  formation.  Here,  then,  the  concentration  was  well 
advanced  as  early  as  Cretaceous  time,  and  there  is  little  doubt,  from  the  similar  relations  of 
the  ores  to  the  Cambrian  in  other  regions,  that  the  ores  of  tiie  Mesabi  district  were  well 
concentrated  even  by  Cambrian  time. 

The  process  of  enrichment  has  undoubtedly  continued  until  the  present  time.  It  there- 
fore appears  that  the  circulating  waters  have  had  eras  in  which  to  perform  their  work;  indeed, 
a  part  of  pre-Paleozoic  time  and  all  of  the  Paleozoic,  Mesozoic,  and  Cenozoic. 

Frequently  during  pre-Cambrian  time  the  ii-on-bearing  formations  were  metamorphosed  by 
igneous  intrusions,  the  principal  effect  of  which  was  to  recrystalUze  the  original  phases  of  the 
iron-bearing  formations,  yet  unaltered,  to  refractory  ampliibole-magnetite  rocks  able  to  resist 
the  ordinary  katamorphic  ore-concentratmg  agencies.  The  alteration  to  ores  of  portions  of  the 
iron-bearing  formations  so  modified  was  practically  stopped  at  the  times  of  the  intrusions. 

In  all  the  districts  since  the  beginning  of  final  concentration  many  thousands  of  feet  of 
strata  have  been  removed  by  erosion.  During  the  process  of  denudation  the  ore  deposits  in 
each  district  began  to  be  secontlarilj-  concentrated  shortly  after  the  iron-bearing  formation  was 
exposed  at  the  surface  and  for  a  long  time  they  continued  to  increase  in  size.  It  is  probable  that 
after  a  sufficiently  long  period  the  growth  of  the  deposits  practically  ceased,  for  denudation 
would  finally  remove  the  ores  at  the  surface  as  fast  as  they  formetl  below  the  surface.  However, 
change  would  not  stop.  The  ore  deposits  formed  would  continue  to  migrate  downward  pari 
passu  with  denudation.  On  account  of  the  pitch,  lateral  migration  would  accompany  downward 
migration.  At  any  given  time  the  masses  of  ore  would  extend  from  the  surface  to  the  depth  at 
which  descending  waters  were  effective.  We  therefore  must  conceive  of  the  secondarily  concen- 
trated iron-ore  deposits  as  slowly  migrating  downward  through  thousands  of  feet,  being  always 
just  in  advance  of  the  plane  of  erosion.  So  far  as  the  original  iron-formation  layers  were  rich 
enough  to  be  ores  without  secondary  concentration,  these  statements  do  not  apply.  The 
amount  of  ore  existing  at  an}^  one  period  tlirough  much  of  preglacial  time  may  have  been 
roughly  constant,  although  there  was  doubtless  considerable  variation  depending  on  topo- 
grapliic  and  climatic  conditions. 

At  times  the  processes  of  denudation  would  go  on  rapidly;  at  other  times  they  would  be 
stayed  for  long  periods,  depending  on  the  post-Keweenawan  history  of  the  Lake  Superior 
region. 

The  important  steps  of  tliis  history  are  (1)  the  great  pre-Cambrian  mountain  making  and 
erosion,  (2)  subsidence  and  Paleozoic  sedimentation,  (.3)  the  post-Paleozoic  uplift  and  denuda- 
tion, (4)  the  deposition  of  Cretaceous  rocks  upon  parts  of  the  region,  (5)  the  post-Cretaceous 
uphft  and  succeeding  denudation,  and  (6)  the  Pleistocene  ice  incursions. 

1.  In  the  pre-Cambrian  period  of  mountam  making  and  denudation  the  ore  deposits 
probably  reached  their  full  development,  and  indeed  they  maj^  during  the  latter  part  of  this 
ancient  time  have  been  of  greater  magnitude  than  they  are  at  present,  although  possibly  not 
so  rich.  In  the  Menominee  district  the  Upper  Cambrian  sandstone  and  the  Ordovician  lime- 
stone cap  the  Huronian  formations  and  even  some  of  the  ore  deposits.     The  upward  extension 


THE  IRON  ORES.  559 

of  the  iron-bearing  formation  was  removed  before  Upper  Cambrian  time.  It  is  clear,  therefore, 
that  the  main  concentrations  of  iron  oxide  for  these  deposits  must  have  taken  place  in  pre- 
Cambrian  time.  The  basal  conglomerates  of  the  Cambrian  carry  ore  fragments  from  previ- 
ously altered  formations.  If,  as  is  probable  (see  below),  Cambrian  and  Ordovician  or  Silurian 
strata  capped  the  beds  in  other  iron-bearing  districts  of  the  Lake  Superior  region,  it  is  all  but 
certain  that  ore  concentration  was  equally  advanced  in  these  other  districts,  although  where 
erosion  has  extended  farther  below  the  Paleozoic  than  in  the  Menominee  district  later  events 
have  had  a  greater  influence  upon  the  present  condition  of  the  ore  deposits.  The  later  stages 
of  this  period  of  denudation  were  marked  by  the  development  of  a  great  peneplain,  over  which, 
it  may  be  assumed,  the  ore-concentrating  processes  acted  slowly. 

2.  After  this  period  of  denudation  the  Paleozoic  sea  encroached  upon  the  Lake  Superior 
region.  Where  the  iron-bearing  formations  were  reached  by  the  sea,  detrital  ores  were  formed 
at  the  base  of  the  Cambrian.  The  entire  region  was  deeply  buried  beneath  the  Paleozoic 
deposits.  Probably  so  long  as  the  region  remained  below  the  sea  the  processes  of  concentra- 
tion practically  ceased  antl  the  mass  of  the  ore  deposits  remained  nearly  stationary.  Sea 
water  does  not  chemically  affect  the  iron  oxides. 

3.  Wlien  after  Paleozoic  time  the  region  was  again  raised  above  the  sea  and  denudation 
began,  little  enrichment  took  place  until  the  major  portion  of  the  Paleozoic  rocks  was  stripped 
from  the  region.  Over  much  of  the  region  these  Paleozoic  rocks  were  entirely  removed,  and 
the  pre-Cambrian  Huroniau  surface  again  emerged  from  below  the  Cambrian  deposits.  In 
the  Menommee  district  and  the  southeastern  part  of  the  Crystal  Falls  district  the  Paleozoic 
deposits  were  not  completely  removed  from  the  iron-bearing  formations,  and  here  considerable 
quantities  of  detrital  ores  are  found  at  the  base  of  the  Cambrian.  In  most  of  the  region 
erosion  did  not  stop  at  the  Paleozoic  but  extended  downward  for  a  greater  or  less  depth  into 
the  Huronian  rocks,  and  it  is  presumed  that  where  this  took  place  the  ore  deposits  migrated 
downward  precisely  as  durmg  the  pre-Cambrian  period  of  denudation. 

4.  Erosion  continued  until  the  end  of  the  Cretaceous  period  of  base-leveling,  when  the 
area  was  again  reduced  nearly  to  an  uneven  plain  and  locally  was  overridden  by  the  sea  and 
capped  by  Cretaceous  rocks,  at  least  as  far  east  as  the  Mesabi  district.  The  basal  strata  of 
these  beds  carry  detrital  iron  ore  from  the  Biwabik  formation.  At  the  end  of  this  period  the 
processes  of  downwartl  denudation  and  concentration  were  greatly  diminished  in  speed. 

5.  Durmg  the  period  of  the  post-Cretaceous  uplift  denudation  and  the  migration  of  the 
ore  deposits  again  went  on,  but  to  what  extent  is  uncertain.  It  is  highly  probable  that  m  the 
Menominee  district  the  topography  of  the  Huronian  rocks  is  largely  pre-Cambrian  and  the 
present  depressions  to  a  large  extent  are  reexcavated  pre-Cambrian  valleys.  The  same  is  true 
of  the  Felch  Mountain  tongue  of  the  Crystal  F^lls  district.  On  the  borders  of  the  Marquette 
district,  also,  Cambrian  deposits  are  found.  However,  it  is  now  a  matter  of  conjecture  as  to 
how  far  the  present  topography  is  redeveloped  pre-Cambrian  topography  and  how  far  it  is 
post-Cretaceous. 

6.  The  last  great  event  in  the  development  of  the  ore  deposits  was  the  glacial  incursion  of 
Pleistocene  time.  So  far  as  the  ore  deposits  are  concerned,  the  work  was  of  two  kinds,  glacial 
denudation  and  glacial  deposition.  The  quantity  of  ore  which  was  removed  during  the  first  stage 
of  Pleistocene  time,  that  of  glacial  erosion,  was  enormous.  Almost  the  entire  zone  of  decom- 
posed rocks  which  must  have  been  adjacent  to  the  ores  has  been  removed.  The  ore  deposits 
were  certainly  truncated  to  at  least  an  equal  depth.  Glacial  erosion  also  in  many  places  cut 
deeper  into  the  soft  ore  bodies  than  into  the  adjacent  hard  rocks,  and  thus  produced  subordinate 
valleys,  as  is  finely  illustrated  in  the  Mesabi  district.  The  abundant  fragments  of  hard  iron  ore 
in  the  glacial  drift  furnish  evidence  of  the  large  amount  of  ore  wliich  has  been  removed  b\'  the 
glaciers.  It  is  certain  that  still  greater  quantities  of  soft  ore  have  been  removed,  although  on 
account  of  its  softness  it  has  been  broken  into  minute  fragments  and  therefore  furnishes  little 
evidence  of  its  removal.  The  foregoing  considerations  lead  to  the  certain  conclusion  that  the 
glacial  truncation  seriously  reduced  the  amount  of  available  iron  ore  in  the  Lake  Superior 
region.     WTiile  the  pi'ocess  of  concentration  has  continued  since  glacial  time  and  has  tended  to 


560  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

enrich  and  deepen  the  deposits,  there  is  no  doubt  that  the  gain  since  the  glacial  incursion  is 
insignificant  as  compared  with  the  loss  of  rich  material  during  the  glacial  period.  Wlien  the 
glaciers  receded,  the  clean-cut  ore  bodies  were  covered  to  a  greater  or  less  depth  by  deposits  of 
glacial  drift.  This  relation  may  be  seen  to  the  best  advantage  in  the  great  open  pits  of  the 
Mesabi  district,  where  the  soft,  clean  ore  extends  directly  to  the  drift,  not  derived  from  the  ore 
but  brought  from  the  north.  The  contacts  in  many  places  are  of  almost  knifeUke  sharpness, 
there  being  practically  no  ore  in  the  basal  layers  of  the  drift. 

It  appears  from  the  foregoing  discussion  that  wliile  the  quantity  of  ore  in  the  Lake  Superior 
region  has  alwaj^s  been  large  since  Cambrian  time,  there  have  been  numerous  vicissitudes  in  its 
history  during  which  the  quantity  of  ore  alternately  increased  and  decreased. 

ORIGIN    OF    MANGANIFEROUS    IRON    ORES. 

Manganese  exists  in  a  series  of  minerals  remarkably  similar  to  and  usually  in  association 
with  those  of  iron.  The  origin  and  secondary  concentration  of  the  manganese  minerals  have 
been  regarded  in  general  as  following  very  closely  those  of  the  iron.  The  subject  has  not  been 
specifically  studied  for  the  Lake  Superior  region.  It  may  be  noted  here  merely  that  the  man- 
ganese tends  to  be  concentrated  in  the  upper  parts  of  the  Lake  Superior  iron-ore  deposits,  and 
that  as  secondarily  concentrated  it  consists  piincipally  of  manganese  dioxide  (pj-rolusite)  and 
subordinately  of  manganese  carbonate.  In  the  general  study  of  the  manganese  deposits  of 
the  Appalachians  and  other  parts  of  the  United  States  it  has  been  found  that  this  is  a  common 
but  not  invariable  relation  of  iron  and  manganese.  In  some  deposits  also  the  relation  is 
reversed,  the  iron  being  above,  the  manganese  below.  Where  they  are  associated  with  clay, 
not  in  the  Lake  Superior  region,  thei'e  seems  to  be  a  tendency  for  the  concentration  of  clay  at 
the  surface  relative  to  the  manganese.  Iron  and  manganese  oxides  and  cla}-  are  the  most 
stable  of  the  common  constituents  of  the  belt  of  weathering,  and  hence  all  of  them  tend  to 
become  residually  concentrated  as  compared  with  other  substances  originally  associated  wnth 
them.  The  vertical  distribution  of  these  three  substances  is  taken  to  be  a  function  of  their 
relative  stabihty  under  various  conditions  of  weathering,  but  the  available  information  does  not 
seem  to  warrant  more  specific  statements. 

PART  OF  THE  METAMORPHIC  CYCLE  ILLUSTRATED  BY  THE  LAKE  SUPERIOR 

IRON    ORES    OF    SEDIMENTARY    TYPE. 

Starting  with  the  ferrous  iron  and  dominance  of  silicates  in  the  original  igneous  rocks,  the 
development  of  the  ore  deposits  is  a  process  of  continuous  katamorphism.  From  the  original 
igneous  rocks  and  their  included  veins  containing  a  small  percentage  of  iron  there  is  developed 
an  iron-bearing  formation — cherty  iron  carbonate  or  greenaUte — containing  25  or  30  per  cent 
of  iron,  which,  on  further  alteration  at  the  surface,  becomes  concentrated  to  50  or  60  per  cent  or 
more.  The  iron-bearing  formation  and  included  ores  may  themselves  be  broken  up  to  yield 
materials  for  later  sedimentary  iron-bearing  formations.  The  upper  Iluronian  iron-bearing 
formations,  the  greatest  and  most  i^roductive  of  the  Lake  wSuperior  region,  ma}-  be  regarded  as 
including  materials  not  only  from  the  chemical  alterations  of  the  older  greenstones  but  from  the 
destruction  of  the  older  iron-bearing  formations  of  the  middle  Iluronian  and  Archean.  These 
formations  have  undei-gone  the  extreme  of  katamorphism.  Nature's  great  concentrating  mill 
has  developed  a  liigh-grade  end  product,  both  chemical  and  mechanical,  through  a  series  of 
concentrations.  The  changes  have  been  those  of  simplification  and  segregation  of  mineral 
compounds,  mai'ked  increase  in  volume,  when  all  substances  entering  into  the  reaction  are 
taken  into  account,  incoherency  of  substance,  and  net  liberation  of  heat,  all  of  them  typical  of 
the  katamorphism  or  destructive  processes  affecting  the  earth's  surface. 

No  sooner  have  the  ores  reached  their  maximum  incoherency  through  katamorphic  changes 
than  constructive  agencies  begin  their  work.  It  may  be  more  correct  to  say  that  they  begin 
before  the  destructive  agencies  have  finished.  The  ores  become  cemented  and  strengthened; 
they  tend  also  to  become  dehydrated  and  more  or  less  magnetic.     As  they  become  buried 


THE  IRON  ORES.  561 

beneath  the  surface,  owing  to  the  deposition  of  later  sediments,  and  as  they  become  folded,  their 
volume  is  decreased  by  an  elimination  of  pore  space  and  moisture,  they  are  recrystalHzed,  are 
shghtly  deoxidized  to  magnetite,  in  small  part  combine  with  siUceous  and  other  impurities  to 
produce  sihcates,  and  are  frequently  rendered  scliistose,  producing  the  hard  specular  ores.  The 
mineralogical  change  is  one  froin  simple  to  less  simple  compounds.  The  net  change  in  energy 
is  loss,  due  to  the  energy  given  off  in  volume  decrease.  The  process  is  a  characteristic  one  of 
anamorphism,  which  affects  all  rocks  under  similar  conditions.  The  anamorphic  changes  in 
the  ores  are  best  shown  in  the  oldest  or  Aixhean  iron-bearing  formations. 

More  marked  anamorphic  results  are  produced  under  the  influence  of  igneous  intrusions. 

The  contrasting  katamorphic  and  anamorphic  changes  affecting  the  ore  deposits  constitute 
a  partial  metamorphic  cycle."  Beginning  with  a  coherent  igneous  rock,  incoherent  ore  deposits 
are  developed  through  kataniorphism  and  in  turn  a  part  are  rendered  coherent  again  through 
anamorphism.  The  mineralogical  changes  are  at  first  from  complex  to  simple  and  later  from 
simple  to  complex.  The  changes  at  first  are  essentially  those  of  simplification  and  segregation 
and  later  this  process  is  arrested  and  on  a  smaller  scale  reversed  in  the  development  of  the 
complex  silicates.  The  ores  are  not  essentially  dispersed  to  again  become  constituents  of  igne- 
ous rocks,  although  certain  of  the  amphibole-magnetite  rocks  associated  with  the  ores  are  not 
easily  distinguishable  from  igneous  rocks.  The  cycle,  therefore,  so  far  as  observation  goes,  is 
not  complete.     There  is  throughout  a  net  loss  of  energy. 

TITAXIFEROUS  MAGNETITES  OF  NORTHERN  MINNESOTA. 

The  great  gabbro  mass  of  Lake  and  Cook  counties,  i\Iinn.,  contains  much  magnetite,  both 
disseminated  and  segregated  into  ore  deposits.  Complete  gradation  may  be  observed  between 
gabbro  carrying  little  magnetite  and  magnetite  carrying  little  of  the  ferromagnesian  con- 
stituents and  feldspars.  The  knowai  deposits  are  extremely  irregular,  with  gradations  between 
themselves  and  the  gabbro  and  containing  within  themselves  much  gabbro  material.  They 
weather  very  much  like  the  gabbro  and  might  be  easily  unnoticed  on  the  weathered  surface. 
There  has  been  little  exploration  for  these  ores.  A  few  drill  holes  have  been  sunk  in  the  region 
south  of  Gunfhnt  Lake,  some  of  them  revealing  depths  of  ore  aggregating  several  hundred  feet. 
The  known  deposits  seem  to  be  distributed  in  irregular  zones  roughly  parallel  to  the  north  or 
basal  margin  of  the  Duluth  gabbro. 

The  composition  of  the  ore  averaged  from  3,556  feet  in  14  drill  holes  is  43.8  per  cent  of  iron. 
The  range  is  from  54  to  20  per  cent.  The  high  titanium  content  renders  the  ores  of  doubtful 
value  for  the  present. 

Where  the  gabbro  comes  into  contact  ^vith  the  iron-bearing  Gunflint  formation  both 
formations  carr}'  magnetite  so  similar  in  texture  that  it  is  difficult  to  tell  one  from  the  other. 
However,  on  analj'sis  the  gabbro  magnetite  is  found  to  be  titaniferous,  while  that  of  the  Gunfhnt 
formation  is  not  titaniferous.  This  fact  seems  to  argue  against  any  considerable  transfer  of 
material  from  the  gabbro  to  the  iron-bearing  formation  during  its  alteration. 

The  titaniferous  magnetites  of  northeastern  Minnesota  are  direct  magmatic  segregations 
in  the  Duluth  gabbro,  according  to  all  geologists  who  have  studied  them,  including  Irving, 
Merriam,  Bayley,  Grant,  Winchell,  Clements,  Van  Hise,  Leith,  and  others.  The  complete 
gradation  from  gabbro  with  a  small  amount  of  original  magnetite  to  a  magnetite  with  small 
amounts  of  amphibole  and  other  gabbro  minerals  can  be  seen  in  almost  any  part  of  the  titanif- 
erous magnetite  deposits.  It  is  scarcely  necessary  to  repeat  the  detailed  petrologic  evidence  so 
fully  given  by  the  writers  named. 

Evidence  is  given  elsewhere  for  the  intrusive  character  of  the  Duluth  gabbro.  It  cooled  far 
beneath  the  surface,  where  there  was  not  easy  escape  for  its  solutions.  This  fact  is  taken  to 
explain  its  retention  of  its  iron  oxides.  It  has  been  argued  under  an  earlier  heading  that  where 
basic  rocks  of  similar  composition  reached  the  surface  large  quantities  of  iron  escaped  and 
became  available  for  ordinary  sedimentary'  deposition. 

o  Leith,  C.  K.,  The  metamorphic  cycle:  Jour.  Geology,  vol.  15,  1907,  pp.  303-313. 
47517°— VOL  52—11 36 


562  GEOLOGY  OF  THE  LAKE  SIJPEIIIOR  REGION. 

IVLVGNETITES  OF  POSSIBLE  PEGMATITIC  ORIGIN. 

The  ore  in  the  Atikokan  district  is  a  magnetite,  higiil^-  iinprcgnatcd  with  amphibolos  and 
sulphides  and  showing  extremcl}'  close  and  intricate  relations  to  associated  diorite.  It  difFers 
from  the  magnetite  of  the  gabbro  of  Minnesota  in  being  nontititniferous  and  in  being  separated 
by  defmite  boundaries — in  many  places  plane  surfaces — from  the  adjacent  wall  rock.  The 
apparent  absence  of  iron-bearing  formation,  the  general  lack  of  banding,  the  high  content  of 
amphibole  corresponding  to  that  in  the  associated  diorite,  the  content  of  sulphides,  and  the 
extremely  intricate  structural  association  with  the  diorite  are  not  easy  to  explain  if  the  ore  is 
sedimentary  and  owes  its  character  to  complex  intrusion  by  the  basic  igneous  masses.  Nowhere 
in  the  Lake  Superior  region  is  intrusion  known  to  completely  destroy  banding,  nor  does  it 
develop  so  much  coarsely  crystalline  amphibole  and  iron  sulphide  with  lack  of  parallel  texture. 
On  the  other  hand,  both  character  and  relations  suggest  pegmatitic  intrusion  or  igneous  after- 
effects, similar  to  those  described  by  Spencer  "  for  the  New  Jersey  magnetites  or  by  Leith  *  for 
certain  western  magnetites. 

The  evidence  for  pegmatitic  origin  of  the  ores  of  the  Atikokan  district  is  weak.  This 
district  lies  outside  of  the  principal  area  studied  in  connection  with  this  report,  but  from  our 
examination  of  it  we  suggest  this  origin  as  a  plausible  one  from  the  facts  available.  Certainly 
this  district  seems  to  show  marked  variations  from  most  of  the  districts  of  the  Lake  Superior 
region — variations  which  seem  to  call  for  another  mode  of  derivation. 

Minute  pegmatitic  veins  of  quartz  or  iron  oxide  or  both  are  common  in  the  ellipsoidal 
basalts  of  the  Vermilion  district.  In  the  coarser  phases  they  may  be  seen  to  be  intimately  and 
irregularly  mixed  with  the  rock,  and  grading  out  toward  the  finer  phases  they  tend  to  take  on 
more  definite  vein  outlines.  In  the  Keewatin  series  as  represented  in  tlie  Vermilion  district  it  is 
in  many  palces  dilhcult  to  determine  whether  the  iron-bearing  formation  is  a  magmatic  segre- 
gation of  greenstone,  a  vein  material  of  a  pegmatitic  nature,  or  an  ordinary  iron-bearing  sediment 
derived  from  them.  In  Plate  XLVIII  are  shown  gradations  from  the  basalt  through  siliceous 
and  jaspery  phases  to  ordinarj'  banded  iron-bearing  formation.  These  intermediate  phases 
seem  to  be  of  a  pegmatitic  nature. 

BROWN  ORES  AND  HEMATITES  ASSOCIATED  T^^TH  PALEOZOIC  AND  PLEISTO- 
CENE DEPOSITS  IN  WISCONSIN. 

ORES    IN    THE    POTSDAM. 

In  the  driftless  portion  of  the  Potsdam  area  north  of  Wisconsin  River  in  western  Wisconsin 
there  are  many  small  patches  of  hematite  and  brown  ore,  closely  associated  with  upper  horizons 
of  the  Cambrian  (Potsdam)  sandstone.  Many  of  these  patches  lie  on  the  tops  and  slopes  of  liills, 
but  some  of  them  follow  the  valleys.  During  the  early  days  of  mining  in  Wisconsin  these  ores 
were  smelted  locally  at  a  furnace  in  Sauk  County,  but  for  30  years  they  have  not  been  mined, 
principally  because  of  the  small-  amounts  available. 

The  origin  of  these  ores  is  not  clear.  Occurring  near  the  upper  horizons  of  the  Potsdam, 
some  of  them  may  represent  residual  accumulations  due  to  erosion  of  the  overh'ing  Ordovician 
limestone.  Samuel  Weidman  "^  believes  that  part  of  them  at  least  are  results  of  later  valley 
filling  by  spring  and  bog  solutions. 

BROWN   ORES   IN    "LOWER   MAGNESIAN "    LIMESTONE. 

At  Spring  Valley,  in  Pierce  County,  Wis.,  are  notlules  and  irregular  masses  of  limonite  in 
clays,  resting  upon  the  eroded  surface  of  the  "Lower  Magnesian"  limestone,  particularly  in  old 
drainage  courses  on  the  surface  of  this  lunestone.     Quoting  from  .Mlcn:<* 

o  Spencer,  A.  C.,  Franklin  Furnace  lolio  (No.  161),  Oeol.  Atlas  U.  S.,  U.  S.  Geol.  Survey,  1908,  pp.  6,  7. 
t  Loilh,  C.  K.,  Bull.  V.  S.  Geol.  Survey  No.  338, 1908,  pp.  75-89. 
«  PersonaU'oiumunication. 

d  Allen,  U.  C,  statement  prepared  for  this  monograph.    See  also  Allen.  R.  C,  The  occurrence  and  origin  of  the  brown  iron  ores  of  Spring 
Valley,  Wisconsin:  Eleventh  Keport.  Michigan  Acad.  Sci.,  1909,  pp.  95-103. 


.' 


PLATE    XL VIII. 


663 


PLATE  XLVIII. 

FeREUGINOUS    chert    or    JASrER,    OF    POSSIBLE    PEGMATITIC    ORIGIN,    IN    BASALT. 

A.  Partly  silicified  basalt  (specimen  2S564)  from  Vermilion  district,  Minnesota.     In  the  ledge  this  is  observed  to 

grade  imperceptibly  into  the  little-altered  basalt  of  the  region. 

B.  Chert,  green  silicate,  and  iron  oxide  (specimen  28565)  from  Vermilion  district,  Minnesota,  more  definitely  seg- 

regated into  bands,  grading  imperceptibly  into  the  rock  sho-mi  in  A  on  the  one  hand  and  into  that  shown  in 
C  on  the  other. 

C.  Same  (specimen  28566),  with  larger  proportion  of  iron  in  bands.     This  is  an  amphibolitic  ferruginous  chert  or 

jaspilite  of  a  type  often  seen  in  the  iron-bearing  formations. 

In  the  ledge  from  which  this  series  of  specimens  was  collected,  it  was  quite  impossible  to  find  any  plane  of 
separation  between  basalt  and  iron-bearing  formations. 

564 


U.   S    GEOLOGICAL  SURVEY 


MONOGRAPH    Lll         PLATE  XLVIII 


FERRUGINOUS  CHERT  OR  JASPER,  OF  POSSIBLE  PEGMATITIC  ORIGIN,  IN   BASALT. 


THE  IRON  ORES. 


565 


Spring  Valley  ia  a  small  town  on  Eau  Galle  River  reached  by  a  spur  from  Woodville,  on  the  Chicago,  St.  Paul, 
Minneapolis  and  Omaha  Railway.  Iron  ores  were  discovered  in  the  vicinity  of  Spring  Valley  about  20  years  ago. 
Thorough  prospecting  developed  a  number  of  deposits,  two  of  which,  known  as  the  Oilman  and  the  Cady  deposits,  are 
being  mined.  The  Oilman  was  opened  about  1890  and  has  been  in  operation  more  or  less  continuously  since  that  time. 
In  1893  a  furnace  was  erected  at  Spring  Valley  tor  utilizing  the  Oilman  ores  and  numerous  charcoal  ovens  were  built 
in  the  vicinity  for  supplying  fuel  for  the  furnace.  Wood  soon  became  scarce  and  coke  supplanted  charcoal  as  a  fuel. 
The  original  plant  has  been  partly  replaced  by  a  more  modern  one. 

GEOLOGY   AND   TOPOGRAPHY. 

The  Upper  Cambrian  sandstone  underlies  the  valleys  and  lower  hill  slopes.  The  uplands  are  formed  by  limestone 
of  Lower  Ordovician  age.     The  strata  are  conformable  and  flat-lying. 

The  topography  is  that  of  the  maturely  dissected  plateau,  and  is  essentially  of  preglacial  origin.  Eau  Galle  River 
and  its  tributary  creeks  are  flowing  through  partly  filled  valleys.  If  the  valleys  were  to  be  filled  to  the  average  height 
of  the  ridges  the  resulting  surface  would  be  a  plain.  A  plain  probably  once  existed  here  as  part  of  a  greater  one  which 
extended  over  a  surrounding  broad  area.  The  present  topography  may  lie  explained  as  resulting  from  the  uplift  of 
this  -ancient  plain,  giving  the  streams  new  erosive  power.  Before  glacial  time  the  streams  had  sunk  their  valleys 
through  the  Ordo\dcian  limestone  and  well  into  the  underlying  Cambrian  sandstone.  During  the  glacial  epoch  the 
valleys  were  partly  filled  by  glacial  wash. 

OILMAN   BROWN-ORE    DEPOSIT. 

The  Oilman  deposit  rests  upon  an  eroded  surface  of  the  Ordovician  limestone,  near  its  base,  on  the  upper  slopes 
of  a  ridge  above  the  valley  of  a  small  creek  tributary  to  Eau  Oalle  River.  It  is  on  the  railroad  and  is  li  miles  west 
of  Spring  Valley.  The  deposit  covers  several  acres  and  in  outline  is  very  irregular,  as  shown  liy  the  mine  workinos 
which  are  open  shallow  excavations,  the  deejjest  being  not  more  than  30  feet.  The  ore  is  a  brown  hj'drated  hematite 
and  occurs  as  nodules  and  concretions  mixed  irregularly  with  ocherous  clay,  sand,  chert  fragments,  and  nodular  con- 
cretions of  sand  and  clay.  Locally  the  deposit  shows  rough  and  irregular  bedding,  but  the  general  absence  of  beddin" 
is  conspicuous.  The  limestone  presents  an  uneven  surface  to  the  bottom  and  sides  of  the  deposit.  In  one  place  a  wall 
of  limestone  some  6  or  8  feet  high,  showing  undoubted  e\ddence  of  having  been  eroded  while  exposed  to  the  air,  abuts 
directly  against  the  ore.  In  places  the  ore  comes  quite  to  the  surface,  but  as  a  rule  it  is  covered  by  a  foot  to  several  feet 
of  clay.  All  the  mining  is  done  by  hand.  The  larger  nodules  of  ore,  called  "rock  "  ore,  are  picked  by  hand  from  the 
clay  and  sand  in  which  they  are  embedded.  Some  of  them  are  very  large  and  need  to  be  broken  up  by  blasting. 
But  most  of  the  ore  in  the  Oilman  mine  is  removed  with  the  impurities  in  which  it  occvu's  and  put  through  barrel 
washers.     The  following  is  the  analysis  of  a  three  months'  sample  of  "rock"  and  "wash"  ore: 


Analysis  of  ore  from  Gilman  mine. 


Fe 43.  6 

SiO. 24.00 

A1263 2.3 

CaO 58 


MgO. 
P.... 

S 

Mn.. 


0.30 
.  14 
.018 
.80 


CADY  BROWN-ORE   DEPOSIT. 

The  Cady  deposit  is  2i  miles  northwest  of  Elmwood  and  about  5  miles  southeast  of  Spring  Valley.  It  covers  several 
acres  on  the  top  and  upper  slopes  of  a  hill  that  rises  steeply  some  200  feet  above  the  valley  of  Cady  Creek.  As  in  the 
Gilman  deposit  the  ore  rests  on  the  Ordovician  limestone.  At  the  time  of  visit  in  1906  the  deposit  had  not  been  opened, 
l)Ut  the  ore  was  exposed  in  numerous  pits  and  trenches.  According  to  W.  H.  Foote,  a  shaft  went  down  through  80  feet 
of  ore  and  struck  a  face  of  limestone  at  that  depth  which  was  at  an  angle  of  60°  with  the  horizontal.  Ore  was  followed 
down  this  face  for  40  feet  more  with  no  bottom.     The  following  analyses  indicate  the  character  of  the  ore  in  this  shaft: 

Analyses  of  Cady  Creek  ore. 


Thickness 
(feet). 

Fe. 

SiO.. 

Mn. 

P. 

10 
10 
16 
22 
2S 
34 
40 
45 
50 
55 
60 
65 

59.12 
49.96 
47.79 
32.96 
46.56 
52.02 
37.91 
55.11 
53.66 
52.02 
52.22 
54.18 

9.0 
14.33 
20.5 
45.25 
22.17 
U.82 
35.34 

2.03 
.83 
1.39 
2.13 
2.47 
2.51 
1.82 
1.73 
2.72 
2!  25 
1.91 
1.33 

Brown  lump  ore.             .          .       .          

073 

Do 

Do                                                                                           ... 

077 

Do 

054 

Do                                                                                 .                               ■     .  .      . 

068 

Do 

Do                                                                                        ..... 

062 

Do 

Do..                 .                                       .          .                   .                   .... 

063 

Do 

566  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

The  ore  contains  a  Bomewhat  higher  percentage  of  iron,  has  a  greater  proportion  of  rock  ore,  and  is  associated  with 
a  less  amoiint  of  impurities  (sand,  clay,  etc.)  than  the  Oilman  ore,  but  is  otherwise  exactly  similar  to  it.  Mining  has 
recently  bc},'uii.  The  oro  is  delivered  to  the  bins  at  the  base  of  the  hill  by  an  aerial  tram.  The  descending  loaded 
buckets  return  the  empties  to  the  to])  of  the  hill, 

ORIGIN  OF   SPRING  VALLEY   BROWN-ORE   DEPOSITS. 

The  ores  near  Sprinj^  A'lilley  are  of  superficial  ori^jjiii,  beiuf^  deposited  upon  the  eroded 
surface  of  limestone  and  other  rocks.  Allen  has  shown,  from  a  consideration  of  the  thickness 
of  the  strata  once  overlying  the  present  ores  and  their  probable  content  of  iron,  that  the  now 
known  dejjosits  were  probabl_y  not  the  result  of  direct  downward  slump  of  residual  materials 
but  are  rather  sediments  transported  laterally  along  drainage  channels  after  the  country  hud 
been  cut  down  to  the  elevation  of  the  ores.  Allen  shows  further  that  since  the  formation  of 
these  deposits  erosion  has  cut  through  them  and  around  them,  with  the  result  that  the  adja- 
cent territory  has  been  lowered,  leaving  the  deposits  on  the  tops  or  slopes  of  hills.  He  con- 
cludes that  the  ore  deposits  of  Spring  Valley  were  laid  down  in  lakes  or  marshes  that  existed 
along  the  drainage  courses  on  the  old  post-Devonian  peneplain,  or  on  the  valley  bottoms,  as  may 
have  been  the  case  in  the  Giiman  and  Cadj-  deposits,  where  the  ore  abuts  directly  against  eroded 
limestone  faces.  The  marshes  and  lakes  were  finally  drained  as  a  result  of  uplift  of  the  land 
which  enabled  the  streams  to  erode  vertically  at  a  greatly  increased  rate.  Narrow  valle3^s 
were  formtd  in  the  older,  broader  ones.  The  outer  margins  of  the  old  valleys  correspond  with 
the  upper  slo]>es  of  the  hills  forming  the  present  valley  sides.  It  is  on  tliese  upper  slopes  that 
the  ores  characteristically  occur.  As  erosion  progressed  ore-covered  areas  would  naturally 
come  to  occupy  liigher  and  higher  relative  elevations,  owing  to  the  resistant  nature  of  the  ore 
beds.     In  this  way  would  result  ore-covered  hilltops,  as  illustrated  by  the  Cady  deposit. 

Weidman,"  who  has  made  a  survey  of  the  region  surrounding  Spring  Valley,  while  accepting 
the  general  view  that  tiie  Spring  Valley  ores  are  of  superficial  origin  and  were  deposited  upon 
the  eroded  surface  of  the  limestone  and  associated  rocks,  is  inclined  to  place  the  date  of  their 
origin  long  after  the  period  of  peneplanation  of  tlie  region.  This  alternate  hj^iiothesis  supposes 
the  ore  to  have  been  formed  in  these  valleys  after  they  were  eroded  to  a  considerable  depth — 
200  to  300  feet — in  the  peneplain  and  perhaps  even  at  the  still  later  stage  when  the  valleys  were 
in  the  process  of  being  filled  again  with  alluvial  material.  The  deposits  lie  in  secondary  and 
tertiary  valleys  and  on  slopes  opening  outward  toward  larger  valleys,  and  the  massive,  lumpy 
character  of  the  ore  indicates  that  it  may  very  well  have  originated  in  the  manner  of  iron-spring 
deposits,  accompanied  by  more  or  less  slope  wash  and  slumping  of  clay  and  sand  wliile  the 
valleys  were  bemg  filled.  Since  the  valley's  were  partly  filled  and  the  ores  formed,  erosion  has 
removed  30  to  40  feet  of  alluvial  material  from  the  valleys  and  a  variable  amount  of  the  ore. 
This  explanation  as  to  the  date  of  origin  of  the  ore — namelj',  at  the  time  when  the  valleys  were 
well  developed — seems  to  apply  very  well  to  the  Giiman  ore  deposit,  where  most  of  the  ore  has 
been  removed  and  where  the  relation  of  the  ore  deposit  to  the  topography  can  be  clearly  observed, 
•  and  it  jirobably  also  applies  equally  well  to  the  Cady  deposit,  where  mining  is  not  sufficiently 
advanced  to  show  the  actual  conditions. 

POSTGLACIAL    BROWN    ORES. 

Postglacial  iron  ores  are  known  in  many  parts  of  the  Lake  Superior  region.  They  are 
ordinary  bog  deposits  to  which  iron  is  being  contributed  in  solution  under  the  influence  of  organic 
material  and  deposited  by  oxidation.  Nowhere  is  their  thickness  known  to  be  over  a  few  feet. 
Lj'ing,  however,  directly  at  the  surface,  they  frequentlj'  attract  attention  and  for  man}'  years 
have  been  subject  to  intermittent  exploration. 


a  Weidman,  Samuel,  Geology  of  northwestern  Wisconsin:  Bull.  Wisconsin  Geol.  and  Nat.  Hist.  Survey.    (In  pi«paration.) 


THE  IRON  ORES.  567 

CLINTON  IRON  ORES  OF  DODGE  COUNTY,  WIS. 
OCCURRENCE    AND    CHARACTER. 

Iron  ores  of  Clinton  age,  similac  to  ores  of  the  same  horizon  in  the  Appalacliian  region, 
appear  in  Dodge  County,  in  soutlieastern  Wisconsin.  The  shipments  to  the  end  of  1909,  wliich 
figure  in  the  total  for  the  Lake  Superior  region,  have  aggregated  570,886  long  tons."^  The  ores 
outcrop  in  a  narrow  belt  extending  for  about  a  mile  north  and  south  on  a  westward-facing  scarp 
caused  by  the  overlying  Niagara  limestone.  The  underlymg  rock  is  Ordovician  shale.  The 
dip  is  eastward  at  the  rate  of  about  100  feet  to  the  mile.  The  beds  are  lens  shaped  along  the 
outcrop  and  range  in  tliickness  up  to  a  maximum  of  37  feet.  Mining  operations  have  followed 
them  400  feet  down  the  dip,  and  they  are  known  by  drilUng  to  extend  farther.  Wells  have 
shown  the  occurrence  of  ore  m  the  southeast  corner  of  the  county  and  near  Hartford  in  tluck- 
nesses  ranging  from  4  to  20  feet,  and  a  diamond-drill  hole  near  Kenosha,  60  miles  to  the  south- 
east, cuts  18  feet  of  ore.  The  iron  beds,  if  continuous  eastward  to  Lake  Michigan,  a  distance 
of  35  miles,  are  nowhere  more  than  800  feet  below  the  surface,  for  the  Niagara  limestone  which 
overlies  them  has  this  thickness  and  it  outcrops  all  the  way  to  the  lake. 

If  we  assume  an  average  tliickness  of  10  feet,  an  extension  down  the  dip  of  2,000  feet,  and 
contuiuous  extension  southward  to  Kenosha  (which  is  doubtful),  the  amount  of  ore  in  these 
deposits  would  be  600,000,000  tons. 

The  ore  is  a  slightly  hydrated  hematite,  running  from  29  to  54  per  cent  m  iron  and  aver- 
aging perhaps  45  per  cent,  high  in  phosphorus,  with  the  typical  granular,  oolitic,  or  flaxseed 
forms  so  characteristic  of  the  ores  of  the  Appalacliian  area.  The  matrix  is  calcite.  Bedding 
is  distinct  and  false  bedding  is  common.  The  granules  he  vnth  their  flat  sides  parallel  to  the 
bedding.  The  individual  granules  have  been  worn  shiny  by  water  action  and  aggregates  of 
them  have  been  rounded  into  pebbles. 

Under  the  microscope  the  iron-oxide  granules  are  found  in  part  to  be  amorphous  and  in  part 
to  have  the  concentric  structure  of  ooUtes.  Clastic  grains  of  quartz  or  of  iron  oxide  commonly 
form  the  nucleus,  surrounded  by  alternate  layers  of  iron  and  siUca.  On  treatment  with  hydro- 
chloric acid  the  iron  is  dissolved,  leaving  little  globular  particles  of  amorphous  siUca,  forming 
at  first  casts  of  oolites  but  on  drying  falling  in,  giving  a  basin-shaped  indentation  on  one  side. 
In  the  Clinton  formation  of  the  East  some  of  the  granules  have  the  structures  of  replaced  marine 
shells,  but  these  have  not  been  noted  in  the  iron-bearing  formation  of  this  horizon  in  Wisconsin. 

The  origin  of  some  of  the  amorphous  granules  is  observed  in  experimental  precipitation 
of  ferric  hydroxide  in  laboratory  solutions  where  the  precipitate  is  allowed  to  settle  slowly. 
There  is  then  observed  a  marked  tendency  fc^r  the  aggregation  of  iron  oxide  into  granules 
identical  in  shape  and  size  with  the  granules  observed  in  the  Chnton  ores.  These  granules 
are  of  the  type  regarded  by  Lehmann''  as  liquid  crystals,  a  globular  form  precedmg  develop- 
ment of  crystal  structure  and  indefinitely  grading  into  it.  In  materials  that  have-  strong 
crystalHzmg  power  this  globular  stage  is  soon  passed  or  is  not  even  observed.  In  substances 
weak  in  crystallizing  power,  such  as  iron  oxide,  the  tendency  inherent  in  the  substance  itself 
to  group  or  crystallize  does  not  go  beyond  this  stage  of  globular  aggregation. 

Along  the  top  of  the  ore  body,  at  the  contact  with  the  overlying  limestone,  is  a  thin  layer, 
rangmg  from  less  than  an  mch  to  6  inches  m  tliickness,  of  a  hard,  compact  bluish  hematite, 
heavier  than  the  oolitic  ore  and  ruiming  about  10  per  cent  higher  in  metallic  iron  than  the  main 
body  of  oolitic  ore.  In  this  hard  bed  there  is  no  trace  of  the  oolitic  structure.  However,  there 
is  an  apparent  gradation  from  one  to  the  other. 

The  contact  between  the  ore  and  the  Niagara  limestone  might  be  termed  a  "knife-edge," 
as  it  is  perfectly  well  defined,  showing  no  gradation  whatever  from  the  iron  into  the  limestone. 
The  lower  contact  of  the  ore  body  mth  the  underlying  calcareous  shale  is  similar  to  the  upper 
contact.     Under  the  microscope  some  of  the  calcite  grains  in  the  limestone  near  the  contact  are 

a  Lake  Superior  iron  ore  shipments  for  1909  and  previous  years,  compiled  bj'  Iron  Trade  Review,  Cleveland. 

'  Lehmann,  O. ,  Fliissige  Kristalle  sowie  Plastizitat  vpn  Kristallen  im  AUgemeinen,  molekulare  Umlagerungen  und  Aggregatzustandsander, 
angen,  Leipzig,  1904. 


568  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

observed  to  be  partly  replaced  by  iron  oxide,  while  other  large  calcite  grains  are  the  result  of 
recrystallizntion.  However,  those  are  not  common.  In  the  lower  contact  the  calcito  Ls  dis- 
colored by  tlie  iron  oxide,  but  where  tliis  iron  stain  occurs  we  do  not  always  find  any  evidence 
of  replacement.  For  the  most  part  the  surface  of  contact  of  ores  and  overlying  limestone  is 
even,  but  locally  the  beds  finger  into  one  another. 

ORIGIN    OF    THE    CLINTON    IRON    ORES. 

For  a  fuller  discussion  of  the  origin  of  the  Clinton  iron  ores  the  reader  is  referred  to  the 
pubUcations  of  the  geologists  who  have  studied  the  Clinton  ores  of  the  Appalachians,  especially 
to  the  recent  work  of  Burchard  in  Alabama."  The  ores  are  not  minetl  on  a  large  scale  in  the 
Lake  Superior  region  and  have  not  been  studied  in  the  same  detail  as  those  of  the  Algonkian 
and  Archcan. 

However,  a  comparison  with  the  ores  of  the  Algonkian  and  Archean  in  the  Lake  Superior 
region  iliscloses  certain  contrasts,  wliich  are  probably  significant  of  origin.  The  Clinton  ores 
constitute  beds  uniform  in  lithology,  with  no  evidence  of  local  concentration  or  replacement  or 
residual  masses  of  unaltered  material,  and  the  adjacent  beds  are  not  altered  or  iron  stained,  as 
they  are  where  secondary  concentration  has  occurred.  The  hematite  is  therefore  probably 
not  the  residual  result  of  the  alteration  of  preexisting  rocks.  On  the  other  hand  the  granules 
and  aggregates  of  granules  making  up  the  ore  are  distinctly  weatherworn  and  he  with  their 
flat  sides  parallel  to  the  strongly  marked  bedding  and  current  bedding,  pomting  strongly  to 
the  deposition  of  the  iron  in  essentially  its  present  mineralogical  condition  in  shalhnv  waters. 

The  Clinton  ores  therefore  differ  from  the  Lake  Superior  Algonkian  and  Archean  ores  in 
being  deposited  as  ferric  hydroxide  under  shallow-water  or  shore  conditions  rather  than  as  some 
ferrous  compounds  in  quiet  water,  as  is  characteristic  of  the  pre-Cambrian  iron  tleposition. 
That  the  waters  were  marine  is  indicated  by  the  character  of  the  beds  both  above  and  below, 
carrying  marine  fossils,  and  also  by  the  similarity  of  these  ores  to  Clinton  ores  of  the  eastern 
United  States,  in  which  marine  fossils  are  plentiful.  It  is  also  clear  from  the  waterworn  granules, 
current  bedding,  and  oohtic  structure  that  the  waters  were  movuig,  suggesting  shore  conditions. 
The  discontinuity  of  the  beds  aiid  their  variation  in  thickness  also  suggest  locally  varying  shore 
conditions.  But  many  features  of  the  history  of  the  deposition  of  these  ores  are  yet  obscure. 
No  satisfactory  answer  has  yet  been  made  to  the  question  why  these  ores  have  developed  at 
this  particular  horizon  in  the  Paleozoic  and  not  at  other  horizons. 

The  final  answer  to  this  problem  must  involve  the  study  of  the  Clinton  ores  of  all  of  North 
America. 

SmiMARY  STATEMENT  OF  THEORY  OF  ORIGIN  OF  THE  LAKE  SUPERIOR  IRON 

ORES. 

The  Lake  Superior  iron  ores  include  the  genetic  types  described  in  the  follo-n-ing  paragraphs. 

1.  Lake  Superior  sedimentary  type:  Iron  brought  to  the  surface  by  igneous  rocks  and  con- 
tributed either  directly  by  hot  magmatic  waters  to  the  ocean  or  later  brought  by  surface  waters 
under  weathering  to  the  ocean  or  other  bodj'  of  water,  or  by  both;  from  the  ocean  deposited  as  a 
chemical  sediment  in  ordinary  succession  of  sedimentary  rocks;  later,  under  conditions  of  weath- 
ering, locally  enriched  to  ore  by  percolating  surface  waters.  To  tins  class  belong  most  of  the 
producing  iron  ores  of  the  Lake  Superior  region,  those  of  the  Michijucoten  district  of  Canada, 
and  most  of  the  nonproducing  banded  iron-bearing  formation  belts  of  Ontario  and  eastern 
Canada. 

2.  Magmatic  segregation  type:  Ores  brought  to  the  outer  part  of  the  earth  in  molten 
magmas  but  were  retained  in  them  during  crystallization,  with  the  result  that  the  ores  form 
part  of  the  rock  itself,  just  as  do  the  feldspar  and  other  minerals.  Such  are  the  titaniferous 
magnetites,  which  contain  refractory  silicates  and  in  places  sid])hur  and  ])hos]diorus  in  dele- 
terious quantities.  Although  these  ores  are  known  in  enormous  quantities  in  the  Duluth  gabbro 
of  northern  Minnesota  they  are  not  mined. 

o  Burchard,  E.  i'.,  The  Clinton  or  red  ores  of  the  Binningham  district,  .Mabaina:  Bull.  U.  S.  Gcol.  Surrey  Xo.  315,  1907,  pp.  130-151. 


THE  IRON  ORES.  569 

3.  Pegmatite  tyjDe:  Ores  which  are  carried  to  or  near  the  surface  in  magmas  and  are 
extruded  from  them  in  the  manner  of  pegmatite  dikes,  after  the  remainder  of  the  magma  has 
been  partly  cooled  and  crystallized.  They  are  deposited  fi'om  essentially  aqueous  solutions 
mixed  in  varying  proportions  with  solutions  of  quartz  and  the  silicates  and  have  had  no  second 
concentration.  To  the  pegmatite  tyjje  are  doubtfully  assigned  the  ores  of  the  Atikokan  dis- 
trict of  Ontario,  and  possibly  also  certain  magnetites  of  the  Vermilion  district.  (See  p.  562.) 
No  detailed  study  of  the  Atikokan  ores  has  been  made.  Ores  of  tliis  type  have  been  mined  in 
small  cjuantity  in  the  Atikokan  district. 

4.  Clinton  setlimentary  type:  Sedimentary  "flaxseed"  ores  deposited  in  shaQow  waters, 
presumably  from  weathering  of  the  land  areas  in  wliich  the  iron  is  either  disseminated  in  igneous 
rocks  or  has  undergone  some  of  the  concentrations  outlined  in  the  three  preceding  paragraphs. 
They  have  suffered  no  essential  second  alteration.  These  are  the  ores  in  the  vicinity  of  Iron 
Ridge,  Wis. 

5.  Brown  or  hydrated  ores,  associated  with  Paleozoic  and  Pleistocene  deposits:  Residual 
or  bog  deposits  in  limestone  as  at  Spring  Valley,  Wisconsin,  or  in  glacial  drift.  Also  abundant 
in  ores  of  the  Lake  Superior  sedimentary  type.  The  associated  substance  is  largely  clay  and 
they  are  therefore  not  susceptible  of  second  concentration. 

Each  of  these  classes  of  ores  has  counterparts  in  ores  mined  elsewhere  in  the  country, 
except  the  Lake  Superior  sedimentary  ores,  the  only  ones  which  have  undergone  a  second  con- 
centration. From  this  class  have  been  produced  99  per  cent  of  the  iron  ores  sliipped  from  the 
Lake  Superior  region  and  annually  80  per  cent  of  the  iron  ores  mined  in  the  United  States,  a 
fact  that  indicates  the  great  importance  of  a  second  concentration. 

All  the  ores  have  been  derived  ultimately  from  the  interior  of  the  earth,  whence  they  were 
delivered  by  igneous  eruptions  to  ])oints  near  or  at  the  surface,  there  to  undergo  various  dis- 
tributions and  concentrations  under  the  influence  of  meteoric  waters  acd  gases.  The  varia- 
tions in  composition,  shape,  and  commercial  availability  of  an  ore  have  been  controlled  by 
variations  of  conditions  untler  which  the  ores  have  reached  the  surface  and  have  been  dis- 
tributed. The  titaniferous  magnetites  rejiresent  ores  brought  nearly  to  the  surface  but  not 
aUowed  to  escape.  The  pegmatites  rejiresent  ores  which  have  been  crystallized  in  the  act  of 
escape.  The  pre-Cambrian  sedimentary  formations  of  the  Lake  Superior  region  were  derived 
largely  from  basic  rocks  of  not  dissimilar  composition  that  reached  the  rock  surface,  though 
usuaUy  under  water,  in  wluch  case  they  crystallized  as  ellipsoidal  basalts. 

The  eruptions  to  which  is  due  primarily  the  introduction  of  most  of  the  known  ores  have 
come  up  along  the  zone  of  the  present  Lake  Superior  basin.  The  copper  ores  of  Keweenaw 
Point  and  the  silver  ores  of  Silver  Islet  have  been  brought  up  by  similar  igneous  rocks  at  a 
httle  later  date  along  the  same  zone.  Along  the  strike  of  the  Lake  Superior  zone  during  Kewee- 
nawan  time  igneous  rocks  also  brought  up  the  cobalt,  nickel,  and  silver  ores  of  Sudbury  and 
Cobalt.  The  minerals  and  petrographic  relations  of  the  Keweenawan,  cobalt,  and  nickel  ores 
bear  many  similarities,  suggesting  possible  differentiation  from  essentially  the  same  magma. 
It  is  suggested  that  the  entire  Lake  Superior  and  Lake  Huron  region  is  a  great  metallographic 
province  from  which  the  early  extrusions  brought  up  iron  salts  and  the  later  extrusions  were 
differentiated  into  the  copper,  silver,  cobalt,  and  nickel  ores. 

OTHER  THEORIES  OF  THE  ORIGIN  OF  THE  LAKE  SUPERIOR  PRE-CA]\ffiRIAN 

IRON  ORES. 

Whitney,"  Wadsworth,*  Winchell,"^  HiUe/  and  others  have  held  the  Lake  Superior  pre- 
Cambrian  ores  of  the  sedimentary  type  to  be  of  igneous  origm.  Winchell's  arguments  are 
nearly  aU  based  on  the  similarity  of  the  textures  of  the  iron-bearing  formations  to  those  of 

a  Foster,  J.  W.,  and  Whitney,  J.  D.,  Report  on  the  geology  and  topography  of  the  Lake  Superior  land  district,  pt.  2,  The  iron  region:  Sen- 
ate Docs.,  32d  Cong.,  special  sess.,  1851,  vol.  3,  No.  4,  406  pp. 

6  Wadsworth,  M.  E.,  Proc.  Boston  Soc.  Nat.  Hist.,  vol.  20, 1881,  pp.  470-479;  Bull.  Mus.  Comp.  Zool.,  Geol.  ser.,  vol.  1, 1880,  p.  75. 
<;  Winchell,  N.  H.,  Structures  of  the  Mesabi  iron  ore:  Proc.  Lake  Superior  Min.  Inst.,  vol.  13, 190S,  p.  203. 
d  Hille,  F.,  Genesis  of  the  Animikie  iron  range,  Ontario:  Jour.  Canadian  Min.  Inst.,  vol.  6, 1904,  pp.  245-287. 


570  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

igneous  rocks.  For  instance,  the  concretions  are  compared  with  bombs,  the  spaces  left  by  the 
leaching  of  silica  are  regarded  as  amygdahiidal  cavities,  tlie  breccias  are  regarded  as  volcanic 
breccias,  the  bedding  is  regarded  as  flow  structure,  the  slump  of  the  ores  in  contact  with  wall 
rocks  is  regarded  as  the  result  of  flow  of  lava  over  the  bluff  represented  by  the  wall  rock. 

In  this  view  Winchell  practically  reaches  a  conclusion  similar  to  that  of  Wadsworth,  who 
believed  that  theores  and  jaspers  are  cliiefl\' eruptive  and  described  the  jasper  and  ore  as  intruded 
into  the  country  rocks  in  wedge-shaped  masses,  sheets,  and  dikes. 

These  resemblances  between  iron  ores  and  igneous  rocks  are  so  superficial  that  they  would 
scarcely  be  taken  seriously  by  most  ol)servers,  and  conclusions  as  to  igneous  origin  ignore  so 
many  fundamental  facts  of  composition,  texture,  and  structural  relations  described  in  these 
reports  that  it  is  not  believed  necessary  to  attempt  to  refute  them. 

In  earlier  reports  Winchell  "  presents  a  different  view  of  the  origin  of  the  ores,  as  follows: 

A  chain  of  active  volcanoes,  having  explosive  emissions,  extended  across  northeastern  Minnesota  about  where 
the  Mesabi  iron  range  is  found.  This  was  near  the  shore  line  of  the  Taconic  ocean,  and  was  accompanied  by  land- 
locked bays  and  perhaps  by  fresh-water  lakes.  Such  marginal  volcanoes  had  a  chemical  effect  on  the  oceanic  water, 
causing  the  precipitation  of  silica  and  probably  of  iron.  Its  basic  lavas  and  obsidians  were  attacked  by  the  hot  waters 
and  were  converted  by  encroaching  silica  into  jaspilite.  Near  the  shore  such  glassy  lavas  were  eroded  by  wave  action 
and  distributed  so  as  to  form  conglomerates  and  sandstones.  Such  action  would  have  distributed  lavas  wholly  silici- 
fied  as  well  as  those  which  were  yet  glassy,  and  the  detritus  of  both  would  necessarily  mingle  with  detritus  from 
the  Archean.  Such  lavas  would  exhibit  great  contortion  and  in  places  great  brecciation,  the  same  as  later  lavas, 
and  these  breccias  must  have  been  mingled  sometimes  with  the  products  of  detrital  action.  After  prolonged  activity 
of  the  volcanoes  most  of  the  deposits  and  of  the  lavas  which  were  submarine  would  be  permeated  by  secondary 
silica,  but  carbonate  of  iron  would  permeate  the  mass  where  carbonic  acid  had  freer  access,  as  in  the  lagoons  into 
which  streams  drained  from  the  land  surface  to  the  north. 

This  view  Winchell  also  applies  to  the  Vermilion  range.  He  argues  that  the  iron  of  the 
iron-bearing  formation  was  first  deposited  as  a  ferric  oxide  and  that  the  ferruginous  cherts 
making  up  the  greater  part  of  the  formation  to-day  are  origmal  oceanic  deposits  laid  down 
essentially  in  the  present  form. 

In  volume  6  of  tlie  "Geology  of  Minnesota"  he  argued  that  the  solutions  formed  from 
the  igneous  rocks  acciunulated  in  the  rocks  to  the  point  of  saturation  and  that  precipitation 
came  later  as  a  result  of  cooling. 

This  discarded  view  of  Winchell  obviously  has  more  points  in  common  vnth  the  theory 
of  origin  outlined  in  the  present  monograph  than  his  more  recent  views,  although  important 
differences  are  still  to  be  noted. 

In  a  report  on  the  Baraboo  range  Weidman  *  reached  the  conclusion  that  the  iron  ores 
of  that  district  were  originally  precipitated  in  bogs  and  shallow  waters  as  limonite  and  hematite 
associated  ^nth  slate,  that  they  were  then  covered  by  the  dolomite,  tilted  up,  and  ernded, 
and  that  the  deposits  to-day  are  essentially  the  same  in  lithology  as  they  were  when  depositeil 
with  the  exception  of  certain  minor  vicissitudes  in  the  way  of  dehydration,  recrystallization, 
etc.  The  deposits  might  under  this  theory  extend  to  indefinite  depths — indeed,  as  far  as  anj' 
of  the  sedimentary  formations  of  the  district — and  in  this  way  the}'  would  contrast  with  the 
distribution  of  the  ores  determined  primarily  by  a  secondary  concentration  from  the  surface. 
In  view  of  the  evidence  of  secondary  concentration  found  in  other  parts  of  the  Lake  Superior 
region  the  burden  of  proof  must  rest  with  one  who  attempts  to  exclude  secondary  concentra- 
tion of  the  Baraboo  ores.  Deep  drilling  in  the  Baraboo  district  has  seemed  to  show  a  diminu- 
tion in  thickness  and  grade  of  ore  beds  and  a  relative  increase  of  iron  carbonate  with  increase 
in  depth,  pointing  to  secondary  concentration  from  the  surface  as  the  agency  which  has  been 
largely  responsible  in  developing  the  ore  bodies.  The  dilVcrence  in  opinion  as  to  tlie  origin 
of  Baraboo  ores  here  indicated  is  really  one  primarilj'  of  emphasis.  Weidman  emphasizes 
the  primary  deposition  in  rich  beds;  we  believe  that  the  primary  deposition,  wliile  a  large 
factor  in  localizing  ores,  has  been  supplemented  by  considerable  secondary  concentration  to 
develop  the  commercial  ore  deposits. 

o  The  geology  of  Minnesota,  vol.  5, 1900,  pp.  997-99S. 

6  Weidman,  Samuel,  Bull.  Wisconsin  Geol.  and  Nat.  Uist.  Siu-vey  No.  13, 1904,  pp.  142-14(3. 


THE  IRON  ORES.  571 

GENETIC  CLASSIFICATION  OF  THE  PRINCIPAL  IRON  ORES  OF  THE  WORLD. 

Iron  ores  are  known  to  have  been  developed  by  a  great  variety  of  igneous  and  metamor- 
phic  processes.  In  almost  any  genetic  classification  of  ore  deposits  iron  ores  will  be  repre- 
sented in  each  of  tlie  divisions,  contrasting  thereby  with  the  less  abundant  precious  metals. 
Moreover,  it  is  likely  that  certain  iron-ore  deposits  would  fall  outside  of  any  such  classifica- 
tion and  others  would  require  assignment  to  two  or  more  of  the  divisions.  The  following 
classificatioa  of  the  iron  ores  of  the  world  has  been  constructed  with  the  idea  of  showing  the 
correlatives  of  the  Lake  Superior  pre-Cambrian  ores  and  the  wide  range  of  conditions  under 
which  the  larger  and  better-known  deposits  have  developed. 

1.  Macmatic   segregations,   usually  in   basic   rocks.     Titaniferous  and   silicated   magnetites, 

weathering  to  limonites,  epidotic  and  chloritic  magnetites.  On  disintegration  yielding  mag- 
netic sands. 

Titaniferous  magnetites  of  northeastern  Minnesota  and  Adirondacks. 

Magnetite  of  Vysokaya  Gora  and  Gorolilagadot  of  the  Uralo,  Russia. 

Silicated  magnetites  and  specular  hematites  of  pre-Cambrian  of  Kiirunavaara,  Gellivare,  etc., 
Sweden. 

Silicated  magnetites  of  Kiirunavaara,  Loussavaara,  and  TuoUavaara,  Sweden. 

Titaniferous  magnetites  in  Taberg,  Sweden. 

2.  Igneous  after-effects,  usually  from  acidic  rocks  (pneumatolytic,  pegmatitic,  etc.),  usually 

deposited  ^^^thin  or  near  parent  igneous  mass. 
Certain  silicated  magnetites  of  Vermilion  and  Atikokan  districts  of  Adirondacks  .and  New 

Jersey,  of  Iron  Mountain.  Missouri,  and  of  Iron  Springs,  Utah. 
Contact-silicated  magnetites  of  Christiania,  suggested  by  Backstrom  and  DeLaunay  to  be 

aqueous  sediments  contriljuted  by  associated  jjorphyries. 

3.  Residual  limonites  resulting  from  weathering  of  igneous  rocks. 

In  this  class  are  most  of  the  laterite  deposits  resulting  from  the  weathering  of  basic  igneous 
rocks  in  tropical  regions.  The  limonites  of  northeastern  Cuba,  constituting  the  weathered 
mantle  of  serpentine  rock,  are  in  enormous  tonnage. 

4.  Sedimentary. 

A.  Iron  oxides,  mainly  syngenetic. 

Crystalline  hematites  of  Minas  Geraes,  Brazil,  the  largest  and  richest  known  deposits  of 
this  type  in  the  world. 

C'ambro-Silurian  micaceous  hematite  and  magnetite  of  Norway. 

Oolitic  limonites,  containing  subordinate  quantities  of  iron-silicate  granules  of  various 
descriptions  and  iron  carbonates,  in  Silurian  Clinton  rocks  of  Wisconsin  and  Appa- 
lachians and  Newfoundland;  in  Jtirassic  of  Luxemlnirg,  Lorraine,  and  elsewhere  in 
Germany  and  in  Cleveland  district  of  England;  in  Tertiary  of  Louisiana,  Texas,  and 
Bavaria. 

Bog  and  lake  limonites,  sometimes  in  granules.     In  glacial  lakes  and  bogs  of  Lake  Superior 
j-egion.     Small  and  nonproductive.     Represented  by  Scandinavian  lake  ores,  Finnish 
lake  ores,  lake  and  bog  ores  of  eastern  Canada,  Massachusetts,  and  elsewhere. 
A  1.  Iron  oxides,  developed  mainly  by  secondary  surface  alterations  of  sedigenetic  carbonates  and 
silicates. 

Pre-Cambrian  hematites  and  limonites  of  Lake  Superior  region. 

Paleozoic  limonites  of  Spring  Valley,  Wisconsin. 

Brown  ores  of  southern  Appalachians,  etc. 
A  2.  Iron  o.ridcs.  resultinr/  from  anamorphic  alterations  of  sedimentary  iron-bearing  formations. 
Specular  hematites   and   silicated   magnetites  derived  from  deep-seated  anamorphism 
of  oxides,  especially  of  carbonates  and  silicates  by  deep  burial,  intrusion,  or  both. 

Marquette  specular  hematites.     Hard  lilue  hematites  of  Vermilion. 

Silicated  magnetites  of  Gunflint  district  of  Minnesota,  eastern  Mesabi,  western  Gogebic, 
western  Marquette,  etc. 

B.  Iron  carbonates.     Usually  associated  with  coal  or  carbonaceous  slates.     Also  various  inter- 

mixtures of  calcium  and  magnesium  carijonates,  with  minor  amounts  of  oxides  and  silicates. 
Huronian  original  iron  carbonates  of  Gogebic,  Marquette,  Menominee,  and  other  districts  of 

Lake  Superior  region,  altering  at  surface  to  limonites  and  hematites,  and  nt  depth  or  by 

igneous  intrusion  to  silicated  magnetites  and  hemitites. 
Carlioniferous  1ilack-band  ores  of  Pennsylvania,  Ohio,  and  Kentucky,  altering  at  surface  to 

brown  ores  or  pot  ores  in  clay. 


572  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Tertiary  black-band  ores  of  Marj-land. 

Carboniferous  lilack-hand  ores  of  Germany. 

Carboniferous  blatk-band  ores  of  Wales  and  Scotland. 

Permian  lilack-liand  ores  of  district  of  Erzberg,  in  the  northern  Alps. 

C.  Iron  silicates.    Greeiialite,  glauconite,  chamosite,  thuringite,  etc.,  with  minor  mixtures  of 

iron  oxides  and  carlionates. 

Hiironian  original  greenalite  rocks  of  Mesabi  district  of  Minnesota,  derived  largely  from  direct 
igneous  contributions,  as  indicated  under  2.  .Mtering  to  hematites  and  limonites  at  sur- 
face and  to  silicated  magnetites  at  depth  or  at  igneous  contacts. 

Lower  Silurian  chamosite  ores  of  central  Bohemia  and  chamosite  and  thuringite  ores  of 
Thuringerwald  and  vicinity,  in  Germany. 

D.  Various  combinations  of  above. 

It  will  be  noted  that  the  Lake  Superior  ores  are  represented  in  most  of  the  princi})al  classes 
here  given.  They  also  constitute  an  important  subclass,  the  greenalite  ores,  developed  by 
ac[ueo-igneous  processes,  not  yet  certainly  idcntilicd  elsewhere. 

Much  the  largest  part  of  the  world's  production  of  iron  ore  has  come  in  recent  years  from 
the  sedimentary  ores.  The  largest  reserves  are  in  that  class.  Also  important  for  the  future 
are  the  resiilual  weathering  ores  of  the  laterite  type,  such  as  are  found  hi  northeastern  Cuba. 
The  highest  grades  are  reached  in  the  sedimentary  ores  which,  in  addition  to  some 'purification 
by  weathering  in  place  in  a  parent  rock,  have  been  sorted  and  segregated  during  transporta- 
tion and  deposition  as  sediments,  and  in  the  Lake  Superior  type,  when  again  exposetl  to  the 
surface,  have  undergone  further  purification  through  katamorphjsm.  These  successive  concen- 
trations have  removed  deleterious  constituents,  broken  up  complex  silicates,  and  left  the  ores 
with  a  porous  texture  better  adapted  for  furnace  reduction  than  the  ores  of  classes  1  and  2. 

The  iron  ores  therefore  illustrate  both  a  wide  range  of  ore-depositing  agencies  and  the 
great  increase  of  values  effected  by  the  reaction  with  meteoric  waters  and  the  atmosphere  in 
the  zone  of  katamorphism. 

One  of  the  most  striking  features  of  the  ore  deposits  of  the  sedimentaiy  class  is  the  preva- 
lence m  them  of  granular  textures,  both  oolitic  and  amorphous.  The  principal  types  of  gran- 
ules are  as  follows : 

Green  ferrous  silicates: 

Greenalite,  Fe(Mg)Si0^.nH20,  amorphous. 

Glauconite,  hydrous  silicate  or  iron  and  potassium,  amorphous,  resembling  earthy  chlorite,  in 

granules. 
Thuringite,  8Fe0.4(Al,Fe)203.6Si02.9H20,  related  to  prochlorite,  massive  and  fresh,  oolitic  when 

altered. 
Chamosite,  SiOj  29  per  cent,  AljOj  13  per  cent,  FeaOj  6  per  cent,  FeO  42  per  cent,  H2O  10  per 

cent.     Related  to  prochlorite.     Oolitic. 
Oolites  with  concentric  rings  of  quartz  and  some  green  silicate,  of  chloritic  nature,  undetermined. 

Found  in  Clinton  and  other  ores. 
Hematite  and  limonite: 

Oolites  consisting  of  concentric  rings  of  silica  and  iron  oxide. 

Amorphous  granules  .representing  oxidation  of  scjme  of  the  ferrous  silicate  granules  mentioned 

above  or  replacing  sliells. 

All  the  above  granules  lie  in  various  cements  of  silica,  iron  oxide,  and  calcium  carbonate. 

The  correlation  and  origin  of  these  various  granular  forms  present  an  interesting  field  for 
monographic  study.  It  is  known  that  some  are  organic,  as,  for  instance,  tite  glauconite  and 
certain  of  the  amorphous  iron-oxide  granules  replacing  shells.  It  is  known  further  that  proba- 
bly the  larger  part  are  inorganic,  including  the  oolites  and  amorphous  greenalite  and  iron 
oxide.  As  shown  in  another  place  (p.  525),  both  the  greenalite  ami  iron-oxide  granides  form 
in  ordinary  chemical  j)recipitates,  and  it  is  further  suggested  that  they  are  perhaps  related  to 
Lehmaim's  liquid  crj'stals.  It  may  be  of  interest  to  note  that  of  the  three  common  iron  com- 
pounds, oxides,  silicates,  and  carbonates,  the  two  former  appear  in  granules,  while  the  last  does 
not.  The  oxides  and  silicates  have  weak  crystallizing  power,  which,  according  to  Lehmann, 
is  usually  a.s.sociated  with  tlic  (Ipvel()|)ment  of  granular  or  amorphous  forms;  the  carbonates 
have  strong  crystallizing  power,  tending  to  give  the  surface  definite  and  angidar  outlines. 


CHAPTER  XVIII.  THE  COPPER  ORES  OF  THE  LAKE  SUPERIOR 

REGION. 


By  the  authors,  assisted  by  Edward  Steidtmann. 


THE   COPPER   DEPOSITS   OF   KEWEENAW  POINT. 
GENERAL    ACCOUNT. 

Although  the  authors  have  studied  the  copper  of  the  Keweenawan  series  in  many  parts 
of  tlie  Lake  Superior  region  and  have  visited  the  copper  deposits  frequently,  they  have  made  no 
systematic  investigation  of  the  ore  deposits  themselves.  Since  the  publication  of  Irving's 
monograph"  on  the  district  by  the  United  States  Geological  Sui'vey,  the  detailed  mapping 
done  by  the  Survey  in  this  region  has  been  confined  to  the  iron  deposits.  It  is  nevertheless 
thought  desirable  to  include  in  this  monograph  a  general  account  of  the  copper  deposits  in 
order  to  summarize,  as  fully  as  possible,  the  present  state  of  knowledge  of  the  geology'  of  the 
Lake  Superior  region.  The  portion  of  tliis  chapter  dealing  with  the  origin  of  the  ores  con- 
tains certain  new  features. 

The  fallowing  description  of  the  ores  is  based  jiartly  on  our  own  observations  and  largely 
on  the  published  descriptions  of  Irving,"^  Rickard, ''  Lane/'  Graton/  and  others. 

The  copper-producing  district  of  Keweenaw  Point  follows  the  axis  of  the  point  in  a  general 
northeasterly  direction  for  70  miles  and  has  a  width  of  3  to  6  miles.  The  richest  portion  of  the 
belt  is  the  central  portion,  in  Houghton  County,  adjacent  to  Portage  Lake  (see  PI.  XLIX),  in 
association  with  the  upper  lava  flows. 

The  copper  is  metallic.  With  the  exception  of  the  comparatively  small  amount  of  coarse 
copper — "mass"  and  "barrel  work" — sorted  out  at  ^he  mines,  all  the  ores  are  subjected  to 
crushing  by  steam  stamps,  followed  by  concentration. 

The  principal  gangue  minerals  of  the  copper  of  this  district  are  calcite,  quartz,  prelmite, 
and  laumontite,  with  smaller  but  still  considerable  quantities  of  analcite,  apophyllite,  natro- 
lite  and  other  zeolites,  orthoclase,  datolite,  epidote,  chlorite  (delessite),  and  native  copper. 
Rarer  associates  are,  according  to  Prof.  A.  E.  Seaman,  of  the  Michigan  College  of  Mines,* 
ailularia,  agate,  anliydrite,  algotlonite,  azurite,  aragonite,  argentite,  amethyst,  annabergite, 
ampliibole,  ankerite,  barite,  braunite,  biotite,  bornite,  cerargyrite,  chalcocite,  chloanthite, 
clu^'socolla,  chalcopyrite,  clilorastrolite,  cuprite,  covellite,  clinochlore  ( ?),  dolomite,  domeykite, 
fluorite,  gypsum,  hematite,  iddingsite,  jasper,  kaolinite,  keweenawite,  limonite,  magnetite, 
martite,  marcasite,  malachite,  melaconite,  muscovite,  mohawkite,  niccolite,  pyrite,  pyrrhotite, 
phillipsite,  powellite,  saponite,  selenite,  stibiodomeykite,  semiwhitneyite,  serj)entine,  silver, 
siderite,  talc,  whitneyite,  thomsonite,  wad,  and  wollastonite.     Though  this  group  of  minerals 

a  Irving,  R,  D.,  The  copper-bearing  rocks  of  Lake  Superior:  Mon.  U.  S.  Geol.  Survey,  vol.  5,  1883. 

b  Rickard,  T.  A.,  Tile  copper  mines  of  tlie  Lake  Superior  region,  New  York,  1905. 

c  Lane,  A.  C,  Tlie  geology  of  Keweenaw  Point — a  brief  description:  Proc.  Lake  Superior  Min.  Inst.,  vol.  12, 1907,  pp.  81-104:  The  geology  of 
copper  deposition:  Am.  Geologist,  vol.  34,  1904,  pp.  297-309. 

ti  Graton,  L.  C,  Silver,  copper,  lead,  and  zinc  in  the  Western  States:  Mineral  Resources  U.  S.  for  1907,  pt.  1,  U.  S.  Geol.  Survey,  1908  (Michigan, 
pp.  496-523;  Copper,  pp.  571-644). 

'  Personal  communication,  1910. 

573 


574 


GEOLOGY  OF  THE  LAIvE  SUPERIOR  REGION. 


is  cliaracteristic  of  the  deposits  in  general,  they  may  vary  in  importance  in  the  difTeront 
tyi)es  as  well  as  in  tiie  difTerent  parts  of  the  distrirt.  Calcite  is  the  most  abundant  associated 
mineral  in  the  transverse  vems  and  conglomerates;  ejjidote  is  the  most  abundant  in  the 
dipping  veins.  The  genetic  sequence  of  these  minerals  is  discussed  uniler  the  origin  of  the  ores. 
The  copper  constitutes  (1 )  veins  intersecting  the  northwestward  dipping  beds  of  the 
Keweenawan  series  described  in  Chapter  XV  and  (2)  beddeil  deposits  formed  by  infiltration  or 
replacement  of  both  the  conglomerate  and  amygdaloidal  beds  of  the  Keweenawan  series, 
chiefly  in  the  beds  below  the  "Great"  conglomerate,  which  is  the  dividing  line  between  the 
lower  part  of  the  Keweenawan,  where  traps  predominate,  and  the  upper  part,  where  sediments 
predominate.  (See  fig.  75.)  Copper  deposits  have  not  been  found  in  felsitic  beds  and  compact 
traps,  except  in  minute  quantities  in  the  latter,  where  they  are  closely  associated  with  amygda- 
loid beds.  Rich  cores  of  native  copper  are  reported  to  have  been  drilled  ,on  the  Indiana 
property,  in  0nt6nagon  County,  from  a  verj^  dense  felsite,  which  appears  to  be  intrusive. 
Development,  however,  has  not  reached  the  productive  stage.  Only  one  bed  above  the  "Great" 
conglomerate  contains  copper,  and  this  is  the  Nonesuch  shale,  which  carries  a  little  disseminated 
copper  throughout  its  extent  and  has  been  worked  m  the  Porcupine  Mountain  district. 

_o*^o°     Basicflows  with 
.  Level  of  Lake  Superior       ,o*,e»interbedded  conglomerate 

'{<  __!;gl,^,;^^!^^tar?— ,■  y j„y^^ yy/Ata^^  Cambrian  sandstone 


Copper-bearing  lodes 
KEWEENAWAN  SERIES 

Figure  75.— Cross  section  of  Keweenaw  Point  near  Calumet,  sliowing  copper  lodes  in  conglomerates  and  amygdaioids. 

The  deposits  earliest  exploited  were  the  veins  transverse  to  the  strike  of  the  beds  in  the 
Eagle  River  area  at  the  northeastern  extremity  of  the  district ;  the  next  were  the  veins  parallel 
to  the  strike,  though  not  uniformly  to  the  dip,  in  the  Ontonagon  area  at  the  southwest  end  of 
the  district.  The  vein  deposits,  especially  those  in  the  Ontonagon  district,  are  characterized 
by  masses  of  copper,  being  in  this  respect  distinguished  from  the  amygdaloidal  and  conglomerate 
copper  deposits,  in  which  the  copper  is,  as  a  rule,  much  more  minutely  disseminated.  The 
amygdaloidal  deposits  were  the. next  to  be  opened,  principally  in  the  central  portion  of  the 
district,  but  also  in  the  Ontonagon  area.  The  conglomerate  deposits  occurring  only  in  a  small 
area  in  the  vicinity  of  Calumet,  in  the  central  portion  of  the  district,  were  the  last  to  be  opened. 
(For  summary  of  history  see  pp.  35-37.)  In  1907  73.1  per  cent  of  the  ore  mined  came  from 
amygdaloidal  lodes  and  26.9  per  cent  from  conglomerate  lodes,  the  vein  deposits  at  present 
being  practically  nonproducing,  although  of  the  total  production  from  the  district  approximately 
3  per  cent  is  sorted  out  at  the  mines  as  coarser  mass  material. 

The  grade  of  the  ores  is  low  and  is  becoming  lower.  In  the  early  days  of  mining  much 
ore  above  3  per  cent  was  mined.  In  1906  the  average  grade  for  the  district  was  1.26  per  cent, 
and  in  1907  it  dropped  to  1.1  per  cent,  and  to  1.05  per  cent  in  1908.  Onlj'  four  mines  in  1908 
worked  ore  yielding  an  average  of  1  per  cent  or  more  in  metallic  copper.  In  190S  the  richest 
iodes  mined  carried  less  than  2  per  cent  metallic  copper,  while  the  poorest  yielded  but  little 
over  0.5  per  cent.  The  grades  and  amounts  mined  from  the  principal  mines  in  1907  are  as 
follows:" 


o  Mineral  Resources  U.  S.  for  1907,  pt.  1,  V.  S.  Geol.  Survey,  1908,  p.  500. 


U.  S.  GEOLOGICAL  SURVEY 


MONOGRAPH   Lll     PL.   XLIX 


Veins 

See  list  below  for 
explanation  of  numbers 


LIST  OF  VEINS 

Lake  lode  (amygdaloid) 

Nonesuch  lode  (conglomerate  and  sandstone) 

Arnold  lode  (ash  bed  amygdaloid)  (Equivalent  to  No. 1 1?) 

Forest  lode  (amygdaloid) 

Branch  lode  (amygdaloid) 

Calico  lode  (amygdaloid) 

Evergreen  lode  (amygdaloid) 

Butler  lode  (amygdaloid) 

Knowlton  lode  (amygdaloid) 

Winona  lode  (amygdaloid) 

Atlantic  lode  (amygdaloid) 

Pewabic  lode  (amygdaloid) 

Allouez  or  Boston  and  Albany  lode  (conglomerate) 

Calumet  and  Hecia  lode  (conglomerate) 

Osceola  lode  (amygdaloid) 

Kearsarge  lode  (amygdaloid) 

Isle  Royale  lode  (amygdaloid) 

Baltic  lode  (amygdaloid) 


R27W 


R26W 


MAP    SHOWING    LOCATION    OF    COPPER-BEARING    LODES    AND    MINES    ON    KEWEENAW    POINT. 

See  page  573. 


THE  COPPER  ORES. 

Ore  output  and  grade  of  the  principal  Michigan  lodes  in  1907. 


575 


Lode. 

Ore 
(tons). 

r.rade 
(per  cent). 

Calumet .■ 

2,400.000 
1,900.000 
2,350.000 
1.250.000 
750.000 

1.835 
1.06 
.87 

Baltic 

Kearsarge 

Pewabic  a 

Osceola '. 

895 

Actual  total  and  average 

8.041,361 
1,250,853 

1  67 

All  other  lodes 

.62 

a  Partly  estimated. 

A  little  native  silver  occurs  with  the  copper  in  some  lodes.  Averaojed  on  the  total  tonnage 
in  1908,  the  silver  yield  was  0.023  ounce  to  the  ton.  Native  silver  is  present  in  all  the  deposits, 
but  is  particularly  characteristic  in  the  veins  of  the  Eagle  River  and  Ontonagon  areas,  where 
also  mass  copper  is  abundant. 

The  amygdaloidal  and  conglomerate  deposits  have  great  extent  along  the  strike,  the 
Kearsarge  lode,  for  example,  being  actively  mined  almost  without  break  for  a  distance  of 
12  miles  and  other  lodes  being  mined  for  2  miles  along  their  strike.  They  have  been  followed 
down  the  dip  to  a  maximum  distance  of  more  than  1^  miles  and  a  vertical  depth  of  about  a 
mile,  making  these  mines  among  the  deepest  in  the  world,  and  are  still  found  to  be  productive, 
although  of  somewhat  lower  grade.  The  depth  to  which  mining  may  be  carried  is  not  yet 
known.  That  it  should  be  possible  to  mine  at  a  profit  ores  as  low  as  0.5  per  cent  at  a  depth 
of  a  mile  is  due  to  the  remarkable  uniformity  and  continuity  of  the  deposits  along  both  strike 
and  dip.  Shoots  of  richer  ore  pitching  parallel  to  the  strike  of  the  beds — as,  for  mstance,  the 
northward-pitchmg  shoot  of  the  Calumet  and  Hecla  conglomerate — are  known  in  a  few  places, 
but  these  are  themselves  so  extensive  that  their  existence  and  alternation  with  leaner  portions 
of  the  beds  have  been  ascertained  only  after  years  of  extensive  mining. 

TRANSVERSE    VEINS    OF    EAGLE    RIVER    DISTRICT. 

The  veins  of  the  Eagle  River  district,  in  the  northern  part  of  Keweenaw  Peninsula,  cut 
vertically  across  the  strike  of  the  betls  of  sediments,  traps,  and  amygdaloids.  The  veins  are 
not  commonly  formed  by  the  filling  of  a  simple  fissure,  but  by  a  large  number  of  subparallel, 
anastomosing  fissures  with  blocks  of  small  rock  inclosed  between,  forming  rather  a  fracture 
zone.  The  productive  zone  is  i-n  the  amygdaloid  beds  immediately  below  the  Allouez  con- 
glomerate and  above  the  greenstone.  The  veins  vary  fi-om  mere  seams  to  those  20  or  30  feet 
wide,  being  widest  where  they  cut  across  loose-textured  amygdaloidal  beds  and  not  exceeding 
a  width  of  3  feet  where  they  are  in  contact  with  compact  traps.  The  greatest  depth  reached  La 
the  minmg  of  transverse  veins  is  1,600  feet,  in  the  Cliff  mine.  The  texture  of  the  rock  traversed 
by  the  veins  also  controls  the  ore  content,  the  veins  being  rich  where  they  cut  porous  amyg- 
daloidal layers  and  poor  where  they  cut  compact  layers.  Many  of  the  amygdaloid  beds  them- 
selves are  rich  enough  to  be  productive  adjacent  to  transverse  veins. 

The  gangue  materials  associated  with  the  copper  of  the  Eagle  River  veins  are  mainly 
calcite,  quartz,  prehnite,  and  laumontite,  but  analcite,  apophyllite  and  other  zeolites,  orthoclase, 
datolite,  epidote,  natrolite,  and  other  minerals  are  found.  Native  silver  is  present.  Veins 
containing  only  calcite  are  generally  bare  of  copper. 

The  copper  is  scattered  through  the  gangue  in  thin  films  penetrating  other  minerals  or  in 
coarser  fragments  filling  interstices  between  other  minerals,  or  occurs  in  lenses,  in  this  occurrence 
usually  with  a  crystalline  form.  Mass  copper  also  is  found  here,  the  masses  ranging  up  to 
many  tons  in  weight  and  many  of  them  containing  fragments  of  wall  rock. 

Irving "  believes  that  these  veins  are  replacements  along  fissured  zones  rather  than  fillings 
of  open  fissures.  As  evidence  he  cites  the  gradation  between  vein  and  wall  rock,  the  replacement 
of  wall  rock  by  copper  masses,  the  occurrence  of  fragments  of  wall  rock  in  the  vein  and  in  the 

1 1rving,  R.  D.,  Mon.  U.  S.  Geol.  Survey,  vol.  5,  1883,  pp.  422-426. 


576  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

copper  masses,  and  the  greater  width  of  the  veins  adjacent  to  amydgaloidal  beds  than  of  those 
in  contact  with  dense  traps.     The  origin  of  tlie  copper  ores  is  discussed  on  pages  580  et  seq. 

Transverse  lissure  veins  are  not  restricted  to  the  Eagle  River  district,  but  are  present 
in  nearly  every  mine  on  Keweenaw  Peninsula.  In  the  southern  districts,  however,  these 
veins,  as  a  rule,  contain  no  copper,  or  at  least  not  enougli  to  make  them  productive.  Many 
of  them  are  barren  even  where  they  cross  productive  beils  of  amygdaloids. 

No  mines  are  now  operating  in  the  Eagle  River  district.     Explorations  have   recently 

been  conducted  there  with  a  view  to  further  mining.     The  mines  which  have   produced  ore 

in  this  district  are  the  ^tna.  Empire,  Delaware,  Amygdaloid,  Copper  Falls,  Central,  Phoenix, 

and  Cliff. 

DIPPINO    VEINS    OF    ONTONAGON    DISTRICT. 

The  dipping  veins  of  the  Ontonagon  district  are  noted  chiefly  for  the  great  amount  of  mass 
copper  that  has  been  removetl  from  them.  Tiie  principal  "mass  "  deposits  are  in  tiie  group 
of  amygdaloids,  traps,  and  conglomerates  corresponding  roughly  to  the  strata  between  Portage 
Lake  and  the  area  covered  by  the  u})per  sediments.  The  veins  are  fillings  of  fractures  following 
the  strike  of  the  beds.  Many  of  those  within  weaker  portions  of  the  bed — for  instance,  along 
amj'gdaloidal  layers — have  a  dip  steeper  than  the  bedding.  Those  that  lie  between  two 
different  beds  are  likel}'  to  dip  at  the  same  angle  as  the  beds. 

The  veins  vary  in  width  from  a  few  inches  to  many  feet.  The  veins  between  different 
beds  are  more  lO'cely  to  be  narrow;  those  cutting  amygdaloidal  beds  may  consist  of  a  wide 
fracture  zone,  with  fi-agments  of  rock  interspersed  with  vein  minerals.  Slickensided  walls 
locally  bound  the  veins,  but  on  the  whole  the  contact  is  irregular.  Irving'^  believed  these 
veins,  as  well  as  the  transverse  veins  of  the  Eagle  River  district,  to  be  largely  replacements  of 
wall  rock. 

Transverse  veins  (crossing  the  strike)  are  present  also  throughout  the  mines  of  the 
Ontonagon  district,  but  they  are  unproductive  except  where  they  cross  dipping  veins. 

The  chief  vein  materials  associated  with  the  copper  are  epidote  and  calcite,  but  the  other 
minerals  above  named  as  generally  associated  with  copper  are  present.  The  copper  occurs  in 
irregular  hackly  masses,  some  of  wliich  are  many  tons  in  weight.  One  mass  found  in  the 
Minnesota  mine  in  1857  weighed  420  tons.  The  large  proportion  of  mass  copper  originally 
mined  in  this  district  gradually  decreased  and  the  production  of  amygdaloidal  copper  increased. 
In  1908  the  production  was  derived  wholly  from  amj'gdaloid  lodes.  The  principal  producing 
mines  are  the  Adventure,  Mass,  Michigan,  and  Victoria.  Recent  explorations  have  shown 
additional  copper  deposits. 

AMYGDALOID    DEPOSITS. 

The  copper  deposits  in  am3'gdaloids  are  by  far  the  most  numerous  and  most  productive  in 
the  Keweenaw  Point  region.  The  amygdaloids  are  the  uj^jier,  and  in  some  places  the  lower, 
vesicular  portions  of  the  many  lava  flows,  vnth  here  and  there  an  interbedded  detrital  la^'er. 
The  thickness  of  the  productive  portion  of  the  amygdaloids  varies  from  a  few  feet  to  35  or  40 
feet.  The  depth  to  which  amygdaloid  beds  are  productive  has  not  been  determined :  the  greatest 
depth  yet  reached  is  shown  in  the  Quincy  mine — 5,280  feet  along  the  incline,  or  4,008  feet 
vertically. 

The  copper  deposits  in  the  amygdaloids,  though  lean  in  places,  are  much  more  continuous 
along  the  strike  than  those  in  the  conglomerates,  several  mines  miles  apart  working  the  same  bed. 
There  are  very  unusual  variations  in  strike  in  the  vicinity  of  the  Baltic,  Trimountain,  and 
Champion  mines. 

The  dip  of  the  amygdaloids  flattens  out  below  and  also  to  the  northeast  along  the  strike. 
The  Quincy  lode  has  a  dip  of  55°  at  the  surface  and  37°  at  a  depth  of  about  5,000  feet  along  the 
inchne.  The  Atlantic  lode  dips  54°,  the  Wolverine  40°,  and  the  "Baltic"  70°,  the  dip  thus 
showing  a  considerable  variation  even  in  a  small  area,  tliough  in  general  being  stec])er  in  the 
southern  part  of  the  region. 

11  Irving,  R.  D.,  Mon.  U.  S.  Oeol.  Survey,  vol.  5,  1883,  pp.  422^36. 


THE  COPPEK  ORES.  577 

In  amygdaloidal  beds  the  copper  occurs  in  cavities  in  amygdules  partly  filled  by  other 
minerals,  alon";  cleava<j;c  or  fracture  phmes  within  these  minerals  or  replacing  tliem  partly  or 
com])letely.  The  minerals  associated  with  the  copper  are  prelmite,  chlorite,  calcite,  and  cjuartz. 
According  to  Irving,"  considerable  portions  of  the  beds  have  lost  all  semblance  to  their  original 
amygdaloidal  structure  and  now  consist  of  clilorite,  epidote,  calcite,  and  quartz  intimately 
associated  or  forming  separate  masses  of  the  most  indefinite  sliape  merging  into  one  another. 
In  places  portions  of  partly  altered  prehnite  occur  associated  with  copper,  but  as  a  rule  prelmite  has 
given  waj'  to  its  alteration  products.  In  these  liighly  altered  masses  cojiper  crystallized  free  wJiere 
it  had  a  chance,  but  more  commonly  it  rei)laced  other  minerals.  In  calcite  bodies  it  formed  those 
irregular,  sohd  brandling  forms  locally  known  as  horn  co])])er,  some  of  them  many  hundred 
]>ounds  in  weiglit;  in  e])idote,  quartz,  and  ])rehnite  bodies  it  occurs  as  thread  and  flakelike 
impregnations;  in  fohaceous,  lenticular  chloritic  bodies  it  forms  flakes  between  cleavage  planes 
and  oblique  jomts,  or  here  and  there — this  is  more  particularly  true  of  fissure  veins — it  replaces 
the  chloritic  selvage-like  substance  till  it  forms  literally  pseudomorphs,  some  of  which  are  several 
hundred  tons  in  weight. 

In  the  Baltic  and  adjacent  mines  are  considerable  quantities  of  black  sulphides  near  the 
surface,  but  even  here  they  are  not  in  sufficient  amount  to  have  economic  value.  The  amount 
of  these  sulphides  decreases  greatly  with  increasmg  depth. 

The  amygdaloids  are  productive  only  where  broken.  Usually  they  have  both  strike  and 
dip  fractures  in  addition  to  very  irregular  fracturing.  Commonly  the  strike  fractures  are  not 
exactly  parallel  to  the  beds  but  cut  across  them  at  acute  angles.  Many  of  these  fractures  show 
shckensiding,  ]iroving  considerable  differential  movements.  At  the  Quincy  and  Baltic  mines  the 
amygdaloid  is  lean  where  there  are  cross  fractures,  but  a  little  distance  away  from  the  cross 
fractures  it  is  rich. 

In  some  places  the  copper  goes  down  into  the  compact  rock  beneath  the  amygdaloid, 
following  zones  of  Assuring,  alteration,  and  replacement. 

In  a  number  of  places  productive  amygdaloid  occurs  below  a  heavy  trap  bed,  as  at  the 
Winona,  Quincy,  Atlantic,  Wolveiine,  and  Baltic  mines. 

The  mines  operating  in  the  amygdaloidal  deposits  and  ]iroducing  60  per  cent  of  the  total 
output  of  the  Keweenaw  Point  district  in  1908  were  the  Calumet  and  Hecla,  Tamarack,  Osceola, 
Quincy,  Centennial,  Wolverine,  Tecumseh,  Franklin,  Isle  Roy  ale,  Atlantic,  Baltic,  Trimountain, 
Champion,  Winona,  Allouez,  Ahmeek,  Mohawk,  Adventure,  Mass,  Michigan,  and  Victoria. 
The  distribution  of  these  mines  and  the  lodes  upon  which  they  are  operating  are  shown  on 
Plate  XLIX  (p.  574).  The  Calumet  and  Hecla,  Osceola,  Ahmeek,  Wolverine,  and  Mohawk  are 
on  the  so-called  Kearsarge  lode,  wliich  has  been  developed  for  an  extent  of  about  14  miles,  the 
largest  deposit  in  the  district.  The  Wolverine  has  the  richest  deposit,  running  about  1.35  per 
cent  of  refined  copper.  The  ore  runs  as  low  as  0.7  per  cent  in  other  mines.  Below  0.7  per 
cent  it  has  not  been  found  profitable  to  mine.  South  of  Portage  Lake  the  only  lode  which  has  a 
large  production  is  the  Baltic.  Its  surface  extent  is  about  4  miles  and  the  yield  averages  about 
1.1  per  cent. 

COPPER    IN    CONGLOMERATES. 

Only  two  workable  beds  of  conglomerate  have  been  found  among  thirty  or  more  beds 
distributed  through  the  Keweenawan  series  untlerneath  the  "Great"  conglomerate — the  Allouez 
("Boston  and  Albany")  conglomerate  and  the  Calumet  and  Hecla  conglomerate — and  even 
these  are  workable  only  in  a  small  area.  A  number  of  other  conglomerate  beds  have  been  found 
to  contain  small  impregnations  of  copijer  but  not  enough  to  be  proiluctive.  The  Allouez 
conglomerate  is  being  worked  by  the  Franklm  Junior  mine  and  the  Calumet  and  Hecla  con- 
glomerate by  the  Calumet  and  Hecla  and  Tamarack  mines.     (See  PI.  XLIX.) 

The  Calumet  and  Hecla  conglomerate  is  the  richest  and  largest  copper  lode  in  the  district 
and  ranks  among  the  fii'st  two  or  three  lai-ge  copper  deposits  of  the  world.  It  is  famous  as  the 
principal  source  of  copper  of  the  Calumet  and  Hecla  Company,  which  has  been  the  greatest 

ttOp.  cit.,pp.  421-422. 
47.517°— VOL  5-2—11 37 


578  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

ilividend  payer  in  the  history  of  mining.  This  conglomerate  thins  both  to  the  north  and  south. 
At  the  North  ITeclti  nime  it  is  not  more  than  8  feet  wide  in  one  place.  It  tliuis  so  ra])i(lly  \o 
the  soutli  that  on  tlie  Osceola  property  it  has  not  been  discovered.  Thus  the  Calumet  and 
Hecla  conglomerate  is  essentially  a  lens. 

The  Calumet  and  Ilecla  conglomerate  bed  is  prochictive  only  in  the  2  rrules  covered  by 
the  Calumet  and  ILecla  and  Tamarack  pioperties.  Nortli  of  this  area  the  bed  was  mined  by 
the  Centennial  mine  without  success,  and  to  the  south  it  was  mined  by  the  owners  of  the  Osceola 
before  they  sunk  down  to  the  Osceola  amygdaloid. 

The  conglomerate  dips  39°  W.  at  the  surface,  flattening  to  36°  with  depth.  It  is  followed 
down  the  dip  from  the  outcrop  to  a  maximum  depth  of  8,100  feet  by  the  Calumet  and  llecla 
Company,  representing  a  vertical  depth  of  4,748  feet.  A  vertical  shaft  belonging  to  the  same 
company  about  a  mile  from  the  outcrop  on  the  hangmg-wall  side  passes  through  the  lode  at  a 
depth  of  3,287  feet  and  goes  to  a  depth  of  4,900  feet.  One  of  the  Tamarack  shafts  reaches  a 
depth  of  5,229  feet,  being  the  deepest  shaft  in  the  world.  The  conglomerate  lode  has  increased 
in  tldckness  from  13  feet  at  the  surface  to  20  feet  in  the  deepest  workings.  The  upper  half 
(stratigraphically)  is  richer  than  the  lower  half  of  the  bed. 

The  copper  content  of  the  Calumet  and  Ilecla  conglomerate  was  formerly  about  4  per  cent 
near  the  surface,  and  now  a  mile  verticall}'  below  the  siuface  is  1  to  H  per  cent  and  averages 
for  the  mine  shipment  1.83  per  cent.  The  copper  is  of  lower  grade  and  less  regularly  dis- 
tributed in  the  Tamarack  part  of  the  same  bed.  This  decrease  in  grade  of  the  ore  worked 
with  increase  in  depth  is  partly  a  real  one  and  jiartly  due  to  improvements  anil  lower  costs, 
enabling  lower-grade  ores  to  be  worked.  The  richer  ores  of  the  Calumet  and  Ilecla  conglom- 
erate constitute  a  shoot  pitching  to  the  north  and  extending  to  the  Centemiial  ground.  The 
upper  half  of  the  conglomerate  bed  is  finer  grauied  than  the  lower  half.  It  contains  more 
interstratified  sandstone  layers,  called  sandstone  bars,  which  are  usually  barren  but  in  places 
are  verj'  rich.  In  some  places  they  separate  the  conglomerate  into  two  parts  ami  in  such  places 
the  values  may  be  either  above  or  below  the  sandstone.  The  conglomerate  is  well  cemented 
to  both  foot  and  hanging  walls. 

Below  the  conglomerate  are  several  amygdaloidal  beds.  Immediately  over  the  conglom- 
erate is  a  trap,  300  or  400  feet  tliick,  which  separates  it  from  the  fu-st  amj'gdaloid.  The  cross- 
section  maps  of  the  formations,  made  from  the  drifts  of  the  deep  shafts  intersecting  the  beds 
for  thousands  of  feet  above  and  below  the  conglomerate,  divide  the  lavas  into  two  classes, 
traps  and  amygdaloids.  The  traps  form  the  greater  part  of  the  sections  and  man}-  of  them  are 
hundreds  of  feet  in  thickness.  The  amygdaloids  compose  a  much  smaller  portion  of  the  sections 
and  are  usually  thin.  The  copper  values  are  very  small  in  the  trap  and  amygdaloids,  both  above 
and  below  the  conglomerate. 

The  rich  portions  of.  the  conglomerate  are  usuall.y  light  colored;  the  poor  portions  are  dark. 
Tliis  is  a  practical  distinction  by  mining  men,  who  speak  of  the  lean  conglomerate  as  "black" 
and  mean  by  tliis  that  wherever  it  is  m  tliis  condition  the  values  are  low  or  lacking.  Tliis 
difference  in  color  is  due  to  the  fact  that  the  alterations,  a  jiart  of  wliich  resulted  in  the  deposition 
of  the  copper,  have  bleached  the  conglomerate.  In  many  jilaces  in  the  rich  conglomerate 
aureoles  of  hghter-colored  material  ma}^  be  seen  at  the  outer  parts  of  pebbles  and  bowldei-s. 

The  Allouez  conglomerate,  worked  by  the  Franklin  Junior  mine,  varies  between  8  and  25 
feet  in  thickness,  with  3  to  4  feet  of  sandstone  at  the  base.  It  is  of  lower  grade  than  the  Calumet 
and  Hecla  conglomerate,  averagmg  about  0.5  per  cent  in  copper. 

The  pebbles  in  the  conglomerates  are  mainly  porphyritic  felsite  with  diabase  and  amvgda- 
loids  m  subordinate  amounts.  Locallv,  as  m  the  Calumet  and  Hecla  conglomerate,  granitic  and 
quartz  porphjny  pebbles  are  abundant.  The  original  cementing  materials  were  siliceous  and 
feldspatlvic  particles,  l)ut  these  have  been  replaced  largely  by  secondary  calcite  and  cpidote, 
with  chlorite,  and  where  the  conglomeiates  are  pro<hictive  the  copper  is  an  important  or  the 
ciuef  cementmg  juaterial.  Copper  also  replaces  pebbles  to  varymg  degrees,  in  this  process 
the  pebble  first  becomes  porous  and  discolored  and  is  altered  to  a  mass  of  epidote  and  chlorite 
with  a  spongelike  skeleton  of  copper  associated  with  calcite.     As  a  rule  epidote  and  clilorite 


THE  COPPER  ORES. 


579 


first  replace  the  matrix  and  porphyritic  feldspars  and  later  copper  replaces  them,  but  often 
copper  replaces  feldspar  directl_y,  penetratmg  m  tliin  fdms  along  cleavaije  planes.  Pebbles  like 
these,  some  of  them  as  large  as  a  man's  head,  are  found  m  both  the  Calumet  and  Pleela  and  the 
Allouez  conglomerates,  being  composed  of  copper  in  various  degrees  up  to  nearly  solid  bowlders 
of  metal.  In  some  pebbles  the  copper  has  almost  entirely  replaced  the  original  material  as  well 
as  tlie  epidote  alteration,  but  more  commoid}^  the  copper  skeleton  contains  in  its  cavities 
unaltered  crystals  of  orthoclase  and  quartz,  surrounded  by  a  crust  of  epidote  and  chlorite. 

Few  fractures  are  noted  in  the  conglomerate,  nor  is  there  evidence  of  slippmg  or  faults  at 
contacts  with  walls.  The  hanging  wall  is  not  safe,  tending  to  fall  down.  Whether  this  is  due 
to  the  weakness  of  the  rock  when  the  stopes  are  taken  out  so  that  new  fractures  are  formed,  or 
whether  incipient  fractures  were  present,  has  not  been  determined.  Certainly  the  distribution 
of  values  is  not  a  fimction  of  exceptional  shattering. 

In  the  Nonesuch  shale  of  the  Porcupine  Mountain  district  copper  is  found  as  a  cementing 
mateiial,  as  a  replacement  of  cementmg  material,  and  as  a  replacement  of  rock  particles.  Many 
of  the  copper  fragments  in  tins  bed  are  pecuHar  in  having  mmute  cores  of  magnetite. 

COMPOSITION    OF    COPPER-MINE    WATERS. 

Lane  has  assembled  a  large  number  of  analyses  of  copper-mme  waters.  He  finds  that 
in  the  upper  levels  of  the  mines,  to  depths  varying  from  500  to  1,000  feet,  the  waters  are  abun- 
dant and  fresh,  though  on  the  whole  somewhat  softer  than  the  river  waters  of  the  Mississippi 
Valley.  Below  these  depths  the  waters  are  much  less  abundant  and  more  highly  concen- 
trated and  contain  principally  chlorides  of  soda  and  calcium.  At  the  deepest  levels  the  cal- 
cium and  soda  may  be  in  about  equal  proportions,  or  the  calcium  may  predominate  over  the 
soda.  At  intermediate  levels  the  soda  predominates  over  the  calcium.  The  contrast  in  com- 
position of  the  upper  and  lower  waters  slightly  resembles  that  of  the  waters  of  the  upper  and 
lower  levels  of  the  iron  mines.  Reference  is  made  on  pages  543-544  to  possible  causes  of  this 
difference  in  composition. 

Of  the  several  analyses  available,  three  are  selected  as  typical  of  the  three  classes  of  water. 

Analyses  of  copper-mine  ivaters.'^ 
[Parts  per  million.] 


Deep 
water. 


Inter- 
mediate 
water. 


3. 

Surface 


Cl 

SO) 

CO3 

PC, 

Na 

K 

Ca 

Mg 

Al 

Fe 

Mn 

SiOs 

(FeAljzOa. 

FeiOa 

AI2O3 


Sum. 
Difference. . 


Total  solids  determined . 


134,910 
2,123 
2,123 

None. 

11,592 

None. 

65,346 
2,12J 

None. 

None. 

None. 
2,123 


702 
75 


414 
91.2 


35 
30 


1,347.2 
2.8 


212,300 


1,350 


'3.0+ 
6  + 
40    + 


''2.3+ 


19 
4 


10 
1.6 


a  Lane,  A.  C,  Mine  waters:  Proc.  Lake  Superior  Min.  Inst.,  vol.  13,  1908,  pp.  74-126. 


b  If  contaminated. 


1.  Water  from  one  of  the  lower  levels  of  the  Quincy  copper  mine,  Hancock,  Mich.  Analysis  by  George  Steiger.  Cited  in  Bull.  U.  S.  Geol. 
Survey  No.  330,  1908,  p.  144. 

2.' Water  from  South  Kearsarge  mine.  Keweenaw  Point,  No.  1  shaft,  ninth  level,  dripping  collected  by  F.  W.  McNair  and  C.  D.  Hohl. 
Analysis  given  by  Lane,  A.  C,  Proc.  Lake  Superior  Min.  Inst.,  vol.  13,  1908,  p.  116. 

3.  Water  from  Tobacco  River,  Michigan.    Analysis  given  by  Lane,  A.  C.,  op.  cit.,  p.  90. 


580  GEOT.OGY  OF  THE  LAKE  SUPERIOIl  REGTOX. 

COPPER  IN  KEWEENAWAN  ROCKS  IN  PARTS  OF  THE  LAKE  SUPERIOR  REGION 

OTHER  THAN  KEWEENAW  POINT. 

Copper  is  known  in  small  quantities  in  the  KeweenaAvan  1ia])s  and  sediments  in  Doii<;las 
County,  Wis.,  in  adjacent  parts  oi'  Minnesota,  on  Isle  Royal,  on  Michipicoten  Island,  and  else- 
where. These  occurrences  are  not  essentially  different  from  those  of  Keweenaw  Point  in  their 
niiiioralogical  and  geologic  associations.  The  copper  occurs  piincipally  in  fissure  veins  cutting 
the  Jjedding  of  the  traps  and  to  a  less  extent  in  the  amygdaloidal  openings  and  interbeddcd 
sediments.  Exploration  has  been  carried  on  intermittently  in  all  the  areas  named.  On  Isle 
Royal  a  considerable  amount  of  metalhc  copper  has  been  mined.  (See  pp.  37-38.)  None  of 
these  districts  are  now  producing  copper. 

ORIGIN  OF  THE  COPPER  ORES. 
COMMON    ORIGIN    OF    THE    SEVERAL    TYPES    OF    DEPOSITS. 

The  copper  ores  of  the  Lake  Superior  region  are  in  part  replacements  of  conglomerates  and 
cementing  material  filling  the  original  openings  in  the  conglomerate,  in  part  fillings  of  amygda- 
loidal openings  and  replacements  in  traps,  to  a  slight  extent  fillings  of  veins  and  replacements 
of  adjacent  wall  rock,  and  finally  cement  and  replacements  of  a  basic  sandstone  liigh  in  the 
series.     The  copper  is  an  integral  part  of  the  cementing  material  of  these  rocks. 

In  a  discussion  of  origin  the  three  types  of  deposits  must  be  considered  as  essentially  a 
unit.     Irving  "  sees  in  them — 

simply  the  results  of  a  rock  alteration  entirely  analogous  to  that  which  has  brought  about  the  deposition  of  copper 
and  its  associated  vein-stone  minerals  within  the  cupriferous  amygdaloids.  They  are  alteration  zones  which  traverse, 
instead  of  following,  the  bedding,  simply  because  the  drainage  of  the  altering  waters  has  been  given  this  direction  by 
the  preexisting  fissures.  *  *  *  Thus  the  differences  in  origin  of  the  several  classes  of  copper  deposits — conglomerate 
beds,  cupriferous  amygdaloids,  epidote  veins  parallel  to  the  bedding,  and  "fissure"  veins  transverse  to  it — which 
at  first  sight  seem  to  l>e  great,  on  closer  inspection  for  the  most  part  disappear. 

That  much  of  the  copper  was  introduced  as  filling  and  replacement  of  wall  rocks  admits 
of  no  doubt.  Several  hypotheses  are  still  open  as  to  the  source  of  the  copper  and  the  manner 
in  which  it  was  transferred  and  redeposited. 

PREVIOUS  VIEWS    OF    NATURE   OF   COPPER-DEPOSITING    SOLUTIONS   AND 

SOURCE    OF    COPPER. 

Irving,''  Wadsworth,''  and  nearly  all  other  geologists  who  have  studied  the  copper-bearing 
rocks  believe  that  the  source  of  the  copper  was  in  the  basic  igneous  rocks,  and  that  so  far  as  it 
was  derived  from  the  sediments,  its  ultimate  source  was  still  the  basic  igneous  roclcs,  because 
tlie  sediments  came  from  those  rocks.  This  belief  is  founded  principally'  on  the  uniform  and 
close  association  of  copper  with  the  basic  igneous  rocks  and  the  known  existence  of  copper 
sulphides  minutely  disseminated  through  some  of  the  coarser  igneous  rocks.  The  source  of 
the  copper  was  believed  by  Pumpelly  '^  to  be  in  tiie  overlying  sediments. 

Smyth «  believed  that  the  ores  did  not  come  from  the  adjacent  wall  rocks  but  from  a  deep- 
seated  source,  the  nature  of  wliich  does  not  appear  from  his  report. 

The  conditions  and  agents  under  which  the  copper  has  been  supposed  to  have  been  taken 
from  the  adjacent  rocks  and  concentrated  have  been  variously  mterpreted.     Irvmg,/  Pumpelly, " 

<"  Irving,  R.  D.,  The  copper-bearing  roelis  of  Lake  Superior:  Men.  U.  S.  Geol.  Survey,  vol.  5, 1883,  pp.  424-428. 

I>  Idem,  pp.  425-420. 

<■  Wadsworth,  M.  E.,  The  origin  and  mode  of  occurrence  of  the  Lake  Superior  copper  deposits:  Trans.  .\ni.  Inst.  Mln.  Eng.,  vol.  27, 1S98, 
pp.  694-090.  See  also  Miiller,  Albert,  Verhandl.  Naturf.  (lesell.  Basel,  1857,  pp.  4U-4.'i.S;  Hauermann,  Hilary,  (Jiiart.  Jour.  Geol.  Soc.,  vol.  22, 1886, 
pp.  448-403;  Wadsworth,  M.  E.,  Notes  on  the  iron  and  copper  districts  of  Lake  Superior:  Bull.  Mus.  Comp.  Zool.  Harvard  Coll.  Geol.  scr.,  vol.  1, 
1880,  p.  126. 

d  Pumpelly,  Raphael,  The  paragenesis  and  derivation  of  copper  and  its  as-sociates  on  Lake  Superior:  Am.  Jour.  Sci.,  3d  scr.,  vol.  2, 1871,  pp. 
188-198;  24.3-258;  347-355. 

'Smyth,  n.  L.,  Theory  of  origin  of  the  copper  ores  of  the  Lake  Superior  district:  Science,  new  ser.,  vol.  3, 1S90,  p.  251. 

/  Irving,  R.  D.,  op.  cit.,  pp.  419-420. 

f  Pumpelly,  Raphael,  op.  cit.,  pp.  353-355. 


THE  COPPER  ORES.  581 

Wadsworth,"  Lane,''  and  others  have  been  inclined  more  or  less  strongly  to  the  theory  of 
concentration  under  the  direct  downward  movement  of  meteoric  waters.  Pumpelly  has  also 
implied  that  concentration  may  have  occurred  when  sediments  were  still  below  sea  level. 
Lane  "^  has  suggested  that  the  waters  were  salt  waters  of  the  type  now  found  in  the  deep  copper 
mines,  and  that  they  represent  fossil  sea  waters  or  fossil  desert  waters,  which  in  the  tilting  of 
the  series  have  migrated  downward.  Van  Hise''  has  argued  that  while  meteoric  waters  have 
done  the  work,  it  has  been  during  their  upward  escape  after  a  long  underground  course.  Smj'th^ 
assigned  the  first  concentration  of  the  ores  to  ascending  solutions  from  a  deep-seated  source 
not  specified. 

OUTLINE    OF    HYPOTHESIS     OF    ORIGIN    OF    COPPER    ORES    PRESENTED    IN 

THE    FOLLOWING    PAGES. 

The  copper  ores  are  characteristically  associated  with  basic  igneous  rocks.  The  source 
of  the  copper-bearing  solutions  lies  in  these  igneous  rocks.  The  original  copper-bearing  solu- 
tions were  hot.  These  solutions  may  be  partly  direct  contributions  of  juvenile  water  from  the 
magma,  partly  the  result  of  the  action  of  meteoric  waters  on  crystallized  hot  rocks. 

ASSOCIATION  OF  ORES  AND  IGNEOUS  ROCKS. 

From  60  to  70  per  cent  of  the  copper  produced  in  this  region  comes  from  the  amygdaloids. 
The  veins  of  mass  copper  also  are  all  in  igneous  rocks  and  these  veins  are  richest  where  they 
lie  parallel  to  or  intersect  amygdaloidal  beds.  The  ore-bearing  rocks  are  characteristically 
near  thick  rather  than  thin  flows.  Barren  conglomerates  are  interbedded  with  productive  flows. 
The  only  productive  conglomerates,  the  Calumet  and  Hecla  and  the  Allouez,  are  associated 
with  thick  flows.     Especially  is  the  overlying  flow  tlaick. 

Not  only  is  the  association  of  the  ores  and  the  igneous  rocks  cons])icuous  in  the  producing 
district,  but  throughout  the  Keweenawan  area  of  Lake  Superior  traces  of  copper  are  widely 
distributed  in  the  igneous  rocks. 

Copper  is  associated  principally  with  basic  igneous  flows,  but  it  is  now  reported  in  drilling 
in  felsite,  supposedly  intrusive,  at  the  Indiana  mine.  Copper  sulphide  is  also  reported  by 
Wright  ^  in  association  with  intrusive  gabbros  and  ophites  of  Mount  Bohemia. 

ORB  DEPOSITION  LIMITED  MAINLY  TO  MIDDLE  KEWEENAWAN  TIME. 

It  is  beheved  that  the  original  deposition  of  the  copper  was  limited  mainly  to  middle 
Keweenawan  time,  or,  if  not,  at  least  to  the  cooling  period  of  the  igneous  rocks  of  that  time. 
As  shown  below,  the  wall-rock  alterations  associated  with  the  ores  seem  to  be  characteristic  of 
hot  water.  Some  of  the  gangue  minerals  are  hot-water  deposits.  Bowlders  of  some  barren 
conglomerate  beds  show  mineralization  wliich  was  developed  before  they  were  broken  from 
the  parent  underlying  ledge.  The  deposition  of  the  copper  was  an  episode  in  the  work  of 
cementation  of  both  sedimentary  and  igneous  rocks,  which  certainly  began  as  soon  as  the  beds 
were  deposited  but  which  continued  to  the  end  of  the  volcanic  period  of  the  middle  Keweenawan 
and  even  longer.  Pumpelly's  work,  mentioned  below,  shows  that  the  copper  was  relatively 
late  among  the  minerals  introduced.  The  same  thing  is  shown  in  some  places  by  the  absence 
of  deformation  effects  upon  the  copper.  The  late  introduction  of  the  copper  is  argued  by 
Smyth  B  from  the  contrast  of  minerals  first  deposited  in  the  copper-bearing  series  with  those 
coming  later  and  carrymg  the  copper,  the  first,  accortlmg  to  him,  bemg  developed  under  condi- 
tions of  weathering  before  the  series  was  folded,  and  the  second  being  developed  after  the  series 
was  folded. 

n  Wadsworth,  M.  E.,  The  originand  mode  of  occurrence  of  the  Lake  Superior  copper  deposits:  Trans.  Am.  Inst.  Min.  Eng.,  vol.  27.  1S9S,  p.  695. 

6  Lane,  A.  C,  The  theory  ot  copper  deposition;   .\ni.  Geologi-st,  vol.  34,  1904,  pp.  297-.'!09. 

(■Lane,  A.  C,  The  chemical  evolution  of  the  ocean:   Jour.  Geology,  vol.  14,  1906,  pp.  221-225. 

d  Van  Hise,  C.  R.,  .\  treatise  on  metamorphism:  Mon.  U.  S.  Geol.  Survey,  vol.  47,  1904,  p.  11.3G. 

t  Op.  cit.,  p.  251. 

/  Wright,  F.  E.,  The  mtrusivc  rocks  of  Mount  Bohemia,  Michigan:   .\nn.  Kept.  Michigan  Geol.  Survey  for  190S,  1909,  pp.  301-.TO7. 

ffSmytii,  H.  L.,  Theory  of  the  origin  of  the  copper  ores  of  the  Lake  Superior  district:  Science,  new  ser.,  vol.  3,  1896,  p.  251. 


582  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Wadsworth  "  cites  tlie  extension  of  copper  in  a  continuous  mass  from  one  flow  to  another 
as  cvidonco  of  introduction  "after  tlie  copppr-l)cann<i;  series  was  complete." 

Wright ''  finds  veins  of  iron  and  copper  suli)iii(Ies  dcveloj)ed  in  the  intrusives  of  Mount 
Bohemia,  but  is  uncertain  whether  they  are  closely  related  in  time  with  the  consolidation  of  the 
intrusives. 

It  may  well  be  that  the  introduction  of  the  copper,  begun  relatively  caily  in  the  middle 
Kcweenawan,  was  to  a  considerable  extent  the  work  of  hot  solutions  after  the  entire  middle 
Keweenawan  was  piled  up,  when  relatively  quiescent  conditions  had  been  reached;  for  the 
lavas,  the  slowly  cooling,  deep-seated  intrusives,  and  the  underlying  reservoir  would  be  sources 
of  heat  and  hot  solutions  for  a  long  time  after  active  volcanism  had  ceased. 

On  the  wliole.  the  evidence  seems  to  indicate  clearly  that  part  of  the  copper  was  deposited 
soon  after  the  extrusion  of  the  associated  igneous  rocks,  but  late  in  the  cycle  of  mineral  deposition 
in  which  copper  was  formed,  and  that  much  of  the  deposition  of  the  copper  followed  the  folding 
and  deformation  of  the  Keweenawan  rocks.  As  this  deformation  undoubtedly  accompanied 
and  immediately'  followed  the  deposition  of  the  Keweenawan  series,  the  fact  that  copper  deposi- 
tion followed  deformation  does  not  necessarily  remove  it  much  in  time  from  the  formation  of  the 
adjacent  rocks.  But,  on  the  other  hand,  there  is  no  evidence  which  fixes  the  close  of  this  period 
of  deposition. 

DEPOSITION  OF  THE  COPPER  ACCOMPLISHED  BY  HOT  SOLUTIONS. 

That  the  copper  was  deposited  by  hot  solutions  seems  to  be  established  b}^  the  facts  stated 
below. 

NATURE    or   GANGUE   MINERALS. 

Prehnite,  epidote,  chlorite,  laumontite,  and  other  gangue  materials  of  the  copper  are 
aluminum  silicates.  Alumina  is  not  ordinarily  transported  by  cold  pluvial  waters,  and.  specifi- 
call}',  it  Ls  not  trans])orted  by  the  fresh  mine  waters  near  the  surface  at  the  present  time  in  any 
but  the  most  minute  quantity.  (See  analyses,  p.  579.)  In  the  deeper,  warmer,  heavily  concen- 
trated chloride  waters  (see  analyses)  alumina  and  ferric  oxide  are  in  larger  though  still  small 
amounts.  The  analyses  report  the  alumina  and  ferric  oxide  together,  and  the  proi)ortion  which 
is  alumina  is  not  known. 

Other  characteristic  associates  of  the  copper  are  datolite,  containing  boron,  and  apophyllite, 
a  fluorine  mmeral — both  substances  which  are  not  ordinarily  ascribed  to  solution,  transportation, 
and  deposition  by  cold  solutions.  Mine  waters  working  on  the  gangue  materials,  which  may 
be  said  to  contain  a  concentration  of  boron  and  fluorme,  even  now  contain  onh'  traces  of  the 
boron  and  fluorine  minerak,  not  enough  to  afford  materials  for  their  precipitation. 

NATURE  OF    WALL-ROCK  ALTERATIONS. 

The  wall  rocks  are  obviously  altereil  by  the  same  solutions  that  have  dei)0sited  the  copper. 
The  bleaching  of  the  wall  rock  is  so  characteristic  of  copper  vems  that  it  is  regarded  as  a  favorable 
sign  in  exploration.  This  bleaching  alteration,  when  measured  quantitatively,  is  found  to  vary 
in  several  important  respects  from  alterations  which  would  be  typical  of  surface  waters  carrying 
the  agencies  of  the  atmosphere.  Below  is  a  table  containing  two  pairs  of  analyses  of  the  fresh 
and  altered  wall  rocks,  selected  carefully  to  eliminate,  so  far  as  possible,  variations  in  original 
composition;  a  group  of  analj'ses  made  under  the  direction  of  Pumpelly,  which  indicate  the 
general  Uend  of  chemical  change  m  the  trappean  beds;  an  analysis  of  an  altereil  Calumet  and 
Hecla  conglomerate  bowlder  described  by  Lane;  a  group  of  analj'ses  by  Lmdgren  illustrating 
the  changes  in  chemical  composition  of  basic  igneous  rocl<s  altered  by  hot  solutions:  and  two 
analyses  of  fresh  and  weathereil  basic  igneous  rocks  given  by  Merrill. 

»  Wadsworth,  M.  E.,Theori!!inandmodeofoccuTrenceof  the  Lake  Superior  copper  deposits:  Trans,  Am.  Inst,  Uin.  Eng.,vol.  27, 189S,  p.  SG.'i. 
b  Wri-lit,  F.  E.,  op,  cit.,  p.  392. 


THE  COPPER  ORES. 

Analyses  of  fresh  and  altered  ivall  rocks  compared  icilh  other  rock  alterations. 


583 


1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

SiOj 

45.83 

18.92 

6.02 

C.24 

8.49 

9.28 

2.10 

.32 

.60 

2.70 

49.40 
16.12 
11.  51 

2.13 

3.52 
10.90 

3.02 
.58 
.10 

2.30 

46.78 
17.04 
7.95 
6.31 
6.31 
0.94 
3.44 
1.10 
.66 
3.62 

46.66 

16.97 

9.52 

4.16 

5.02 

9.37 

4.08 

.44 

.91 

2.79 

47.74 

16.75 

2.55 

6.31 

8.32 

11.40 

1.93 

.14 

(    2.73 

42.71 

14.93 

7.45 

3.48 

2.70 

22.76 

.54 

.04 

/    3.56 

42.83 
16.58 
4.42 
3.81 
6.96 
14.11 
1.29 
1.39 
f    6.48 

46.32 
15.95 
2.86 
8.92 
4.08 
10.28 
3.56 
1.23 
f    3.25 

49.20 

AljOs           

16.00 

Fe^Oj                             .        ..          

3.03 

FeO    

7.10 

MeO                            

6.98 

CaO 

3.44 

NaiO                   

5.05 

K2O 

1  31 

H2O—       

f      4.51 

HsO+ 

TiO. 

1.02 

^    1.29 

^    1.36 

2.78 

2.26 

PjOs 

COs      

.10 

.59 

.08 

.02 

S 

1 

SOa 

1 

Cii                          

.017 

.04 



MnO 

.52 

.22 

.87     .     .89 

1.17 

FeS-i 



' 

10. 

11. 

12. 

13. 

14. 

15. 

16. 

17.      1      18 

19. 

SiOz 

AljOa     

52.83 
Ifi.  30 
9.60 
2.48 
3.98 
2.98 
0.54 
2.49 
1     2.76 

31.42 

10.83 

15.58 

12.08 

3.36 

2.84 

1.98 

1.04 

f  14.52 

45.70 

20.44 

9.50 

8.95 

2.24 

7.46 

.80 

.28 

.35 

2.78 

1.10 

46.22 

10.  22 

12.88 

7.45 

.84 

15.56 

.18 

1.04 

.58 

3.91 

.95 

45.50 

14.15 

11.20 

9.83 

6.76 

2.30 

1.57 

1.18 

.23 

4.84 

1.11 

.14 

3.04 

37.01 

12.99 

.43 

3.57 

5.49 

9.78 

.13 

4.02 

.13 

1.92 

.85 

.06 

15.04 

61.01 

11.89 

1.57 

6.08 

8.87 

10.36 

4.17 

.15 

.24 

2.09 

.98 

.17 

45.74 

5.29 

.13 

2.06 

.94 

23.85 

.11 

1.29 

.22 

1.07 

.36 

.07 

18.91 

47.00 
15.70 
4.78 
9.96 
6.36 
8.96 
2.77 
1.23 

{'3.' 24' 

42.50 

17  00 

FeO  

2  70 

CaO 

4.20 

1  50 

KjO 

70 

H2O—  

{■■■g.'so 

H2O+ 

PjOs 

S 

SO3   

.03 

.04 

Cu 

Trace. 

Trace. 

1 

MnO 

.25 
7.86 

.24 
7.99 

■       1            

FeSj               

1.  specimen  475006.  country  rock 70  feet  Irom  the  lode,  seventh  level  of  Winona  mine,  Keweenaw  Point,  Mich.  Analysis  by  R.  D.  HaII» 
University  of  Wisconsin,  1909. 

2.  Specimen  47499,  center  of  lode,  same  locality.    Analysis  bv  R,  D.  Hall,  University  of  Wisconsin.  1909. 

3.  Specimen  47506,  12  feet  from  footwall  of  sLxtv-third  level  of  Quincy  mine,  Keweenaw  Point,  Mich.  Analysis  bv  R.  D.  Hal!,  University  of 
Wisconsm.  1909. 

4.  Specimen  47505,  footwall  near  lode,  same  locality.     Analysis  by  R.  D.  Hall,  University  of  Wisconsin,  1909. 

5.  ilelaphyre,  lower  zone  of  bed  64,  Eagle  River  section,  Mich.  Pumpelly,  Raphael,  Metasomatic  development  of  the  copper-bearing  roclvs 
of  Lake  Superior:  Proc.  Am.  Acad.  Arts  and  Sci.,  vol.  13,  1878,  p.  293. 

6.  Prehnitized  upper  zone  of  bed  64,  same  locality.    Idem. 

7.  Pseudo-amygdaloid,  middle  zone  of  bed  64,  same  locality.     Idem. 

8.  Bottom  of  bed  87,  same  locality.    Idem.,  p.  285. 

9.  Middle  of  bed  87,  same  locality.     Idem. 

10.  Diabase  porphyrite.  regarded  by  Lane  as  the  original  of  the  altered  conglomerate.  Lane,  A.  C,  The  decomposition  of  a  bowlder  in  the 
Calumet  and  Hecla  conglomerate:  Econ.  Geology,  vol.  4,  1909,  p.  161. 

11.  Altered  conglomerate.    Idem. 

12.  Fresh  basaltic  rock  from  center  of  flow,  15  feet  from  lode.  Dingle  Creek  mine,  Douglas  County,  Wis.  Analysis  by  W.  G.  Wilcox,  LTniversity 
of  Wisconsin,  1910. 

13.  Superjacent  amygdaloidal  lode,  same  flow.    Analysis  by  W.  G.  Wilcox,  University  of  Wisconsin,  1910. 

14.  Amphibolite  schist.  Mina  Rica  vein,  Ophir,  Placer'County,  Cal.  Fairly  fresh,  but  contains  some  calcite  and  pyrite.  Lindgren,  Waldemar, 
Metasomatic  processes  in  fissure  veins:  Trans.  Am.  Inst.  Min.  Ehg.,  vol.  30,  1901,  p.  666. 

15.  Completely  altered  amphibolite  schist,  Conrad  vein,  Ophir,  Placer  County,  Cal.    Idem. 

16.  Fresh  diabase,  Grass  Valley,  Cal.     Idem. 

17.  Altered  diabase.  North  Star  mine,  Grass  Valley,  Cal.     Idem. 

18  and  19.  Average  of  five  fresh  (16)  and  weathered  (17)  basic  igneous  rocks— diabase  from  Spanish  Guiana,  diabase  from  Medford,  Mass.,  basalt 
from  Bohemia,  basalt  from  Crouzet,  France,  and  diorite  from  Albemarle  County,  Va.  Calculated  from  analyses  given  by  G.  P.  Merrill  in  Rocks, 
rock  weathering,  and  soils. 

In  the  following  table  the  fu'st  two  pairs  of  analyses  given  above  are  calculated  as  closely 
as  possible  into  the  minerals  actually  observed  in  the  rocks : 

Mineral  compositions  of  fresh  and  altered  wall  rocks  calculated  from  first  two  pairs  of  analyses  given  above. 


Minerals. 

47500B. 

47499. 

47506. 

47506. 

1.67 
17.82 
25.02 
9.30 
27.09 
13.60 
.50 
.01? 
5.00 

2.78 
25.15 
20. 29  ■ 

6.67 
28.82 
22. 24 

2.00 

30.74 

12.90 

.66 

2.22 

Albite  molecule      

33.54 

25.02 

Olivine 

Augite                                                             

1  88 

Chlorite 

13.41 

20.58 

Water         .                                 .      .  .          ... 

.91 

.04 
5.28 
15.22 
7.66 
1.40 
9.33 

Hematite 

4.64 

2.88 

Prehnite 

Epitlote 

13.05 

Calcite..               .                               .           .                   ...... 

.20 

.05 

100.01 

100.73 

100.87 

100. 13 

584  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

Analj'ses  representing;  ordinary  weathering  alterations  indicate  a  uniformit}'^  of  results 
which  may  serve  as  a  basis  for  comparison  with  alterations  of  unknown  cause.  Comparin^i 
the  alterations  of  the  wall  rocks  of  the  copper  deposits  of  unknown  origin  with  the  known 
results  of  weathering  of  similar  types  of  basic  rocks  and  the  results  of  the  alteration  of  similar 
basic  igneous  rocks  by  thermal  solutions,  tlic  following  conclusions  seem  justified: 

1.  The  changes  in  the  chemical  composition  of  the  Lake  Superior  copper  rocks  lack 
uniformity. 

The  changes  in  the  composition  of  basic  igneous  rocks  by  thermal  solutions  lark  uniformity. 
The  weathering  of  basic  igneous  rocks,  as  well  as  tlie  weathering  of  all  other  rocks,  causes 
certain  changes  in  the  chemical  composition,  which  are  almost  rigidly  uniform. 

2.  The  changes  effected  in  the  silica  content  of  the  Lake  Superior  (•oj)per  rocks  adjacent 
to  the  deposits  are  not  governed  by  silicate  ratios  in  the  original  rock. 

The  changes  effected  in  the  silica  content  of  basic  igneous  rocks  elsewhere  by  their 
alteration  by  thermal  solutions  are  not  controlled  by  silicate  ratios  in  the  original  rocks. 

The  changes  effected  in  the  silica  content  of  basic  igneous  rocks  by  weathering,  as  well 
as  those  effected  ])y  the  weathering  of  all  other  silicate  rocks,  are  governed  by  the  silicate 
ratios  in  the  original  rocks. 

3.  The  changes  in  the  chemical  composition  of  the  Lake  Superior  copper  rocks  show  local 
concentration  of  lime  or  alkahes,  depending  on  the  stage  of  mineral  paragenesis  represented 
by  the  analysis.  There  has  been  an  increase  in  the  ferric  iron  and  water  content  throughout ; 
FeO  appears  to  have  been  consistently  removed  or  rather  oxidized  in  place.  There  Ls  evidence 
of  both  the  removal  and  the  introduction  of  AI2O3. 

The  changes  in  the  chemical  composition  of  the  rocks  altered  by  thermal  solutions  elsewhere 
show  a  consistent  increase  in  KjO  and  a  consistent  decrease  in  Na,0,  FejOs,  and  MgO.  AljO, 
has  suffered  considerable  decrease  in  some  rocks.  In  others  the  eAadence  for  the  decrease, 
■increase,  or  stability  of  ALO3  is  not  very  clear. 

The  changes  m  the  chemical  composition  of  basic  rocks  by  weathering  consist  in  a  uniform 
decrease  in  CaO,  MgO,  NajO,  KjO,  and  SiOj;  AI2O3  remains  nearlj^  constant  and  water  is 
introduced. 

It  follows  from  the  facts  presented  that  the  changes  in  the  chemical  composition  of  the 
Lake  Superior  copper  rocks  effected  by  their  alteration  adjacent  to  the  copper  deposits  are 
fundamentally  different  from  those  which  are  known  to  have  been  caused  by  the  action  of 
weathermg  solutions.  On  the  other  hand,  these  changes  present  similarities  to  the  changes 
effected  by  thermal  solutions. 

4.  Coincident  with  the  changes  in  the  chemical  composition  of  the  Lake  Superior  copper 
rocks,  the  tendency  of  their  mineralogical  alteration  is  not  in  accoixl  with  the  change  produccii 
by  weathering,  but  it  is  in  harmony  with  the  mineralogical  alterations  effected  by  thermal 
solutions. 

The  weathering  of  basic  igneous  rocks  results  in  the  development  of  kaolin,  quartz, 
carbonates,  and  other  sunple  compounds  and  the  decomposition  of  all  minerals  not  in  the  list 
of  secondary  minerals.  The  development  of  kaolin  in  the  absence  of  the  tlevelopnient  of  other 
secondar}^  silicates  is  one  of  the  best-esta])lished  criteria  of  the  decomposition  of  rocks  untler 
the  influence  of  meteoric  solutions. 

The  mineralogical  changes  caused  by  the  alteration  of  the  Lake  Su|ierior  copper  rocks 
adjacent  to  the  deposits  can  be  generalized  as  a  progressive  development  of  cldorite.  prelmite, 
epidote,  quartz,  and  the  alkaline  silicates  in  succession,  witli  more  or  less  overhi]),  in  place  of 
the  origmal  mineral  constituents  of  these  rocks — plagioclase  feldspars,  augite,  some  magnetite, 
olivine,  and  otiier  accessory  minerals. 

The  mineralogical  changes  of  the  basic  igneous  rocks  altered  elsewhere  by  hot  solutions 
consist  in  the  development  of  sericite  and  calcite  from  augite,  hornblende,  epidote,  biotite,  and 
feldspars.  The  ferromagnesian  minerals  alter  also  to  chlorite,  which  in  turn  ciianges  to 
muscovite.  Magnetite  alters  to  siderite,  and  ilmenite  to  rutile.  The  final  product  is  essentially 
sericite,  carbonates,  quartz,  and  sulphides. 


THE  COPPER  ORES.  585 

The  results  of  the  alteration  of  tlie  copper-bearing  rocks  by  the  solutions  which  deposited 
the  copper  and  gangue  minerals  have  been  found  to  contrast  with  the  efTects  of  weathering 
in  a  manner  similar  to  the  results  of  the  alteration  of  certain  basic  igneous  rocks  by  thermal 
solutions  elsewhere.  A  fuller  discussion  of  these  contrasts  will  be  found  in  a  paper  by 
Steidtmann." 

PARAGENESIS   OF   COPPER   AND    GANGUE   MINERALS. 

A  study  of  the  paragenesis  of  copper  and  gangue  materials,  according  to  Pumpelly,'' 
discloses  the  following  order  of  deposition  of  the  minerals:  (1)  Cldorite  and  some  laumontite; 
(2)  laumontite;  (3)  laumontite,  prehnite,  epidote;  (4)  quartz  and  a  green  earth  mineral; 
(5)  calcite;  (6)  copper  and  calcite;  (7)  calcite,  alkaline  minerals,  orthoclase,  analcite,  apoph- 
yllite,  datolite.  The  members  of  tlus  order  overlap  one  another.  Copper  was  largely  deposited 
after  the  development  of  ferrous  iron-bearing  minerals,  chlorite,  and  epidote  and  is  more 
intimately  associated  with  these  iron-bearing  minerals  than  with  non  iron-bearing  minerals, 
except  prehnite. 

The  phenomenon  of  mineral  paragenesis  in  deposits  derived  from  solutions  indicates  that 
the  parent  solutions  have  experienced  certain  definite  physical  and  chemical  changes.  Depo- 
sitional  cj^cles  are  as  certainly  related  to  changes  in  concentration  of  the  solutions,  changes  in 
temperature  or  pressure,  changes  in  chemical  composition,  etc.,  as  cycles  of  sedimentary  depo- 
sition are  related  to  certain  definite  changes  in  physiographic  conditions.  It  is  difficult  to  see 
how  present  pluvial  waters  could  develop  such  a  depositional  cycle. 

Smyth '^  argues  that,  of  the  above-named  series  of  minerals,  the  first  deposits,  principally 
chlorite  with  other  nonalkaline,  hydrous  silicates,  were  developed  by  ordinary  weathering 
immediately  after  the  igneous  rocks  were  extruded  at  the  surface;  that  the  copper  and  later 
associated  minerals  were  introduced  later,  after  the  folding  of  the  Keweenawan  series;  that 
therefore  the  succession  of  minerals  does  not,  as  Pimipelly  ^  supposed,  represent  a  continuous 
march  of  alteration.  The  minerals  of  the  second  period  are  sharph'  separated  from  the  alter- 
ation products  of  the  first  period,  which  they  often  replace,  by  their  richness  of  alkalies  and  by 
the  presence  of  fluorine  and  boron.  Smyth's  ^  argument  is  essentially  that  copper  was  intro- 
duced Ln  solutions  contrasting  with  ordinary  meteoric  solutions  and  of  later  origin.  Still  fur- 
ther, Smyth "  cites  the  occurrence  of  copper  under  impervious  layers  of  greenstone  as  evidence 
of  arrest  of  solutions  coming  from  below. 

CONTRAST   WITH   PRESENT   WORK   OF   METEORIC    SOLUTIONS. 

In  general,  the  kind  of  work  done  by  the  waters  which  de])osited  the  copper  contrasts  with 
that  being  accomplished  to-daj-  by  meteoric  waters.  It  is  true  that  the  minute  quantities  of 
sulphides  in  the  basic  igneous  rocks  are  oxidized  to  the  sulphates  of  copper,  transported,  and 
redeposited  by  coming  into  contact  with  ferrous  solutions  in  the  presence  of  alkaline  carbonates. 
Evidence  of  solution  at  the  surface  is  to  be  seen  at  many  places  in  the  stains  of  carbonates  of 
copper,  and  yet  there  is  no  evidence  that  ground  or  surface  waters  are  at  present  segregating 
copper  deposits  firom  tlie  country  rocks.  Pluvial  solutions  now  active  in  the  copper  ores  are 
not  known  to  carry  copper.  The  concentrated  solutions  of  the  deep  mines,  which  can  not, 
under  any  hypothesis  of  deposition  from  meteoric  solutions,  be  regarded  as  the  normal  meteoric 
solutions  from  which  the  co]i])er  was  derived,  are  known  to  deposit  small  amounts  of  copper 
on  mine  tools,  but  analyses  of  these  waters  show  only  a  very  small  percentage  of  copper.  It 
seems  evident  that  if  pluvial  solutions  are  so  inefficient  in  carrying  copper  from  the  concentrated 
materials  at  the  present  time,  their  inefficiency  in  leaching  the  sparsely  disseminated  primary 

a  Steidtmann,  Edward,  A  graphic  comparison  of  the  alteration  of  rocks  by  weathering  with  their  alteration  by  hot  solutions:  Econ.  Geology, 
vol.  3,  1908,  pp.  381-109. 

6  Pumpelly,  Raphael,  Paragenesis  and  derivation  of  copper  and  its  associates  on  Lalje  Superior:  .\m.  Jour.  Sci.,  3d  ser.,  vol.  2,  1871,  p.  350. 
'  Smyth,  H.  L.,  Theory  of  origin  of  the  copper  ores  of  the  Lake  Superior  district:  Science,  new  ser.,  vol.  3,  1S9G,  p.  251. 
J  Pumpelly,  Raphael,  op.  cit. 
'Op.  cit.,  p.  251. 


586  GEOT.OGY  OF  THE  LAXE  SUPERIOR  REGION. 

copjxT  of  the  igneous  I'ocks  of  the  series  would  be  a  thousandfold  greater.  Waters  away  from 
the  miiu's,  even  wliere  tiu\v  are  running  through  the  basic  igneous  rocks,  are  found  to  be  nearly 
if  not  quite  lacking  in  cojipor. 

SOURCE    OF    THERMAL    SOLUTIONS. 

THREE   HYPOTHESES. 

Three  hy])Otheses  as  to  tlie  source  of  the  thermal  solutions  suggest  themselves — that  they 
were  juvenile  solutions,  a(|ueous  or  gaseous,  given  off  by  tlie  igneous  rocks  on  cooling;  that 
they  were  meteoric  waters  heated  by  contact  with  igneous  rocks;  that  they  were  some  com- 
bination of  the  two.  That  both  juvenile  and  meteoric  sources  contributecl  to  the  thermal 
solutions  would  be  expected  from  the  general  conditions  of  sedimentation  of  the  Keweenawan 
series.  Lava  beds  were  piled  one  above  another  at  comparatively  short  intervals,  separated 
by  the  dejjosition  of  coarse  fragmental  sediments,  probably  developed  subaerialjy.  Simulta- 
neously, or  later,  intrusives  penetrated  the  interbedded  igneous  and  sedimentary  rocks,  both 
parallel  to  and  across  the  bedding.  The  waning  of  igneous  activity  allowed  sediments  to  accu- 
mulate in  thicker  beds  and  finally,  in  the  upper  Keweenawan,  without  interru])tion.  The 
igneous  rocks  may  be  supposed  to  have  carried  with  them  the  usual  complement  of  magmatic 
waters  and  vapors.  They  were  fluid  and  became  amygdaloidal.  Such  solutions  would  be 
speedily  mixed  with  surface  meteoric  waters.  The  rapid  jiiling  up  of  beds  would  imprison  both 
juvenile  and  meteoric  waters  under  conditions  that  would  cause  them  to  lose  their  heat  only 
slowly.  The  maximum  bleaching  and  cementation  of  both  igneous  rocks  and  sediments  and 
the  simultaneous  deposition  of  copper  may  be  supposed  to  have  occurred  at  this  time.  Tilting 
of  the  beds,  with  accompanymg  fractures  and  faults,  began  early  in  Keweenawan  time  and 
continued  throughout  the  period.  The  tilting  may  be  supposed  to  have  slowly  moved  the  con- 
tained solutions,  and  when  erosion  had  beveled  the  beds,  access  for  more  meteoric  waters  was 
given.  At  tliis  time,  when  the  elevations  were  certainly  mountainous  and  the  openings  in  the 
rocks  not  cemented,  as  at  present,  meteoric  solutions  would  have  a  vigorous  and  deep  circu- 
lation. These  general  facts  lay  the  burden  of  proof  heavily  on  anyone  attempting  to  show 
that  the  thermal  solutions  were  juvenile  or  meteoric  alone.  They  seem  to  show  that  the 
meteoric  solutions  were  in  the  greater  abundance.  They  do  not  show  whether  the  distinctive 
work  of  copper  deposition  accomplished  by  these  solutions  was  due  to  the  juvenile  or  the 
meteoric  contributions,  or  both. 

This  leads  us  to  the  question  whether  the  copper  was  contributed  directly  in  hot  juvenile 
solutions  escaping  from  the  igneous  rock,  or  whether  it  was  leached  from  crystalline  wall  rocks 
by  hot  solutions  of  both  juvenile  and  meteoric  nature.  In  the  nature  of  the  case,  quantitative 
evidence  with  which  to  answer  this  question  is  difficult  to  obtain. 

The  view  that  at  least  some  of  the  ore-bearing  solutions  were  magmatic  is  favored  b}' 
the  evidence  cited  on  foregoing  pages  that  the  ores  are  associated  in  place  and  time  with 
igneous  extrusions,  that  the  ore-depositing  solutions  were  hot,  that  they  carried  fluorine  and 
boron,  and  that  the  ores  were  deposited  in  mineral  cycles  showing  rapidly  changing  conditions. 

Of  similar  import  is  the  apparent  scarcity  in  the  crystaUized  wall  rocks  of  copper  wliich 
could  be  leached  and  concentrated  in  sufficient  amounts  to  explain  the  present  deposits.  Cop- 
per has  been  found  most  sparingly  as  a  primary  constituent  in  the  fresh  igneous  rocks.  It 
has  not  been  reported  from  microscopic  examination  of  the  fine-grained  surface  rocks,  and 
in  only  a  few  cases,  in  minute  quantities,  have  sulpliides  of  copper  been  found  in  the  coareer 
igneous  rocks.  No  evidence  has  been  thus  far  adduced  that  such  minutely  ilisscmmated  cop- 
per is  more  abundant  in  the  igneous  rocks  in  the  copper-bearing  areas  than  in  igneous  rocks 
outside  the  copper-bearing  areas.  The  few  copper  determinations  which  have  been  made  in 
the  analyses  of  fresh  igneous  rocks  show  either  no  copper  or  but  little  more  than  a  trace.  On 
the  other  hand,  analyses  are  few,  and  the  final  word  as  to  the  original  copper  eontent  of  the 
fresh  igneous  rocks  can  not  be  saitl  until  more  analyses  are  available. 


THE  COPPER  ORES.  587 

There  is  no  reason  to  believe  from  present  known  facts  that  the  unleached  wall  rocks  are 
any  richer  in  copper  in  the  vicinity  of  productive  lodes  than  they  are  in  other  parts  of  the 
Keweenawan  series  throughout  the  Lake  Superior  region.  In  northern  Minnesota  and  other 
known  nonproductive  areas  there  seems  to  be  fully  as  much  copper  in  the  igneous  rocks  as  in 
those  of  Keweenaw  Point.  If  it  is  assumed  that  the  copper  deposits  have  been  concentrated 
entirely  by  the  action  of  meteoric  waters  on  the  basic  igneous  rocks,  it  is  difficult  to  account 
for  the  absence  of  deposits  tliroughout  mucli  of  the  Keweenawan  and  also  in  certain  porous 
■strata  witliin  the  producing  district.  The  exti-eme  localization  of  the  deposits  in  time  and 
place  seems  to  be  something  more  characteristic  of  highly  concentrated  magmatic  solutions 
than  of  a  universally  acting  agent  like  meteoric  waters  working  down  from  tne  surface. 

But  granting  that  the  fresh  igneous  rocks  contain  minute  quantities  of  copper,  which  may 
have  been  picked  up  and  concentrated  by  meteoric  solutions  later,  is  not  their  pi-csence  in 
these  wall  rocks  evidence  that  during  the  cooling  of  these  lavas  ccmcentrated  copper-bearin" 
solutions  were  present,  some  of  which  may  have  escaped  from  the  parent  rock  during  crystal- 
hzation  ?  The  inherent  probabihty  of  such  an  origin  of  the  solutions  is  increased  by  consid- 
eration of  the  evidence  derived  from  certain  western  copper  deposits,  where  a  fairly  good  case 
has  been  made  out  for  the  direct  contribution  of  copper  salts  in  juvenile  solutions,  as,  for 
instance,  in  the  Clifton  district  of  Arizona  by  Lindgren. 

\one  of  the  evidence  above  citeil  for  the  direct  contribution  of  copper  salts  in  juvenile 
solutions  entirely  excludes  the  hypothesis  that  meteoric  waters,  aided  by  the  heat  of  the 
lavas,  may  have  accomplished  the  result  by  leacliing  of  wall  rocks.  From  the  known  conditions 
of  extrusion  of  the  lavas  and  the  association  of  the  sediments  it  is  practically  certain  that 
meteoric  waters  were  present,  that  they  were  hot,  and  that  therefore  they  were  able  to  accom- 
plish some  alterations.  To  what  extent  they  may  have  concentrated  copper  we  have  no 
apparent  means  of  knowing.  They  were  probably  effective  in  rearranging  the  copper  to  give 
the  present  variation  in  grade  with  depth.  This  change  in  depth  is  the  one  fact  which  seems 
to  be  more  closely  related  to  the  activity  of  meteoric  solutions  than  to  the  deposition  from 
juvenile  solutions. 

On  the  whole  the  evidence  is  taken  to  point  to  a  probable  original  concentration  of  cop- 
per by  hot  solutions  largely  of  juvenile  contribution,  but  more  or  less  mixed,  necessarily,  with 
meteoric  waters  and  a  later  working  over  of  the  deposits  by  waters  dominantly  of  meteoric 
source.  In  any  case  there  is  a  high  degree  of  probabihty  that  the  associated  basic  igneous 
rocks  are  the  source  of  the  copper  deposits.  The  doubt  arises  only  as  to  the  manner  of  their 
derivation  from  these  wall  rocks — whether  they  are  due  to  the  escape  of  solutions  of  a  juvenile 
nature  before  or  during  the  crystalhzation  of  the  lavas,  or  whether  on  the  breaking  up  of  the 
crystallized  rocks  by  katamorphic  alterations  the  minute  portions  of  copper  they  contained 
were  concentrated  in  t!ie  deposits. 

WERE  THE  THERMAL  SOLUTIONS  DERIVED  FROM  EXTRUSIVE  OR  FROM  INTRUSIVE 

ROCKS? 

The  attempt  to  ascertain  the  particular  igneous  rocks  from  which  the  copper  ores  were 
contributed  and  the  conditions  favoring  the  release  of  the  copper  solutions  leads  first  to  a 
scrutiny  of  the  conditions  under  which  the  igneous  rocks  associated  with  the  copper  ores  cooled. 
Most  of  the  igneous  rocks  containing  the  copper  deposits  or  associated  with  them  are  clearly 
surface  flows,  with  typical  surface  textures,  interbedded  with  other  flows  and  with  setliments. 
So  clear  is  tliis  origin  and  so  uniform  the  bedded  succession  that  it  has  been  commonly  assumed 
that  most  of  the  igneous  rocks  associated  with  the  ores  are  flows,  yet  some  undoubted  intrusive 
rocks  are  known  and  some  of  the  bedded  traps  lack  specific  e\ndence  of  extrusive  character 
and  may  possibly  be  sills  or  laccohthic  intrusives,  such  as  are  known  to  be  present  in  other 
parts  of  the  Keweenawan  of  the  Lake  Superior  region.  Certain  irregularities  in  the  strikes, 
dips,  and  thickness  of  the  igneous  beds  may  be  thus  explained. 

Was  the  copper  brought  in  by  the  extrusive  rocks  which  are  interbedded  with  the  sechments, 
or  was  it  subsequently  introduced  by  intrusives*     The  evidence  available  is  not  conclusive. 


588  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

There  are  perhaps  IVwcr  jjarallels  elsew^here  of  the  deposition  of  jactisllic  ores  in  quantity  from 
surface  extrusive  rocks  than  from  intrusives,  though  it  has  been  shown  definitely  that  some 
ores  have  been  derived  from  cxtrusivcs. 

At  the  base  of  certain  barren  conglomerates  occur  copper-bearing  amygdaloidal  pebbles 
that  are  apparently  ich-ntieal  in  character  with  an  underlyint;  amygdaloidnl  flow,  from  which 
they  seem  to  have  been  derived.  In  such  cases  mineralization  has  evidently  taken  place  before 
the  development  of  the  conglomerate,  which  points  to  the  effusive  rock  as  the  source  of  the 
copper-bearing  solutions,  for  the  conglomerates  closely  followed  the  lavas  in  deposition. 

Copper-bearing  amygdaloidal  traps  have  been  found  in  jUaska"  in  which  a  similar  hne 
of  evidence  points  to  the  trap  as  the  direct  source  of  the  copper. 

SIGNIFICANCE   OF   SULPHIDES   OF   COPPER  IN  THE   INTBUSIVES   AND  LOWER  EFFUSIVES. 

The  intrusive  rocks  carrying  sulpliides  are  possibly  the  deep-seated  equivalents  of  the 
lavas  which  carry  metallic  copper.  Wright  so  regards  the  intrusives  of  Mount  Bohemia.  The 
absence  or  subordination  of  native  copper  in  tlie  intrusives  may  be  due  to  the  temperature 
conditions  of  tlie  rocks  when  copper  deposition  took  place.  If  hot  cuj)rous  sulphates  were 
dehvered  from  the  intrusive  rocks,  they  may  have  been  deposited  as  sulphide  in  the  highly 
heated  intrusive,  and  partly  as  native  copper  in  the  equivalent  traps,  where  there  was  more 
rapid  cooling  and  where  a  lower  temperature  prevailed.  (Sec  p.  5S9.)  Another  speculation 
is  that  the  extraordinary  differential  concentration  of  native  copper  in  the  upper  lavas  may 
have  been  due  to  a  process  of  magmatic  differentiation.  The  intimate  relation  of  the  ores  with 
basic  igneous  rocks  anrl  their  general  absence  from  the  felsites  is  further  .suggestive  of  this. 

CONCLUSION    AS   TO    SOURCE    OF   COPPER-BEARING    SOLUTIONS. 

It  is  concluded,  therefore,  that  the  copper-bearing  solutions  were  hot,  that  they  were  both 
juvenile  and  meteoric,  that  the  copper  probably  was  in  part  contributed  directly  in  juvenile 
waters  and  in  part  by  leaching  of  wall  rocks  bj'  the  hot  solutions,  the  evidence  developed  being 
as  yet  insuliicient  to  enable  quantitative  statements  as  to  the  relative  importance  of  the  two. 

It  is  known  that  magmas  expel  on  consolidating  all  constituents  which  can  not  assume 
stable  mineral  form  under  the  existing  chemical  and  physical  conditions.  Water,  copper,  and 
numerous  other  substances  belong  to  this  class.  Such  a  source  for  ore-bearing  solutions  has 
been  repeatedly  appealed  to  in  the  search  for  the  origin  of  western  copper  and  other  ores.  Posi- 
tive evidence  for  such  contribution  is  in  the  nature  of  the  case  extremely  elusive.  As  a  rule 
the  best  that  can  be  done  is  to  present  evidence  ehniinating  the  hypothesis  that  meteoric  waters 
are  accomplishing  the  work,  and  to  show  that  direct  igneous  contribution  is  a  possible  alter- 
native source.  For  the  Lake  Superior  copper  this  explanation  of  source  meets  the  objections 
which  have  been  cited  against  tlic  deposition  of  the  copper  by  meteoric  solutions  and  best 
explains  the  transportation  of  abundant  alumina  silicates,  fluorine,  and  boron,  the  remarkable 
concentration  of  copper  as  compared  with  other  constituents,  the  cycles  of  mineral  deposition, 
the  pecuhar  alterations  of  the  wall  rocks,  the  facts  that  the  period  of  ore  deposition  was  largely 
limited  to  middle  Keweenawan  time  and  that  ore  deposition  at  the  present  time  is  almost  nil, 
and  finally  the  extreme  localization  of  the  copper  lodes,  a  localization  which  seems  to  be  char- 
acteristic of  the  association  of  ores  of  all  kinds  with  igneous  rocks.  The  conclusion  tliat  the 
ore-depositing  solutions  have  been  contributed  hot  by  the  igneous  rocks  does  not  exclude 
the  cooperation  of  hot  and  cold  nu'teoric  waters,  either  in  the  primary  deposition  of  the  ore  or 
in  further  segregation  and  modification  of  it. 

It  is  suggested  later  that  the  present  deep  mine  waters  rejiresent  the  residuum  or  brine  of 
tliese  solutions,  possibly  more  or  less  mixed  with  jiluvial  waters.  We  are  unable  to  follow  Lane  *" 
in  his  conclusion  that  the  waters  represent  fossil  or  connate  waters  either  of  the  Keweenawan 
sea  or  of  the  arid  conditions  under  whicii  tlie  Keweenawan  nuiy  iiave  been  deposited. 


II  Knopf,  Adolph,  The  copper-bearinK  amygdaloids  of  the  White  River  region.  Alaslia:  Econ.  Geolog}-,  vol.3, 1910,  p.  251. 
6  Lane,  A.  C,  The  chemical  evolution  of  the  ocean:  Jour.  Geolos.v,  vol.  14, 1900.  pp.  221-225. 


THE  COPPER  ORES.  589 

CHEMISTRY    OF    DEPOSITION    OF    COPPER    ORES. 

The  uncertainty  of  the  conditions  of  deposition  of  tlie  copper  of  course  requires  tliat  any 
discussion  of  the  chemistry  of  the  deposition  of  these  ores  be  tentative  and  that  a  witle  range 
of  processes  be  taken  into  consideration.  A  hypothesis  of  the  chemical  processes  of  copper 
deposition  may  be  based  on  the  postulates  that  the  hot  solutions  which  deposited  the  copper 
derived  part  of  their  constituents,  notably  boron,  fluorine,  and  perhaps  copper,  directly  from 
the  igneous  rocks  as  magmatic  emanations;  that  they  may  have  partly  derived  the  alkalies, 
alkahne  earths,  alumina,  silica,  and  perhaps  some  copper,  from  the  decomposition  of  the  wall 
rocks,  affected  by  the  thermal  solutions,  and  that  these  solutions  probably  carried  the  copper 
as  the  chloride  and  possibly  as  the  sulphate.  The  sparseness  of  sulphides  in  the  deposits  seems 
to  imply  that  the  primary  solutions  were  either  lean  in  sulphates  or  else  the  conditions  were 
unfavorable  to  the  deposition  of  sulphides.  The  abundance  of  chlorides  in  deep-mine  waters 
of  possibly  residual  origin  suggests  that  the  copper  was  carried  as  chloride. 

The  deposition  of  metallic  copper  from  such  solutions  has  been  accomplished  experi- 
mentally in  these  ways  and  perhaps  others.  First,  Fernekes  succeeded  in  precipitating  metallic 
copper  from  a  cupric  chloride  solution  with  ferrous  chloride  at  a  temperature  of  200°  to  250°  V., 
in  the  presence  of  prelinite  and  other  silicates,  which  neutralized  the  hydrochloric  acid  resulting 
from  the  reaction."  Second,  Stokes  obtained  metallic  copper  by  the  cooling  of  a  hot  solution  of 
cuprous  and  cupric  sulphate;  by  the  action  of  ferrous  sulphate  on  cupric  sulphate  at  200°  C;  and 
by  the  action  of  hornblende  and  siderite  on  cupric  sulphate  at  200°  C*  Third,  Biddle  suc- 
ceeded in  throwing  down  copper  from  a  solution  of  ferrous  and  cupric  chlorides  in  the  presence 
of  an  excess  of  alkaline  carbonate  at  ordinary  temperature.'^  Fourth,  Sullivan''  finds  that 
various  silicates — feldspar,  biotite,  shale,  prehnite,  augite,  amphibole,  etc. — will  throw  out  copper 
from  copper  sulphate  solutions  by  an  act  of  double  decomposition.  The  bases  of  the  silicates 
pass  into  solution  in  very  nearly  the  same  proportion  as  copper  is  taken  out.  The  mineral  form 
of  the  copper  deposited  in  this  manner  is  unknown,  but  the  process  may  bear  some  relation  to 
the  problem  in  hand. 

Lane*  suggests  that  eleotrochemical  action  between  the  copper  solutions  and  the  wall 
rock  may  have  caused  the  precipitation  of  copper.  Pumpelly^  regards  the  intimate  associ- 
ation of  copper  with  protoxide  silicates,  in  which  the  replacement  of  alumina  by  ferric  oxide  is 
especially  favored,  as  indicative  of  a  close  genetic  relation  between  the  ferric  condition  of  the 
iron  oxide  in  the  associated  silicates  and  the  metallic  state  of  the  copper,  and  believes  that  the 
higher  oxidation  of  the  iron  was  effected  through  the  reduction  of  the  oxide  of  copper  at  the 
expense  of  the  oxygen  of  the  latter.  Van  Hise^  believes  that  the  reducing  agents  which  pre- 
cipitated native  copper  were  ferrous  solutions  derived  from  the  iron-bearing  silicates  and  fer- 
rous compounds  in  the  solid  form,  magnetite  and  silicate.  This  view  is  in  accord  with  the 
findings  of  Sullivan. 

The  geologic  relations  of  the  copper  which  are  especially  applicable  to  the  problem  are 
these:  First,  copper  is  intimately  associated  with  and  preceded  by  ferrous  silicate  minerals; 
second,  it  was  deposited  with  calcite,  it  is  known  to  replace  cjuartz,  and  its  deposition  was 
usually  followed  by  the  development  of  alkaline  silicates. 

A  tentative  hypothesis  of  the  chemistry  of  the  deposition  of  the  ore  may  be  built  on  the 
preceding  postulates  as  follows: 

Hot  solutions  containing  copper  chlorides,  boron,  and  fluorine  compounds,  CO2,  and  pos- 
sibly other  magmatic  emanations  entered  the  porous  parts  of  the  formations,  where  they  began 

a  Econ.  Geology,  vol.  2, 1907,  p.  580. 

6  Stokes,  n.  N.,  Experiments  on  the  solution,  transportation,  and  deposition  of  copper,  silver,  and  gold:  Econ.  Geology,  vol.  1.1906,  pp.  644-650. 
c  Biddle,  II.  C,  The  deposition  of  copper  by  solutions  of  ferrous  salts:  Jour.  Geology,  vol.  9,  1901.  pp.  430-436. 
d  Sullivan,  E.  C,  The  interaction  between  minerals  ami  water  solutions:  Bull.  U.S.  Geol.  Sur\'ey  No.  312.  1907.  p.  64. 
«  Ann.  Kept.  Michigan  Geol.  Survey  for  1903, 1905,  p.  249:  Econ.  Geology,  vol.  4, 1909,  p.  170. 

/  Pumpelly,  Raphael,  The  paragenesis  and  derivation  of  copper  and  it«  associates  on  Lake  Superior:  Am.  Jour.  Sci.,  3d  ser.,  vol.  2,  1871,  pp. 
353-3.54. 

B  Van  Hise,  C.  R.,  A  treatise  on  metamorphism:  Men.  U.  S.  Geol.  Survey,  vol.  47,  1904,  p.  1136. 


590  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

the  work  of  deposition  and  tlic  solution  and  replaccnu-nt  of  tlie  wall  rock.  Hot  solution';,  in 
•general,  remove  lime  and  soda  with  great  rapidity.  Pressure  and  CO^  alone  could  eause  tlie  solu- 
tion of  alumina."  In  general,  there  would  lie  a  tendency  for  the  decomposition  of  all  minerals 
in  the  wall  rocks,  and  a  consequent  enrichment  of  the  solutions  in  the  constituents  taken  out  of 
the  wall  rocks.  However,  as  Lane*  suggests,  the  calcium  silicate  and  sodium  silicate  in  the 
solution  would  tend  to  keep  magnesium  out  of  solution.  These  processes  would  tend  to  develop 
chlorite  by  replacement  and  to  keep  magnesia  permanently  out  of  solution.  In  general, chlo- 
rite is  the  most  stable  mineral  form  which  magnesia  assumes  in  the  presence  of  hot  solutions.'^ 
This  first  step  in  the  cycle  of  deposition  thus  accounted  for  left  the  solution  rich  in  lime,  iron, 
and  aluminum  silicates.  Changing  conditions,  perhaps,  of  concentration,  heat,  and  pressure 
brouglit  about  the  saturation  of  these  constituents,  and  a  generation  of  laumontite,  prehnite, 
and  epidote  followed  the  development  of  chlorite.  Silica  became  insoluble  in  this  solution 
after  the  deposition  of  the  lime-aluminum  silicates,  which  resulted  in  the  precipitation  of  c|uartz. 
Tlie  individualization  of  cjuartz  was  followed  by  the  deposition  of  copper.  It  is  suggested  that 
the  solutions  were  relatively  rich  in  alkalies,  probably  the  carbonates  and  chloride  both,  when 
the  period  of  copper  deposition  began,  for  the  deposition  of  copper  accompanied  the  solution 
of  ((uartz  and'  was  followed  by  the  deposition  of  alkaline  silicates.  Under  these  conilitions 
copper-bearing  solutions  reacting  with  the  ferrous  silicates  of  the  wall  rock  and  perhaps  with 
ferrous  salts  in  solution  in  presence  of  alkaline  carbonates  caused  the  precipitation  of  copper. 
It  is  furthermore  suggested  that  the  deposition  of  calcite,  coeval  with  the  deposition  of  copper, 
was  due  to  the  interaction  of  alkaline  carbonate  and  calcium  chloride.  As  calcium  chloride 
is  a  solvent  of  copper,  its  precipitation  as  a  carbonate  would  give  additional  impetus  to  the 
precipitation  of  copper. 

It  is  quite  possible  that  the  progress  of  the  cycle  of  deposition  of  the  copper  outlined  on 
page  585  was  accompanied  by  a  gradual  loss  of  heat  and  pressure  and  that  tlus  loss  was 
greatest  where  the  solutions  were  nearest  the  surface.  According  to  Soret's  principle  "*  when 
two  parts  of  a  solution  have  different  temperatures,  there  will  be  a  concentration  of  the  dissolved 
parts  in  the  cooler  portion.  This  concentration  tends  to  bring  about  deposition  of  some  of  the 
dissolved  parts.  This  is  shown  also  by  the  work  of  Stokes  «  on  the  interaction  of  cuprous  and 
cupric  sulphates.  Consequently,  deposition  of  copper  may  have  begun  in  the  upper  zones 
of  the  solution  and  gradually  extended  downward.  Diffusion  and  convection  currents  would 
tend  to  keep  the  composition  of  the  solutions  uniform.  However,  when  the  deposition  of 
copper  began  at  the  lower  horizons  the  richness  of  the  solutions  was  diminished.  It  is  possible 
that  such  a  process  caused  the  gradual  diminution  of  the  richness  of  the  ore  deposits  with 
increase  in  depth. 

Tliroughout  this  process  there  was  a  continual  concentration  of  the  alkalies  in  the  solutions. 
After  the  deposition  of  copper  these  alkahes  became  partly  insoluble  in  the  solution  under  the 
existing  physical  and  chemical  conditions  and  were  tlirown  out  as  analcite,  orthoclase,  lluorine- 
bearing  apophyllitc,  and  other  alkaline  zeoUtes.  The  boron  silicate,  datolite,  was  thrown  out  at 
the  same  time.  It  is  also  possible  that  the  major  precipitation  of  the  datolite  ami  ap()i)hylUte 
took  place  in  the  upper  zones,  as  appears  to  have  been  the  case  with  copper,  for  Lane  and 
other  observers  are  incUned  to  believe  that  certain  alkaline  silicates  are  more  abundant  in  the 
upper  levels  of  the  mines. 

This  closes  the  cycle  of  deposition  of  the  copper  and  gangue  minerals.  The  general  results 
of  the  process  suggest  that  tiie  present  deep  mine  waters  represent  the  more  or  less  modified 
residuum  or  brine  of  the  solutions  that  accomplished  the  deposition  of  the  copper. 

oGawalowski,  A.,  Chem.  Centraltil.,  pt.  1,  1900.  p.  640. 
6Lanc,  .\.  C.  Eeon.  Geology,  vol.  4.  1109.  p.  l(l(i. 

c  Steidtmann,  Edward,  A  graphic  comparison  of  the  alleralions  of  rocks  liy  weathering  with  their  alterations  by  hot  solutions:  Econ.  Geology, 
vol.  3,  1908,  p.  398. 

d  Sorct,  Charles,  .Vnnales  chim.  phys.,  ."ith  ser.,  vol.  22,  1881,  p.  293. 
'  Stokes,  U.  N.,  Econ.  Geology,  vol.  2,  1907,  p.  580. 


THE  COPPER  ORES.  591 

CAUSE    OF    DIMINUTION    OF    RICHNESS    AVITH    INCREASING    DEPTH. 

There  is  little  in  the  Lake  Superior  copper  deposits  in  the  nature  of  the  oxide  or  weathered 
zone  so  characteristic,  of  sulphide  deposits.  Possibly'  glaciation  has  removed  marked  evidences 
of  surficial  change.  In  a  few  places  the  upper  few  hundred  feet  of  the  lodes  is  less  rich  than  the 
parts  below,  suggesting  a  leaching  from  the  upper  part  of  the  lode.  Below  this  the  ores  very 
gradually  and  uniformly  diminish  in  richness  with  increase  in  depth,  a  diminution  which  is 
caused  mainly  be  slight  changes  in  proportions  of  minerals  rather  than  by  differences  in  kinds 
of  minerals  in  the  ores. 

The  relation  of  the  richer  portions  of  the  lotle  to  the  erosion  surface  suggests  at  once  a 
do^\^lward  concentration  by  meteoric  waters  such  as  has  been  demonstrated  to  explain  this 
relation  in  so  many  mimng  districts.  But  this  explanation  presents  many  difficulties.  The 
present  underground  waters  near  the  surface  do  not  carry  copper  in  abundance,  nor  can  we 
suggest  any  probable  chemical  reaction  which  would  explain  the  solution  of  metallic  copper. 
The  ore  diminishes  in  value  far  below  the  depth  of  the  present  meteoric  circulation.  There 
is  no  sharply  discriminated  oxide  zone.  The  kinds  of  minerals  are  essentially  the  same  from  the 
top  down,  and  the  changes  in  proportions  and  values  are  much  more  gradual  than  the  changes 
ordinarily  ascribed  to  secondary  concentration  from  the  surface.  The  diminution  of  value 
with  increase  in  depth  has  been  demonstrated  so  generally  to  be  the  result  of  concentration 
from  dowmward-moving  meteoric  waters  that  one  hesitates  to  offer  any  other  explanation 
except  on  most  decisive  proof.  Such  decisive  proof  is  here  lacking.  But  nevertheless  another 
hypothesis  seems  to  us  reasonable — that  the  richness  of  the  ore  near  the  surface  was  due  to  a 
precipitation  of  copper  from  primary  solutions  near  the  surface,  where  they  were  cooled  under 
less  pressure  and  became  mingled  with  oxidizing  waters.  It  would  be  necessary  to  assuiiie 
that  convection  and  diffusion  would  tend  to  equalize  the  concentration  of  the  copper  solutions, 
thus  causing  some  migration  of  copper  salts  toward  the  zone  of  precipitation  and  thus  diminishing 
the  amounts  of  salts  precipitated  from  solutions  lower  down.  The  oxidation  of  cuprous  salts 
in  solution,  effected  by  the  mingling  of  meteoric  waters,  would  develop  cupric  salts  wliich  in 
moving  down  and  by  reacting  with  the  cupric  salts  would  deposit  metallic  copper,  as  noted 
on  page  5S9. 

Tliis  hypothesis  avoids  the  difficulty  of  getting  the  copper  into  solution  from  the  metallic 
form,  which  would  have  to  be  assumed  on  the  hypothesis  of  a  downward  concentration  by 
meteoric  waters. 

In  this  hypothesis  of  deposition  of  richer  copper  ores  by  primary  solutions  near  the  surface, 
there  is  no  emphasis  on  the  direction  of  movement  of  the  solutions.  It  is  conceivable  that  they 
may  have  been  upward-movuig  waters,  that  at  the  time  of  deposition  the  waters  may  have  been 
moving  little  or  none  at  all,  or  possibly  that  the  waters  had  begun  to  take  a  downward  movement 
as  a  result  of  the  cooling  and  contraction  of  the  lavas.  Lane  "  has  estimated  such  downward 
movement  as  amounting  to  possibly  a  mile  do\via  the  dip  in  the  vicinity  of  the  present  erosion 
surface.  So  far  as  the  currents  were  downward  moving,  there  may  have  been  upward  artesian 
flow  through  fissures  in  impervious  beds  overlying  pervious  betls.  Wadsworth ''  cites  as 
evidence  of  dowiiward-moving  waters  the  occurrence  of  spikes  of  copper  and  calcite  which 
extend  from  one  bed  down  into  others,  with  the  small  end  downward,  like  an  icicle. 

RELATION  OF  COPPER  ORES  TO  OTHER  ORES  OF  THE  KEWEENAWAN. 

It. is  an  interesting  and  significant  fact  that  rocks  of  probable  Keweenawan  age  are  closely 
associated  with  a  considerable  variety  of  ores  on  the  north  and  east  sides  of  Lake  Superior.  On 
Silver  Islet,  on  the  northwest  side  of  the  lake,  and  thence  westward  on  the  main  shore  are  igneous 
dikes  of  probable  Keweenawan  age  cutting  the  slates  of  the  Animikie  group  and  carrying  native 
silver  and  other  minerals.  (See  pp.  593-594.)  On  the  north  shore  of  Lake  Huron  basic  igneous 
bosses  and  dikes  of  probable  Keweenawan  age  are  associated  with  quartz  veins  carrying  chal- 

oLane,  A.  C,  Econ.  Geology,  vol.  4, 1909,  p.  164. 

6  Wadsworth,  M.  E.,  The  origin  and  mode  of  occurrence  of  the  Lake  Superior  copper  deposits;  Trans.  Am.  Inst.  Min.  Eug. .  vol.  27,  1898,  p.  695. 


592  GEOLOGY  OF  THE  LAKE  SUPERIOK  REGION. 

copyrite,  which  is  tlie  source  of  the  copper  ores  of  the  Bruce  mines  and  many  small  jirospects 
in  this  district.  In  the  Sudbury  (Ustrict,  to  the  northeast,  basic  i<;;neous  rocks  of  probable 
Kewcenawan  age  are  closely  associated  with  the  nickel  deposits;  and  still  farther  to  the  northeast 
basic  igneous  rocks,  probably  of  Kcweenawan  age,  are  associated  with  cobalt-silver  deposits. 
The  main  structural  lines  in  all  these  districts  trend  north  of  east  and  south  of  west,  correspond- 
ino-  to  the  axial  line  of  tlie  Jjake  Superior  syncline.  All  these  districts  have  certain  ore-bearing 
minerals  in  common.  The  difference  is  primarily  a  difFerence  in  proportion.  For  instance,  the 
Lake  Superior  copper  deposits  are  associated  with  metallic  silver  and  a  minute  amount  of  cobalt 
and  nickel.  The  silver  (lei)osits  of  Silver  Islet  carry  small  amounts  of  copper,  cobalt,  and  nickel. 
The  Sudbury  nickel  deposits  carry  considerable  copper  and  a  small  amount  of  co})alt  and  native 
silver.  In  the  Cobalt  district  the  native  silver  and  cobalt  ores  carry  considerable  amounts  of 
nickel  and  copper.  In  the  discussion  of  general  geology  (Chapter  XX)  it  will  be  shown  that  this 
general  region  was  probably  a  geosynchne  of  deposition  during  pre-Cambrian  time,  affected 
by  repeated  foldings  along  axes  parallel  to  the  shore,  and  a  locus  for  igneous  activity.  The 
distribution  and  character  of  the  ores  through  this  general  zone  further  suggest  the  generaliza- 
tion that  here  is  a  metallographic  province  along  which  igneous  rocks  have  brought  u])  cpiite 
different  but  still  related  ores,  these  ores  taking  a  considerable  variety  of  structural,  mineralog- 
ical,  and  chemical  characteristics,  partly  because  of  original  differences  in  the  composition  of 
the  ore-bearing  sohitions  in  these  different  districts  and  partly  because  of  the  different  con- 
ditions under  which  they  approached  the  surf  ace,  those  in  Canada  remaining  as  intrusive  beneath 
the  surface  and  those  at  Keweenaw  Point  coming  largely  to  the  surface. 

Still  further,  in  pre-KeM^eenawan  time  this  same  general  region  was  a  shore  line  of  deposition 
with  repeated  outbursts  of  volcanism.  The  attempt  has  been  made  to  connect  the  iron-ore 
de})osits  of  tlie  Lake  Superior  region  with  this  volcanism.  Thus  along  this  great  geosyncline 
earlier  volcanism  was  associated  with  extrusion  of  iron  salts  and  later  volcanism  with  a  variety 
of  cobalt  and  silver,  copper,  and  nickel  salts. 


CHAPTER  XIX.   THE  SILVER  AND  GOLD  ORES  OF  THE  LAKE 

SUPERIOR  REGION. 

SILVER  ORES. 

PRODUCTION. 

Mention  has  already  been  made  of  the  mming  of  silver  with  the  Keweenaw  Point  copper 
ores.     The  total  value  of  silver  thus  mined  from  1887  to  the  end  of  1909  is  $1,805,308.50. 

In  addition  to  this,  vems  in  the  slates  of  the  Animikic  group  on  the  northwest  side  of 
Lake  Superior  have  yielded  silver  ores,  principally  from  Silver  Islet,  as  follows: 

Silver  produced  from  veins  on  northwest  side  of  Liil-e  Superior.'^ 

Produced  by  Silver  Islet,  from  commencement  to  close  of  mining ^3,  250,  000 

Produced  by  the  mainland  group  to  1903,  including  the  Shuniah,  Rabbit  Mountain,  and 
Silver  Mountain  groups  of  mines 1,885,  681 


5, 135,  681 
SILVER    ISLET. 

The  following  account  of  Silver  Islet  is  largely  quoted  from  Ingall.*  »Silver  Islet  is  a  small 
island  of  nearly  flat-lying  Animikie  slates  about  a  mile  out  in  Lake  Superior  off  Thunder  Cape. 

The  silver-bearing  vein  cuts  the  Animikie  slates  and  a  diorite  dike,  but  its  principal  value 
is  found  within  the  diorite  dike.  This  dike  dips  from  60°  to  75°  SE.  The  dikes  in  the  AnimUvie 
of  this  part  of  the  Lake  Superior  region  are  connected  with  the  Logan  sills,  of  Keweenawan  age. 
The  vein  strikes  N.  35°  W.  and  tlips  70°  to  80°  SE.;  its  thiclvness  averages  about  8  to  10  feet, 
but  in  some  places  it  has  shown  from  20  to  30  feet  of  solid  vein  stuff.  Two  bonanzas  were 
found  in  the  vein;  the  first,  yielding  over  -12,000,000,  was  shaped  like  an  irregular  pear  with 
its  large  end  down;  the  second  bonanza,  found  considerably  later,  was  shaped  like  an  inverted 
cone.  The  gangue  of  the  vein  consists  of  calcite,  quartz,  and  dolomite,  the  dolomite  varying 
in  color  from  cream  to  pmk  according  to  the  varymg  amounts  of  manganese  it  carries.  The 
relative  quantity  of  calcareous  and  siliceous  matter  varies,  however,  in  different  parts  of  the 
vein,  and  in  places  streaks  of  cjuartz  have  preponderated  to  such  an  extent  as  to  make  some  of 
the  ore  highly  siliceous.  The  metallic  minerals  are  native  silver,  argentite,  galena,  blende, 
copper,  and  iron  pyrites,  with  marcasite.  Macfarlane  also  mentions  tetrahedrite,  domeykite, 
niccolite,  and  cobalt  bloom,  the  two  latter  probably  o.xidation  products  of  a  peculiar  mineral 
called  macfarlanite,  containing  arsenic,  cobalt,  nickel,  and  silver.  Two  new  minerals  are  also 
said  to  have  been  found  in  the  ore  by  Wurtz,  called  by  him  huntelite  and  animikite.  The 
three  minerals  last  named,  according  to  Lowe,  "are  now  [October,  1881]  the  principal  produc- 
ing silver  ores  of  the  mine."  Besides  the  above,  Courtis  found  in  the  ore  shipped  to  the  Wyan- 
dotte smelting  works  rhodochrosite,  annabergite,  antimonial  silver,  and  cerargyrite,  the  last 
"where  the  rock  has  been  decomposed."  The  native  silver  is  generally  disseminated  through 
the  ore  in  more  or  less  dendritic  masses,  the  points  of  native  silver  forming  nuclei  for  the  deposit 
of  niccolite  and  sulphurets.  Graphite  also  occurs  in  considerable  quantity  and  seems  to  be 
connected  in  some  way  with  the  occurrence  of  the  silver.  Silver  does  not  occur  without 
graphite,  but  graphite  may  be  present  without  silver.     Out  of  the  whole  series  of  twenty-one 

n  Eighteenth  Ann.  Rept.  Ontario  Bur.  Mines,  pt.  1,  1909,  p.  12. 

b  Ingall,  E.  D.,  Report  on  mines  and  mining  on  Lalte  Superior:  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  of  Canada  for  18S7-S8,  vol.  3,  new 
ser.,  pt.  2,  1888,  pp.  27H-40H. 

47517°— VOL  52—11 38  593 


594  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

dikes  cut  by  the  vein  the  Silver  Islet,  carrying  the  ore,  is  the  only  one  impregnated  strongly' 
with  grapliite  and  ])yrito. 

A  curious  feature  of  the  vein  is  the  combustible  gas  which  has  been  encountered  in  large 
quantities  in  the  workings.  This  gas  is  accompanied  by  water  containing  calcium  chloride  in 
solution.  The  gas  and  water  are  confined  ]5rinci[)ally  in  large  vugs  oi'  cavities  in  tlie  vein, 
under  great  pressure  in  the  deepest  workings.  Above  tiie  eighth  level  all  water  infiltrating 
into  the  mine  is  pure  lake  water.     An  analysis  of  the  water  is  as  follows: 

Analysis  of  ndlcr  from  Silver  Islet  minefi 

Chloride  of  potassium 0.  4582 

Chloride  of  sodium 16.  8098 

Chloride  of  calcium 17.  0807 

Chloride  of  magnesium 1.  2937 

Sulphate  of  lime 0672 

Carbonate  of  lime 2936 

Silica 0540 

GENERAL    ACCOUNT    OF    SILVER    IN    TIIE    ANIMIKIE    GROUP. 

Passing  over  Ingall's  detailed  description  of  mines  and  prospects,  his  summary  of  the 
occurrence  of  silver  in  the  Animikie  group  northwest  of  Lake  Superior  is  partty  as  follows : 

The  veins,  as  regards  their  strike  directions,  resolve  themselves  into  three  series — a  north- 
west series,  a  northeast  series,  and  an  east-west  series.  The  northwest  direction  of  strike 
characterizes  the  Coast  group  of  mines,  of  which  the  famous  Silver  Islet  vein  is  the  most 
striking  example.     The  vein  of  the  Beaver  mine  also  has  this  trend. 

All  the  veins  of  the  Rabbit  Mountain  group  of  mines,  Math  the  exception  of  the  Beaver, 
may  be  classed  as  northeast;  the  vein  in  the  Thunder  Bay  mine  also  belongs  to  tlus  series. 

The  veins  of  the  third  series  do  not  run  in  general  due  east  and  west,  but  a  httle  north  of 
east  and  south  of  west.  To  tliis  series  belong  nearly  all  the  chief  veins  of  the  Port  Arthur 
mines,  with  the  exception  of  the  Thunder  Bay,  just  mentioned,  and  nearly  all  the  Silver  Mountain 
group  of  mines. 

The  vein-fUhng  minerals  consist  in  general  of  quartz,  barite,  calcite,  and  fluorite  consti- 
tuting the  basis  of  the  gangue,  in  which  occur  the  different  metallic  minerals: — blende,  galena, 
pyrites  of  several  species,  and  here  and  there  some  sulphurets  of  copper;  the  silver  in  the  orey 
parts  occurs  as  argentite  and  in  the  native  state,  the  former  being  the  more  common.  At  some 
places  the  veins  carry  a  dark-green,  probably  chloritic  material  which  on  some  surfaces  has  a 
bright,  waxy  luster.  Locally  a  soft,  greasy  talcose  material,  probably  saponite,  accompanies 
the  ore,  notably  at  the  Beaver  nunc  and  to  a  lesser  extent  at  one  or  two  other  places.  Carbon 
in  various  forms  has  also  been  found  here  and  there.  In  some  of  the  vugs  in  the  veins,  which 
have  been  found  near  the  surface,  stiff  clay  and  ocherous  material  have  sometimes  been  obtained, 
along  with  nuggets  of  argentite,  the  former,  however,  having  evidently  been  washed  in  from 
the  surface  and  having  thus  embedded  the  silver  minerals  already  existing  in  the  vugs. 

The  Silver  Islet  vein  was  somewhat  exce{)tional  in  carrying,  besitles  the  minerals  above 
noted,  various  arsenical  and  antimonial  ores  of  silver,  with  compounds  of  nickel  and  cobalt 
and  other  metallic  minerals  which  have,  so  far,  not  been  found  in  the  rest  of  the  veins.  Other 
salient  features  of  this  vein  were  the  pink  and  cream-colored  dolomite  spar  which  formed  a 
characteristic  and  jirominent  constituent  of  much  of  the  gangue  of  the  rich  ore  and  the  pre- 
ilominance  of  native  silver  in  the  rich  parts,  whereas  in  the  rest  of  the  veins,  though  native 
silver  occurs  in  considerable  quantity  at  some  places,  yet  argentite  seems  to  be  the  form  hi 
which  it  is  generally  fount!. 

It  is  interesting  to  note  that  both  the  mineral  waters  and  the  mflammable  gixs  that  were 
found  in  opening  the  Silver  Islet  mine  have  also  been  encountered  at  other  points  in  the  dis- 
trict. Inflammable  gas  comes  up  at  several  points  in  and  around  Thunder  Bay,  causing  con- 
siderable ebullition  in  the  water  and  keeping  it  open  all  winter.     At  one  of  these  points  has  been 

a  Ingall,  E.  D.,  op.  cit.,  p.  29H. 


THE  SILVER  AND  GOLD  ORES.  595 

placed  a  small  tank  connected  with  an  inverted  funnel  anchored  on  the  bottom,  and  it  affords 
sufficient  gas  to  keep  a  good-sized  light  burning.  At  the  Rabbit  Mountain  mine,  in  one  of  tlie 
lower  levels,  water  running  over  the  breast  of  the  drift  gave  off  a  faint  odor  of  sulphureted 
hydrogen  and  was  depositing  a  white  flocculent  material,  and  both  here  and  at  the  Beaver 
mine  it  was  reported  that  small  quantities  of  inflammable  gas  had  been  struck. 

ORIGIN    OF    SILVER    ORES    IN    THE    ANIMIKIE    GROUP. 

The  origin  of  the  silver  ores  in  the  Animikie  group  has  not  been  studied  by  the  writers. 
The  ores  have  been  regardetl  by  some  observers  as  brouglit  up  by  thermal  waters  accompanying 
the  trap  intrusions.  All  the  ore  bodies  found  so  far  occur  near  or  within  a  moderate  distance 
of  trap  in  some  form,  either  in  dikes,  as  in  the  Coast  group  of  mines,  or  in  sheets,  as  in  the  other 
groups.  Many  similarities  to  the  Sudbury  and  Cobalt  ores  further  suggest  this  origin.  Ingall 
argues,"  on  the  other  hand,  that  as  the  fissures  intersect  and  dislocate  the  trap  sheets  and  dikes 
equally  with  the  other  rocks  the  traps  must  have  been  formed  and  solidified  long  before  the 
fissures.  He  suggests  that  the  silver  may  be  derived  from  the  traps  through  decomposition  of 
some  of  their  mineral  constituents  carrying  minute  quantities  of  silver  by  waters  infiltrating 
do^\'nward  through  all  their  joints  and  pores,  and  that  these  waters,  passing  onward  and  soaking 
into  the  permeable  parts  and  minerals  in  the  gangue  of  the  veins,  have  there  deposited  their 
silver  contents,  the  various  forms  of  carbon  present  in  the  sedimentary  rocks  having  had  some 
influence  in  effecting  this  preci])itation.  The  presence  of  the  soft  talcose  and  the  various 
chloritic  materials  in  tliis  connection  he  regards  as  favorable  to  this  assumption. 

GOLD  ORES. 

Low-grade,  free-milling  gold-quartz  ores  have  a  widespread  distribution  in  the  Lake 
Superior  region.  The  best  Ivnown  of  them  are  the  Rainy  Lake  deposits,  the  Ropes  gold  mine 
in  the  Marquette  district  of  Michigan,  and  many  gold  prospects  on  the  northeast  shore  of  Lake 
Superior,  includmg  the  Grace  mine  near  Michipicoten.  The  gold-bearing  quartz  veins  of 
Ontario  are  principally  in  the  Laurentian  and  to  a  less  extent  in  the  Keewatin  series.  Cole- 
man'' classifies  them  as  follows  (his  "Huronian"  includes  Keewatin): 

1.  True  fissure  veins. 

a.  In  granite  and  gneiss. 

b.  In  Huronian  ma.ssive  or  schistose  rocks. 

2.  Bedded,  lenticular,  or  segregated  veins. 

a.  In  gneissoid  rocks. 
6.  In  Huronian  schists. 

3.  Contact  deposits  between  granite  or  gneiss  and  Huronian  rocks. 

4.  Fahlbands  in  Huronian  schists. 

5.  Dikes  of  porphyry  or  felsite  with  associated  quartz,  mainly  in  Huronian  rocks. 

6.  Eruptive  masses. 

7.  Placer  deposits  of  Pleistocene  age. 

The  Rainy  Lake  and  Michipicoten  gokl  ores  are  mainly  in  rocks  of  the  Laurentian  series. 
Though  containing  rich  shoots,  the  ores  are  as  a  whole  of  low  grade,  yielding  less  than  $12  a 
ton,  and  theii-  mass  is  not  large  enough  for  profitable  mming  of  ores  of  this  grade.  A  large 
number  of  mines  and  plants  have  been  equipped  at  much  expense  for  the  mining  and  extraction 
of  these  ores,  but  thus  far  none  have  apparently  been  put  on  a  reasonably  profitable  basis. 
Mining  began  in  1S91  and  reached  its  maximum  in  1899,  since  which  it  has  waned  and  is  now 
almost  •  abandoned. 

Gold  mining  on  the  northeast  side  of  Lake  Superior  is  yet  in  the  exploratory  state,  no  con 
siderable  shipments  having  been  made. 


a  Op.  Cit.,  p.  H3H. 

IiColeman,  A.  P.,  Third  report  on  the  west  Ontario  gold  region:  SLxth  Rept.  Ontario  Bur.  Mines,  for  1890, 1897,  p.  115. 


596  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

The  total  production  of  gold  in  Ontario  since  1891  has  been  $2,281,292." 
Tvikc  the  gold  oi'PS  of  the  north  shore  of  Lake  Superior,  the  ores  at  the  Ropes  mine  consist 
of  inctaUic  ^old  in  quaitz  veins  in  peridotitc  in  the  Laurentian  rocks.  Their  grade  is  low  and 
it  is  doubtful  whether  there  has  been  a  profit  on  the  ore  taken  out.  Mining  was  conducted 
intermittently  at  the  Ropes  mine  from  1882  to  1897.  During  this  period  of  activity  the  mine 
produced  gold  (with  some  silver)  to  the  value  of  about  •SO.'JO.OOO.  Sinc'e  that  time  some  gold 
has  been  taken  out  of  the  tailings. 

A  small  amount  of  gold  has  been  taken  out  of  similar  quartz  veins  in  the  pcridotitcs  at  the 
Michigan  mine,  about  .3  miles  west  of  the  Ropes. 

a  Report  on  the  mining  and  metallurgical  industries  of  Canada,  1907-8,  Dept.  of  Mines,  1908,  p.  307. 


CHAPTER  XX.   GENERAL  GEOLOGY. 
INTRODUCTION. 

In  the  early  chapters  of  this  volume  the  general  geography  and  physiography  of  the  Lake 
Superior  region  have  been  treated,  and  a  liistory  of  the  development  of  knowledge  concerning 
the  region,  as  well  as  the  views  of  various  authors,  has  been  given.  In  the  chapters  on  tlie 
individual  distiicts  have  been  considered  the  geologic  succession,  topography,  deformation,  and 
the  lithology,  metamorphism,  relations,  and  thickness  of  each  of  the  formations.  Also  the 
formations  have  been  classified  by  groups  and  series,  and  the  relations  of  these  groups  and 
series  have  been  discussed.  Finally,  a  resume  of  the  geologic  history  of  each  district  has  been 
given.  In  short,  each  chapter  treating  of  a  district  is  substantially  independent,  giving  briefly 
a  complete  discussion  of  the  geology. 

It  therefore  remains  for  this  closmg  chapter  to  consider  the  broader  features  of  the  Lake 
Superior  geology,  and  especially  the  comparative  features.  The  fundamental  thought  of  this 
chapter  will  be  the  comparison  of  the  tlifferent  districts  with  one  another  from  several  points 
of  view.  This  comparison  will  be  essentially  confined  to  the  principal  ore-producing  districts 
which  have  been  studied  in  detail  by  the  United  States  Geological  Survey.  Several  outlying 
areas,  including  north-central  Wisconsin  and  the  Baraboo  and  Minnesota  River  districts,  are 
so  isolated  that  attempts  at  correlation  are  largely  speculative  in  the  present  state  of  knowledge. 
These  districts,  therefore,  will  be  referred  to  only  incidentally  in  the  following  discussion,  and 
the  reader  is  referred  to  Chapters  IX  and  XIV  for  the  available  information  concerning  them. 

In  Bulletm  360  of  the  United  States  Geological  Survey  the  reasons  are  fully  given  which 
lead  to  the  major  division. of  the  pre-Cambrian  rocks  mto  Archean  and  Algonkian.  The  dis- 
cussion leading  to  this  conclusion  will  not  be  here  repeated.  Those  who  are  interested  in  it 
may  refer  to  that  volume. °  The  general  succession  of  series  of  the  Lake  Superior  region  pro- 
posed by  the  United  States  geologists  has  been  agreed  to  by  an  international  committee  of 
geologists.  (See  p.  84.)  This  succession  has  been  established  in  the  Lake  Superior  region 
since  1904.  It  has  now  been  found  to  apply  to  parts  of  Canada,  and  has  been  recently  applied 
by  Adams  ^  to  the  entire  Canadian  shield.  But  Canadian  geologists  have  not  grouped  the 
series  into  the  major  systems  of  Archean  and  Algonkian,  as  has  been  done  for  the  Lake  Superior 
region. 

It  remains  to  distribute  the  formations  that  occur  m  each  of  the  districts  of  the  Lake 
Superior  region  between  the  broader  divisions  of  Archean  and  Algonkian,  and  to  correlate  the 
series  and,  so  far  as  possible,  the  formations  which  occur  in  one  district  with  those  found  in 
another.  Our  classification  and  correlation  of  the  Lake  Superior  pre-Cambrian  rocks  are  given 
in  the  accompanying  table. 

PRINCIPLES  OF  CORRELATION. 

The  lowest  rocks  found  in  the  region  are  those  of  the  Archean  system  or  basement  com- 
plex, consisting  of  the  Keewatin  and  Laurentian  series,  with  their  characteristic  features  and 
relations.  This  system  gives  us  a  horizon  from  which  to  work  upward.  At  the  top  of  the  pre- 
Cambrian  is  the  Keweenawan  series,  which  occurs  mainly  in  a  great  continuous  area,  and  which 
gives  us  a  horizon  from  wliich  to  work  downward.     Between  the  Archean  and  the  Keweenawan 

a  Van  Hise,  C.  R.,  and  Leith,  C.  K.,  The  pre-Cambrian  geology  of  North  America:  Bull.  U.  S.  Geol.  Survey  No.  360,  1909,  pp.  19-25. 

i>  Adams,  F.  D.,  Jour.  Geology,  vol.  17,  190!),  pp.  1-18. 

597 


598  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

is  the  Iluronian  series.  The  Kewecnawan  and  Huronian  series  together  make  up  the  Algon- 
kian  system.  In  certam  districts  the  Iluronian  is  separable  into  three  divisions,  marked  by 
uucotd'ormities;  in  other  districts  it  is  separable  into  two  divisions,  and  in  still  other  districts 
it  is  not  yet  divisible.  The  most  serious  questions  therefore  arise  in  the  correlation  of  the 
Huronian  formations  of  the  several  districts. 

In  correlating  the  Huronian  rocks  the  following  principles  are  used: 

1.  Relations  to  series  or  groups  of  known  age;  that  is,  to  recognizetl  horizons.  In  using 
this  criterion  the  relations  of  the  Huronian  to  the  Arcliean  and  to  the  Keweenawan  are  especially 
helpful,  for  these  rocks  are  readity  recognizable  and  afford  datum  planes  from  which  to  work 
up  and  down.  The  upper  Huronian  (Animikie  group)  adjacent  to  Lake  Superior,  being  con- 
tinuous through  so  much  of  tlus  region,  is  also  very  helpful  as  a  recognizable  datum  plane. 

2.  Unconformities.  The  unconformities  between  the  divisions  of  the  Iluronian  are  of 
great  assistance  in  correlation.  Wliere  all  of  the  Huronian  is  pi'esent,  separated  by  two  uncon- 
formities, there  is  naturally  no  difficulty  in  separating  it  into  lower,  middle,  and  upper.  T\niere, 
however,  the  Huronian  has  only  one  unconformity  or  where  in  a  disconnected  district  only  one 
division  of  the  Huronian  is  present,  the  unconformities  fail  to  be  a  determinmg  factor. 

3.  Lithologic  likeness  of  the  formations.  Tliis  criterion  is  of  assistance,  but  it  clearly  has 
severe  Imiitations,  because  again  and  again  the  geologic  conditions  have  been  the  same,  pro- 
ducing like  formations  at  different  times.  This  is  illustrated  by  the  remarkable  similarity  of  the 
iron-bearing  formations  of  the  upper  Huronian,  middle  Huronian,  and  Archean.  The  natural 
behef  that  they  were  of  the  same  age  long  acted  as  a  bar  to  progress. 

4.  Like  sequence  of  formations.  Similar  sets  of  formations  in  the  same  order  are  of  much 
greater  unportance  as  a  criterion  for  correlation  than  the  likeness  of  single  foimations.  But 
conditions  producmg  similar  sets  of  formations  have  frequently  recurred  tluring  geologic  time. 
For  instance,  when  a  sea  transgresses  over  a  land  area,  there  are  normally  formed  in  ortler  a 
psephite,  a  psammite,  a  pelite,  and  a  nonclastic  formation. 

5.  Subaerial  or  subaqueous  origm.  Closely  connected  with  the  third  and  fourth  criteria  is 
the  question  whether  the  deposits  were  formed  under  air  or  under  water.  It  is  clear  that  the 
conditions  of  the  formation  of  these  two  classes  of  deposits  are  so  different,  and  therefore 
the  nature  of  the  formations  wliicli  maybe  contemporaneous  so  variable,  that  there  is  difficulty 
m  correlating  the  two.  Also  it  is  plain  that  the  difficulties  in  correlating  disconnected  conti- 
nental deposits  are  scarcely  less  great.  On  the  other  hand,  the  correlatmg  of  subaqueous 
deposits  with  one  another  is  relatively  easy. 

6.  Relations  with  intrusive  rocks.  The  older  the  series  the  more  intricately  it  is  likely  to 
be  cut  by  intrusive  rocks,  and  this  relation  is  of  assistance  in  connection  with  the  other  criteria. 
However,  as  there  have  been  igneous  intrusives,  both  acidic  and  basic,  in  great  quantities  up 
to  middle  Keweenawan  time,  this  criterion  has  relativeh'  small  utility  in  the  correlation  o^  the 
Lake  Superior  Huronian. 

7.  Deformation.  The  amount  and  nature  of  deformation  are  of  assistance  in  correlation. 
On  the  whole  the  older  the  series  the  greater  and  more  intricate  the  deformation.  Thus  in 
this  respect  the  Archean  rocks  exceed  all  later  series.  The  Keweenawan  is  much  less  deformed 
than  the  other  pre-C'ambrian  series.  But  the  differences  in  deformation  of  the  Huronian  divi- 
sions may  not  be  so  marked  in  a  single  district  as  to  give  unportant  assistance  in  the  discrimi- 
nation of  these  divisions  from  one  another.  AJso  a  particular  division  of  the  Huronian  may 
be  much  deformed  in  one  district  and  not  in  another. 

8.  Degree  of  mctamorphism.  The  degree  of  mctamorphism  is  of  some  assistance  in  cor- 
relation. On  the  whole  the  older  rocks  are  more  metamorphoseii  than  the  younger  rocks,  but 
this  criterion  has  limitations,  since  witliin  comparativeh'  short  distances  the  closeness  of  folding 
and  the  quantity  of  igneous  intrusions  may  greatly  vary,  and  tliese  are  very  important  factors 
in  ])roducing  metamorphism. 

The  criterion  relied  on  more  than  all  others  in  the  correlatioii  of  the  Cambrian  and  post- 
Caml)rian  formations — that  of  fossils,  showing  similaiity  of  the  life  on  the  earth  at  the  time 
the  equated  formations  were  laid  down — is  not  available  for  the  pre-Cambrian  rocks  of  the 


Camlation  of  ]irt-C<nnlirianTOcki  of  the  Lake  Superior  Ttffion. 


1 

fl 

B«i4aaad(r«Dp. 

Uimanw-uitncL 

CryiMinUxlbMeL 

SturtKiD  lUlUlCI. 

Fdcb  Uounitln  dji- 
utei. 

CaJiiinaldWrtfl 

Honjne«  dlrtrlel. 

Iioo  m*or  dUlrlct. 

PanokwOogDlilc 
dtelrlct. 

K.M^hAntrnl  wiHWB  '  UarTttn,    ItiuK,    and 

vldnUf. 

Nccodnh.  NoflU 
JJluD.  >Dd  Dlack 
Klvuraiou 

«„^».». 

Waterloo  ana,  WIkod- 

»1Q, 

¥vx  River  valley. 

Utnbldliulct. 

QiinnirK  Lake  db- 
trkt. 

PlEton  Point. 

Anlmlkleor  Loon  Lake 
dUirtct. 

Cnyuoa  dUlrfel. 

VmnlUoodlilttei        's^^r^SS    UlehlplcwtendltirtBi 

North  gaw^arUta 

Uppw, 

Not    MHilinnJ.    tiul 
gtnulvM  In  upper 

Nol  Idmlinw] 

Uoulilfullr  pnuDi 

boubltully  isoenl 

A  twill 

OnaSiaOl. 

it). 

Abunt. 

AbKOt. 

AbMM. 

Ab«nt 

AbMnt. 

AbMnf, 

Ab«n>. 

Abral. 

Oabbro*.   dlaba«j, 
»lc. 

EmtMrru*  gnniu  (In- 
mislvf). 

Dululli  EBbbro. 

Unoootmrnlly 

Acldlfi  and  butc  In- 

vStsr 

BIwjblk      rormalloti 
(Iran    beaiUic   and 

Pokeg^^iarUlt*. 

ilisntinioiEPCnuille, 
Imnitlvc  Into  rocks 

Sutltomli    (slalp. 

KluiDcroU-i!     whJrh 

or  IhF   Kulrt   Lakr 
slfllp   and    OgUhlic 
caiiKlunieniti'  ol  llie 
Vermilion  dUttlot. 

C'aiiriL™ 

(Diii'iiFbj.  *° 

Onbluo   and    "red 

Conglomerole,    saad- 

Bailo  and  aeldic  la- 
trutlva  and  oitru- 

Ulddlft. 

AbK'Dl. 

"tS-^u'^Tt.^ 

1 

a 

Lo-». 

1  onelamaraloi. 

troup). 

and  uiruilta. 
MlcUcuom.-    .J.  10 
To  0)*  toulh  pully 
npLuwl  b>  IhB  vot 

nuranlu  CUiki- 
0«^<*  qiurtill* 

rlnvoilonglalnulra 
and  nlnirira 

iHvlnf  mtmbor. 

VuiuantonMlloo 

UklUfunmtilaK. 
FakhK^lil 

IninuKai  ond  ot- 

MIcftlBuniiiB  (-nan- 

V'ulrw)  fontia  Una  .ni  ti- 
dlvldsl     loiu     Ida 

J^bar'.Tii'iirijI.W 
m»nb«,  and  Tiad- 

Uncntonnlty 

(julnueicT  viii>i, 

;,',ria!.m,'r- 

MlflilncnniBilBla.  Ill- 

du.lliwinwulWioI 

dOUllthll     BfT.    utd 

IIIB    r.t.d    OIlIW 

Mlu'lilcuniiig  alalB. 
Inrludlnx  \nleui 
Iron-bwrlniniBin- 

Uncnitone     Inlnj. 
Htm  and   oxtru- 

Tjl^ilate. 

North    Moond    con- 
ElomcialB    and 

Arpln     roDflODionto 
andqua^ulte. 

Uarahaliumconclam- 

Uualhan  conttotnef- 
alo. 

- —  rnwrnfonnlly^ 

niironUin  <|iiarltlli4. 
Paalbly    U.H„dloi( 
KeAOHiiiwoniiiiniU 

jlbly     (Ubdlvldwl 

Huronlan  qunrliltm. 

Buronlan     qUBrti- 

KB.™-'- 

<)iiulill><     (uprar 

Oninlli-,     Inlnulvo 

Fnwloni  dolomlto, 
maliilv  ituroRilIu, 
iDcliiJlne  lion- 
liMrlnsiiuinibiirla 
Its  lowr  liorlion, 

Swloy  slnie. 

DantlMu  i]iiar1>1lu. 

IVAterlw      ijuartilu; 
piadblymlddlallii- 

(Iran  bautni). 

lultrbnldcdquani- 
Itnamltlates. 

Black  Ilale 

Vlrtlnl«("BvU)uli-l 
9law.lacludlncl>»r- 

aunntui  ransailds 
(Iron   bearlog.  but 
nan  prod  uctlvB). 

AbBDl. 

\bwll. 

SodlMiU   uat   Sod- 

UMdlalluroDlui, 

NaiBuno'    totmiiJon 

lion) 
BJuaa  sJsts. 
AJIblk  qunritlt.. 

NfcaiiDH  a)  (utma- 
llaii(lmnl>wrtn(j. 
AJlblk  quuldlf 

ivokMilo)  •lllilrun- 
Wrtu  (falo  nivrn- 
bm  aw  lop, 

RaDdvfltv  daloiiillA 

— I'mmnfonnliyr — 

lUndvlIlF  ilolomlK. 
einrunn  quaruJi*. 

^lotonlonnliy^ 

Abwnt. 

— Dnmnronullyt- 

RandrtUfdolamll* 
SluiKOoa  quaniil* 

^Itneonlofmlljr^ 

AUanl. 
BEurtnfi  quartili*. 

<juarUlu;  In  moil  ot 
dQlonilla. 

NotlilmllOtd 

Nulldruimud, 

Alaani. 

— Uoconlornitty^ 

Had     tllror    11m*- 

tlone), 
Sunday  qufltUIU. 

Galibio-dMrlle  s(r- 

Ithyomojorln 
IntniiiTu     nlo- 
UoH'. 

Poivore       Bill  11 

quanilte. 
Ilamlmic  ilnUi. 

UnoooloriD  Uy 

nrenlK-  Inlnulvn. 
fJniywiiuke 

Oianlle  and  gretn- 
Hone.Inlnulvelnto 
rociw  b»low. 

Btole.myMraoke.and 

pbyrloi.dolaWtra, 
lamnropbyres,     In- 
tnulie    liito    rocks 

KnltoLflkcsbjle- 
ApwmormBtlon  (Iron 

Wriiic.    but   000- 

ptoductlvsj, 
Oftsbke  MDglomenile. 

UnuUtraaadalbjerU- 
Inulve  rocks- 

sodlincnu.  taalnly 
<|B1«  and  mils 
whbts,  wllb  In- 
imjln-  Uld  ox- 
truilvD  nckf. 

lawar-mldJlo  Hu- 
ronlan   ("lipvr 
ColorojimndWUI- 
CntnllD     and 
lotT^va  Inlo 
Ooti   ™H«lom. 

BMLmanU.  wlib  In- 
iruilre  aud  ailni' 
ii™  rocks 

,„.,.-. 

UVirr  tla'f 
KODB  dokitnlii- 
Uanaril  ijuartuia 

KMidvlllodolaiiillo- 

MMiamllflwl 

Houndaii  (onnmitin 

irlW  and  quarU- 
Ita.  lAtlBinl  to  >m 
til*  «i|iilva1«nl  'If 

ooilla  and  fliur- 
Eton  quutilU). 

.^u 

lAurooUui  Hirln  ilo- 
tniiltt    lolo    K*- 

wlllDj. 

OnnliF,  •vMillr.  tut- 

Idoill* 
FBimM  snelM. 

dUbuaSlkai. 

(iranHa  and  psnl- 

a».,.-^. 

rsjs;  '■■"'■ 

Aelillrfolcanlcrwin. 
probably      l^uita- 

Granitej  uid  pocphy- 

UranllsindcnrUin. 
inlnulTo  into  K^ 
waltn. 

OrBnK.aDdp-1-s.  "r^-^ne^KS: 

1 

hatVDiln  ivliii 

»lil>l.  tht  JBllir 
bnaati  ofiii  In  a  tn 

n^JToar    haiuU    of 

bot^lonnailaD- 

anwn  rUIil*. 

"assss/ffl" 

UrMutDoa      and 

Goclo  and  kIiHW. 

fiitlidTwilito.  SSd 

OrMnichliu.FMO- 
glnncHndnuuIied 

Umn  Khtita.  tnta- 
porpli>rl»* 

Soadan  lormatlaD 
>li0D   btarlne    and 

J^SddSr'pStrt 

boilD    Icneoos  and 
iMjalyvolewIorwik 

OrwD  KhEiu  and 
<bi«'  riid    Ireo- 

bcallne  lockl. 
Slw."l 

rl<mtr  Huronlan" 

Hon     1  l°i  o  n 
bnilnc    and 

tV^ira  lull 
Ofoi  (lap  raen- 
lEonn. 

lions  and  crasti 
-■hUt.,    maoped   by 

17517«— VOL  52— n     (Tf.  Inrt.  paRi-  59»  I 


GENERAL  GEOLOGY.  599 

Lake  Superior  region.  Recognizable  fossils  have  not  yet  been  discovered  in  these  rocks, 
although  the  carbonaceous  slates  which  extend  back  to  the  Archean  seem  to  show  that  life 
existed  at  the  tmie  of  the  earliest  pre-Canibiian  series. 

The  correlation  of  the  pre-Cambrian  roclcs  is  not  as  definite  as  that  for  fossiliferous  rocks — 
a  fact  which  makes  it  desirable  to  retain  local  names  to  an  extent  not  warranted  were  fossil 
evidence  available.  The  use  of  local  names  is  desirable  in  this  region  also  for  the  reason  that 
the  districts  in  which  they  are  applietl  are  separated  by  considerable  areas  that  are  much  less 
known.  The  geologist  or  mining  engineer  is  seldom  interested  in  the  general  correlation  of 
formations  and  finds  it  convenient  to  have  local  terms  which  have  areal  as  well  as  stratigraphic 
significance.  Ivocal  terms  have  been  used  in  the  six  preceding  reports  on  this  region  by  the 
United  States  Geological  Survey,  have  become  permanently  entrenched  in  the  literature,  and 
are  known  and  understood  by  the  local  mining  n^en.  To  discard  these  terms  in  favor  of  more 
general  terms  would,  it  is  believed,  introduce  confusion  and  perhaps  vagueness.  At  some 
future  time,  when  explorations  shall  have  made  the  areal  connection  so  definite  that  there  can 
be  no  question  about  correlation,  a  sweeping  change  in  names  might  be  introduced  to  advan- 
tage. At  present,  with  so  many  unsolved  problems  and  with  possibilities  for  changes  in  views 
of  correlation,  there  can  be  little  advantage  in  substituting  general  names,  even  for  formations 
whose  correlation  is  regarded  as  reasonably  well  established. 

GENERAL  CHARACTER  AND  CORRELATION  OF  THE  ARCHEAN. 
The  Archean  system  comprises  the  Keewatin  series  and  the  Laurentian  series. 

KEEWATIN    SERIES. 

The  Keewatin  series,  wherever  it  is  found  in  a  relatively  unchanged  condition,  is  remark- 
ably uniform  in  its  general  character,  and  even  the  portions  that  have  been  metamorjihosed 
show  features  that  are  consistent  with  the  theory  that  before  metamorphism  they  had  the 
same  general  character  as  the  less  altered  portions.  The  Keewatin  comprises  two  great  forma- 
tions, a  dominant  igneous  formation  and  a  subordinate  sedimentary  formation.  It  is  found  in 
its  most  typical  facies  in  a  comparatively  unaltered  condition  in  the  Vermilion  and  Lake  of  the 
Woods  districts.  The  characterization  of  the  Ely  greenstone  for  the  Vermilion  district,  given 
on  pages  119-122,  might  be  applied  to  each  of  the  other  districts  without  important  changes. 
The  Keewatin  is  a  great  volcanic  series,  composed  dominantly  of  basalts  and  intermediate 
roclvs.  For  all  of  these  regions  where"  the  rock  is  lava  and  is  least  metamorphosed,  a  peculiar 
ellipsoidal  or  ])illow  structure  is  characteristic.  It  has  been  pointed  out  (see  pp.  510-512)  that 
this  structure  and  the  relations  of  the  Keewatin  to  the  iron-bearing  roclvs  are  evidences  that 
the  eruptions  were  at  least  in  part  subaqueous.  Associated  with  the  lavas  are  vast  quantities  of 
volcanic  fragmental  rocks.  In  some  districts — for  instance,  the  Marcpiette  and  the  Menominee — 
the  tuffaceous  variety  of  greenstone  appears,  but  tlie  ellipsoidal  structure  is  not  common. 
However,  in  these  districts  the  Keewatin  is  much  noetamorphosed  by  dynamic  action  and  by 
later  intrusions,  so  that  the  ellipsoidal  structure  would  have  been  largely  destroyed  even  if  it 
once  existed,  as  it  has  been  where  the  Keewatin  rocks  lie  close  to  similar  large  intrusive  masses 
on  the  north  shore.  Barring  the  changes  due  to  metamorphism,  there  is  a  very  remarkable 
likeness  of  the  dominating  igneous  portion  of  the  Keewatin  in  the  different  parts  of  the  I^ake 
Sujjerior  region. 

A.ssociated  with  the  igneous  roclcs  of  the  Keewatin  are  subordinate  masses  of  sediments. 
These  sediments  comprise  slates,  iron-bearing  formation,  and  dolomite.  The  slates  that  have 
been  metamorphosed  are  so  similar  to  the  schistose  phases  of  the  greenstone  itself  that  they 
are  in  places  difficult  to  recognize.  They  have  been  found  in  almost  every  district.  The 
iron-bearing  formation  is  the  prominent  sedimentary  one  in  the  Vermilion,  Atikokan,  Kamin- 
istikwia,  and  Michipicoten  districts.  It  occurs  very  subordinately  in  the  Marquette  district. 
la  the  Lake  of  the  Woods  district  the  iron-bearing  formation  has  not  been  found,  but  small 
masses  of  dolomite  occur.     In  all  the  districts  the  iron-bearing  formation  and  the  dolomite  are 


600  GEOLOGY  OF  THE  I^AKE  SUPERIOR  REGION. 

associated  with  the  slates.  It  is  beUeved  that  in  areas  where  these  sedimentary  formations 
are  of  considerable  extent  and  thickness  they  represent  later  Keewatin  time.  The  chief  reason 
for  this  belief  appears  in  the  Vermilion  district,  wliere  the  iron-bearing  format  if  )n  is  thick  and 
large  masses  of  it  occur  adjacent  to  the  Huronian,  in  both  the  Lake  Vermilion  and  the  Ely 
areas,  showing  that  the  main  mass  of  this  formation  was  at  the  top  of  the  series  at  the  beginning 
of  Huronian  sedimentation. 

The  Keewatin  is  the  oldest  series  in  the  Lake  Superior  region.  It  is  dominantly  volcanic. 
Moreover,  the  lavas  were  poured  out  mainly  below  the  water.  Thus  for  the  earliest  time  of 
wliich  we  have  record  in  this  region  the  surface  conditions  were  those  of  submarine,  regional 
volcanism.  Sedimentation  was  local  and  subordinate,  and  the  sediments  of  the  Keewatin  are 
believed  to  have  a  close  genetic  connection  with  the  associated  volcanic  rocks.     (See  pp.  126-127.) 

The  Keewatin  locally  is  schistose  and  the  original  textures  and  structures  are  discernible 
with  difficulty.  Where  not  schistose  the  Keewatin  may  be  separated  into  distinct  lava  flows  and 
beds  of  pyroclastic  rocks,  with  interleaved  sedimentary  beds  for  which  strike  and  dip  are  deter- 
mined. The  folding  is  usuall}'  close  and  the  beds  stand  at  steep  attitudes.  To])ographically 
the  Keewatin,  though  rough  in  detail,  has  on  the  whole  less  bold  rehef  than  the  Algonkian 
sediments.  The  schistose  phases  are  in  general  less  resistant  to  weathering  and  stand  lower 
than  the  massive  phases.     The  Keewatin  occupies  a  part  of  the  great  Archean  peneplain. 

LAURENTIAN    SERIES. 

The  Laurentian  series  is  dominantly  represented  by  great  masses  of  granite,  granitoid 
gneiss,  and  syenite — all  acidic  rocks.  Intermediate  and  basic  rocks  are  subordinate.  The 
Laurentian  intrudes  the  Keewatin  series,  the  intrusive  masses  ranging  from  great  bathohths 
many  miles  in  diameter  to  dikes  and  minute  injections,  even  to  "ht  par  lit''  intrusions.  The 
great  batholiths  are  perhaps  best  illustrated  and  have  been  most  accurately  described  for  the 
region  north  of  Lake  Superior,  particularly  near  Lake  of  the  Woods  and  Rainj-  Lake,  by  Law- 
son.°  These  intrusive  rocks,  in  connection  with  the  concurrent  dynamic  action,  have  pro- 
duced profound  metamorpliic  effects  in  the  older  Keewatin  rocks,  which  in  consequence  have 
been  changed  over  extensive  areas  to  hornblende  schist  and  hornblende  gneiss.  In  many 
considerable  areas  the  Keewatin  and  Laurentian  are  so  intimately  mixed  that  it  would  be 
difficult  to  give  an  estimate  of  the  relative  proportions  of  the  two.  In  some  places  there  is 
evidence  that  the  Keewatin  has  actually  been  absorbed  to  some  extent  and  thus  modified 
the  composition  of  the  Laurentian  intrusives.  This,  combined  with  the  intricate  relations 
along  the  contacts  of  the  two  formations,  gives  locally  an  appearance  of  gradation  from  the 
massive  Laurentian  granite  through  the  gneiss  containing  various  mixtures  of  intrusive  and 
intruded  rocks  to  the  extremely  metamorphosed  variety:  These  facts  have  led  Lawson  *  to 
his  theory  of  subcrustal  fusion,  according  to  which  all  the  acidic  material  is  but  the  fused 
Keewatin.  From  our  point  of  view,  however,  the  evidence  for  such  a  conclusion  is  inadequate, 
the  most  fundamental  point  against  its  correctness  being  the  very  great  difference  in  chemical 
composition  of  the  Laurentian  and  Keewatin.  T\Tiere  not  influenced  by  each  other,  the 
Laurentian  has  the  chemical  composition  of  ordinary  acidic  igneous  rocks,  whereas  the  Keewatin 
has  the  average  composition  of  a  basic  rock  of  the  basalt  type.  If,  as  Daly  "■  suggests,  the 
Laurentian  is  supposed  to  have  invaded  the  Keewatin  by  a  process  of  overhead  stoping  and 
the  slope  blocks  have  sunk,  the  contrast  in  composition  near  contacts  is  explained.  In  the 
nature  of  the  case,  evidence  for  tliis  kind  of  subcrustal  fusion  is  chiiicult  to  obtain,  and  so  far 
as  the  Lake  Superior  region  is  concerned  this  hypothesis  ma\-  be  noted  merel}-  as  an  interesting 
guess. 

dLanrson,  A.  C,  Report  on  the  geology  of  the  Lake  of  the  Woods  region,  with  special  reference  to  the  Keewatin.  (Huronian)  belt  of  the 
.Vrchean  rocks;  .\nn.  Repl.  Cieol.  and  Nat.  Hist.  Survey  Canada  for  !S8o,  new  ser.,  vol.  1,  ISSfi,  pp.  5-131  cc,  with  geologic  map;  Report  on  the 
geology  of  the  Rainy  Lake  region;  Idem  for  1887-88,  new  ser.,  vol.  3,  pt.  1,  18S8,  pp.  1-182  r,  with  2  maps. 

t>  Op.  clt.,  1S8.S,  p.  i:u  F. 

cDaly,  R.  A.,  The  mechanics  of  igneous  intrusion:  .\ni.  Jour.  Sci.,  4th  ser.,  vol.  26, 1908,  p.  30.  . 


GENERAL  GEOLOGY.  601 

The  Laurentian  of  the  Lake  Superior  region  as  a  whole  is  characterized  by  both  massive 
and  schistose  phases.     It  is  perhaps  surprising  that  so  large  a  proportion  should  be  massive. 
I    It  is  topographically  rough  in  detail,  the  massive  parts  usually  standing  somewhat  higher  than 
the  schistose  parts,  but  altogether  it  forms  a  part  of  the  Archean  peneplain. 

GENERAL    STATEMENTS     CONCERNING    THE    ARCHEAN     SYSTEM. 

Both  Laurentian  and  Keewatin  rocks  appear  in  each  of  the  important  districts  that  have 
been  considered  in  the  detailed  chapters.  Manifestly  the  wide  and  irregular  distribution  of 
the  Archean  is  a  natural  consequence  of  the  fact  that  these  rocks  constitute  the  basement  com- 
plex upon  which  later  formations  were  laid  down.  Whether  or  not  they  are  now  at  the  sur- 
face at  any  particular  locahty  depends  on  subsequent  deposition,  folding,  and  denudation — 
that  is,  it  depends  on  whether  geologic  agencies  have  brought  them  to  the  surface. 

If,  in  the  future,  erosion  should  cut  the  Lake  Superior  region  to  a  depth  of  several  thousand 
feet  below  the  present  surface,  it  would  probably  be  seen  that  much  the  larger  part  of  the  area 
would  be  occupied  by  the  Archean,  and  it  is  believed  that  the  Archean  everywhere  underhes 
all  later  rocks. 

It  appears  from  the  foregoing  characterization  of  the  Keewatin  and  Laurentian  that  the 
Archean  as  a  whole  was  a  period  of  regional  igneous  activity.  All  succeeding  series  contain 
sedimentary  rocks  in  large  or  dominant  proportions;  they  are  treated  essentially  as  sedimen- 
tary series  and  the  igneous  rocks  are  considered  with  reference  to  the  sediments.  In  the 
Archean,  on  the  other  hand,  the  igneous  rocks,  which  make  up  more  than  90  per  cent  and 
probably  more  than  95  per  cent  of  the  area,  are  primarily  considered,  and  the  subordinate 
masses  of  sedimentary  rocks  are  discussed  in  reference  to  the  igneous  rocks. 

The  igneous  activity  of  Archean  time  was  both  plutonic  and  volcanic  on  a  tremendous 
scale.  Probably  at  present  the  plutonic  igneous  rocks  of  the  Archean  occupy  a  much  larger 
area  at  the  surface  than  the  volcanic  rocks,  but  this  is  doubtless  due  in  large  measure  to  the 
very  profound  erosion  which  has  taken  place  since  Archean  time,  and  which  has  in  consider- 
able measure  removed  the  volcanic  rocks  and  exposed  the  plutonic  rocks.     . 

A  very  characteristic  feature  of  the  Archean  of  the  Lake  Superior  region  is  its  likeness 
from  one  district  to  another,  and  this  is  so  whether  the  lithologic  types  of  rocks  or  their  relations 
are  considered.  The  foregoing  description  of  the  intrusive  relations  between  the  Laurentian 
and  Keewatin  is  applicable  with  scarcely  a  change  to  each  of  the  several  districts.  If  a  set  of 
specimens  from  the  Laurentian  or  Keewatin  south  of  Lake  Superior  were  unlabeled,  they  could 
not  be  disci'iminated  from  a  set  of  specimens  from  the  Archean  northwest  or  east  of  the  lake. 
There  are,  of  course,  some  exceptional  types  of  rocks  which  occur  only  locally,  but  these  are 
extremely  subordinate  in  their  mass.  This  extraordinary  paralleUsm  of  phenomena  of  the 
Archean  of  one  part  of  the  Lake  Superior  region  with  that  of  another  part — and,  for  that  matter, 
with  the  Ai'chean  of  other  parts  of  the  world — has  led  to  the  phrase  that  the  Archean  is  "homo- 
geneous in  its  heterogeneity" — that  is,  while  it  is  heterogeneous  for  any  one  district,  it  shows 
the  same  kind  of  heterogeneity  in  each  of  the  other  districts. 

Topographically  also  the  Archean  is  a  unit.  Though  rough  in  detail  it  is  a  great  peneplain, 
the  UTCgularities  of  wliich  do  not  constitute  regular  lineaments,  and  it  is  thus  m  contrast  to  the 
Algonkian  rocks,  part  of  which  usually  stand  above  the  peneplain  surfaces  with  conspicuous 
linear  features. 

Whether  or  not  it  is  generally  accepted  that  the  Archean,  as  the  term  is  here  used,  can  be 
safely  correlated  with  similar  rocks  of  other  geologic  provinces,  it  can  hardly  be  doubted  that 
the  Archean  rocks  of  the  different  districts  of  the  Lake  Superior  region  form  parts  of  a  single 
great  system.  This  conclusion  is  supported  by  substantially  all  the  criteria  in  reference  to  cor- 
relation given  on  pages  597-599.  The  system  wherever  it  occurs  is  in  a  basal  position.  It  rests 
unconformably  below  all  the  series  with  which  it  comes  into  contact.  The  general  lithologic 
hkeness  of  the  heterogeneous  mass  is  remarkable.  The  Keewatin  rocks  are  largely  submarine. 
The  complexity  of  intrusives  is  greater  than  that  m  any  other  series.     The  deformation  is 


602  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

greater  than  in  other  pre-Camhrian  series.  The  metamorphism  is  profound.  Similarity  of 
sequence  of  formations  in  difTerent  areas  of  the  Kcewatin  is  lacking,  but  in  place  of  this  are 
the  prevalent  intrusive  relations  which  exist  between  the  Kecwatin  and  Laurcntian. 

It  is  of  interest  to  note  that  the  oldest  recognized  Archean  rocks  are  basalts,  with  tex- 
tures indicating  both  subaqueous  and  subaerial  extrusion.  The  basement  upon  which  they 
rest  has  not  been  identified.  It  is  natural  to  turn  to  the  Laurentian  granites  and  gneisses,  but 
wherever  these  are  found  in  contact  with  the  Kecwatin  they  are  intrusive  into  it.  "VMietlier 
some  parts  of  the  Laurentian  represent  the  original  basement  or  whether  the  Laurentian  as  a 
whole  has  formed  the  basement  and  has  been  subsequently  fused,  there  is  no  evidence  to 
show. 

GENERAL  STATEMENTS  CONCERNING  THE  ALGONKIAN  SYSTEM. 

CHARACTER    AND    SUBDIVISIONS. 

The  Algonkian  system  on  the  whole  contrasts  with  the  Archean  in  being  dominantly  sedi- 
mentary rather  than  dominantly  igneous,  in  being  less  metamorphosed,  in  having  distinctly 
recognizable  stratigraphic  sequence,  and  in  topography.  The  sediments  are  largely  water 
assorted  and  deposited  but  in  part  are  probably  subaerial.  The  iron-bearing  formations  are 
regarded  as  having  an  exceptional  character,  being  derived  partly  from  submarine  volcanic 
rocks  either  in  magmatic  solutions  or  by  the  reaction  of  hot  volcanic  material  with  sea  water, 
or  both.     (See  p.  516.) 

The  Algonkian  system  comprises  in  its  fullest  development  in  the  Lake  Superior  region  four 
unconformable  divisions — lower  Huronian,  middle  Huronian,  upper  Huronian,  and  Keweenawan. 
The  Keweenawan  series  is  essentially  a  unit  geographically  and  lithologically  and  is  considered 
as  such  in  the  following  discussion.  The  Huronian  series,  especially  the  lower  and  middle 
Huronian,  presents  such  variation  in  lithology  and  succession  as  to  require  its  consideration 
under  two  main  geographic  subprovinces — (1)  the  northern  subprovince,  including  the  north 
shore  of  Lake  Superior  and  westward  extension  into  Minnesota,  and  (2)  the  southern  subprov- 
ince, including  the  Gogebic  and  Marquette  districts  of  the  south  shore  of  Lake  Superior  and  the 
continuation  of  this  belt  eastward  to  the  north  shore  of  Lake  Huron,  and  the  Menommee, 
Crystal  Falls,  and  Iron  River  districts  of  Michigan. 

NORTHERN    HURONIAN    SUBPROVINCE. 

LOWER-MIDDLE   HTJRONIAN. 
LITHOLOGY    AND    SUCCESSION. 

The  Huronian  rocks  unconformably  above  the  Archean  and  unconformably  below  the  upper 
Huronian  (Animikie  group)  of  the  north  shore  of  Lake  Superior  are  extensive  and  thick.  The 
unconformities  above  and  below  are  great.  At  many  places  the  comparatively  flat-lying 
Animikie  group  may  be  seen  resting  upon  the  steeply  inclined  or  vertical  truncated  edges  of 
the  middle  or  lower  Huronian.  The  latter  rocks  consist  mainly  of  conglomerates,  graywackes, 
slates,  and  mica  schists.  In  some  places  it  is  possible  to  divide  them  into  two  formations,  the 
lower  consisting  dominantly  of  conglomerates  and  the  upper  dominantly  of  graywackes  and 
slates  and  their  metamorphosed  equivalents. 

The  most  characteristic  and  widespread  of  these  rocks  are  the  conglomerates  of  the  lower 
formation.  Those  which  lie  near  the  subjacent  rocks  from  which  they  are  derived  are  commonly 
coarse  bowlder  conglomerates.  Their  fragments  vary  in  lithology,  depending  on  the  under- 
lying formation.  They  may  be  dominantly  from  granite,  from  greenstone,  or  from  gneiss, 
or  mixtures  of  these  three  in  viirious  proportions  and  also  with  other  materials.  Many  of 
the  conglomerates  at  .higher  horizons  have  a  fine-grained  matrLx.  Some  of  them  have  a 
slate  matrix  through  which  very  nunu>rous  isolated  i)ol)l)los  and  bowlders  are  scattered  in 
an  irregular  manner.     These  have  bt'en  called  slate  conglomerates.     In  certain  localities  the 


GENERAL  GEOLOGY.  G03 

slate  conglomerates  are  the  only  rocks  found.  Associated  with  the  slate  conglomerates  in 
many  places  are  beds  of  well-laminated  slate  and  schist. 

As  has  been  intimated,  the  upper  formation  consists  commonly  of  pelites.  The  most 
extensive  areas  of  pelite  are  those  of  the  Vermilion,  Rainy  Lake,  and  Hunters  Island  districts.. 

At  the  west  end  of  the  Vermilion  district,  between  the  conglomerate  (there  called  the  Ogishke 
conglomerate)  and  the  slate  (known  as  the  Knife  Lake  slate)  is  a  thin  iron-bearing  formation 
(called  the  Agawa  formation)  which  appears  to  grade  toward  the  southwest  into  a  calcareous 
slate.  The  latter  is  the  only  known  representative  of  a  limestone  in  the  lower  or  middle  Huro- 
nian  of  the  northern  subprovince.  At  this  particular  locality  the  succession  is  in  certain  respects 
similar  to  that  of  the  middle  Iluronian  of  the  Marquette  district,  but  by  far  the  greater  areas 
and  masses  of  these,  rocks  in  the  northern  subprovince  exhibit  no  close  analogy  in  succession 
or  lithology  with  either  the  lower  Iluronian  or  the  middle  Huronian  of  the  south  shore. 

IGNEOUS    ROCKS. 

During  the  time  of  the  deposition  of  the  rocks  under  discussion  there  were  very  great 
outbreaks  of  igneous  rocks,  basic  and  acidic,  plutonic  and  volcanic.  Contemporaneous  volcanic 
detritus  is  mingleil  in  varying  proportions  with  ordinary  sedimentary  material,  from  a  subor- 
dinate to  a  dominant  amount,  as  at  Kekekabic  Lake.  The  contributions  of  volcanic  material 
were  so  great  as  to  make  them  quantitatively  very  important.  Some  of  the  larger  of  the  plutonic 
masses  are  the  intrusive  granites  in  the  Mesabi  and  Vermilion  districts.  The  slates  that  have 
been  intruded  by  great  masses  of  granites  and  have  been  deformed  have  become  pelite  schists 
(mica schists).  This  phase  is  extensively  illustrated  in  the  Rainy  Lake  and  Namakan  Lake 
areas.  The  conglomerates  under  similar  circumstances  are  metamorphosed  to  psephite  schists 
or  gneisses,  as  illustrated  by  the  schistose  conglomerates  adjacent  to  the  Snowbank  granite  in 
the  Vermilion  district. 

CONDITIONS    OF    DEPOSITION. 

Coleman"  holds  that  the  lower  Huronian  slate  conglomerate  at  one  locality  in  the  Cobalt 
district  of  Ontario  is  a  glacial  till.  He  points  out  the  likeness  of  the  great  masses  of  the  slate 
conglomerate  to  modern  glacial  till  and  to  the  Dwyka  glacial  deposits  of  South  Africa,  and  con- 
cludes that  they  are  all  till.  However,  even  if  the  glacial  origin  of  the  conglomerate-bearing 
striated  and  grooved  bowlders  at  Cobalt  is  accepted — geologists  are  not  all  agreed  as  to  tliis — 
it  does  not  follow  that  the  Huronian  conglomerate  of  the  northern  subprovince  as  a  whole  is  of 
this  origin,  because,  among  other  reason's,  the  Cobalt  area  is  a  long  way  east  of  the  Lake 
Superior  region. 

Wliether  or  not  Coleman's  conclusion  as  to  origin  applies  to  the  lower-middle  Huronian 
in  tliis  subprovince,  it  is  regarded  as  likely  that  these  rocks  are  essentially  of  terrestrial  tleposi- 
tion  because  of  their  unassorted  character,  being  made  up  principally  of  conglomerate  and 
graywacke,  lacking  quartzite  and  limestone;  because  of  the  recurrence  of  conglomerates  at 
many  horizons  through  several  hundred  feet;  because  the  extensive  conglomerate  beds,  like 
the  Ogishke,  have  a  thickness  and  extent  which  are  more  easily  explained  by  terrestrial  than 
by  subaqueous  sedimentation,  which,  according  to  Barrell,''  is  not  likely  to  produce  conglom- 
erates over  100  feet  tlnck;  and  finally  because  the  part  of  the  lower-middle  Huronian  nearest 
the  granite  or  greenstone  of  the  Archoan  is  locally  a  recomposed  rock,  which  has  not  been  sorted 

CORRELATION. 

The  criteria  under  which  the  formations  under  discussion  are  classed  as  middle  or  lower 
Huronian  are  the  following:  They  rest  upon  the  Archean  and  are  below  the  Animikie  group, 
or  upper  Huronian;  they  are  separated  from  these  rocks  by  unconformities;  they  are  exten- 
sively cut  by  both  basic  and  acidic  igneous  rocks;  they  are  similar  in  their  deformation  and 


oColeman,  A.  P.,  The  lower  Huronian  ice  age:  Jour.  Geology,  vol.  16,  1908.  p.  154. 

6  Barren.  Joseph,  Relative  geological  importance  of  continental,  littoral,  and  marine  sedimentation:  Jour.  Geology,  vol.  14. 1900,  pp.  433-446: 
also  personal  communication. 


GO-t  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

degree  of  metamorphisiii.  It  thus  appears  that  the  assignment  of  the  rocks  under  discussion 
to  the  general  place  of  lower  Iluronian  and  middle  Iluronian  is  unquestioned.  But  as  large 
portions  of  these  rocks  may  be  land  formations,  they  can  not  be  exactly  correlated  with  the 
aqueous  de])osits  of  the  middle  and  lower  Huronian  to  the  south.  The  deposition  of  land  sedi- 
ments may  well  have  begun  earlier  than  that  of  the  aqueous  deposits  or  it  may  have  continued 
later.  On  earlier  maps  jjublished  by  the  United  States  Geological  Survey  the  rocks  here  named 
lower-middle  Iluronian  appear  as  lower  Huronian.  As  earlier  continental  deposits  are  likely 
to  be  removed  by  later  erosion,  however,  it  is  probable  that  part,  probably  the  larger  j)art,  of 
these  rocks  are  of  middle-Huronian  age.  It  has  already  been  noted  that  in  northeastern 
Minnesota  there  is  a  similarity  in  succession  to  the  middle  Huronian  of  the  Marquette  district. 

UPPER  HTJBONIAN   (ANIMIKIE   GROUP). 

LITHOLOGY    AND    SUCCESSION. 

The  upper  Iluronian  of  the  northern  subprovince  extends  from  a  point  some  distance  east 
of  Nipigon  Bay,  on  the  north  shore  of  Lake  Superior,  westward  through  Thunder  Bay  to  the 
Mesabi  cUstrict  of  Minnesota,  thence  southwest  and  south  to  the  Cuyuna,  Little  Falls,  ("arlton, 
Cloquet,  and  St.  Louis  River  cUstricts  of  Minnesota.  The  belt  extending  from  Nipigon  Bay  t^) 
the  Mesabi  district  consists  from  the  base  up  of  the  following  rocks: 

L  Conglomerate,  quartz  slate,  and  quartzite.  These  reach  a  tliickness  of  200  feet  on  the 
Mesabi  range.  Farther  east,  in  the  vicinity  of  Gunflint  Lake  antl  Thunder  Bay,  the  tluckness 
becomes  only  a  few  inches  or  a  few  feet. 

2.  Iron-bearing  formation,  700  to  1,000  feet  thick  in  the  Mesabi  district  and  thinning 
somewhat  toward  the  east  and  west. 

3.  Slate,  best  exposed  in  the  Thunder  Bay  district.     Thickness  unknown,  but  large. 
Throughout  the  northern  part  of  this  belt  the  sediments  are  gently  inclined  to  the  south  at 

angles  ranging  from  5°  to  20°  and  locally  even  up  to  4.5°,  with  pitches  of  gentle  minor  folds  in 
the  same  direction.  In  general  the  upper  Huronian  is  not  schistose  but  has'  suffered  contact 
metamorphism  where  it  is  in  contact  with  the  Keweenawan  gabbro  and  granite  and  other  large 
intrusive  masses.  It  rests  unconformably  against  the  older  rocks  to  the  north,  the  unconformity 
being  marked  by  areal  relations,  differences  in  steepness  of  dip,  amount  of  schistosity,  kinds 
of  metamorphism,  relations  to  intrusive  rocks,  basal  conglomerates,  and  topogra])hy.  The 
unconformity  is  one  of  the  most  conspicuous  in  the  Lake  Superior  region.  The  line  of  contact 
is  easily  recognized  by  casual  field  observation.  That  the  essential  continuity  of  the  upper 
Huronian  is  obvious  is  indicated  by  the  early  use  of  the  term  Animikie  not  only  for  the  upper 
Huronian  rocks  on  Thunder  Bay,  but  for  those  in  the  Mesabi  chstrict. 

In  the  area  southwest  of  the  Mesabi  district,  in  the  St.  Louis  River  and  Cuyuna  districts  and 
the  country  to  the  west,  the  upper  Iluronian  consists  principally  of  slate,  carrying  lenses  of  iron- 
bearing  formation,  with  many  intrusive  and  possibly  extrusive  rocks  and  certain  rare  quartz- 
ites,  the  horizon  of  which  is  not  satisfactorily  determined  but  wliich  are  probably  basal  to  the 
division.  The  upper  Huronian  in  this  area  contrasts  markedly  with  that  along  the  Mesabi  range 
and  farther  east  in  being  closely  folded,  in  the  abundance  of  its  intrusive  rocks,  and  in  possession 
of  cleavage,  as  well  as  in  the  differences  in  lithologic  character  just  noted.  It  is  suggested 
i])]).  214,  528,  611)  that  the  structural  differences  may  be  related  in  some  way  to  proximity  to 
the  axis  of  the  Lake  .Superior  syncline,  or  that  the  Mesabi  and  eastward  belt  of  the  upper 
Huronian  may  represent  a  shore  phase  of  deposition,  while  the  upper  Huronian  of  the  Cuyuna 
area  to  the  south  may  be  an  offshore  phase. 

IGNEOUS    ROCKS. 

Intrusive  into  the  upper  Huronian  are  the  great  Duluth  gabbro  of  northern  Minnesota, 
the  basic  siUs  of  the  Gunflint  and  Animikie  Bay  liistricts  (Logan  sills),  a  few  basic  dikes  and 
possibly  sills  in  the  Mesabi  district,  a  granite  mass  on  the  east  end  of  the  Mesabi  range,  and 


GENERAL  GEOLOGY.  605 

more  abundant  basic  and  intrusive  masses  in  the  Cuyuna  district.     Most  of  the  intrusives  are 
of  Keweenawan  age.     Contemporaneous  volcanic  rocks  have  not  been  recognized. 

Extrusive  rocks  rest  on  the  Animikie  in  the  Cuyuna  district.  It  has  been  shown  that 
many  of  the  capping  chabases  of  the  Nipigon  area  may  be  extrusive.  These  are  doubtless 
middle  Keweenawan,  but  some  of  them  may  be  late  Animikie. 

CONDITIONS    OF    DEPOSITION. 

The  upper  lluronian  is  a  unit  for  the  region,  hence  the  conditions  of  deposition  are  dis- 
cussed on  pages  612-614,  after  the  southern  subprovince  has  been  treated. 

CORRELATION. 

The  correlation  of  the  upper  Huronian  of  the  northern  subprovince  with  that  of  the 
southern  subprovince  is  discussed  on  page  610. 

SOUTHERN    HURONIAN    SUBPROVINCE. 

LOWER   HXrnONIAN. 
LITHOLOGT    AND    SUCCESSION. 

The  lower  Huronian  of  the  southern  subprovince  reaches  its  fullest  development  in  the 
Marquette  district,  where  it  consists,  from  the  base  up,  of  the  Mesnard  quartzite,  Kona  dolomite, 
and  Wewe  slate.  In  the  Gogebic  district  the  lower  Huronian  includes  similar  quartzite  and 
doloinite  named  respectively  the  Sunday  quartzite  and  the  Bad  River  limestone,  but  the  slate 
overlying  the  limestone  is  absent. 

Although  the  north  shore  of  Lake  Huron  does  not  fall  within  the  area  covered  by  this  report, 
it  is  desirable  to  consider  the  position  of  the  series  there  because  that  is  the  district  to  which 
the  term  Huronian  was  first  applied.  The  lower  Huronian  of  the  north  shore  of  Lake  Huron 
includes  a  great  clastic  formation  above  wliich  is  a  limestone.  In  most  places  the  clastic  forma- 
tion comprises  a  conglomerate  at  the  base,  above  this  a  quartzite,  and  above  this  a  slate.  In 
other  places  the  conglomerate  is  almost  immediately  overlain  by  the  Umestone.  The  succession 
is  very  similar  in  its  essential  features  to  that  of  the  lower  Huronian  of  the  Marquette  district. 

The  lower  Huronian  is  represented  in  the  Menominee,  Iron  River,  and  adjacent  districts 
of  Micliigan  and  Wisconsin.  It  consists  of  a  quartzite  (the  Sturgeon  quartzite)  followed  by  a 
dolomite  (the  Randville  dolomite);  but  in  the  Iron  River  district  the  quartzite  and  dolomite 
are  interbedded  and  for  them  the  new  name  Saunders  formation  has  been  introduced. 

The  lower  Huronian  partakes  of  the  major  structure  described  for  each  of  the  districts. 
As  a  whole  the  folding  is  not  as  intense  as  in  the  Archean.  Cleavage  is  usually  lacking,  jointing 
is  abundant,  and  bedtling  is  easily  discerned. 

The  quartzite  of  the  lower  Huronian  of  tliis  subprovince  represents  a  cleanly  assorted  sand, 
now  strongly  indurated,  more  or  less  iron  stained,  and  locally  showing  fracturing  and  rock 
flowage,  but  retaining  its  original  bedding  structure  as  a  conspicuous  feature.  It  therefore 
contrasts  in  many  respects  with  the  lower  Huronian  of  the  northern  subprovince.  The  dolomite 
overlying  the  quartzite  is  very  cherty  and  shows  more  evidence  of  deforn^ation  than  the  quartzite. 
The  weathering  of  this  dolomite  emphasizes  the  folded  and  brecciated  chert  layers  and  serves 
to  make  the  formation  easily  identifiable. 

IGNEOUS    ROCKS. 

In  the  areas  which  are  certainly  known  to  be  lower  Huronian,  contemporaneous  igneous 
activity  was  not  important.  This  applies  to  all  the  districts  south  of  Lake  Superior,  as  well  as 
to  the  area  north  of  Lake  Huron.  In  this  respect  the  lower  Huronian  contrasts  with  the  miildle 
and  upper  Huronian  and  to  a  more  marked  degree  with  the  Archean.     The  contrasts  between 


606  GEOLOGY  OF  THE  hAKE  SUPERIOR  REGION. 

the  Archean  and  tlie  li)\ver  Huronian  in  tliis  respect  are  contributory  evidence  of  the  uncon- 
onuity  l)et\veeii  tlie  two.     (See  pp.  617-018.)     The  volcanic  activity  of  Archeun  time  appar- 
ently liad  died  out  comi)letely  in  this  Huronian  subprovince  before  tlie  dej)osition  of  the  rocks 
unquestionably  belonging  to  the  lower  Huronian. 

Later  intrusive  rocks  cut  the  lower  Huronian  in  small  dikes.  The  [)ost^Huronian  or 
Keweenawan  granites  of  the  Florence  district  of  Wisconsin  doubtless  also  cut  the  lower  Huronian, 
but  exposed  contacts  are  only  those  of  the  granite  and  upper  Huronian. 

CONDITIONS    OF   DEPOSITION. 

It  has  appeared  that  the  lower  Huronian  south  of  Lake  Superior  and  on  the  north  shore  of 
Lake  Huron  comprises  first  a  great  clastic  formation  consisting  from  the  base  up  of  conglomerate, 
rpiartzite,  and  slate.  Over  this  is  a  largely  nonclastic  formation  now  represented  by  a  dolomite, 
and  localy  above  tliis  in  the  Marquette  district  is  another  clastic  slate  formation. 

The  essential  subaqueous  origin  of  the  lower  Huronian  is  believed  to  be  showTi  by  the 
cleanly  assorted  nature  of  the  sediments,  the  ripple  marlcs  of  a  shore  rather  than  a  stream  type, 
and  extensive  beds  of  hmestone.  It  remains  to  be  proved  that  such  thick  and  continuous 
Umestone  formations  may  be  produced  as  terrestrial  formations.  Finally  the  conglomerate 
at  the  base  of  the  group  contrasts  stronglj^  with  the  arkose  and  thick  conglomerate  masses  at 
the  base  of  the  middle-lower  Huronian  of  the  north  shore,  and  is  beUeved  to  be  more  character- 
istic of  aqueous  sedimentation. 

It  therefore  appears  that  at  the  beginning  of  lower  Huronian  time  the  conditions  in  the 
southern  subprovince  had  become  those  of  normal  sedimentation  in  which  the  material  destroyed 
by  the  epigene  agents  was  sorted  and  hiid  down  in  beds  one  upon  another,  the  lithologic  character 
varying  from  time  to  time.  This  is  evidence  that  the  erosive  forces  of  air  and  water  were 
working  as  at  ])resent.  Moreover,  as  emphasized  by  Chamberlin  and  Salisbur}',"  it  is  evidence 
that  the  weathering  processes  possessed  their  full  efficiency,  and  this  would  favor  abundant 
vegetation.''  With  the  beginning  of  Huronian  time  at  the  latest  commences  the  part  of  the 
history  of  the  world  to  which  Lyell's  principles  of  uniformity'^  are  apj)licable.  These  ancient 
Huronian  rocks  have  no  lithologic  peculiarity  which  can  discriminate  them  from  the  rocks  of 
much  later  age;  indeed,  there  is  notliing  to  indicate  that  when  they  were  laid  down  the  con- 
ditions were  in  any  respect  different  from  those  which  prevail  to-day,  ^\-ith  the  sole  negative 
point  that  fossils  have  not  been  found. 

CORRELATION. 

The  Gogebic,  Marquette,  and  original  Huronian  districts  are  approximately  in  an  east-west 
line  and  the  prevailing  strikes  of  the  lower  Huronian  in  all  but  the  Crystal  Falls  and  Iron  River 
districts  are  in  the  same  general  direction,  favoring  the  correlation  of  the  rocks  of  the  different 
districts. 

In  each  district  the  lower  Huronian  rests  with  profound  unconformity  upon  the  underlying 
Archean  or  basement  complex,  the  unconformity  being  marked  where  ex])osed  by  differences 
in  lithology,  by  metamoqahism,  and  by  the  presence  of  a  basal  conglomerate,  and  being  shown 
also  by  the  areal  relations  and  relations  to  intrusive  rocks. 

The  lower  Huronian  is  overlain  unconformably  by  the  upper  Huronian  (Animikie  group) 
in  all  the  districts,  and  by  the  middle  anil  upper  Huronian  in  the  Marquette,  Crystal  Falls, 
original  Huronian,  and  Menominee  districts. 

The  lower  Huronian  of  the  southern  sub])rovince  has  no  counteipart  in  the  northern  sub- 
province,  though  it  occu]Hes  the  same  general  position  in  the  succession  as  the  lower-middle 
Huronian  of  the  northern  subprovince. 

a  Chamberlin,  T.  C,  and  Salisbury,  R.  D.,  Geology,  vol.  2,  1900,  pp.  KS-ltB. 

b  Van  Hise.  C.  R..  A  treatise  on  metamorphism:  Mon.  U.  S.  Geo!.  Survey,  vol.  47,  1904,  p.  477. 

<:  Lycll,  Charles,  Principles  of  geology,  vol.  1.  10th  ed.,  lSii7,  pp.  305-326. 


GENERAL  GEOLOGY.  607 

MIDDLE   HURONIAN. 
LITHOLOGY    AND    SUCCESSION. 

The  middle  Huronian  is  represented  in  the  ilarquette,  original  Huronian,  Crystal  Falls, 
and  ilenominee  districts.  In  the  Marquette  district,  where  it  was  iirst  discriminated  and  is  best 
developed,  it  consists  from  the  base  up  of  the  Ajibik  quartzite,  Siamo  slate,  and  iron-bearing 
Xegaunee  formation  (nonclastic).  On  the  north  shore  of  Lake  Huron  the  broader  features  of  the 
middle  Huronian  are  analogous  with  those  of  the  Marquette  district — that  is  to  say,  the  rocks 
comprise  a  clastic  formation  below,  consisting  of  a  conglomerate  at  the  base  and  over  this  a 
quartzite,  both  so  thick  and  extensive  that  they  have  been  mapped  separately,  antl  above  these 
clastic  formations  a  clierty  limestone. 

In  the  Crystal  Falls  district  the  middle  Huronian  is  represented  principally  by  the  volcanic 
Hemlock  formation,  containing  iron-bearing  slate  near  the  top.  The  iron-bearing  Xegaunee 
formation  is  doubtfully,  present;  the  Ajibik  quartzite  is  present  near  the  northeast  corner  of 
the  district,  near  the  Marquette  district.  Volcanism  seems  to  have  intervened  between  the 
deposition  of  the  lower  Huronian  antl  the  u])per  Huronian,  making  lithologic  correlation  diffi- 
cult. It  is  to  be  noted,  however,  that  the  Clarksburg  volcanic  rocks  of  the  Marquette  district 
began  to  be  extruded  in  middle  Huronian  time,  and  tljese  are  therefore  to  be  partly  correlated 
with  the  Hemlock  volcanic  rocks  of  Crystal  Falls. 

In  the  Menominee  tlistrict  the  miildle  Huronian  is  taken  to  be  represented  by  cherty  quartz- 
ite, heretofore  not  separated  from  the  Randville  dolomite  of  the  lower  Huronian.  There  is 
evidence  also  in  the  jasper  and  iron  pebbles  in  tlie  conglomerate  at  the  base  of  the  upper  Huronian 
that  an  iron-bearing  formation  corresponding  in  position  and  character  to  the  Negaunfie  was 
present  in  the  district  before  upper  Huronian  time,  but  no  remnants  of  this  are  now  known. 

IGNEOUS    ROCKS. 

In  the  Marquette  district  the  middle  Huronian  is  associated  with  part  of  the  Clarkslaurg 
formation  of  basic  intrusive  and  extrusive  rocks.  In  the  original  Huronian  district  igneous 
rocks  are  lacking  in  the  middle  Huronian.  The  presence  of  igneous  rocks  in  the  middle  Huronian 
of  the  Marquette  district  and  their  absence  in  the  middle  Huronian  of  the  original  Huronian 
district  may  perhaps  be  correlated  with  the  presence  in  the  former,  and  the  absence  in  the  latter, 
of  an  iron-bearing  formation.     (See  pp.  506-507.) 

Hemlock  volcanic  rocks  form  the  princijjal  part  of  the  middle  Huronian  in  the  Crystal 
Falls  district.  In  the  IMenominee  district  volcanic  rocks  are  absent  from  the  division.  The 
Keweenawan  ( ?)  granites  of  Florence  County  doubtless  also  cut  the  midtlle  Huronian,  though 
they  nowhere  come  into  contact  with  it  at  the  surface. 

CONDITIONS    OF   DEPOSITION. 

The  extensive  formations  of  cleanly  assorted,  well-rounded,  ripple-marked  sands,  now 
quartzites,  of  the  middle  Huronian,  both  south  of  Lake  Superior  and  north  of  Lake  Huron, 
point  toward  subaqueous  deposition.  The  pure  nonclastic  iron-bearing  formation  south  of 
Lake  Superior  and  the  cherty  limestone  formation  north  of  Lake  Superior  point  in  the  same 
direction.  Still  further  is  this  shown  by  the  association  of  these  rocks  with  partly  subaqueous 
volcanic  rocks  of  the  Clarksburg  formation.  The  iron-bearing  formation  and  possibly  some 
of  the  associated  slates  have  a  close  genetic  connection  with  some  of  the  associated  volcanic 
rocks. 

In  the  Crystal  Falls  district  the  middle  Huronian  was  principally  a  time  of  extrusive 
volcanism,  partly  subaqueous.  The  volcanic  rocks  are  interbedded  with  the  slates  and  iron- 
bearing  rocks,  subaqueously  deposited.  In  the  Menominee  district  the  middle  Huronian  is 
represented  only  by  shreds  of  quartzite  and  perhaps  by  the  iron-bearing  Negaunee  formation. 


608  GEOLOGY  OF  THE  LAKE  SLTERIOR  REGION. 

The  quartzite  is  very  cherty,  as  if  derived  from  decomposition  of  the  Randville  dolomite, 
against  which  it  rests.  It  is  well  betlded  and  well  assorted.  At  one  locality  there  seems  to  l)e  a 
conglomerate  with  well-rovmdcil  bowlders  near  its  base. 

On  the  whole  the  evidence  favors  subaqueous  deposition  of  the  middle  Huronian. 

CORRELATION. 

The  middle  Huronian  rocks  in  the  Marquette  and  original  Huronian  districts  are  correlated 
on  the  basis  of  similar  succession  of  clastic  and  nonclastic  rocks,  similar  relations  to  the  lower 
Huronian,  similar  east-west  trend,  similar  metamori^hism,  and  the  fact  that  they  are  subaqueous 
in  both  districts.  They  diiVer  in  that  the  nonclastic  formation  of  the  Marfiuette  di.strict  is  an 
iron-bearing  formation  and  that  of  the  original  Huronian  district  a  limestone,  that  associated 
igneous  rocks  are  present  in  the  Marcjuette  district  and  not  in  the  original  Huronian  district, 
and  that  in  the  Marquette  district  the  overlying  rocks  are  upper  Huronian  and  in  the  original 
Huronian  district  no  upper  Huronian  is  present,  although  to  the  northeast  in  the  Sudbury 
basin  rocks  probably  to  be  correlated  with  the  mitldle  Huronian  are  overlain  unconformably 
by  rocks  with  upper  Huronian  characteristics. 

The  middle  Huronian  of  the  Crystal  Falls  district,  being  largejy  volcanic,  may  be  correlated 
lithologically  with  the  lower  part  of  the  Clarksburg  formation  of  the  Marquette  district.  So 
far  as  the  Ajibik  and  Negaunee  formations  are  present  in  this  district  they  are  correlated 
directly  with  formations  of  the  same  names  in  the  Marquette  district.  They  occur,  however, 
in  the  northeast  corner  of  the  Crystal  Falls  district,  the  area  nearest  to  the  Marquette  •district, 
and  the  correlation  is  of  little  aid  in  correlating  the  middle  Huronian  as  a  whole.  The  middle 
Huronian  of  the  Crystal  Falls  .district  is  principally  a  great  assemblage  of  volcanic  rocks  Ij'ing 
between  the  lower  Huronian  and  upper  Huronian  and  differing  from  the  dominantly  sedi- 
mentar}^  middle  Huronian  of  other  districts.  Its  correlation  is  therefore  based  principally  on 
its  position  in  the  geologic  column. 

The  middle  Huronian  of  the  Menominee  district  is  correlated  with  the  middle  Huronian 
of  other  areas  almost  entirely  on  the  basis  of  its  stratigraphic  position,  unconformably  above  the 
lower  Huronian  and  unconformably  below  the  upper  Huronian.  As  it  consists  only  of  a  rem- 
nant of  quartzite,  lithologic  comparison  with  the  middle  Huronian  of  other  districts  is  of  no 
value. 

The  equivalents  of  the  middle  Huronian  have  not  been  identified  in  the  other  districts  of 
the  Lake  Superior  region,  though  it  is  possible  that  future  work  may  result  in  its  identification 
in  the  Florence  and  Iron  River  districts. 

trPPER  HURONIAN   (ANIMIKIE   GROUP). 

LITHOLOGY    AND    SUCCESSION. 

The  upper  Huronian  of  the  southern  subprovince  consist  mainly  of  a  thick  slate  foimation 
carrying  two  or  more  iron-bearing  beds  or  lenses  near  its  base  ami  possibly  othei-s  higher  in  the 
group. 

In  the  Gogebic  district  it  consists  from  the  base  up  of  the  Palms  formation,  the  iron-bearing 
Ironwood  formation,  and  the  Tyler  slate. 

In  the  Marquette  district  it  consists  from  the  base  up  of  the  Goodrich  quartzite,  the  iron- 
bearing  Bijiki  schist,  and  the  Mchigamme  slate. 

In  the  Menominee  district  the  lower  iron-bearing  part  of  the  upper  Huronian  is  called  the 
Vulcan  formation  and  the  upper  slate  the  Michigamme  ("Hanbury")  slate.  The  Vulcan 
formation  is  subdivided,  from  the  base  up,  into  the  Tradere  iron-bearing  member,  the  Brier  slate 
member,  and  tlie  Curry  iron-bearing  member. 

In  the  Crystal  Falls  district  a  similar  subdivision  into  Vulcan  and  ^lichigamme  is  made, 
but  there  not  only  are  the  members  of  the  Vulcan  formation  not  discriminated,  but  the  forma- 
tion is  iuterbcdded  near  the  base  of  the  slate  and  is  treated  as  a  member  of  the  .Micliigamme 


GENERAL  GEOLOGY.  609 

and  not  as  a  distinct  formation,  although  it  is  mapped  separately.  On  former  maps  of  the  Crys- 
tal Falls  district"  the  iron-bearing  rocks  were  not  given  a  separate  name,  but  were  mapped  with 
the  slate  as  upper  Huronian.  In  this  report  they  are  correlated  with  the  Vulcan  formation 
and  called  the  Vulcan  iron-bearing  member. 

In  the  Calumet  district  the  upper  Huronian  is  divided  into  the  Michigamme  slate,  the 
Vulcan  formation,  and  a  third  formation  at  the  base,  the  Felch  schist.  The  Vulcan  formation 
is  subdivided  into  three  iron-bearing  beds  and  two  slate  beds. 

In  the  Felch  Mountam  district  the  slate  is  absent  except  where  the  district  opens  out  to  the 
west;  the  Vulcan  formation  is  not  subdivided  and  the  Felch  schist  forms  the  base  of  the  upper 
Huronian.  The  Vulcan  and  Felch  formations  of  this  district  correspond  respectively-  with  the 
"Groveland"  and  "Mansfield"  formations  of  the  earlier  mapping  of  the  district.  The  reasons 
for  the  change  of  names  are  given  on  ])agcs  303-305. 

In  the  Iron  River  district  the  upper  Huronian  is  represented  by  the  Michigamme  slate,  inter- 
bedded  near  the  base  of  which  is  an  iron-bearmg  member  that  has  been  correlated  with  the 
Vulcan  formation,  although  the  evidence  is  not  conclusive  that  certain  iron-formation  bands 
classed  as  Vulcan  may  not  belong  stratigraphically  higher  than  the  Vulcan  formation  as  typically 
developed  in  the  Menominee  district.  The  same  remarks  may  be  made  concerning  the  Florence 
district  in  Wisconsin. 

Throughout  the  southern  subprovince  the  Michigamme  slate  is  closely  folded  and  in  much  of 
the  area,  especially  in  the  vicinity  of  the  intrusive  rocks  it  has  a  strongly  developed  cleavage. 
Bedding  is  usually  to  be  observed  except  in  places  where  there  has  been  exceptionally  good 
development  of  cleavage.  The  iron-bearing  formations  and  quartzites  also  have  been  closely 
folded,  but  lack  cleavage. 

IGNEOUS    ROCKS. 

Basic  intrusive  and  extrusive  rocks  in  the  upper  Huronian  are  represented  in  this  subprov- 
ince by  the  Clarksbuj'g  formation  of  the  Marquette  district;  by  the  Prescjue  Lsle  area  of  the 
Penokee-Gogebic  district,  where  volcanic  rocks,  lavas,  and  tuffs  were  built  up  during  the  larger 
part  of  uppei'  Huronian  time,  and  by  basaltic  schists  of  the  Menominee^  Crystal  Falls,  Iron  River, 
and  Florence  districts.  In  individual  occurrences  it  has  not  been  found  possible  to  determine 
whether  these  basic  igneous  rocks  are  intrusive  or  extrusive  or  even  to  exclude  the  possibility 
of  the  rocks  being  pre-Huronian.  Some  of  the  intrusive  rocks  are  probably  of  Keweenawan 
age.  Granites  of  probable  Keweenawan  age  intrude  the  upper  Huronian  and  associated  basaltic 
extrusives  in  the  Florence  district. 

CONDITIONS    OF    DEPOSITION. 

The  conditions  of  dei^osition  of  the  upper  Huronian  ui  this  subprovince  are  discussed  on 
pages  612-614. 

CORRELATION. 

There  can  be  little  doubt  about  the  correlation  of  the  upper  Huronian  in  the  several  districts 
of  the  southern  subprovince.  The  rocks  as  a  whole  are  easily  eroded  and  heavily  drift  covered 
and  therefore  have  few  outcrops,  with  the  residt  that  areal  connections  have  not  been  every- 
where traced,  although  they  probably  exist.  The  upper  Huronian  of  the  Marquette  district 
opens  on  the  west  and  southwest  into  a  gi-eat  slate  area,  which,  so  far  as  Icnown,  is  the  same 
slate  area  as  that  surroundmg  the  Crystal  Falls  district,  and  thence  extends  south  and  south- 
west into  the  Menominee  and  Iron  River  districts.  Tlu-oughout  the  subprovince  the  greater 
part  of  the  upper  Huronian  is  slate  and  the  u'on-bearing  formation  is  characteristically  near 
the  base  of  the  gi'oup.  In  metamorphism,  folding,  amount  of  intrusive  rocks,  and  relations  to 
intrusive  rocks  the  upper  Huronian  within  the  province  is  a  unit. 

a  Mon.  U.  S.  Geol.  Survey,  vol.  36,  1899. 
47517°— VOL  52—11 39 


610  GEOLOCIY  OF  THE  LAKE  SUPERIOR  KEGTOX. 

From  a  study  of  the  structural  facts  alone  it  may  not  be  aflirincd  tliat  the  unconformity  at 
the  base  of  the  upper  Iluronian  of  the  southern  subprovince  represents  a  considerable  time 
interval.  However,  when  this  unconformity  is  considered  in  connection  with  the  deep  erosion 
and  local  absence  of  the  middle  Iluronian  between  two  divisions,  which  are  identified  on  satis- 
factory evidence,  as  upper  Iluronian  and  lower  Iluronian,  it  is  evident  that  the  time  break 
represented  may  be  a  large  one.  Great  time  intervals  are  known  to  be  represented  in  other 
parts  of  the  geologic  column,  as,  for  mstauce,  between  the  Paleozoic  and  Mesozoic  in  parts  of 
the  West,  where  structural  evidence  is  slight. 

The  correlation  of  the  upi)er  Huronian  of  the  southern  and  northern  subprovinces  is  scarcely 
less  clear.  In  each  subprovmce  the  basal  member  is  quartzite  and  slate,  followed  by  an  iron- 
bearing  formation  and  then  by  thick  slate.  The  differential  metamorphism  Ls  similar  m  the 
two  subprovmces.  In  both  the  upper  Huronian  rests  with  strong  unconformity  upon  Archean 
or  middle  or  lower  Iluronian.  In  both  it  is  unconfonnably  beneath  the  Keweenawan.  On 
the  north  shore  it  dips  gently  to  the  south  under  the  Lake  Superior  syncLine ;  in  the  northern 
part  of  the  southern  subprovince  the  upper  Huronian  of  the  Gogebic  district  dips  steeply  to 
the  north  under  the  same  syncline.  The  identity  in  the  succession  of  formations  in  these  two 
subprovinces,  their  position  immediately  below  the  Keweenawan,  and  their  general  structural 
alliances  with  that  series  give  such  strong  evidence  of  equivalence  that  no  one  can  seriously 
doubt  that  the  upper  Iluronian  of  the  two  regions  is  essentially  contemporaneous. 

If  one  saw  the  flat-lymg,  little-altered  upper  Huronian  at  one  locahty  and  the  most  folded 
and  metamor])hosed  phases  at  another  far  distant  and  had  not  proved  their  continuity,  he 
might  think  that  the  rocks  of  the  different  localities  belonged  to  different  divisions,  but  in 
many  places  the  various  metamorphosed  and  unmetamorphosed  phases  have  been  found  to 
connect. 

GENERAL      REMARKS      CONCERNING      THE      UPPER       HURONIAN      (ANIMIKIE 
GROUP)     OF    THE    LAKE    SUPERIOR    REGION. 

CHARACTER. 

The  Animikie  is  the  only  group  that  is  continuous  throughout  the  Huronian  subprovinces. 
It  is  the  principal  iron-bearing  group.  Although  it  has  been  described  m  connection  with  each 
of  the  subprovinces,  a  further  general  description  is  here  presented  to  emphasize  its  unity  over 
the  Lake  Superior  region. 

The  upper  Huronian  was  deposited  on  a  remarkably  uniform  peneplain.  Remnants  of 
this  peneplain  appear  from  beneath  the  upper  Huronian  hi  the  Mesabi,  Anhiiikie,  and  Gogebic 
districts.  The  post-Animikie  and  post-Keweenawan  folding  have  resulted  in  the  tdting  of  this 
plam  to  various  angles  and  it  is  truncated  by  later  peneplains.  In  each  of  the  districts  in 
which  a  full  succession  is  found  there  is  a  clastic  formation  at  the  bottom,  a  niiddle  iron-bearing 
formation,  and  an  upper  slate  formation.  The  bottom  chxstic  formation  consists  of  a  con- 
glomerate at  the  base,  which  m  the  northern  subprovince  and  the  northern  part  of  the  southern 
subprovince  passes  up  into  a  shale  or  slate  and  in  most  places  linally  into  a  quartzite.  In  the 
different  districts,  and  in  the  same  district,  the  relative  proportions  of  conglomerate,  quartzite, 
and  slate  vary,  as  does  also  the  particular  phase  which  Ls  adjacent  to  the  iron-bearing  formation. 
For  instance,  m  the  Marquette  district  conglomerate  and  quartzite  are  domuiant  hi  the  Goodrich 
quartzite  and  there  is  comparatively  little  slate.  In  the  Penokee-Gogebic  district  conglomerate 
and  slate  are  dominant  in  the  Palms  formation  and  the  quartzite  is  a  thin  formation  at  the 
top.  In  the  Mesabi  district  the  Pokegama  quartzite  is  somewhat  similar.  In  the  Animikie, 
Menominee,  and  Crystal  Falls  districts  the  clastic  formation  is  very  tlihi  hideed. 

Over  the  clastic  formation  is  the  iron-bearmg  formation,  which  hi  the  Marquette  district 
is  known  as  the  Bijiki  schist,  in  the  Menommee  district  as  the  ^■ulcan  formation,  in  the  Crystal 
Falls,  Iron  River,  ami  Florence  districts  as  the  Vulcan  iron-bearing  member,  in  the  Gogebic 
district  as  the  Ironwood  formation,  in  the  Mesabi  district  as  the  Biwabik  formation,  and  in  the 
Cuyuna  district  as  the  Deerwood  iron-bearmg  member.     This  u'on-bearuig  formation  is  by  far 


GENERAL  GEOLOGY.  611 

the  most  persistent  and  important  oi'  those  of  the  Lake  Superior  region.  In  it  are  probaljly  9.5 
per  cent  of  the  known  ore  reserves.  It  is  not  a  pure  nonchistic  formation,  but  has  interstratified 
slaty  hi  vers  of  variable  thickness.  A  number  of  these  layers  have  been  recognized  in  the  Mesabi 
and  Gogebic  districts.  In  the  Menominee  district  one  of  them  is  of  sufficient  thickness  to 
constitute  a  distmct  member  of  the  formation  and  is  known  as  the  Brier  slate  member;  it  sepa- 
rates the  two  ore-bearing  momljers  of  the  Vulcan,  the  Curry  and  Traders.  The  maximum 
tliickness  of  the  u'on-bearing  formation  for  the  region  is  1,000  feet. 

In  parts  of  the  region  the  iron-bearing  formation  does  not  lie  at  a  definite  hoi-izon  l^etween 
the  coarse  clastic  sediments  at  the  base  and  the  shales  aljove,  but  appears  as  more  or  less  isolated 
and  overlapping  lenses  entirely  withm  the  slate  which  forms  the  upper  part  of  the  upper 
Huronian.  This  is  the  characteristic  occurrence  of  the  iron-bearing  formation  of  the  upper 
Huronian  m  the  great  area  extending  south  and  west  from  the  Mesabi  and  St.  Louis  River 
districts,  including  the  new  Cuyuna  range,  and  of  the  triangular  area  of  Michigan  between  the 
Marquette,  Menominee,  and  Gogebic  districts,  mcluding  the  Florence,  Iron  River,  and  Crj^stal 
Falls  districts.  The  iron-bearing  formation  in  this  relation  to  the  slate  appears  also  in  the 
western  part  of  the  Marquette  district.  Iron-bearing  lenses  of  this  kind  seem  on  the  whole  to 
be  more  numerous  near  the  base  of  the  slate  than  elsewhere,  but  in  many  places  it  is  not  known 
what  their  stratigraphic  position  really  is,  the  rocks  both  above  and  below  them  being  slate. 
It  will  be  noted  that  the  sharply  delimited,  extensive  ii-on-bearmg  formations,  occurrmg  at  a 
defuiite  horizon  above  the  lower  clastic  formations  of  the  upper  Huronian,  border  the  okler 
formations  on  the  northwest  and  southeast  sides  of  the  Lake  Superior  syncline,  and  that  the 
discontinuous  lens-shaped  parts  of  the  formations  in  the  slate  are  located  far  from  the  contacts 
with  the  older  formations.  The  suggestion  is  made  that  this  ilifl'erence  may  be  due  to  original 
difference  of  conditions  of  deposition  near  the  old  shore  against  which  the  upper  Huronian 
sea  washed,  as  compared  with  the  conditions  off  shore. 

Above  or  associated  with  tiie  iron-l^earing  formation  is  the  upper  slate  formation  known 
as  the  Michigamme  slate  in  the  Marquette,  Crystal  Falls,  Calumet,  Menominee,  Iron  River, 
and  Florence  districts,  the  Tyler  slate  in  the  Penokee-Gogebic  district,  the  Virginia  slate  in 
the  Mesabi,  Cuyuna,  and  adjacent  districts,  and  the  Rove  slate  in  the  Vermilion  district.  It 
occupies  a  large  area  in  Michigan  south  of  Lake  Superior,  an  immense  area  west  of  Lake  Supe- 
rior extending  far  into  central  Minnesota,  and  a  very  large  area  about  Thunder  Bay  and  vicinity. 
It  probably  extends  westward  beyond  the  western  boundary  of  Minnesota  and  widens  out  in 
this  direction.  It  is  entirely  possible  that  this  formation  wiU  ultimately  be  found  to  connect 
beneath  the  later  formations  with  the  slates  of  the  Black  Hills  of  South  Dakota  and  even  with 
the  Belt  series  of  Montana.  Indeed,  the  areal  extent  of  this  formation  is  far  greater  than  that 
of  all  the  other  Huronian  sediments  of  the  Lake  Superior  region. 

The  formation  being  for  the  most  part  a  slate  and  so  soft  as  to  be  extensively  covered 
by  the  drift,  exposed  sections  in  which  to  measure  its  thickness  are  rare.  Also  cleavage  in 
these  sections  has  so  obscured  bedding  that  estimates  are  worth  little.  In  the  Penokee-Gogebic 
district,  where  such  a  section  is  exposed,  the  possible  maximum  thickness  has  been  estimated 
at  about  12,000  feet,  but  this  is  probably  too  large.     Seaman  and  Lane  "^  suggest  4,000  feet. 

The  rocks  of  this  formation  in  the  Mesabi  and  Animikie  districts  are  principally  shales. 
Elsewhere  they  are  principally  slates.  At  Carlton  and  Cloquet,  on  St.  Louis  River,  tlie  forma- 
tion is  niuch  folded  and  has  a  slaty  cleavage,  and  farther  to  the  southwest,  where  intruded  liy 
masses  of  granite  and  diorite,  it  locally  becomes  so  metamorphosed  as  to  pass  into  a  schist. 
A  like  change  is  noted  in  the  character  of  the  upper  formation,  the  Tyler  slate,  at  the  west  end 
of  the  Penokee  district,  where  it  is  intruded  by  igneous  rocks. 

Conspicuous  in  the  slate  at  many  horizons  are  seams  and  lenses  of  pyritiferous  and  gra- 
phitic slates.  These  are  so  characteristically  associated  with  some  of  the  discontinuous  non- 
bearing  lenses,  originally  iron  carbonate,  as  to  serve  as  guides  in  exploration. 

u  Lane,  A.  C,  and  Seaman,  A.  E.,  ^ioles  on  the  geological  section  of  Michigan:  Jour.  Geology,  vol.  15,  1907,  p.  686. 


612 


GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 


The  slate  as  a  wliole  gives  evidence  by  its  composition  of  being  less  leached  of  its  bases 
than  average  slates  or  residual  clays.  The  cojnposition  also  sliows  that  it  must  have  been 
derived  from  rocks  on  an  average  more  basic  than  granites.  In  figure  76,  prepared  Ijy  S.  H. 
Davis,  the  mineralogical  composition  of  the  upper  Huronian  slates,  calculated  from  chemical 
composition,  is  compared  graphically  with  that  of  a  variety  of  other  clays  and  soils. 

The  upper  Huronian  slate  and  ii'on-bearing  formations  arc  interbe(hled  locally  with 
abundant  basaltic  extrusive  rocks,  partly  subaqueous,  and  tuffs  in  the  southern  subprovince. 
In  the  northern  subprovince  these  are  yet  known  definitely  only  in  the  Cuyuna  district  of 
Minnesota. 

QUARTZ 


CLAY  AND  FERRIC  OXIDE  SILICATES 

Figurl;  7ti.  — Triangular  diagram  comparing  the  amounts  of  undecomposed  silicates,  quartz,  and  residual  weathered  products,  siit-li  as  clay  and 
ferric  oxide,  in  dilTerent  kinds  of  muds,  shales,  and  weathered  rocks.  For  description  of  method  of  platting  see  page  182.  The  mineral  com- 
positions are  calculated  from  chemical  analyses.  Dotted  lines  with  arrows  indicate  the  progressive  change  in  proportions  of  constituents 
between  the  unaltered  and  altered  rocks.  The  diagram  brings  out  clearly  the  fact  that  the  upper  Uuroniau  shale  represents  the  little- 
decomposed  ddbris  of  a  basic  igneous  rock. 


CONDITIONS   OF   DEPOSITION  OF   THE   UPPER   HXTRONIAN   (ANIMIKIE    GROTTP). 

Any  hypothesis  of  the  conditions  of  deposition  of  the  upper  Huronian  must  be  built  around 
the  following  salient  facts: 

The  succession  of  a  thin  fragmental  base,  an  iron-bearing  formation,  and  a  thick  mud 
deposit,  and  the  tiiinness,  evenness,  and  wide  extent  of  the  basal  conglomerate  and  quartzite. 

The  fact  that  the  upper  Huronian  rests  upon  a  Hat  plane  beveling  alike  hard  and  soft, 
resistant  and  nom'esistant  rocks,  without  residual  or  terrestrial  deposits  at  the  base. 


GENERAL  GEOLOGY.  6L3 

The  association  of  discontinuous  iron  carbonate  lenses  with  graphitic  slates  at  different 
horizons,  pointing  strongly  to  bog  or  lagoon  conditions. 

The  lack  of  sorting  or  decomposition  in  the  slates  as  shown  by  analyses. 

Contemporaneous  volcanism,  partly  submarine,  probably  relatetl  to  the  deposition  of  the 
ore,  so  associated  with  the  upper  Huronian  as  to  indicate  subaqueous  origin  for  at  least  a  part 
of  it. 

The  hypothesis  which  seems  to  fit  this  group  of  facts  better  than  others  which  have  suggested 
themselves  to  us  is  this: 

1 .  The  first  upper  Huronian  event  was  the  advance  of  the  upper  Huronian  sea  to  a  shore  line 
somewhere  north  of  the  present  northern  boundary  of  Lake  Superior.  In  the  area  of  Michigan 
and  Wisconsin  it  passed  over  middle  and  lower  Huronian  rocks  which  were  nearly  flat-lying  and 
perhaps  not  much  eroded.  On  the  north  shore  it  passed  over  middle  and  lower  Huronian  rocks 
which  had  been  closely  folded  and  deeply  eroded.  This  advance  was  perhaps  accompanied 
by  some  planation  or  scouring  of  the  land  area,  as  suggested  by  the  evenness  of  this  plane 
and  the  manner  in  which  it  bevels  alike  soft  and  hard  formations  and  by  the  absence  of  residual 
or  terrestrial  deposits  beneath  the  cleanly  assorted  fragmental  base  of  the  upper  Huronian. 
Had  the  land  been  base-leveled  by  terrestrial  erosion  prior  to  the  advance  of  the  sea,  that  advance 
would  seem  likely  to  have  flooded  the  river  mouths  and  required  them  to  build'  up  to  grade, 
resulting  in  the  development  of  terrestrial  deposits,  including  much  mud,  in  advance  of  the 
encroaching  sea,  to  be  ultimately  covered  by  it,  and  not  removed.  It  is  entirely  conceivable 
that  farther  to  the  south  the  upper  Huronian  sea  may  actually  have  advanced  over  this  zone 
of  terrestrial  deposition,  but  that  in  the  Lake  Superior  region  the  sea  had  encroached  upon 
the  upper  portions  of  the  rivers  and  was  cutting  into  the  rock.  There  seems  to  be  an  absence 
of  sea  cliffs  to  the  north  of  the  present  upper  Huronian  beds,  but  this  may  be  explained  by 
later  erosion. 

The  advance  of  the  sea  over  a  gently  sloping  surface  was  accompanied  by  deposition  of  a 
thin  conglomerate  and  sand  formation  spread  evenly  over  a  large  area.  Barrell  "  has  shown 
that  \\'ith  tlie  low  gratlient  characteristic  of  such  advance  the  conglomerate  at  the  base  is  likely 
to  be  very  tliin,  if  not  altogether  lacking,  being  worn  out  by  littoral  abrasion,  and  in  modern 
instances  being  observed  to  disappear  a  short  distance  from  the  shore.  Conglomerates  of  this 
sort  may  be  thick  and  coarse  only  around  monadnocks  standing  above  the  plane  of  transgression. 
The  deposit  of  the  upper  Huronian  sea  seems  to  be  similar  to  the  thin  fragmental  base  of  the 
Cambrian,  which  was  laid  down  by  the  Paleozoic  sea  advancing  also  from  the  south  over  a  flat 
surface.  The  absence  of  conglomerate  in  the  Cambrian  except  around  monadnocks  is  well 
known. 

2.  Then  came  the  deposition  of  the  iron-bearing  material.  Tliis  is  a  chemical  precipitate 
requu'ing  either  quiet  conditions  of  deposition  or  extreme  rapidity  of  deposition  to  account  for 
the  lack  of  interbedded  coarse  fragmental  sediments.  It  has  been  argued  in  another  place 
(pp.  506  et  seq.)  that  the  thick  iron-bearing  formations  near  the  base  of  the  upper  Huronian, 
such  as  those  of  the  Mesabi,  Gogebic,  and  Menominee  districts,  find  their  essential  explanation 
in  their  genetic  relation  with  basic  volcanism,  wluch  furnishes  sources  for  unusually  abundant 
deposition  of  iron  salts.  The  abruptness  of  the  change  from  quartz  sand  to  iron-bearing  forma- 
tion and  the  usual  lack  of  any  fragmental  material  in  the  iron-formation  layers  seem  to  imply 
some  unusual  change  of  conditions,  probably  not  related  to  topographic  or  climatic  changes. 

3.  The  advance  of  the  upper  Huronian  sea  overlapped  the  Lake  Superior  region  but  may 
not  have  progressed  much  farther  north.  We  fuid  no  record  of  it  farther  north,  though  allow- 
ance must  be  made  for  much  erosion.  The  flatness  of  the  plane  would  require  that  planation  or 
scouring  should  l)e  weakened  diu-ing  the  northward  transgression.  The  rivers  would  then  be 
able  to  hold  their  own  against  the  sea,  and  deposition  of  river  alluvium  in  the  form  of  great 
deltas  may  be  supposed  to  have  predominated  over  marine  fragmental  deposition.     Then  were 

o  Barren,  Joseph,  Relative  geological  importance  of  continental,  littoral,  and  marine  sedimentation:  Jour.  Geology,  vol.  14,  1906,  pp.  433-446; 
also  personal  communication. 


614  GEOLOGY  OF  THE  LAKE  SLTPERTOIl  REGION. 

built  up  the  thick  masses  of  mud  deposits  characterized  by  discontinuous,  pyritiferous,  {):raphitic 
seams,  and  iron-car))()iia(o  lenses  at  different  horizons,  wliich  seem  better  explained  by  delta 
and  lagoon  contlitions  than  by  any  other  hypothesis  tliat  has  been  suggested.  A.ssociation 
with  subaqueous  extrusions  is  thus  explained.  So  far  as  deltas  are  terrestrial  the  upper  Huron- 
ian  muds  are  terrestrial. 

The  lack  of  tlecomposition  of  the  muds  and  the  graphitic  material  associated  with  iron 
carbonate,  indicating  the  probable  existence  of  peaty  material  associated  with  bog  deposits, 
favor  the  view  tliat  the  climate  may  have  been  contimiously  cool  and  wot,  for  nowhere  are 
the  conditions  for  hick  of  decomposition,  bog  formation,  and  absence  of  oxidation  of  carbon  so 
well  developed  as  in  a  district  where  a  continuous  covering  of  water  prevents  the  access  of 
oxj'gen.  In  warmer  regions  or  in  those  in  whicii  a  part  of  the  year  .is  hot  and  dry  the  organic 
material  is  likely  to  be  oxidized,  giving  an  abundance  of  carbon  dioxide  for  attack  of  the  rocks. 

Contemporaneous  basic  igneous  extrusions,  so  abundant  in  the  upper  Hiu-onian,  doubtless 
furnishetl  an  unusual  source  for  mud,  by  their  decomposition  when  hot,"  through  the  agencies 
of  acid  solutions,  through  the  agencies  of  the  atmosphere  acting  upon  sulpliides  and  thereby 
freeing  sulphuric  acid  for  attack  on  the  adjacent  rock,  and  finally  perhaps  by  reaction  of  the 
hot  lavas  with  sea  water.  In  figure  76  (p.  612)  is  indicated  the  direction  of  alteration  of  basalt 
by  hot  sulphuric-acid  solutions  of  the  Hawaiian  Islands.  The  most  altered  pliase  represents 
rock  which  has  not  been  transported.  It  is  to  be  noted  that  the  direction  of  alteration  is  some- 
what dilTerent  from  that  of  weathering.  It  is  entirely  possible,  if  not  probable,  from  the  posi- 
tion of  upper  Huronian  slates  in  the  diagram,  that  they  have  been  derived  from  the  katamorphism 
of  basic  igneous  rocks,  both  by  ordinary  weathering  and  by  the  unusual  alteration  of  hot  acid 
solutions  associated  with  the  igneous  rocks  themselves. 

The  upper  Huronian  sediments  are  therefore  regarded  as  the  combined  result  of  an  advanc- 
ing sea  scouring,  perhaps  cutting  the  old  surface,  of  a  source  in  which  basic  volcanic  rocks  form 
a  distinctive  part,  and  of  the  final  deposition  of  a  great  mud  delta. 

The  building  up  of  the  upper  Huronian,  developing  terrestrial  conditions  toward  the  close, 
fm-nishes  an  appropriate  setting  for  the  inauguration  of  the  great  Keweenawan  period  of  terres- 
trial sedimentation  which  followed  after  an  interval  of  erosion. 

KEWEENAWAN  SERIES. 

As  the  Keweenawan  is  a  unit  to  a  greater  extent  than  the  Huronian  or  the  Archean,  being 
located  along  the  border  of  Lake  Superior  with  large  inland  extensions,  and  as  the  general  out- 
line of  the  history  of  the  Keweenawan  has  been  given  in  Chapter  XV,  we  give  here  only  the 
briefest  summary  of  the  salient  features  of  the  series. 

LITHOI.OGT    AND    SUCCESSION. 

It  has  been  seen  tliat  the  Keweenawan  is  separable  uito  three  divisions,  a  lower,  middle, 
and  upper.  The  lower  Keweenawan  was  formed  during  a  period  of  sedimentation  and  con- 
sists of  conglomerates,  sandstones,  shales,  and  limestones.  This  division  of  the  Keweenawan 
is  not  very  thick,  but  it  is  widespread.  The  maximum  measurement  is  1,400  feet.  The  michlic 
Keweenawan  represents  a  time  of  combined  sedimentary  and  igneous  action,  containing  many 
alternations  of  sedimentary  and  igneous  deposits.  In  general  the  igneous  activity  greatly  domi- 
nated in  the  early  part  of  middle  Keweenawan  time,  but  was  less  dominant  in  the  later  part. 
Upper  Keweenawan  time  was  again  a  period  of  normal  sedimentation.  At  the  base  of  the 
upper  Keweenawan  are  thick  conglomerates,  which  are  overlain  by  shales  and  these  bj-  a  very 
thick  sandstone  formation. 

As  contrasted  with  the  Huronian  the  Keweenawan  sediments  are  dominantly  clastic.  Xon- 
clastic  sediments  are  found  only  in  one  locality,  in  the  Nipigon-Black  Bay  district.  Moreover, 
the  clastic  formations  are  coarse,  being  dominantly  either  psepiiitic  or  psammitic.     Only  sub- 


a  Maxwell,  Walter,  Lavas  and  soils  of  the  Hawaiian  Islands:  BiUl.  A,  Exper.  Sta.  Hawaiian  Sugar  Planters  Assoc.,  1905,  pp.  8-22. 


GENERAL  GEOLOGY.  615 

ordinately  are  pelites  present,  the  single  important  representative  being  the  shale  of  the  upper 
Keweenawan. 

Another  feature  in  which  the  Keweenawan  sediments  contrast  with  the  Huronian  is  tliat 
they  are  largely  derived  from  the  igneous  rocks  of  the  series  itself. 

IGNEOUS    ROCKS. 

Tlie  igneous  rocks  of  the  middle  Keweenawan  are  both  plutonic  and  volcanic.  They  include 
basic,  acidic,  and  intermediate  varieties,  the  basic  rocks  being  dominant.  As  the  detritus  of  the 
middle  and  upper  Keweenawan  is  derived  largely  from  the  igneous  rocks  of  the  period  itself,  in 
arriving  at  an  estimate  of  the  mass  of  igneous  intrusions  and  extrusions  of  this  time  we  must 
consider  not  only  the  original  igneous  rocks,  but  the  sediments  wliich  are  derived  fi-om  them. 
The  mass  of  the  Keweenawan  volcanic  and  ])lutonic  facies  is  enormous. 

CONDITIONS    OF    DEPOSITION. 

It  is  probable  that  the  sediments  of  the  Keweenawan  were  largely  land  deposits.  (See 
pp.  416-418.)  The  principal  arguments  for  this  conclusion  are  their  prevailing  red  color,  their 
little-assorted,  feldspatliic  nature,  and  their  rapid  alternation  with  abundant  extrusive  rocks 
having  textures  that  are  ordinarily  associated  \vith  subaerial  cooling,  in  contrast  with  the  tex- 
tures of  subaqueous  cooling  so  common  in  the  volcanic  rocks  of  the  lower  Huronian  and  the 
Keewatin.  But  it  is  also  probable  that  a  portion  of  them  were  deposited  under  water.  In 
the  discussion  of  orogeny  (pp.  622-623)  it  is  shown  that  the  Lake  Superior  basin  was  formed 
largely  in  Keweenawan  time,  and  it  is  highly  probable  that  this  basin  contained  water. 

CORRELATION. 

The  correlation  of  the  different  areas  of  the  Keweenawan  is  a  simple  problem.  The  great 
area  of  Keweenawan,  extending  from  Keweenaw  Point  through  northern  ^Michigan  into  Wiscon- 
sin ami  Minnesota  and  thence  northeastward  to  the  Thunder  Bay  district  and  Lake  Nipigon,  is 
almost  continuous.  Therefore  the  only  problem  of  correlation  so  far  as  the  general  series  is  con- 
cerned is  that  of  the  rocks  of  Isle  Roj'al,  Michipicoten,  and  the  areas  on  the  east  coast  of  Lake 
Superior.  The  placing  of  these  rocks  in  the  Keweenawan  is  based  on  their  position  at  the  top  of 
the  pre-Cambrian,  the  unconformity  at  their  base,  and  their  remarkable  likeness  in  litliology, 
succession,  deformation,  and  metamorphism  to  the  rocks  of  the  main  Keweenawan  area. 
Though  all  these  points  bear  on  the  question,  it  was  the  likeness  of  the  lavas  of  these  areas  to 
those  of  the  main  area  and  their  interstratification  with  red  sandstones  and  conglomerates 
which  led  the  earlier  geologists  who  worked  in  the  Lake  Superior  region  to  recognize  the  identity 
of  the  separated  areas  of  Keweenawan  rocks. 

The  problem  of  fixing  the  exact  relations  of  the  Keweenawan  and  Cambrian  is  not  so 
simple  The  evidence  as  given  in  Chapter  XV  is  in  favor  of  the  Algonkian  age  of  the  main  part 
of  the  Keweenawan. 

PALEOZOIC   ROCKS. 

The  Keweenawan  is  the  latest  period  which  this  monograph  treats  in  detail.  On  the  gen- 
eral geologic  map  (PI.  I,  in  pocket)  the  Paleozoic  and  later  rocks  are  shown  as  covering  a  large 
part  of  the  area  south  of  Lake  Superior,  but  they  are  all  represented  l)y  one  color,  for  it  is  not 
our  purpose  to  consider  the  post-Algonkian  formations  separately.  The  Paleozoic  rocks  are 
mentioned  only  in  so  far  as  they  are  related  to  the  Proterozoic — that  is,  the  Algonldan  and 
Archean. 

For  the  most  part  the  formation  which  overlies  the  Proterozoic  rocks  is  a  sandstone,  which 
is  generally  recognized  as  of  Cambrian  age.  Its  basal  portion  where  in  contact  mth  iron-bearing 
formations  consists  of  detrital  ferruginous  rocks.  This  formation  is  everywhere  in  a  substan- 
tially horizontal  attitude,  thus  conti-asting  strongly  with  the  Proterozoic  rocks.  In  general  the 
relations  between  this  sandstone  and  the  Proterozoic  rocks  are  those  of  most  profound  uncon- 


616  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

formity,  luid  tliis  is  true  whichever  of  the  more  ancient  series  undeilics  the  sandstone.  The 
manner  in  which  tiu^  ('ambrian  sandstone  cuts  unconformahly  across  tiie  several  series  of  the 
pro-Cumbrian  is  well  illustrated  on  tlie  east  side  of  the  ])re-Cumbrian  area  of  the  Upper  Penin- 
sula of  Michigan  and  northern  Wisconsin.  Here  the  Cambrian  is  fossiliferous.  The  uncon- 
tormnblo  relation  to  the  Arcliean  is  splendidly  illustrated  alono;  the  Lake  Superior  shore  north  of 
Mar<iuotte.  The  discordant  relations  with  the  Iluronian  are  shown  at  many  localities  in  the 
Menominee  district.  At  some  localities  in  northern  Wisconsin  tiic  sandstone  rests  upon  the 
Kewoenawan.  The  discordant  relation  between  the  Cambrian  sandstone  and  tlie  Keweenawan 
is  particularly  well  seen  at  Taylors  Falls,  on  St.  Croix  River.  Here  the  sandstone  rests  upon  the 
tilted  edges  of  the  lower  Keweenawan.  At  this  locality  the  sandstone  has  been  found  to  con- 
tain fossils  wliicii  have  been  determined  by  Berkey  °  to  combine  "to  a  certain  tlegree  character- 
istics of  both  the  ;\Iiddle  and  Upper  Cambrian,"  wliich  "do  not  as  a  whole  present  a  primitive 
faunal  aspect."  Apparently  the  earliest  Paleozoic  rocks  here  are  either  those  of  the  upper  part 
of  the  Mitldle  Cambrian  or  the  lower  part  of  the  Upper  Cambrian,  or  both. 

Adjacent  to  Lake  Superior  is  an  area  of  sandstone,  about  the  age  and  relations  of  which 
there  is  room  /or  difference  of  opinion.  Tiiis  area  of  sandstone  is  south  of  the  west  end  of 
Lake  Superior,  making  the  shore  of  Chequamagon  Bay,  the  south  shore  of  the  west  end  of  Lake 
Superior,  and  the  Madaline  Islands  (Lake  Superior  sandstone).  In  this  area  the  Lake  Superior 
sandstone  does  not  carry  fossils  and  is  possibly  Keweenawan,  but  it  has  been  regarded  by  alias 
pro])ably  the  efjuivalent  of  the  fossiliferous  Upper  Cambrian  to  the  south  and  is  so  treated 
in  this  monograj)h.  That  these  sandstones  liave  relations  to  the  Archean  and  Iluronian  hke 
those  described  for  the  more  extensive  areas  of  Cambrian  sandstone  to  the  south  has  been 
agreed  to  by  all  observers  from  early  days.  It  is  also  agreed  that  tliese  sandstones  are  certainly 
later  than  and  have  unconformable  relations  with  the  midtUe  and  lower  Keweenawan.  Follow- 
ing Irvuig,  we  have  incUned  to  the  view  that  the  same  relation  exists  between  these  sandstones 
and  the  upper  division  of  the  Keweenawan.  However,  Lane  and  Seaman  ''  believe  tliat  the 
Upj)er  Cambrian  sandstone  of  Checjuamagon  Bay  and  the  Madaline  Islands  grades  down  into 
upper  Keweenawan  sandstone  (which  they  call  Freda).  Tliis  has  been  confirmed  by  recent 
work  of  Thwaites,  of  the  Wisconsin  Geological  Survey  (unpublished).  The  significance  of  tiiis 
relation  in  the  correlation  of  the  Keweenawan  series  is  discussed  in  the  chapter  on  the  Keweena- 
wan (pp.  415-416). 

The  Paleozoic  rocks  of  this  region,  except  in  the  area  above  noted,  contain  marine  fossils 
at  several  horizons  and  are  therefore  in  large  part  submarine  deposits.  They  form  a  portion 
of  the  great  series  of  Paleozoic  rocks  which  has  been  traced  in  continuous  overlap  over  a  large 
part  of  the  North  American  continent.  That  any  of  them  are  of  terrestrial  origin  is  not  proved, 
though  it  is  not  impossible  that  part  of  the  samlstone  bordering  the  southwest  shore  of  Lake 
Superior  may  be  terrestrial.  The  abundant-  partly  decomposed  feldspar  in  the  Cambrian  of  the 
Lake  Superior  region  is  probably  derived  largely  from  the  Keweenawan  below,  which  is  beheved 
to  be  in  part  a  terrestrial  deposit. 

CRETACEOUS  ROCKS. 

In  northern  Michigan,  Wisconsin,  and  eastern  Minnesota  the  Paleozoic  are  the  fossiliferous 
formations  that  rest  upon  the  pre-Cambrian,  but  in  northern  Minnesota  there  are  local  ])atches 
of  Mesozoic  (Cretaceous)  rocks  which  have  tliis  position.  These  show  that  in  such  areas  either 
Paleozoic  rocks  were  never  deposited  over  the  pre-Cambrian,  or  else,  and  tliis  is  more  probable, 
they  were  deposited  and  removed  by  erosion  before  Cretaceous  time.  The  Cretaceous  carries 
marine  fossils.     Its  basal  portion    contains  detrital  ferruginous  sediments. 

a  Berkey,  C.  P.,  Geology  of  the  St.  Croix  Dalles:  Am.  Geologist,  vol.  21,  1898,  p.  292. 

^  Lane,  A.  C,  and  Seaman,  .\.  E.,  Notes  on  the  geological  section  of  Michigan;  pt.  1,  The  pre.Ordovician:  Jour.  Geology,  vol.  15, 1907,  pp.  680-69S. 


GENERAL  GEOLOGY.  617 

PLEISTOCENE  DEPOSITS. 

The  Pleistocene  deposits  of  the  Lake  Superior  region  are  separately  treated  in  Chapter  XVI 
(pp.  427-459)  and  will  not  be  summarized  here. 

PRE-CAJMBRIAN  VOLCANISM. 

It  is  by  contrasting  the  volcanism  of  the  different  pre-Cambrian  periods  tliat  we  gain  an 
idea  of  their  relative  importance.  The  volcanism  of  the  Archean  is  unicjue,  botii  as  to  volume 
and  as  to  extent.  If  we  may  presume  that  the  Archean  which  is  buried  is  of  the  same  character 
as  that  which  is  now  at  the  surface — and  this  has  been  shown  in  some  })laces  by  drilling — it  would 
follow  that  the  Archean  rocks  not  only  once  covered  the  entire  Lake  Superior  region  but  extended 
to  an  indefinite  distance  in  all  directions.  They  are  composed  dominantly  of  igneous  rocks, 
volcanic  and  plutonic.  The  mass  of  igneous  rocks  of  this  time  is  immeasurably  greater  than  that 
of  any  succeeding  pre-Cambrian  epoch;  indeed,  much  greater  than  tiiat  of  all  of  them  put 
together.  Evidence  has  also  been  given  (pp.  510-512)  in  favor  of  the  idea  that  some  of  the 
basic  volcanic  rocks  are  submarine. 

In  the  Iluroniau  also  there  are  intrusive  and  extrusive,  rocks  of  both  basic  and  acidic 
character  in  vast  volume,  but  far  less  in  amount  than  the  enormous  masses  of  the  Archean. 
The  basic  extrusive  rocks  are  abundant  in  the  middle  and  ui)per  Huronian  of  the  southern 
subprovince  and  especially  in  the  upper  Huronian,  but  their  distribution  is  local.  Like  the 
extrusive  rocks  of  the  Keewatin,  those  of  the  Huronian  are  partly  submarine.  They  are  repre- 
sented by  the  Clarksburg  formation  of  the  Marquette  district,  the  Hemlock  formation  of  the 
Crystal  Falls  district,  the  volcanic  rocks  of  Brule  River  in  the  Florence  district,  the  volcanic 
rocks  of  Presque  Isle  in  the  Gogebic  district,  and  many  unnamed  greenstones  in  the  Crystal 
Falls  and  Iron  River  districts.  In  the  lower  Huronian  basic  extrusive  rocks  are  subordinate. 
Granites  are  extensively  intruded  into  the  Huronian. 

The  Keweenawan  was  a  time  of  volcanism,  plutonic  and  surface,  wliich  extended  over  the 
entire  Lake  Superior  basin  and  to  varying  distances  inland — indeed,  a  time  of  regional  volcanism 
which  can  be  fairly  compared  with  the  outbreaks  of  Tertiary  volcanoes  in  parts  of  the  western 
United  States.  In  northern  Canada  and  in  the  southwestern  United  States  are  large  areas 
showing  many  volcanic  rocks  which  may  belong  to  this  same  period.  The  basic  extrusive 
rocks  of  the  Keweenawan  contrast  with  those  dominant  in  the  Huronian  and  Keewatin  in 
exhibiting  textures  peculiar  to  subaerial  cooling  instead  of  textures  characteristic  of  subaqueous 


cooling. 


pre-ca:vibrian  life. 


No  fossils  definitely  recognizable  as  such  have  yet  been  found  in  the  pre-Cambrian  of  the 
Lake  Superior  region.  The  greenalite  granules  of  the  Mesabi  district,  first  called  glauconite 
and  thought  possiblj'  to  be  of  organic  origin,  are  now  known  to  be  chemical  precipitates.  The 
carbon  that  is  so  abundant  in  the  shales,  a  part  of  it  in  the  form  of  hydrocarbon,  is  probably 
of  organic  origin.  The  limestones  give  no  decisive  proof  one  way  or  the  other,  but  they  are 
evidence  of  extensive  carbonation  of  the  rocks,  which  is  now  largely  accomplished  by  the 
assistance  of  organisms.  The  probable  existence  of  life  is  also  indicated  by  the  well-assorted 
nature  of  the  sediments  of  some  of  the  series,  implying  the  presence  of  vegetable  life  to  assist 
in  rock  decay. 

unconformities. 

UNCONFORMITY  BETWEEN  THE  ARCHEAN  AND  LOWER  HURONIAN. 

Unconformity  may  signify  discordance  of  structure  and  intervening  erosion  with  or  without 
great  time  lapse,  or  great  time  lapse  with  or  without  great  discordance  in  structure  or  erosion, 
or  both.  The  lapse  of  time  may  be  measured  by  the  extent  of  the  intervening  deformation 
and  erosion  or  by  the  absence  of  beds  known  to  have  been  deposited  elsewhere  during  the  interval, 


618  GEOLOGY  OF  THE  LAI^E  SUPERIOR  REGION. 

which  iiiav  or  iiiav  nut  have  covered  the  area  in  question.  "Great  unconformity"  as  the  term 
is  onhnarily  used  means  structural  discordance,  deep  erosion,  long  time  interval,  and  lack  of 
deposition  of  sediments  known  to  be  deposited  elsewhere,  or  some  coint)inati<jn  of  these 
conditions.  Of  these  criteria,  the  first  three  are  the  ones  here  emphasized.  Tlie  correlation 
of  the  pre-Caml)rian  between  widely  separated  areas  is  still  so  uncertain  in  tiie  lack  of  fossils 
that  the  last  criterion  can  not  be  satisfactorily  used. 

Wlierever  the  lower  Huronian  is  distinctly  recognized  as  such,  there  is  an  unconformity 
at  its  base.  The  rocks  on  the  two  sides  of  the  unconformity  contrast  ^\■idely.  Those  on  one 
side  of  it  are  dominantly  igneous  rock§,  partly  plutonic;  tliose  on  the  other  side  are  doniinantly 
sedimentary.  During  the  time  represented  by  tliis  unconformity  what  seems,  from  present 
evidence,  to  have  been  a  great  world  period  of  volcanism  ceased  and  the  conditions  became 
such  that  normal  sedimentary  rocks  were  formed. 

Contrast  in  the  character  of  the  rocks  on  the  two  sides  of  the  unconformity  is  correlated 
with  other  evidence  of  the  greatness  of  the  break.  Before  the  lowest  Huronian  was  deposited, 
the  Keewatin  and  in  places  the  Laurentian  rocks  had  taken  on  a  schist osity.  The  plutonic 
rocks  of  Archean  time  had  been  brought  to  the  surface  by  erosion.  The  basal  conglomerate 
beds  of  the  lower  Huronian  rest  upon  the  Keewatin  schists  at  various  angles.  It  is  not  easy  to 
conceive  of  a  physical  break  more  indicative  of  lapse  of  time  than  that,  for  instance,  which  is 
shown  w-ith  diagrammatical  sharpness  at  the  east  end  of  the  Gogebic  district  between  the  lower 
Huronian  Sunday  quailzite  and  the  Keewatin  scliists.  Of  course  where  the  folding  has  been 
close  and  the  shearuig  between  the  Huronian  and  the  Archean  very  great,  the  evidence  of 
unconformity  may  have  been  paitly  obliterated  and  the  two  series  appear  to  grade  into  each 
other — for  instance,  at  certain  places  near  Teal  Lake,  but  even  here  the  unconformity  may  be 
recognized. 

On  the  north  shore  of  Lake  Superior  the  unconformity  at  the  base  of  the  Huronian  is  not 
conspicuous  but  is  as  certainly  existent  as  that  south  of  I^ake  Superior.  In  the  'S'ermilion  district 
the  Huronian  series  is  a  definite  succession  beginning  \\ith  conglomerates  and  passing  up  mto 
slates.  The  discrunination  between  the  basal  complex  and  the  Huronian  is  an  easy  one.  The 
break  usually  has  the  aspect  of  one  of  the  first  magnitude.  In  some  YermiUon  localities,  espe- 
cially where  only  the  conglomeratic  and  arkosic  faciesof  the  Huronian  are  found,  and  these  are 
largely  composed  of  the  immediately  underlying  rock,  the  break  could  not  l)e  asserted  to  be  of 
great  magnitude.  If  the  suggestion  is  correct  that  in  lower  Huronian  tune  the  region  north  of 
Lake  Superior  was  in  large  measure  a  land  rather  than  an  oceanic  area,  tliis,  combuied  -with  the 
fact  that  basaltic  tuffs  and  conglomerates  occur  in  the  Archean,  is  sufficient  to  explain  the 
confusion  and  the  apparent  insignificance  at  some  places  of  the  unconformity  at  the  base  of  the 
Huronian.  It  is  entirely  possible  that  mistakes  have  been  made  in  the  placmg  of  certam  con- 
glomerates in  the  Huronian.  If  it  is  admitted  that  there  may  be  locaUties  in  which  the  relation 
is  confused,  whereA'er  the  Huronian  is  represented  by  a  great  series  of  sediments,  as  m  the 
Vermifion  district,  tlie  Michipicoten  district,  and  the  Cobalt  tlistrict,  there  is  no  difliculty  what- 
ever in  discriminating  between  the  Archean  and  the  Huronian  as  a  whole  and  in  proving  that 
a  profound  unconformity  separates  the  two. 

UNCONFORMITY    BETWEEN    THE    LOWER    AND    MIDDLE    HLTRONIAN. 

E-vidence  of  the  unconformity  between  the  lower  Huronian  and  the  middle  Huronian  is 
plain  in  the  Marquette  district,  where  the  basal  conglomeratic  formation  of  the  middle  Huro- 
nian cuts  diagonally  across  all  the  formations  of  the  lower  Huronian  and  downi  to  the  Archean. 
This  means  that  after  the  lower  Huronian  was  deposited  antl  before  middle  Huronian  time 
the  lower  Huronian  Was  indurated  and  brouglit  to  the  surface,  and  difi'erential  erosion  occurretl 
sufficient  to  cut  througli  the  entire  division  into  tlie  Archean.  The  disconlance  of  strike  jmd 
dip  between  the  two  divisions  at  any  one  locality  is  shght. 


GENERAL  GEOLOGY.  619 

In  the  Ciystal  Falls  district  the  mitldle  Iluronian  is  composed  mainly  of  volcanic  rocks. 
So  far  as  its  structure  can  be  worked  out  it  is  nearly  accordant  with  tlie  lower  Huronian,  but 
in  the  nature  of  the  case  conformity  or  unconformity  is  difficult  to  prove.  In  tlie  Menominee 
district  the  middle  Iluronian  cjuartzite  rests  on  the  Kaudville  dolomite  of  the  lower  Huronian 
with  sUght  though  distinct  structural  discordance.  Conglomerate  is  found  at  one  locality  in 
this  cUstrict. 

UNCONFORMITY     AT     THE     BASE     OF     THE     UPPER     HURONIAN     (ANIMIKIE 

GROUP). 

The  unconformity  at  the  base  of  the  upper  Iluronian  is  easily  recognized  as  extending  over 
the  Lake  Superior  region.  The  upper  Huronian  rests  at  different  locahties  on  each  of  the 
more  ancient  tUvisit)ns  of  middle  Iluronian,  lower  Huronian,  and  Archean,  tnnicating  and 
derivmg  detritus  from  whichever  of  these  divisions  it  overlies.  Tiie  erosion  precedmg  upper 
Huronian  time  apparently  reduced  the  larger  part  of  the  Lake  Superior  region  to  a  peneplain. 
The  best  illustration  of  this  is  furnished  by  the  Penokee-Gogebic,  Mesabi,  and  Animikie  dis- 
tricts, m  each  of  which  the  c^uartzite  at  the  bottom  of  the  upper  Iluronian  does  not  vary  more 
than  200  feet  m  tliickness  for  a  distance  of  more  than  80  miles  and  is  in  contact  here  with  the 
Keewatm,  there  with  the  Laurentian,  and  in  still  other  places  with  the  lower  Huronian.  This 
shows  that  the  maxinuim  elevations  of  these  heterogeneous  rocks  at  the  time  of  the  encroach- 
ment of  the  upper  Huronian  sea  did  not  exceed  a  few  hundred  feet.  Even  after  the  deformation 
wliich  the  deposition  plane  has  undergone,  it  is  still  to  be  recognized  in  the  Mesabi  and  Gogebic 
districts  as  a  remarkabh'  even  surface. 

In  the  Crystal  Falls  district  the  relations  of  the  upper  Huronian  (Animilde  group)  to  the 
underlying  rocks  are  obscured  by  the  fact  that  the  immediately  underlying  rocks  are  those  of 
the  volcanic  Hendock  formation,  and  lack  of  exposures  makes  it  extremely  difficult  to  ascertain 
the  relations,  but  there  is  some  evidence  of  unconformity.  (See  p.  300.)  The  unconformity 
between  the  upper  Iluronian  and  underlymg  rocks  in  the  Menominee  district  is  marked  bj- 
discordance  of  structui-e,  basal  conglomerates,  and  overlap. 

On  the  south  shore  of  Lake  Superior  the  discordance  m  strike  and  dip  between  the  upper 
Huronian  and  the  middle  and  lower  Iluronian  is  not  strong,  but  nevertheless  is  distmct.  On 
the  north  shore  of  Lake  Superior  before  upper  Iluronian  time  the  earlier  Huronian  had  been 
closely  folded  and  a  nearly  vertical  schistosity  developed,  so  that  at  many  places  the  upper 
Iluronian  (Animikie  group)  rests  upon  the  edges  of  the  metamorphosed  Huronian  below. 

UNCONFORMITY  AT  THE  BASE  OF  THE  KEWEENAWAN. 

The  upper  Huronian  and  Keweenawan  have  an  approximately  similar  strike  and  dip, 
and  it  was  oidy  slowh'  recognized  tJiat  the  two  series  are  discordant.  The  best  evidence  of 
tliis,  so  far  as  contacts  are  concerned,  is  found  on  tha  north  shore  of  I^ake  Superior,  in  the 
Thunder  Bay  and  Nipigon  districts,  where  the  basal  Keweenawan  contains  abundant  detritus 
derived  from  the  upper  Iluronian  (Animikie  group)  and  rests  upon  its  eroded  edges,  showing 
that  the  older  series  was  deposited,  indurated,  and  eroded  after  having  been  formed  before 
Keweenawan  time.  However,  the  depth  of  erosion  between  the  two  is  best  shown  on  the 
south  shore  of  Lake  Superior,  where,  in  the  Penokee-Gogebic  district,  the  differential  erosion 
of  the  upper  Huronian  apparently  amounts  to  several  thousand  feet  witliin  a  few  miles. 

UNCONFORMITY    AT    THE    BASE    OF    THE    CAMBRIAN. 

All  the  pre-Cambrian  formations  are  in  a  more  or  less  tilted  position,  the  dip  varymg  from 
a  few  degrees  in  the  newest  parts  of  the  Keweenawan  to  veiticality  m  jxirts  of  the  Huro- 
nian and  Archean.  The  Cambrian,  on  the  otlier  hand,  is  horizontal  or  nearly  so.  More- 
over, the  Cambrian  is  nowhere  cut  by  igneous  rocks.  These  relations  give  evidence  that  the 
.orogenic  movements  and  igneous  intrusions  so  characteristic  of  the  pre-Cambriau  had  ceased, 


620  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

and  tliat  tlie  coiHlitions  liail  iurivcd  wliich  marked  the  n;reat  Cambrian  transgression.  Tlie 
Cambrian  rests  upon  a  roniari<ably  iinifonn  ])ro-Canibrian  ])ene])lain,  wliich  is  known  to  extend 
far  to  the  north  and  soutii  of  the  Lake  Superior  region.  Ths  uncoiiforniity  at  tlie  base  of  the 
Cambrian  is  evidently  one  of  the  great  breaks  in  the  geologic  coiunin. 

A  possible  exception  to  the  above  general  statements  may  exist  in  the  relations  of  the 
Cambrian  and  upper  sandstone  beds  of  the  Keweenawan.  From  the  bottom  to  the  top  of 
tiie  Keweenawan  there  is  progressively  less  tilting.  The  structural  discordance  between  the 
Canibiian  and  the  middle  and  lower  Keweenawan  is  therefore  much  more  cons|)icuous  than 
that  between  the  Cambrian  and  ui)i)er  Keweenawan,  wliicii  are  perhaps  conformable.  The 
significance  of  this  local  conformity  is  discussed  on  pages  415—116. 

DEFORIVIATION  AND  META.MORPHISM. 
GENERAL    CONDITIONS. 

From  the  preceding  section  on  unconformities  it  is  evident  that  during  or  after  the  forma- 
tion of  each  of  the  pre-Cambrian  series,  or  both,  there  was  a  time  of  orogenic  movement  which 
produced  folding,  faulting,  and  metamorphism.  Of  the  several  periods  of  deformation,  three 
stand  out  conspicuously — that  at  the  close  of  the  Archean  throughout  the  region,  that  at  the 
close  of  the  lower-middle  Huronian,  mainly  on  the  north  shore,  and  that  at  the  close  of  the 
Keweenawan  on  the  south  shore.  As  a  result  of  these  successive  deformations  the  Lake 
Superior  region  is  essentially  an  asymmetric  s3Ticlinorium  with  nearly  east-west  axis. 

The  amount  of  deformation  in  each  series  is  partly  a  function  of  the  age  of  the  series,  as 
after  its  formation  each  series  was  subjected  to  all  the  movements  wliich  followed.  One  would 
expect  the  complexity  of  structure  to  be  the  greatest  in  the  Archean  and  least  in  the  Keweena- 
wan, and  such  are  tlie  facts.  But  it  does  not  follow  that  each  series  has  a  characteristic  degree 
of  deformation  and  metamoiphism  corresponding  to  its  position  in  the  geologic  column.  The 
difference  in  deformation  of  different  parts  of  the  Huronian  series  may  be  nearly  as  great  as  the 
difference  between  the  deformation  of  the  Archean  and  that  of  the  Keweenawan. 

The  Lake  Superior  region  exhibits  every  varietj'  of  folding,  from  the  most  intricate  plica- 
tion of  the  Archean  and  lower  Huronian  to  the  broadest  open  folding  of  the  upper  Keweenawan. 
The  major  structure  of  the  region  is  unrjuestionably  controlled  by  folding  rather  than  by  fracture 
deformation,  but  the  latter  is  not  unimportant.  Every  district  which  has  been  considered  in 
detail  shows  faults  of  greater  or  less  magnitude  and  exhibits  innumerable  joint  fractures.  For 
the  most  part  these  faults  are  comparatively  small  and  do  not  greatly  modifj'  the  general  dis- 
tribution of  tlie  formations,  although  many  produce  considerable  displacements  which  are  impor- 
tant in  the  detailed  geology  of  the  districts.  Exceptions  to  the  above  statement  are  to  be  made 
in  reference  to  the  great  faults  of  the  Keweenawan,  of  which  one  runs  tlirough  the  center  of 
Keweenaw  Point  and  another  extends  along  the  northern  part  of  northern  Wisconsin  and  is 
believed  to  be  continuous  between  Isle  Royal  and  the  mainland  of  Minnesota.  These  faults 
result  in  the  repetition  of  the  Keweenawan  rocks  and  give  them  a  wider  present  distribution. 

The  competent  strata  controlling  the  deformation  of  the  pre-Cambrian  rocks  of  this  region, 
whether  by  foldmg  or  bj'  faulting,  have  been  the  quartzites  and  the  plutonic  rocks,  especially 
the  granites.  These  rocks  show  on  the  whole  more  simple  folding  and  faulting  than  the  softer 
beds  associated  with  them.  The  slate  formations  especially  have  accommodated  themselves 
to  tliis  control  by  close  folding  and  development  of  cleavage.  For  instance,  in  the  ilarquette 
district  the  Archean  and  the  overlying  quartzites  are  folded  into  a  broad  composite  .syncline 
with  considerable  faulting.  The  'intervening  and  overlying  slates,  on  the  other  hand,  ap])car 
in  close  folds,  characteristic  of  mcompetent  strata.  Their  deformation  has  been  obviouslj- 
controUed  by  the  readjustments  between  them  and  the  quartzites.  It  may  be  assumed  that, 
so  far  as  the  competent  quartzite  is  concerned,  the  develo|)ment  of  the  ^larciuette  syncline 
has  required  movement  of  the  upper  beds  upward  and  outward  from  the  syncline  as  compared 
with  the  lower  beds,  as  indicated  by  arrows  in  figure  35  (p.  253).  The  major  readjustment  has 
resulted  in  the  overthrust  or  drag  folds  in  the  slate.     This  is  the  essential  explanation  of  the 


GENERAL  GEOLOGY.  621 

abnormal  fan-shaped  folding  of  the  Marquette  district-  Simihir  drag  folds  in  the  soft  laj^ers 
between  the  competent  strata  may  be  found  in  almost  any  part  of  the  Lake  Superior  region 
where  competent  and  incompetent  layers  have  been  folded  together. 

In  coimection  with  the  folding,  faulting,  and  intrusions  there  have  developed  slaty,  schistose, 
and  part  of  the  gneissose  structures.  All  these  structures  are  common  in  the  Archean  and  are 
widespread  in  the  lower  and  middle  Iluronian.  In  the  upper  Huronian  slatiness  occurs  rather 
extensively  and  schistosity  is  common  where  the  rocks  have  been  intruded  by  granite,  as  in 
northern  ^Minnesota  and  Florence  County,  Wis.  The  Keweenawan  does  not  exhibit  any  of  these 
secondary  structures. 

Thesii  structures  are  all  characteristic  of  the  zone  of  flowage,  in  wliicli  the  alterations  are 
anamorphic.  During  their  formation  all  the  phenomena  of  granidation  and  crystallization  of 
the  iuthvidual  mineral  constituents  are  exhibited  in  very  diverse  rocks  and  m  widely  varj'mg 
degrees,  from  moderate  changes  to  complete  recrystallization.  Where  the  I'ocks  have  been 
imder  deep-seated  conditions  and  these  secondary  structures  are  not  found  the  changes  may  be 
moderate,  but  on  the  other  hand  they  may  be  extreme. 

In  connection  with  the  faults,  joints,  and  other  fractures  all  the  alterations  of  the  zone  of 
katamorphism  have  taken  place.  These  are  perhaps  best  illustrated  in  the  Keweenawan 
series. 

In  general  in  the  zone  of  observation  more  or  less  extensive  katamorphic  changes  are  super- 
imposed upon  the  anamorphic  changes  above  mentioned,  for  the  once  deep-seated  rocks,  now 
near  the  surface,  have  long  been  in  the  zone  of  fracture,  where  the  changes  are  katamorpliic. 
It  thus  appears  that  various  combinations  of  the  alterations  of  the  zones  of  anamorpliism  and 
katamorpliism  are  exhibited  b\'  the  same  rocks. 

PRINCIPAL    ELEMENTS    OF    STRUCTURE. 

The  major  axes  of  the  orogenic  movements  in  this  region  have  been  in  general  parallel  to 
Lake  Superior,  producing  a  synclinoriimi.  But  the  eastern  and  western  parts  of  Lake  Superior 
show  a  difference  in  trend,  the  dividing  north-south  line  bemg  at  about  88°  longitude,  wliich 
cuts  Keweenaw  Point  a  few  miles  west  of  its  end.  West  of  tliis  line  the  trend  of  the  axis  of  the 
lake  is  about  .30°  north  of  east.  East  of  it  the  trend  of  the  axis  is  south  of  east.  To  these 
trends  the  strike  of  the  rocks  corresponds  almost  exactly  for  the  west  half  of  Lake  Superior  and 
approximately  for  the  east  half.  The  average  strike  for  the  region  is  nearly  east  and  west. 
This  prevailing  structure  is  represented  in  the  Lake  Superior  trough  itself,  m  tlie  IMesabi  and 
Aniniikie  monocline,  in  the  repeated  fokls  of  the  Cuyuna,  Iron  River,  Crystal  Falls,  and  Florence 
districts,  in  the  Gogebic  monocline,  and  in  the  Marquette,  Menominee,  Felch  Mountain,  Calumet, 
and  Sturgeon  s\aiclinoria. 

The  strikes  of  the  Lake  Superior  rocks  are  in  accord  with  those  in  the  greater  part  of  the 
pre-Cambrian  sliield  to  the  northwest,  north,  and  northeast,  even  as  far  north  as  the  Hudson 
Bay  region. 

Locally  the  strikes  varj'  greatly  from  the  genei'al  (hrections  mdicated,  and  they  may  be 
even  north  and  south,  as  is  conspicuously  illustrated  in  the  Republic  trough  and  the  Archean 
oval  of  the  Fence  River  area  in  the  Crystal  Falls  district.  The  local  variations  in  strike  and  dip 
are  partly  explained  b^'  original  configuration  of  the  basement  rocks.  They  are  more  largely 
explained  by  the  cross  folding  of  the  region.  The  axis  of  one  great  cross  fold  runs  north  and 
south  through  Keweenaw  Pomt  and  the  eastern  part  of  the  Crystal  Falls  district.  Another 
crosses  the  Marquette  district  in  a  north-south  direction  ui  the  vicinit}'  of  Ishpeming.  Others 
cross  the  Mesabi  district  from  northeast  to  southwest. 

The  intrusive  rocks  are  also  important  factors  in  producing  the  variations  of  the  strike  of 
the  folds  from  the  major  Lake  Superior  structure.  Very  commonly  the  strikes  of  the  strata 
a})out  massive  laccoliths,  bosses,  or  batholiths  are  much  influenced  by  the  igneous  rocks,  there 
being  a  marked  tendency  for  the  strikes  to  be  peripheral  or  tangential  to  the  intrusives.  This 
relation  is  illustrated  in  all  the  districts  m  wliich  the  mtrusive  rocks  occur  m  large  masses,  but 
is  best  exemplified  in  the  region  northwest  of  Lake  Superior.     Here  the  intrusive  masses  are 


622  GEOLOGY  OF  THE  LAKE  SLTERIOR  REGION. 

so  large  as  to  be  the  most  important  factor  iir  the  control  of  the  local  strikes  and  dips,  although 
even  here  there  is  an  unquestioned  tendency  for  the  general  east-west  strike  of  the  region  to 

mamtain  itself. 

The  cross  sections  A-A.  and  B-B  on  the  general  map  (PI.  I,  in  pocket)  give  the  best  conception 
of  the  dips  of  the  formations.  It  will  be  noted  that  the  Archean,  lower  ITuronian,  and  middle 
Huronian  beds  have  steep  dips  in  northerl}-  or  southerly  directions,  that  tlie  upper  Iluronian 
beds  are  as  a  whole  less  steeply  inclined  and  have  in  part  a  definite  relation  to  the  synclinal 
structure  of  the  T^ake  Superior  region,  and  that  the  Keweenawan  beds  are  still  less  steeply 
inclined  and  arc  entirely  in  accord  with  the  synclinal  structure  of  the  basin.  All  tliis  ileforma- 
tion  was  complete  before  Cambrian  time  except  tiie  faulting.  There  is  no  evidence  that  the 
faulting  was  not  much  later  than  the  Cambrian — possibly  post-Cretaceous. 

THE    LAKE    SUPERIOR    BASIN. 

The  structure  of  the  Lake  Superior  basui  is  well  shown  by  the  cross  sections  on  the  general 
map  (PI.  I,  m  pocket).  The  basin  is  essentially  an  east-west  asymmetric  synclmorium  with 
steeper  dips  on  the  south  than  on  the  north  side,  shown  principally  in  the  Keweenawan  and 
upper  Huronian  rocks  bordering  the  lake. 

The  upper  Huronian  of  the  Penokee  district  dips  north,  toward  Lake  Superior,  at  a  steep 
angle,  varying  considerably  but  for  the  most  part  between  55°  and  80°.  The  upper  Huronian 
of  the  north  shore  in  the  ]\Iesabi  and  the  Animikie  districts  dips  .south,  toward  Lake  Supeiior, 
at  comparatively  flat  angles,  ranging  from  substantial  horizontaUty  up  to  45°,  the  more  common 
dips  bemg  between  8°  and  20°. 

The  folding  of  the  Keweenawan  has  a  ver}"  close  relation  to  the  I^ake  Superior  trough.  On 
the  south  shore  of  Lake  Superior  the  Keeweenawan  rocks  dip  northwest,  toward  the  lake,  at 
angles  var\Tng  from  as  high  as  80°  in  the  lower  part  of  the  series  to  extremely  flat  angles  in  the 
upper  part  of  the  series.  On  the  north  shore  of  Lake  Superior  and  on  Isle  Royal  they  dip  at 
moderate  angles  to  the  southeast,  toward  Lake  Superior.  Thus  the  Keweenawan  at  Keweenaw 
Point  and  its  extension  to  the  southwest  and  the  Keweenawan  of  the  north  shore  constitute 
a  clearly  marked  synclinal  trough  which  extends  inland  in  ^Michigan,  Wisconsin,  and  Minnesota. 
The  axis  of  this  syncline  lies  about  halfwa}'  between  Isle  Royal  and  Keweenaw  Point.  It  does 
not  ran  to  the  head  of  the  lake,  but  to  the  head  of  Chequamegon  Bay  and  thence  far  inland  to 
the  southwest.     Here  it  carries  minor  folds. 

To  the  southeast  of  the  synclinorium  in  ]\Iicliigan  there  is  one  great  fault,  and  probably  two. 
To  the  northwest  of  it  in  Wisconsin  there  is  another  great  fault,  and  this  fault,  or  another,  is 
supposed  to  continue  between  Isle  Royal  and  Thunder  Bay.  The  Keweenawan  rocks  nearest 
the  axis  of  the  sjmcline  are  on  the  upthrow  sides,  and  the  "eastern"  and  "western"  sandstones 
southeast  and  northwest  of  the  synclinorium,  respectively,  are  on  the  downthrow  sides  of  the 
faults.  Consequently  the  Keweenawan  of  northern  Wisconsin  and  Isle  Royal  is  probably 
repeated,  at  least  in  part,  on  the  Minnesota  coast,  and  below  the  rocks  thus  repeated  the  Miiuie- 
sota  Keweenawan  extends  down  to  the  base  of  the  series.  Similarl}-  a  part  of  the  Keweenawan 
rocks  of  Keweenaw  Point  are  probably  repeated  in  the  "South  Range." 

At  Michipicoten  the  dips  are  to  the  south.  On  the  east  shore  of  Lake  Superior  the  Kewee- 
nawan dips  westward  toward  the  lake.  Some  thrusting  and  buckling  have  accompanied  the 
faulting  along  the  north  side  of  the  synclmorial  axis  in  Wisconsin. 

Wherever  there  is  a  thick  succession  of  Keweenawan  rocks  the  dips  are  steeper  at  the  base  and 
grow  flatter  at  the  top.  This  is  best  illustrated  by  Keweenaw  Point,  Isle  Royal,  and  Michipi- 
coten. Elsewhere  the  changes  of  dip  are  not  .so  great.  This  general  lessening  of  the  dip  of  the 
Kew-ecnawan  in  passing  from  lower  to  higher  horizons  is  regarded  as  evidence  that  the  folding 
of  the  series  which  caused  the  Lake  Superior  basin  took  place  largely  during  Keweenawan 
time. 

The  development  of  the  Lake  Superior  basin  in  Keweenawan  lime  has  tended  to  give  paral- 
lelism of  strike  to  all  the  rocks  of  the  region  but  is  not  regarded  as  sufficient  to  explain  the 
remarkable  parallelism  actually  observed  in  older  rocks.     The  trend  of  the  axial  lines  of  the 


GENERAL  GEOLOGY.  623 

Lake  Superior  s\nicline  is  in  accord  with  that  of  tlie  folds  tliroiip;li  a  hirge  part  of  the  pre- 
Cambrian  shiekl  of  Canada.  It  is  cK)ul)tful  if  the  Keweenawan  fohUng  ever  affected  a  hirge 
part  of  tliis  sliield.  Much  of  the  folding  is  unquestionably  earlier.  Therefore  it  is  reasonable  to 
assume  that  the  dominant  trend  in  the  Lake  Superior  folds,  as  well  as  in  the  pre-Cambrian  of 
Canada,  was  pi'obal)!}'  establisiied  before  Keweenawan  time. 

Bordering  the  main  Lake  Superior  basin  are  minor  synclinal  folds  of  the  Marquette,  Felch 
Mountain,  Calumet,  Menominee,  and  Mesabi-Cuyuna  districts,  with  axes  nearly  parallel  to  the 
longer  axis  of  the  I^ake  Superior  basin.  These  districts  present  evidence  that  they  were  folded 
to  their  present  attitude  in  considerable  part  at  the  same  time  as  the  main  Lake  Superior  basin; 
that  is,  in  Keweenawan  time.  Another  fact  seems  to  relate  them  even  more  closely  to  the  Lake 
Superior  basin  of  Keweenawan  age.  The  minor  synclinoria  are  asymmetric  and  tend  to  have 
their  steeper  sides  toward  the  lake.  This  may  be  observed  in  the  Marquette,  Felch  Mountam, 
and  Calumet  districts,  principally  in  the  upper  Huronian  rocks.  The  upper  Huronian  of  the 
nortli  shore  has  a  gentle  soutiiward  dip  along  the  Alesabi,  Gunflint,  antl  Animikie  ranges,  wiiich 
changes  to  steeper  dips  near  the  axis  of  the  basin  in  tlie  Cu3'una  district.  The  Vermilion  district 
shows  evidence  of  a  similar  structure  in  the  Keewatin  schists.  The  normal  development  of  a 
great  syncline  of  the  Lake  Superior  type  would  ])e  accompanied  by  a  differential  slipping  between 
the  competent  layers,  whereby  the  upper  .layers  would  move  up  and  away  from  the  syncline  as 
compared  with  the  lower  layers,  just  as  has  been  already  described  for  the  Marcjuette  district. 
So  far  as  there  is  failure  of  the  beds  taking  part  in  tliis  movement,  it  is  likely  to  lie  influenced  by 
this  differential  movement  and  to  result  in  minor  folds  with  steeper  sides  toward  the  axis.  It  is 
believed  that  the  accordance  of  structure  of  the  districts  mentioned  with  the  requirements  of 
this  general  principle  is  more  likel}^  to  be  a  natural  and  necessary  secjuence  than  a  coincidence. 
It  may  be  noted  that  the  displacement  of  the  beds  in  this  type  of  fokling  has  nearly  the  same 
direction  as  the  displacement  along  the  main  fault  lines  already  mentioned. 

The  departures  from  this  control  of  minor  folds  by  the  Keweenawan  fold  of  the  Lake  Supe- 
rior basin  are  due  to  original  configuration  of  basement,  to  plutonic  intrusive  rocks,  or  to  cross 
folding. 

The  asymmetric  character  of  the  Lake  Superior  basin  itself  may  have  interesting  significance 
as  to  the  general  orogen}-  of  the  region.  If,  as  suggested  in  the  following  pages,  the  Lake  Superior 
basin  has  been  essentially  the  locus  of  a  shore  zone  against  the  continental  area  to  the  north,  then 
the  gentle  southward  slope  of  the  north  limb  of  the  Lake  Superior  syncline  is  awav  from  the  old 
shore  line  and  the  steeper  dip  of  the  south  limb  of  the  S3aicline  faces  the  shore,  as  if  there  had  been 
a  thrust  from  the  south  toward  the  continental  area  to  the  north. 

The  extrusions  of  the  volcanic  rocks  along  the  borders  of  the  lake  on  an  extensive  scale  of 
themselves  gave  opportunity  for  subsidence  elsewhere.  It  may  be  that  the  depression  of  the 
center  of  the  Lake  Superior  basin  was  a  correlative  of  the  outflows  of  lava  along  the  border,  the 
two  together  and  tlie  inciting  or  attendant  epeirogenic  and  orogenic  movements  lowering  the 
center  of  gravity  of  the  Lake  Superior  masses.  This  movement  of  subsidence  for  tlie  basin  would 
be  assisted  by  tiie  deposition  of  lava  beds  and  of  setliments  within  the  basin,  although  these  are 
regarded  as  accessory  rather  than  as  prime  factors  in  the  development  of  the  basin. 

RESUJiIE  OF  HISTORY. 

TJiis  monograph  may  close  with  a  brief  resume  of  the  great  events  of  the  pre-Cambrian 
period  in  the  Lake  Superior  region.  Early  in  the  history  of  the  earth,  when  the  Lake  Superior 
region  was  at  least  in  part  below  the  sea,  there  were  great  outbreaks  of  volcanic  rocks,  which 
continued  for  an  indefinite  time  and  as  a  result  of  which  the  Keewatin  was  mainly  built  up. 
Subordinate  masses  of  sediment — conglomerate,  slate,  and  iron-bearing  rocks — were  laid  down, 
especially  late  in  Archean  time.  The  beginning  of  the  Keewatin  period  we  can  not  even  dimly 
see,  for  we  do  not  recognize  the  basement  on  which  the  Keewatin  rests.  Later,  or  contempo- 
raneously at  least  with  the  later  stages  of  the  vast  regional  extrusions,  which,  as  has  been 
explainedj  were  of  a  magnitude  never  subsequently  approached,  was  the  intrusion  of  enormous 
ma.sses  of  aciilic  rocks,  including  great  batholiths,  bosses,  stocks,  and  dikes.     These  rocks  consti- 


624  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

tute  the  Laurentian  series.  For  some  unknown  reason  the  extrusive  rocks  of  the  Keewatin  were 
dominaiilly  of  the  basic  and  intermediate  tyjics,  and  the  Laurentian  intrusive  rocks  dominantly 
of  the  acidic  type,  althougii  all  tliese  types  occur  both  as  oxtrusives  and  intrusives. 

In  Archean  time  the  Keewatin  rocks  were  greatly  deformed  and  extensively  metamorphosed, 
largely  imder  the  influence  of  the  Laurentian  intrusions.  It  is  extremely  probable  that  during 
Archean  time  at  many  places  the  land  was  raised  above  the  sea  l)y  the  upbuilding  of  the  lavas, 
the  intrusion  of  the  batholiths,  and  the  folding.  But  the  Keewatin  rocks  now  observable  are 
largely  submarine,  and  contemporaneous  sediments  knowTi  to  result  from  erosion  are  rare  or 
lacking. 

Finally  there  came  a  time  when  a  general  epeirogenic  movement,  perhaps  in  connection 
with  the  orogenic  movements  and  intrusions,  raised  the  entire  Lake  Superior  region  above  the 
sea.  This  gave  the  conditions  for  deep  denudation  which  removed  a  great  but  unknown  thick- 
ness of  the  Archean  rocks,  exposing  the  schistose  Keewatin  rocks  and  the  coarse,  massive  tex- 
tures of  the  Laurentian  batholiths. 

The  lower  Iluronian  sea  then  advanced  over  the  region  fi-om  the  south,  extending  at  least 
as  far  north  as  the  present  south  shore  of  Lake  Superior.  Lender  the  water  of  the  advancing 
sea  the  lower  Huronian  sediments  were  laid  down.  These  are  of  a  normal  subaf[ucous  U~pe, 
consisting,  first,  of  a  psepliite,  followed  by  a  ])sammite,  next  a  pelite,  in  places  carlionaceous,  and 
finally  a  limestone.  The  character  of  these  sediments  proves  beyond  question  that  at  the 
time  of  their  deposition  the  conditions  had  become  similar  to  those  whicli  prevail  to-day,  both 
as  to  the  agents  concerned  in  erosion  and  as  to  those  concerned  in  deposition. 

Tlie  de])ositi()n  of  the  lower  Huronian  was  followed  by  an  uplift  and  recession  of  the  sea. 
The  area  of  the  southern  Huronian  subprovince  and  ])erhaps  also  that  of  the  northern  subprov- 
ince  were  gently  folded.  Erosion  locally  cut  through  the  lower  Huronian  of  the  Marquette 
district,  but  in  most  of  the  southern  subprovince  it  has  not  gone  through  the  Randville  dolo- 
mite. The  next  period  of  deposition  was  that  of  the  middle  Iluronian,  evidence  of  which  is 
found  only  in  the  southern  Huronian  subprovince.  The  middle  Huronian  here  consisted  of 
subaqueous  sediments — psephites,  psammites  peUtes,  and  a  nonclastic  iron-bearing  formation — 
indicating  that  the  sea  had  again  advanced. 

It  is  believed  that  durmg  lower  and  middle  Huronian  time  the  sea  did  not  advance  over 
the  area  north  of  the  present  Lake  Superior,  that  this  was  a  land  area,  and  that  the  great  rivers 
flowed  to  the  south  into  the  Iluronian  sea.  On  this  northern  liighland  were  deposited  an  exten- 
sive and  peculiar  slate  conglomerate  and  httle-assorted  graywackes,  slates,  and  conglomerate, 
which  in  their  general  characteristics  and  associations  are  taken  to  be  subaerial  and  delta 
deposits. 

After  middle  Huronian  time  the  northern  and  southern  Huronian  suljprovinces  were  raised 
above  the  sea,  folded,  and  eroded.  The  northern  subprovince  was  much  more  affected  at  this 
time  than  the  southern  subprovince.  The  advance  of  the  upper  Huronian  sea  from  the  south 
across  the  area  brought  about  the  deposition  of  upper  Iluronian  sediments  upon  a  remarkably 
plane  surface,  with  elevated  areas  that  were  perhaps  not  covered  in  several  places  in  northern 
Wisconsin  and  south  of  the  Manjuette  district  of  Michigan.  To  what  extent  this  surface  was  one 
of  previous  base-leveling  by  subaerial  erosion  and  to  what  extent  by  marine  planat ion  is  not 
known.  The  fresh  surface  of  contact,  the  manner  in  which  the  plane  truncates  hard  and  soft 
rocks,  tlie  lack  of  residual  soils  or  sediments,  and  the  thimicss,  evenness,  and  wide  area  of  the 
fragmental  base  of  the  upper  Iluronian  seem  to  favor  the  view  that  the  surface  may  have  been 
finally  cleared  by  marine  scouring,  whatever  the  extent  of  earlier  erosion.  In  the  southern 
Huronian  subprovince  the  rocks  had  not  been  ]ireviously  folded  as  mucii  as  in  the  northern 
subprovince,  in  consequence  of  which  erosion  and  ])lanation  accom])lisheil  less  conspicuous,  or 
less  easily  identified  results,  though  erosion  seems  to  have  removed  nearly  all  of  the  middle 
Huronian  in  the  Menominee  district.  The  upper  Huronian  was  thus  laid  down,  ^vith  conspic- 
uous unconformity  in  the  nortliern  part  of  the  I'cgion,  because  of  the  folding  of  earlier  rocks, 
and  with  far  less  discordance  in  the  southeiii  ])art  of  the  southern  Iluronian  subprovince,  where 
the  earlier  rocks  hat!  not  been  so  much  folded. 


GENERAL  GEOLOGY.  625 

The  shore  deposits  of  the  advancing  upper  Huronian  sea  were  thin  psephites,  which  were 
followed  by  psammites  or  ])elites,  and  these  very  extensively  by  an  iron-bearing  formation, 
locali}^  alternating  with  pelites.  This  is  the  formation  containing  the  great  deposits  of  Lake 
Superior  ores,  in  the  Mesabi,  Penokee,  Menominee,  Cuyuna,  and  other  districts.  The  depo- 
sition of  the  iron-bearing  rocks  to  a  tliickness  of  nearly  a  thousand  feet,  with  so  little  frag- 
mental  sediment,  is  not  duplicated  elsewhere  in  the  geologic  record  and  seems  to  require  some 
unusual  condition.  The  explanation  is  believed  to  lie  in  the  great  basic  extrusions  both  pre- 
ceding and  accompanying  the  upper  Huronian  deposition,  furnisliing  an  unusual  source  of  mate- 
rials for  the  iron-bearing  formation.     (See  pp.  51.3  et  seq.) 

The  alternations  of  iron-bearing  formation  and  ]ielite  were  followed  by  a  very  tliick  pehte, 
the  greatest  of  the  Huronian  formations.  The  contUtions  allowing  this  unusual  accumulation 
of  mud  may  have  been  those  of  delta  deposition.  The  sea  seems  not  to  have  advanced  much 
farther  north  than  the  Lake  Superior  region,  and  it  is  conjectured  that  when  the  advance  stopped, 
the  rivers  were  able  to  make  headway  against  the  sea  and  build  u])  great  delta  and  mud  deposits 
over  the  jireviousl}'  deposited  iron-formation  sediments.  The  character  of  these  deposits  is 
perhaps  related  to  volcanism.  The  existence  of  abundant  discontinuous  pyritiferous  and 
gi'apliitic  lenses  in  the  slate,  associated  with  lenses  of  iron  carbonate,  seems  to  be  evidence  of 
lagoon  conditions  accompanying  delta  deposition.  As  in  most  deltas,  a  considerable  part  of  the 
deposits  may  be  regarded  as  terrestrial. 

At  the  close  of  upper  Huronian  time  the  land  was  raised  or  built  above  the  surface  by 
delta  dejDosition  and  the  upper  Huronian  beds  were  very  gently  folded  and  deepl^^  eroded,  the 
differential  erosion  amounting  apparently  to  thousands  of  feet.  Then  followed  the  events  of 
Keweenawan  time,  which  were  first  those  of  terrestrial  de])osition,  associated  with  enormous 
extrusions  of  igneous  rock,  merging  later  into  conditions  of  subaqueous  deposition  in  the  Lake 
Superior  syncline. 

During  the  time  tlie  Keweenawan  series  was  being  built  up  the  Lake  Superior  basin  was 
formed,  resulting  in  marked  diminution  of  dip  in  passing  from  lower , to  upper  Keweenawan. 
The  folding  of  the  lower  Keweenawan  and  middle  Keweenawan  rocks  which  produced  this 
basin  deformed  also  tlie  adjacent  rocks,  especially  the  upper  Huronian  of  the  west  half  of  the 
basin,  so  that  they  share  in  the  synclinorial  structure.  The  antecedent  movements  were 
probably  along  axes  parallel  to  that  of  the  Lake  Superior  syncline,  but  the  present  marked 
parallelism  of  axes  of  folds  in  all  of  the  jire-Cambrian  is  probably  due  largely  to  Keweenawan 
folding. 

At  the  end  of  the  Keweenawan  period  the  land  was  raised  for  the  fourth  time  above  the  sea 
and  the  long-continued  denudation  preceding  the  Cambrian  period  took  place,  developing  a 
peneplain  that  is  even  yet  largely  preserved.  The  Cambrian  transgression  began  far  to  the  south 
and  finally  overrode  the  entire  Lake  Superior  region.  The  floor  for  the  Cambrian  deposition 
was  composed  of  tilted  rocks  with  the  exception  of  the  upper  Keweenawan  sandstone.  The 
structural  and  lithologic  accordance  of  the  Cambrian  with  tlie  upper  Keweenawan  beds  raises 
the  question  whether  the  deposition  of  the  upper  Keweenawan  sediments  in  the  Lake  Superior 
basin  did  not  continue  until  the  Cambrian  sea  readied  them  and  gradually  merged  into  Cam- 
brian deposition.     The  Cambrian  was  succeeded  by  the  later  Paleozoic  deposits. 

After  Paleozoic  time  the  region  was  again  raised  above  the  sea  and  eroded.  Since  then 
there  have  been  many  episodes  of  uplift,  subsidence,  and  warping.  At  one  time  the  Cretaceous 
sea  covered  the  western  part  of  the  region.  Erosion  has  removed  all  but  small  patches  of  the 
Cambrian  from  the  uplands,  and  reexhumed  and  modified  the  pre-Cambrian  topography. 
Faults  developed  in  ])Ost-Cambrian  and  perhaps  in  post-Cretaceous  time. 

It  thus  appears  that  in  the  Lake  Suj)erior  region,  from  the  earliest  time  to  the  Cambrian, 
there  were  five  great  periods  of  rock  formation  separated  and  followed  l>y  jjeriods  of  epeirogenic 
movement,  orogenic  movement,  and  erosion,  each  of  these  intervals  being  markctl  by  an 
unconformity.  The  first  of  these  unconformities,  tliat  at  the  top  of  the  Archean,  is  the  most 
conspicuous,  represents  a  strong  lithologic  contrast,  and  lias  been  by  all  geologists  taken  as  an 
47517°— vol52— 11 40 


626  GEOLOGY  OF  THE  LAKE  SUPERIOR  REGION. 

essential  datum  plane  in  mapping  and  working  out  the  geologic  history.  The  unconformities 
separating  the  divisions  of  the  Iluronian  and  the  Iluronian  from  the  Keweenawan  are  of  differ- 
ing value,  but  all  represent  important  structural  and  time  breaks.  The  unconformity  at  the 
base  of  the  Cambrian  is  one  of  the  first  magnitude  and  is  coextensive  with  the  great  uncon- 
formity at  this  horizon  outside  of  the  Lake  Superior  region. 

Of  the  five  periods  of  deformation,  three  stand  out  consi)icuously — that  at  the  close  of  the 
Archean  tliroughout  the  region,  that  at  the  close  of  the  lower-middle  Huronian  principally  on  the 
north  shore,  and  that  at  the  close  of  the  Keweenawan  priiicipally  along  the  axis  of  the  Lake 
Superior  basin  and  on  the  south  shore.  These  areas  of  folding  had  been  shore  zones  of  heavy 
Huronian  and  Keweenawan  deposition.  As  is  common,  the  shore  zone  was  a  place  of  recurrent 
upheaval  and  subsidence,  marked  orogenic  movement,  igneous  activity,  and  sedimentation. 
To  these  many  causes  combinetl  is  due  the  complexity  of  the  geolog}'  of  the  region. 

These  shore  conditions  may  bear  some  relation  to  the  tact  that  part  of  the  Lake  Superior 
region  south  of  the  international  boundary  is  one  of  the  great  iron-producing  areas  of  the  world. 
It  has  been  a  source  of  surprise  to  many  that  the  adjacent  Canadian  region,  in  which  the  geology 
seems  to  be  in  a  general  way  similar,  has  not  been  found  to  bear  iron  ore  in  anytlung  hke  the 
abundance  of  the  States  to  the  south.  But  by  far  the  larger  ]>art  of  the  iron-ore  deposits  in  the 
States  occur  in  the  middle  Huronian  and  dominantly  in  the  upper  Huronian  formations.  The 
middle  Huronian  is  known  in  a  general  belt  fringing  the  main  pre-Cambrian  area  of  Canada  along 
the  north  shore  of  Lake  Superior  and  extending  northeastward  tlirough  Lake  Tiniiskaming. 
It  may  exist  also  farther  north  in  the  interior  of  the  Canadian  pre-Cambrian,  but,  to  judge  prin- 
cipally from  the  facts  observed  on  the  north  shore  of  I^ake  Superior,  the  interior  jjre-Cambrian 
region  of  Canada  was  probably  above  the  sea  during  middle  and  upper  Huronian  and  Kewee- 
nawan time  and  only  continental  deposits  were  formed  in  it.  The  up])er  Iluronian,  the  {)rin- 
cipal  iron  producer,  is  but  scantily  represented  along  the  soiithem  margin  of  the  Canadian  pre- 
Cambrian.  The  only  iron-bearing  formation  which  has  an  extensive  occurrence  in  the  great 
pre-Cambrian  shield  of  Canada  is  tliat  of  the  Keewatin  series.  The  Keewatin  iron-bearing 
formation  has  not  been  largely  productive.  If  the  apparent  scarcity  of  middle  and  upper 
Huronian  rocl<s  over  this  area  is  a  real  one,  which  is  not  yet  finally  proved,  it  can  not  be 
expected  that  in  the  central  highlands  of  Canada  will  be  found  iron-bearing  formations  that  are 
to  be  paralleled  with  those  of  the  middle  and  upper  Huronian  south  of  T.,ake  Superior. 

But  it  may  be  that  in  Huronian  time  the  central  highland  area  had  a  shore  zone  to  the  north 
as  well  as  to  the  south.  The  occurrence  of  probable  late  Algonkian  sediments  on  the  east  coast 
of  Hudson  Bay  and  In  the  Copper  Mine  region,  on  the  west  side  of  the  liay,  give  color  to  this  sug- 
gestion. The  Hudson  Bay  region  seems,  from  available  facts,  to  be  another  geosyncline  of 
sedimentation  and  folding  corresponding  somewhat  to  that  of  the  Lake  Superior  and  Lake 
Huron  regions.  An  iron-bearing  formation,  remarkably  similar  to  the  Animikie,  is  here  known." 
If  this  hypothesis  proves  to  be  true,  this  northern  region  warrants  careful  exploration  for  min- 
eral wealth. 

Attention  has  been  called  on  pages  591-592  to  the  possible  genetic  association  of  the  Lake 
Superior  copper  ores  with  the  Georgian  Bay  copper  ores,  Silver  Islet  silver  ores,  Sudbury  nickel 
ores,  and  Cobalt  silver-cobalt  ores.  This  suggests  an  hypothesis  as  a  reasonable  basis  for 
further  geologic  work.  Huronian  and  Keweenawan  rocks  seem  to  be  more  abundantlv  present 
in  the  Lake  Superior,  Georgian  Bay,  and  Tiniiskaming  areas  than  farther  to  the  north.  They 
have  been  folded  along  })arallel  axes  in  all  of  these  districts.  Volcanism  has  been  an  impor- 
tant accompaniment  of  this  deformation.  Earlier  volcanism  has  been  associated  with  the 
(lejiosition  of  unique  iron-bearing  formations  owing  their  wide  distribution  to  the  agency  of 
sedimentation  intervening  between  their  contribution  by  igneous  masses  and  their  final  deposi- 
tion. Later  volcanism  developed  copper,  nickel,  cobalt,  and  silver  ores,  showing  some  evidence 
of  genetic  relationship.  The  Lake  Superior  region,  then,  may  be  regarded  broadly  as  a  ]iart  of  a 
great  metallographic  province  containing  a  variety  of  ores  associated  with  volcanism,  which  mav 
be  related  with  folding  along  an  old  shore  zone. 

a  Leith,  C.  K.,  An  Algonkian  basin  in  Hudson  Hay— a  comparison  with  the  Lalte  Superior  basin:  Econ.  Geology,  vol.  5,  1910.  pp.  227-246. 


INDEX. 


A.  Page. 

Abbott,  C.  E.,  mine  sections  by 138 

Acl;nowledgment5  to  those  aiding 30, 427, 573 

Adams,  Cuyler,  discovery  by 44 

Adams,  F.  S.,  on  geology  of  Cnyuna  range 219 

Agassiz,  Lalie.  description  of 442. 443 

Agawa  formation,  correlation  of l35, 598 

distribution  and  character  of 132, 603 

topography  of 94 

Agawa  Lake,  geology  near 132 

Ajiijik  Hills,  geology  at 259 

Ajibik  quartzite,  correlation  of 598, 007, 608 

deposition  of 501 

distribution  and  character  of 252. 259-201, 287, 2S8, 289, 007 

relations  of 200-261, 204 

structure  of 259 

topography  of 105 

Alden,  W.  C,  on  glaciation 439 

Algonkian  rocks,  correlat innof 598-599, 602-015 

distribution  and  character  of 85, 123-137. 

146-1.50, 169, 161-178. 198-208. 212-215, 226-235, 251-252, 
256-269. 283-323, 339-345, 366-357,  360-361,  598,  002-015 

igneous  rocks  in 344-345 

iron  horizons  in 460, 508 

Algonquin.  Lake,  description  of 440-447 

map  of ■- 447 

Allen,  R.  C,  on  Iron  River  district 308-320 

on  Spring  Valley  ores 505-506 

on  Woman  River  district 555 

Alloa,  geologj'  at 360 

Allouez  conglomerate,  copper  in 575, 577, 578 

production  from 37 

Alteration.    5fc  Secondary  concentration:  Weathering,  etc. 

Amasa,  geology  near 293. 298, 299. 323. 607 

u-on  near 324 

Amicon  River,  geology  on 379 

Amphibole-magnetite  rocks,  banding  of 551-552 

formation  of 558 

sulphur  in 552 

surface  alteration  of 553-554 

Amygdaloid  deposits,  character  of 670-577 

copper  in 574, 576-677 

distribution  and  character  of .578-579 

plate  showing 472 

production  from 37 

.\mygdules,  filling  of 512 

Anderson,  T.,  on  ellipsoidal  structure 511 

Angeline  Lake,  geology  at 271 

Animikie  district,  correlation  of 598 

description  of 208-209 

extension  of 209 

geology  of 205-208,611 

iron  ores  of 206-207. 209-210 

analyses  of 210 

concentration  of 210 

map  of 200 

physiography  of 208-209 

reserves  of 492 

structure  of 208 

Animikie  group,  correlation  of 213-214, 598, 608-614 

deposition  of 176-177, 278, 610, 612-614 

distribution  and  character  of 130,149, 

159,163-177. 198-201. 204-209, 212-214. 225. 229-234, 251-2,52, 
265-268, 283-290. 298-299. 306-307. 309. 311-323. 410, 608-liI4 

intrusions  in 197, 208, 215. 290, 318, 370, 372-375. 426 

iron  in 42,46,206-207,460-461,507,515,610 

naming  of 42 


Page. 
Animikie  group,  relations  of.. . .  170, 203-203, 234,318, 370-371, 414,513,019 

silver  in 46, 593-595 

origin  of 595 

structure  of , 175-176 

unconformity  at  base  of 619 

See  also  Iluronian.. 

Antoine  Lake,  geology  near 333, 336 

Apostle  Islands,  geology  of 3711, 449 

Aqueous  sediments,  character  of 501-502 

Aragon  mine,  geology  near 339 

sections  of,  figure  showing 347, 348 

Aragon  region,  cross  sections  in,  plate  showing 346 

Archean  system,  correlation  of 599-602 

deposition  of 623-624 

distribution  and  character  of 85, 118- 

129, 144-154. 100-101 , 198. 205, 225-227. 252-253. 283. 287-288, 291- 
293, 300-302, 30(i,  .309-310, 329, 331. 356, 357, 300,  597, 699, 601-002 

igneous  rocks  of 601, 617 

iron  of 460 

relations  of 230, 617-618 

structure  of 161 

unconformity  at  top  of 617-618 

view  showing  rocks  of 112 

.\rchean  time,  volcanism  in 617 

-Vrpin  quartzite,  correlation  of 598 

distribution  and  character  of 366. 357 

topography  on 108 

Ashland ,  geology  near 376, 379, 415, 421 ,  452 

Ashland  mine,  section  of,  figure  showing 237 

Atikokan  district,  geology  of 149.599 

iron  ore  of 149,  <00, 561, 569 

.\tikokan  mine,  history  of 46 

Atikokan  River,  iron  on 149 

Atikokan  series,  distribution  and  character  of 148 

Atkinson,  geology  near 313, 317 

section  near,  figure  showing 318 

Atlantic  mine,  geology  at 238, 576 

history  of 36 

iron  ore  of,  analysis  of 238 

Aurora,  geology  near 178, 236 

Austin  mine,  geology  at 285 

B. 

Bad  River,  geology  near 228, 232, 378, 413 

Bad  River  limestone,  correlation  of 598, 605 

distribution  and  character  of 225, 228, 605 

relations  of 228, 230 

Bad  Vermilion  Lake,  geology  at 146 

Balsam,  geology  near 299 

Baltic  amygdaloid,  description  of 37.576,577 

Banding,  cause  of 551-552 

Baraboo  district,  correlation  of 598 

description  of 359 

geology  of 360-362 

iron  ores  of 362-364, 570-571 

analyses  of 362, 363, 491 

production  of 461 

reserves'of 492 

secondary  concentration  of 363-304 

manganese  in 488 

map  of 359 

mining  in,  history  of 45 

section  of,  figure  showing 360 

Baraboo  quartzite.  correlation  of 357.598 

distribution  and  character  of ? 360-361.354 

Bare  Hill,  geology  at 382,385,410 

627 


628 


INDEX. 


rage. 

Barrel],  Joseph,  on  deposition IJI3 

Basalt,  decomposition  of 50;i,514 

Basement  Complex,  definition  of 2'A 

Basswood  Lake,  geology  near 119,121, 128-129 

graniteof. 12S-129 

relations  of -. 129 

topography  of ; 128 

Batchewanung  Hay ,  geology  at 42.'i 

Banldry  Lake,  geology  near 1.13 

Bayley,  W.  S., on  Keweenawan series 398, 400-401, 410 

on  Menominee  district 90,100-107 

Bayley,  W.  S.,  and  Van  Hise,  C.  R.,  on  Maniuettedistrict. . .,  96, 105-100 
Bayley,  W.  S.,  Clements,  J.  M.,  and  Smyth,  H,  L.,  on  Crystal 

Falls  district 96-97, 107 

Beaches,  old,  elevation  of 451-452 

formation  of 449-451 

Beaver  Hay.  geology  at 373-374 

laccolith  at 373 

Beck,  Richard,  on  limonite 521 

Becker,  ll.  F.,  on  iron  ores 509 

Bell,  J.  M.,  on  region  northeast  of  Lake  Superior 95, 155-1.56 

Belted  plain,  origin  of 109-110 

Berkey,  C.  P..  on  Cambrian  series 616 

on  glaciation 4.37, 4.'i9 

on  Keweenawan  series 377 

Bessemer,  geology  near 234, 236, 243 

Bessemer  ore,  definition  of 478 

exhaustion  of 495 

Bessie  mine,  geology  at 279 

Bibliography  of  region 73-84 

Biddle,  H.  C,  work  of 589 

Bigsby,  J.J..  on  glacial  erratics 432 

Bijiki  schist,  alteration  of 279 

correlation  of 598. 608,fllft-Cll 

distribution  and  character  of 251, 266-267, 283, 285. 608 

garnet  in 260 

iron  ore  in 266,270,272,275,278,283,280,460,603 

relations  of 265, 266-267. 208 

topography  of 106 

Bingoshick  Lake,  geology  on 202 

Birch  Lake,  geology  near .' 126, 162, 164, 177 

Birch  Lake  area,  mining  in,  history  of 42 

Bittern  Lake,  Iron  near 207 

Biwabik  formation,  circulation  in,  figure  showing 180 

correlation  of 598, 010-<U1 

distribution  and  character  of 159. 164-171 

folding  in,  plate  showing 180 

greenalite  in 105-108 

intrusions  In 171-172 

iron  ore  of 104-172, 400 

plates  showing 408, 548 

paint  rock  in 171 

phosphorus  in ,* 194,195 

relations  of 174 

section  of,  plate  showing 180 

Biwabik  mine,  geology  near 160. 170 

history  of 43 

Black  Bay,  geology  at 367-370, 393-394, 422 

section  at 367 

Black  River,  correlation  near 598 

geology  near 359, 377, 379, 384-385, 411, 413, 423, 424, 425 

section  on 388 

Black  Sturgeon  River,  geology  at 367 

Bogs,  iron  in 502-503, 516, 569 

Bohemia,  geology  near 385 

Bohemia  conglomerate,  distribution  and  character  of 381 

Bone  Lake,  geology  near 294 

Bone  Lake  schist,  topography  on 107 

Bowen,  N.  L,,  on  igneous  rocks 410-^111 

Bowen,  C.  F.,  and  Corey,  G.  W.,  on  Menominee  district 345 

Boyer  Lake,  geology  at 153, 157-158 

iron  at 150 

mud  and  rock  at,  analyses  of 157.1.'iS 

Braal'.,  iron  ores  of 495 

Breccia,  dist ribiit ion  and  character  of 127 

Brier  1 1  ill ,  geology  at 339, 342 

Brier  slate  member,  correlation  of ". 598. OH 

distribution  and  character  of 335-340 


Page. 

Broken  Bluffs,  geology  at 259 

Brooks,  A.  II.,  on  Marquette  district 96 

Brooks  Lake,  geology  near 153 

Brotherton  mine,  geology  at 236,238 

Brown  iron  ores,  mining  of,  lilstory  of 45 

occurrence  and  chanicter  of 479, 564-506,569 

Bruce  mines,  copper  ores  of 592 

Brule  River,  geology  near 310,313,317,319.321,322-323.017 

Burchard,  E.  F.,  on  production  of  manganiferotis  iron  ores 488 

Burntside  Lake,  geology  near 129 

Burt,  \V.  A.,  discoveries  of 38 

Burt  mine,  iron  ore  from,  analyses  of 193 

Burwash,  E.  N.,  on  Michipicoten  Island 390-391 

By-products,  recovery  of 48 

C. 

Cacaquabic  granite,  distribution  and  character  of 136 

intrusions  by 136 

topography  of 94 

Cache  Bay,  geology  at , 131 

Cady  deposit,  iron  ores  of 565 

iron  ores  of,  analysis  of 565 

Calumet  and  Hecla  conglomerate,  distribution  and  character  of. .  382, 575 

copper  in 577-578 

Calumet  and  Hecla  mine,  production  from 37 

Calumet  district,  geology  of 306-307, 609,611 

iron  ores  of 327 

analysis  of 327 

secondary  concentration  of 328 

volumetric  diagram  of 353 

copper  ores  in 574 

correlation  in 598, 609 

location  and  area  of ' 32, 306 

map  of ■ 306 

section  of,  figure  showing 574 

stmcturein 623 

Cambrian  system,  deposition  of ■ 625 

distribution  and  character  of 225,235,251,269,283,285,291, 

300. 302. 304, 306, 307,329-330, 332, 345-346,355, 356, 360, 015-616 

iron  ores  at  base  of 352-353 

relations  of 384, 415-416, 615-616, 619-6-20 

unconformity  at 619-620 

Canadian  shore,  geologic  work  on,  history  of 70 

Cape  Choy ye,  geology  at 391, 393 

Carbonate  ores,  greenalite  and,  relations  of 525 

nature  of 520-^521 

occurrence  and  character  of 479 

origin  of 502,503,507,514,516,519-520,571 

secondary  development  of 552 

Carbon  dioxide,  source  of 527 

Carlton  district,  geology  of 213, 214, 215.375 

Cariton  Peak,  geology  of 92, 374 

Carp  River,  fault  at,  map  showing 252 

geology  at 253. 259 

Caspian  mine,  geology  at 316 

Chamberlin,  T.  C,  on  glaciation 439 

on  Wisconsin  topography 98, 100 

Chamberlin,  T.  C,  and  Irving,  R.  D.,  on  physiography 115 

Champion,  geology  near 268 

Champion  location,  discovery  of 37 

Chandler  mine,  iron  ores  of 480,553 

section  of,  figure  showing 138 

Chapin ,  iron  ores  at 347, 348 

Chequamcgon  Bay,  geology  near 376, 413 

Chert,  definition  of 462 

distribut ion  and  character  of 462, 529 

Chert,  ampliibolitic,  photomicrographs  of 548 

Chert,  ferruginous,  composition  of 528 

leaching  of S37-S39 

photomicrographs  of 524. 5.34.548 

plates  showing 466. 468. 470. 542. 564 

Chester,  A.  U.,  on  Giants  Range  ores 42 

Chicago  Lake,  formation  of 446 

map  of 446 

Chicago  mine,  iron  ore  from,  plate  showing 468 

Chicagon  mine,  geology  at 315 

Chippewa  R  i  ver,  deposition  by 438 

geology  on 357 


INDEX. 


629 


rn,-  T-  Page. 

l^Bippewa- Keweenaw  lobe  of  Labrador  glacier,  extent  of. . . .  427, 428, 441 

CSnrinnati  mine,  history  of '  43 

ore  from,  photomicrograph  of; 524 

Clarksburg  formation,  correlation  of 007, 609, 017 

distribution  and  character  of 251 , 2(5si  607^  609 

iron  ores  in '  cq-, 

"''^t'^sof "-'^^'!'!'!;;;;;;;;;;;;'265,268 

topography  on. jgg 

Clements.  J.  M.,  on  Crystal  Falls  district -'.. '....^294^295,510 

ouglaciation 431  435 

on  physiography !'!  91,' 92,93-94,98!  102 


on  Vermilion  district. 


„,  40-41 

t  lements,  J.  M.,  and  Smyth,  H.  L.,  on  Crystal  Falls  district.  39^0. 107 
Clements,  J.  M.,  Smyth,  H.  L..  and  Bayley,  W.  S.,  on  Crystal 

Falls  district 9g_97  jq^ 

Cleveland  mine,  geology  at "  '271 

historyof... ^ 

575 

36 

460 


45 
568,569 


,  on  Michipicoten  ranges. 


end  mine,  description  of 

historyof, 

Clinton  iron  ore,  deposits  of 

distribution  and  character  of. 567-568 

historyof 

origin  of. 

photomicrographs  of "536 

shipmentsof ..,_ 

_  567 

Chnton  Point,  geology  near 376,379  4'>1 

Cloquet  district,  geology  of 213. 214,215  375 

Cobalt  district,  geology  of. '       '       'g^j 

^  °"'''"-; :!:;;!:::!:::::::::;:'592,626 

Coke,  use  of .,  ,„ 

^  ,,         .  .  4/-4S 

Colby  mme,  geology  at 03^ 

Cole,  G.  A.  J,,  and  Gregory.  J.  W.,  on  ehipsoidal  structure 511 

Coleman,  A.  P.,  on  gold  ores -nj 

on  lake  basihs 

on  northwestern  Ontario 1  jo. 

Coleman,  A.  P.,  and  Willmott,  A.  B 

Coleraine,  building  of 

Collins,  W.  H.,  on  region  north  of  Lake  Superior 

Commonwealth,  geology  near 

Commonwealth  district.    See  Florence  district. 

Competition  in  mining,  effects  of 

Concentration,  mechanical,  process  of 539-540 

Concentration,  secondary.    See  Secondary  concentration;  Weath. 
ering. 

Conglomerates,  copper  in 

Copper  ores,  area  of,  extent  of V-Za 

area  of,  map  showing '      c- . 

association  of,  with  igneous  rocks 5gj 

chapter  oa 

See  also  Keweenaw  Point,  copper  ores  of. 

character  01 -„„ 

0/3 

deposition  of,  chemistry  of 589-590 

"<'^'^°' ■..'.'-.'.'..'....  582-586 

,     t™"^"' : 5S1-5S2 

deposits  of,  types  of jjp 

depth  and,  relations  of -„, 

e.xtentof „. 

0/5 

grade  of ^^^ 

mine  waters  from c-q 

mineralogy  of '"!-'"!!!;!;!;!!!;;"573^574,582 

mining  of,  history  of 3j_3g 

occurrence  of 3„, 

modeof '      „, 

origin  of ! .  569 

production  of 


431 

-150, 603 

95,160 

44 

95 

.       321 

41 


577-57! 


573-593 


Cretaceous  rocks,  deposition  of        .  ^fjs 

detritaiorcsm '.y^'.'.'.'.'. ::.'::::.:.::::  i9^m 

analysis  of 

distribution  and  character  of '.'.■.■.■  'i59,'l'78^i79,'2U,'21S,  616 

"°°-°': ■. 178-179, 196-197, 460*503 

phosphorus  m jg  ' 

Crj'stal  Falls,  geology  near 295  323 

Crystal  Falls  district,  correlation  in 


exploration  in . 


.TOS,  606, 610-011,617 

484 

eO^^OSyot 291-300, 441,  i;07-609, 619 

map  showing ,^, 

iron  ores  of,  alteration  of 

character  of 


546 

323,325 

composition  of 324-32' 

magnetic  phases  of ^g 

plate  showing .„ 

proportion  of 

relations  of 

reserves  of 


462 

501 ,  507,  .i59 

489 

secondary  concentration  of 326  5.39 

volumetric  diagram  of 

location  and  area  of ... ; 

map  of 


580-592 
575 

relations  of,  to  other  ores 691-S9'> 

wall  rocks  of,  alteration  of 582-585 

Copper  Lake,  hon  at jj„ 

Copper  River,  geologj-  near 378-379 

Cordierite  homstone,  distribution  and  cliaraoter  of.  ■. 173-174, 200 

Corey,  G.  W.,  and  Bowen.  C.  F.,  on  Menominee  district '345 

Correlation,  chart  showing 59g 

details  of.    See  particular  systems,  etc. 

principles  of .„- 

Courtis.  W.  M.,  on  silver  minerals 

Coutchiching  schists,  correlation  of 

occurrence  and  character  of •j.jg 


599 
593 
598 
147 


Cox,  G.  H.,  on  hj'dration  of  ores 556^557 

Creosote,  recovery  of 48 


353 

- 32,291 

mining  in,  history  of 3„     ' 

physiography  of !!!!!!!"!! 90^97  ,07 

Cuba,  iron  ores  of 495 

CuB  mine,  geology  near „„g 

Current  River,  geology  near "" 205  206 

Curry  member,  correlation  of 'ggg 

distribution  and  character  of.  335^340  347 

,-,  "•™°'-<'<" ;::;::;;:::;;: '345 

(.  iirry  mine,  geology  near 339 

Cuyuna  district,  correlation  in ; 698  610-611 

description  of " '        '  32  211 

exploration  in 484^485 

S™l°Sy<" !!!!!!. '211-216,' 375. 604-605 

iron  ores  of,  alteration  of  ,,„ 

,  „  646 

analyses  of 220 

character  of '  2I9_2')o 

composition  of 2''0-223 

figure  showing 221  222 

distribution  of 'jig 

magnetic  phase  of 217-219  486 

phosphorus  in '  oon 

■"'^■''"^sof !!!!!!!!!!!!!!;!;;;;'5oi,507 

reserves  of ,„„ 

J  489 

secondary  concentration  of 223-224  639 

section  of,  figure  showing '210 

structure  of 216  223 

topography  on '^^g 

manganese  in "      ^^ 

maps  of 212 

mining  in,  history  of ^^  ^g 

phosphorus  in "      224 

physiography  oi jn 

213-214 

623 

D. 

Daly,  R.  A.,  on  intrusion 

on  lavas 

Uam  Lake,  geology  near Xil 

Darling,  J.  n.,  on  earth  movement 

Davis,  C.  A.,  on  Marquette  district 

Davis,  W.  M.,  on  physiography 

Dead  River  area,  description  of 

geology  of ^y_; 

,     '"'^Po' ■■ -.'^''''^''^^:'...      286 

Deception  Lake,  geology  near 208  370 

iron  near ..!...!...  207 

Deerhunt  mine,  geology  at 

Deer  Lake,  geology  near 

Deerwood,  iron  near nig 

Deerwood  member,  alteration  of 223-224 

correlation  of ..!!.'.'.'.'.'.'.'."598,610-611 

distribution  and  character  of 212-216 

'■■™.i° -!!!!!!!'!!!;!;;;;;;;;..  "460 

magnetic  rocks  in 216-219 

structure  of ,^7 


rocks  of,  correlation  of. . 
structure  of. 


600 
510-511 


451 

96 

110 

287 


301 
254 


630 


INDEX. 


Page. 

Deformation,  changes  due  to ^^ 

description  of 620-621, B2Wafi 

Deltas,  formation  of 45^459 

Doming,  A  C,  aid  of ^" 

Density,  relation  of ,  to  cubic  feet  of  ore,  diagram  showing 480 

Dialjase,  composition  of • ■"' 

Diamonds,  discovery  of  in  drift *^^ 

Diemer,  M.  E.,  work  of ^^^ 

Dilces,  distribution  and  character  of 136,236,411 

Dip  needle,  use  of,  in  prospecting ■'■' 

Disappointment  Lake,  geology  at 129, 131, 133, 134, 199 

Disappointment  Mountain,  geology  at H^ 

Districts,  ore-bearing,  list  of 31-32 

Docks,  ore,  list  of ■•* 

Mewol *^ 

Dog  River,  geology  on \^ 

Dori5  conglomerate,  correlation  of °^^ 

distribution  and  character  of 150, 151, 154-155 

petrography  of 154-lo5 

Douglas  County,  Wis.,  copper  of ^5*0 

Drag  folds,  description  of 347-348 

figure  showing 350 

Drainage,  character  of 8' 

description  of 33-.J4 

Drainage,  ancient,  character  of 8G-87 

modification  of 113. 435 

figures  showing 1'3 

Drift,  ages  of 435-436 

areas  of 4o4-45o 

deposition  of ^'■-  435-437 

distribution  of 439-441 

drainage  of  areas  of ^5 

obscilration  by ■*30 

stratification  of *27 

topography  of 454-455 

modification  of 455-459 

See  aim  Pleistocene;  Glaciers:    Moraines;   Kamcs:   Outwash 
plains,  etc. 

Driftless  Area,  lakes  in 438 

location  of ^^ 

map  showing ^8 

physiography  of 454 

view  of ^"i 

Drilling,  exploration  by 484-485 

Drumlin.s,  formation  of 433-434 

Dubois,  II.  W.,  and  Mixer,  C.  T.,  analysis  by 281 

Duluth.  geolog>-  near 452-453. 458-459 

physiography  near 458 

Duluth,  Lake,  formation  of 444-445 

map  of : 445 

Duluth  escarpment,  description  of 112-115 

glaciation  of 431 

view  of If 2 

Duluth  gabbro,  correlation  of 598 

distribution  and  character  of 137, 

159, 177, 198. 201-202, 372-373, 410, 414-415 

dikes  of 137 

intrusion  by 1.31,137,372-373,426,561 

metamorphism  by - 546 

plate  showing 548 

relations  of 202-203,372-373 

segregations  in 561 

view  of .- 112 

Dumortierite.  occurrence  of 515 

E. 

Eagle  TI  arbor,  geology  near 413 

mining  near 36 

Eagle  U  iver  district,  copper  veins  of 575-576 

geology  of 425 

section  in 380 

silver  In 575 

Eames,  II.  H.,  on  Mcsabi  district 42 

Eastern  sandstone,  relations  of 388-389 

Eeliel,  E.C.,  on  production  of  manganiferous  iron  ores 4.S8 

Eleanor.  Lake,  geologj-  near 153, 154 

Eleanor  slate,  correlr tion  of 598 

distribution  and  character  of 160, 154 


Page. 

Elevations,  height  of , 33,86,94 

EHtman,  ,\.  TI.,  on  Minnesota  geology 371,:i7.')-376 

Ellipsoidal  structure,  occurrence  and  character  of 120, 148, 151 

origin  of 502,511-512 

plate  showing 120 

significance  of.  In  ore  genesis 510-512 

Ely,  geologj- near 119,122,123,124.126 

iron  ores  near 1.37-138 

character  of 140 

composition  of 139, 140 

secondary  concentration  of 142-143 

changes  in 142-143 

figure  showing 143 

Ely  greenstone,  acidic  flows  interliedded  with 121 

age  of 127-128 

clastic  rocks  associated  with 121 

correlation  of 598 

distribution  and  character  of 119-122 

intrusions  in 121-122, 128-129 

mineral  composition  of 120-121 

relation  of,  to  Soudan  formation 124-128 

topography  on 93, 119 

Ely  Lake,  geology  near 122 

Embarra,ss  granite,  correlation  of 598 

distribution  and  character  of 159, 178, 415 

intrusions  l)y 178 

Embarrass  Lake,  geology  near 160, 176 

Emerald  Lake,  geology  near 122,123,126 

England,  iron  ores  of 495 

English  Bay,  geologj'  at 368 

Epsilon  Lake,  geology  near 136 

Erosion,  amount  of 89-90, 109,558-559 

relation  of,  to  iron-bearing  sediments 50S-506, 5.58-559 

topography  due  to 86,98-99,109 

Eruptive  rocks,  iron  in 512-513, 569 

mineralization  of 569 

relations  of,  to  iron  ores 506-516 

solutions  from 587-588 

See  also  Lavas;  Igneous  rocks. 

Escarpments,  age  of 116 

description  of -• 112-116 

distribution  and  character  of 110-111 

origin  of 111-112, 117 

structural  relations  of 116-117 

Eskers,  formation  of 434 

Exploration,  cost  of 47 

methods  of 484-1S6 


Fall  Lake,  geology  at 119 

Faulting,  description  of 620 

evidence  of .' 87, 104, 114, 117 

Influence  of,  on  physiography 87, 98-99, 101 ,  104, 112-115, 1 17 

figure  showing ! 112 

Fault  scarps.    Sec  Escarpments. 

Fay  Lake,  geology  near 131 

Felch,  iron  ore  near 327 

Felch.Mountain  district,  correlation  in 598,609 

geology  of 302-305,386,609 

iron  ores  of 326-327 

analysis  of 327 

relations  of 501 

secondary  concentration  of 328 

■     volumetric  diagram  of 353 

location  and  area  of 32, 302 

physiography  of 107 

stnut lire  in 623 

Felch  schist,  correlation  of 327, 609 

distribution  and  character  of 302, 303,306,307, 609  ' 

relations  of 305 

Felsite,  copper  in 574 

Fence  River  district,  geology  of 293,295,296-298 

Fence  River,  physiography  of 107 

Fenner,  C.  N.,on  igneous  rocks 511 

Fernckes,  G . ,  work  of SS9 

Field  work,  correlation  of  laboratory  e.xperiments  and 527-529 

Fish  Creek,  geology  on 379, 415 

Flambeau  River,  geology  on 357 


INDEX. 


631 


Page. 

Florence  district,  correlation  in 598,610-611,617 

exploration  in 484 

geology  of 320-323, 376, 379-380, 606 

iron  ores  of,  alteration  of 546 

character  of 323-  325 

composition  of 324-325 

production  of 461 

relations  of ,' 601 ,  507 

reserves  of 492 

secondary  concentration  of 326 

structure  of 475 

volumetric  diagram  of 353 

location  and  area  of 32, 320 

map  of ! Pocket. 

mining  in,  history  of 39-40 

Flowage,  rock,  alteration  by 554-555 

Fluor  I.'iland,  geology  on 368 

Folding,  extent  of 123,  C20-622 

,  figure  showing 123 

Fond  du  Lac,  physiography  near 452 

Foster.  J.  AV.,  and  Whitney,  J.  D..  on  Marquetle  district 96 

Fourfoot  Falls,  geology  at 345 

Fox  River  valley,  correlation  in 598 

geology  of 365 

map  of . .  .^ 359 

Freda  sandstone,  deposition  of 426 

distribution  and  character  of 384, 414, 417, 426 

Freedom  dolomite,  correlation  of 598 

distribution  and  character  of 360-361 

iron  in 460 

Freight  rules,  relation  of.  to  grade  of  ores 494 

Fuel,  nature  of 47-48 

Fumee.  Lake,  geology  near 336 

G. 

Gabbro.  metamorphism  by 546 

Gabbro  plateau,  character  of 91-92 

monadnocks  on 92 

Galiimichigami,  Lake,  geology  near 131, 136, 199, 202 

Gary,  E.  n.,  on  iron  ore  and  freight  rates 494 

Gate  Harbor,  geology  near 413 

Geikie,  A.,  on  igneous  rocks 511 

on  iron  ores 508-509 

Geography,  maps  showing 31, 32 

outline  of 30-32 

Geologic  history,  rgsumfi  of 023-026 

Geologic  map  of  Lake  .Superior  region Pocket. 

Geography,  physical,  account  of ■ 85-117 

Geologic  knowledge,  growth  of 72-73 

Geologic  work,  history  of 70-84 

Georgian  Bay.  copper  ores  at 626 

Geology,  general 597-1)26 

Germany,  iron  ores  of 495 

Giants  Range,  definition  of 41 

description  of 169 

geology  of .' 160, 172-173, 176, 177, 178, 179 

Iron  of 165 

mining  on,  history  of 42 

physiogiaphy  of 103-105 

structure  of 175 

Giants  Range  granite,  correlation  of J9S 

distribution  and  character  of 135-136, 162 

intrusions  of 170 

phosphorus  in 194 

relations  of 162-163 

topography  on 94 

Gilbert,  G.  K.,  on  glaciation 450 

Gilman  deposits,  iron  ores  of 565 

iron  ores  of,  analysis  of 565 

Glacial  deposits,  distribution  and  character  of 179, 

216, 308-309, 355, 559-560 
See  also  Pleistocene  deposits;  Drift;  etc. 

Glacial  epoch,  description  of 427-453 

See  also  Pleistocene. 

Glacial  lakes,  beaches  of 449-452 

deposits  in 452-453, 455 

distribution  and  character  of 441-448 


Page. 

lilacial  lakes,  formation  of 427  4,3^ 

tilting  of,  etiect  of 448^449 

Glaciation,  effects  of 33,91,92,98,106,114-116,427-453 

erosion  by ■_ 427 

period  of 427 

Glaciers,  advance  of 427-429 

advance  of,  map  showing 428 

effects  of 427 

contrasts  in 43Q 

constructive  work  of 4.33-441 

See  also  Moraines,  etc. 

erosion  by 43(^32 

melt  ing  of 435 

retreatot !'!  429-435 

soiu-ce  of 427 

transportation  by 432-433 

See  also  Drifts. 

Glauconite,  relation  of,  to  iron 503 

Goetz  Lake,  geology  near 153 

Gogebic  district,  correlation  of (joe  617 

geology  of 214, 380, 423!  600 

iron  ores  of,  alteration  of. 54(5 

analyses  of 4gi 

magnetic  phase  of ^gQ 

production  of 49-51,69,461 

relations  of 601  .507 

reserves  of 489^92 

secondary  concentration  in 475  539 

structure  of 475  4gg 

manganese  in 4gg 

mine  waters  in,  analysis  of ,-. 543 

mining  in,  history  of 40 

See  also  Penokee-Gogebic. 

Gogebic  Lake,  geology-  near 385,  .388 

Gold  ores,  distribution  and  character  of 595-596 

Gold,  mining  of,  history  of 46 

Goldthwait,  J.  W.,  on  glaciation 451 

Goodrich  mine,  geology  near 265 

Goodrich  quartzite,  correlation  of. 598, 608 

distribution  and  character  of 251, 265, 283, 285, 288, 289, 608 

iron  ores  in 270-272 

relations  of 264, 265, 266-267, 268 

topography  of 106 

Goose  Lake,  geology  near 258 

Gordon,  A.  T,,  analysis  by 179, 191, 193 

Gordon,  W.  C  on  Black  River  geology 384-385,388 

Graben  faulting,  description  of 112 

figure  showing 112 

Grace  mine,  gold  in 595 

Grand  Portage  Bay,  geology  at 370 

Grand  Rapids,  geology  near _ .      164 

iron  near 164 

Grand  Traverse,  physiography  near 433 

Granite  Bluff,  geology  at 306 

Granite  Island,  geology  at 414 

Grant,  U.  R.,  map  by 355 

on  Gunflint  Lake  district 201 

on  Keweenawan 398-399. 400-401 

on  physiography 91, 92, 9,S-99, 100 

on  Wisconsin  geology 378 

Grant^burg,  glaciation  at 4.37 

t  treat  conglomerate,  deposition  of 425 

distribution  and  character  of 381-387, 390, 413, 416, 418 

relations  of 574 

Great  Lakes,  drainage  to 33-34 

history  of 448-449, 456-459 

maps  showing 457, 458 

structural  relations  of m 

transportation  on 490-497 

Great  Palisades,  geology  near 371-372 

section  at,  figure  showing 371 

Greenalite,  alkaline  solutions  producing,  source  of 525 

alteration  of 187, 197, 210, 530, 5.37 

chemistry  of 550-551 

plate  showing 532, 534 

analyses  of 1G7 

carbonate  ores  and,  relations  of 526 


632 


INDEX. 


Page. 

Greenalite,  carbon  dioxide  producing,  source  of 527 

conii»os:tion  of 528 

dei)osition  of 503. 521-522 

distribution  and  character  of 165-108,  •102, 572 

iron  in 10.^108 

nature  of 522. 525 

oxidation  ancl  hydration  of 530,537 

phosphorus  in ^^^ 

photomicrofiraphs  of 524.  .'532 

plates  showint; 120.474 

Green  Bay  lobe  of  Labrador  glacier,  extent  of 428 

Greens'one  conglomerate,  correlation  of 59S 

distribution  and  character  of '-^^ 

iron  in ^'  - 

Oreenwater  Lake,  iron  of '50 

Gregory,  J.  W.,  and  Cole,  G.  .\.  J.,  on  ellipsoidal  structure 511 

Gros  Cap.  geology  at 151, 152, 153, 154. 393 

Gros  Cap  greenstone,  correlal  ion  of 598 

distribution  and  character  of 150-151 

intrusions  of '54 

petrography  of 151 

Grout,  W.  F.,  on  the  Kewecnawan 376 

Groveland,  iron  ores  near 327 

Gi-ovcland  formation,  distribution  and  character  of 296, 304, 305 

topography  of -      '^^ 

Gunflint  formation,  analysis  of 204 

correlation  of 598 

distribution  and  character  of 198, 199-200 

iron  ores  in 200, 203-204, 460. 480 

magnetite  in 501 

metamerphism  of 200 

phosphorus  In 1^5 

relations  of 203 

section  of,  figure  showing 199 

structure  of 199-200 

topography  of 102 

GunHint  Lake  district,  correlation  in 598 

description  of 198 

geology  of 136, 172, 177, 198-203. 209. 604 

iron  ore  from 203-204 

photomicrograph  of 524 

physiography  of 101, 1U2 

H. 

Hall,  C.  W.,  on  Cuyuna  district 213 

on  glaeiation "155 

on  Kewecnawan  series .- 377 

Hall,  R.  D.,  analysis  by 158, 173, 518 

Hamburg  slate,  distribution  and  character  of 356 

topography  of 108 

Hanbury  n  ill,  geology  at 335 

Hanbury  slate,  correlation  of 267, 307. 330. 340. 598 

topography  of 1"^ 

Hancock  mine,  geology  at 307 

Hanging  valleys,  submerged,  occurrence  of 114 

Hartford  mine,  ore  from,  washing  tests  on 281 

Hawkins  mine,  folding  at,  view  of 180 

Helen  fonnation,  correlation  of 598 

distri  bution  and  character  of 150, 152-153, 155 

intrusions  in 154 

iron  in 152,155.460 

petrography  of ■ 153 

relations  of 153-154 

Helen  mine,  description  of 150. 157 

geology  near 152, 154 

hanging  valley  near,  view  of 432 

history  of 45-16 

Hematite,  deposition  of 527 

mining  of,  history  of 45 

occurrence  and  character  of 479, 534-566. 572 

Hematite  Mountain,  glaeiation  near 431 

Hematitic  chert,  plate  showing 406 

Hemlock  formation,  correlation  of 598, 007,617 

distribution  and  character  of 291,294-296,323,607 

relations  of 297,300,507 

topography  of ' 107 

Hcnnansvllle  limestone,  distribution  and  character  of 306, 

307,330,345-346 
Heron  Bay,  topography  near 95 


Page. 

nibbing,  Minn.,  geology  near 105,10 

iron  ores  near 476 

section  near,  figure  showing 180 

Highlands,  elevations  in 86 

rocks  oJ 85 

subdivisions«f 85 

topographic  development  of 85-89 

Sec  a^so  Uplands;  Monadntxjks;  Peneplain. 

History,  geologic,  rfoumfi  of 623-026 

History  of  mining 35-<J9 

Holyokemine,  geology  near 254.287 

Hot«hkiss,  \V.  O.,  work  of 320,345 

Houghton,  Douglass,  in  vest  igal  ions  by 35.38,71 

Hubbard.  L.  L.,on  Kewecnawan  rocks 381-382, 

3S5, 398. 400-401 .  404-405. 418, 421 

Hudson  Bay,  drainage  to 34 

geology  of 626 

glaciers  from 427, 428 

iron-bearing  rocks  in  region  of 026 

Ifuinboldt,  geology  at 265 

Hunt,  T.  S.,on  N'ipigon  Bay 368 

Hunters  Island,  geology  of 122, 126, 130 

physiography  of "■  -  94-95 

Hunters  Island  series,  distribution  and  character  of 118 

Huron  Creek,  view  of ; 434 

Huronian  series,  correlation  of 305, 598-599. 002-614 

defonnation  of - 624-^)25 

deposition  of 176-177. 603.  (iOO-OOS.  024-fi25 

distribution  and  character  of 129-137. 146-1 50, 

159, 101- 177. 198-201 .  205-207. 212-214, 224-234, 
251-269,283-323.329-344.350-357,598.602-014 

igneous  rocks  in -. 176, 

178, 197, 206, 268-269, 299, 304, 318, 322-323, 603, 609, 614,617 

iron  horizons  in 460-461 

iron  ores  of 501.504.505,513 

relations  of 170, 230,300, 318.372, 617-619 

map  showing 292 

structure  of 175-176,225,286 

unconformities  at  and  in 617-619 

topography  of 92,94,98.102-108 

view  of 112 

Huronian,  upper.    S{c  .\nimikie. 

Hydration,  variation  in.  cause  of 555-557 

I. 

Ice.    Sue  Glaeiation:  Glaciers:  etc. 

Igneous  rocks,  alteration  by  intrusions  of 545-554 

association  of.  wit  h  copper  ores 581 

with  iron  ores 506-510 

calcium-magnesium  content  of 506 

copper  in 581 .  588 

derivation  of  iron  from 500,506-518,568.571 

distribution  of 85, 507-508 

erosion  of 507 

iron  in ? -       505,512-513,566 

metamorphism  by 545-554, 558 

mineralization  by 569 

nomenclature  of 395-407 

physiography  of 108 

solutions  from 587-588 

Sec  also  .Solutions. 

weathering  of 503-505, 514-516 

See  also  Eruptive  rocks:  Intrusive  rocks;  Lavas. 

Illinois  mine,  section  of,  figure  showing 304 

Indiana  claim ,  copper  on 574 

Indians,  use  of  copper  by 35 

Industry,  changes  in.  history  of 47 

influence  of  physiography  on 48 

Ingall.  E.  D..  on  .■silver  ores 593-595 

International  Geological  Committee. cited 145,147.597 

Intrusive  rocks,  distribution  and  character  of 135-1.30.614 

relation  of.  to  structure 621-<i22 

solutions  from 587-588 

See  also  Igneous  rocks. 

Iron,  salts  of 51S-519 

source  of 502,518 

conclusions  on 516 

transportation  of S3* 


INDEX. 


633 


Page. 

Iron-bearing  formations,  alteration  of 500, 529 

alteration  of,  by  igneous  rocks 545-554 

chemistry  of 550-551 

analyses  of 491-492 

association  ^ f.  with  igneous  rocks , 602-5IS,  611 

character  of 461-462, 549-550 

composition  of 462, 500 

correlation  of 610-011 

deformation  of 600 

deposition  of 499, 50O-5I8, 013. 625 

character  of 500-506 

distribution  and  character  of 620 

horizons  of 460, 491-492,  lila-611 

magnetism  in _ 4S(W488 

ore  in,  proportion  of 462. 611 

outcrops  of 476^  477 

topography  of 470 

weathering  of 502-505, 516. 539-540 

See  also  particular  formation.-. 

Iron  Belt  'nine,  geology  at 236 

Iron  carbonate.    See  Carbonate  ores. 

Iron  II  ill.  geology  near 333, 334-335, 340, 341, 343 

map  and  cross  section  at 335 

Iron  ores,  analyses  of 477 

bodies  of.    See  Ore  bodies 

chapter  on 460-571 

character  of 480-484 

chemical  composition  of 477-479 

representation  of Igo 

figures  showing 182,221,223,478.480 

chemical  origin  of 500-518 

contents  of 481-484 

determination  of 481-482 

diagram  for 480 

deposition  of 499-51S 

chemistry  of 518-528 

order  of 501 

genetic  classification  of 571-572 

grade  of 493-495 

figure  showing 493 

horizons  of 460 

hydration  of,  variation  in 555-557 

localization  of 518, 544-545 

magnetic  phases  of 185-186. 486-488 

magnetitic  phases  of 480 

manganese  in 488 

mechanical  concentration  of 540-541 

metamorphism  of , 500, 549-550. 560-561 

mineralogy  of 479-480 

mine  waters  of,  composition  of 540,543-544 

mining  of,  history  of 38-69 

methods  of 497-499 

occurrence  of 236-237 

in  pitching  troughs,  figure  showing 236 

origin  of 499-570 

summary  of 568-569 

plates  showing 480, 542 

production  of 49-79, 490-491 

by  grades 479 

figure  showing 49, 490 

proportion  of ,  in  formations 462 

reserves  of 488-495 

availability  of 488-490, 491-492 

comparison  of,  with  other  regions 492 

grade  of 493-495 

life  of 490-491 

royalties  on 499 

secondary  concentration  of.    See  Secondary  concentration, 
structure  of.    See  particular  districts. 

texture  of 480 

chart  showing 481 

transportation  of,  cost  and  methods  of. 495-497 

by  glaciers 432. 559-560 

value  of - 499 

views  of 480 

volume  of,  relation  of.  to  cubic  feet  of  ore,  chart  showing 480 

5ff  fl/w  Hematite;  Limonite:  Clinton  ores;  Brown  ores,  etc.; 
particular  districts. 


Page. 

Iron  ores,  foreign,  consumption  of 495 

Iron  ores,  manganiferous,  origin  of. 488, 5ti0 

Iron  River,  geology  near 313, 315-3I6, 507 

Iron  River  district,  correlation  in 598,606,609-«ll,617 

exploration  in 484 

geology  of 309-320,605,009 

iron  ores  of,  alteration  of 540 

character  of 323-325 

composition  of 324-325. 501 

relations  of 501.507 

secondary  concentration  of 326 

structure  of 475 

volumetric  diagram  of 35;{ 

location  and  area  of 32  308 

mapoS Pocket. 

mining  in,  history  of ' 39-40 

physiography  of 308-309 

Iron  sulphide,  deposition  of 527 

Ironwood,  geology  near 243, 394 

Ironwood  formation,  alteration  of 243-245 

alteration  of,  figure  sliowing 245 

correlation  of 598.608-611 

distribution  and  character  of 225. 230-232 

'■•on  of 235-247, 250, 460 

phosphorus  in 247-249 

figin-es  showing _.  248. 249 

relations  of 232. 385 

structure  of 235-236. 250 

Irving.  R.  D.,  on  copper  ores 575,577,580-58 

on  glaciation 439 

on  Keweenawan  series 366, 372-378.  .381,385-389, 394, 397, 409, 414 

on  Keweenaw  district loO 

on  Lake  .Superior  basin 421.457 

on  physiograpliy 97, 98, 102-103. 104 

work  of 29. 30, 71 

Irving.  R.  D.,  and  Chamberlin,  T.  C,  on  Keweenaw  Point...  115,384 

Irving,  R.  D.,and  Van  Ilise,  C.  R.,  on  physiography 103 

Ishpeming.  geology  near 263, 265, 267, 269, 270, 271 

jaspi tite  from,  plate  showing 464 

Ishpeming  formation,  topography  of 106 

Isle  Royal,  copper  ores  of 530 

escarpment  of 115-116 

geology  of 389-390, 408, 418, 421, 425 

correlation  of 615 

mining  on 530 

history  of 37-38 

physiography  of. 99. 1 16. 457 

ridge  on.  view  of 90 

Isle  Royal  location,  history  of 36 

Issati,  Lake,  formation  of 443 

J. 

Jacobsville  sandstone,  distribution  and  character  of 385 

Jasper,  analyses  of 139, 140, 505 

distribution  and  character  of 124-125. 461-462 

origin  of 650 

plates  showing 464, 466, 472, 564 

section  of,  figure  showing 123 

Jasper  Bluff,  jaspilite  from,  plate  showing 464 

Jasper  conglomerate,  plate  showing 542 

Jasper  Lake,  geology  at 126 

Jasper  Peak,  description  of 90 

geology  of 122 

view  of 88 

Jaspilite.    See  Jasper. 

Joseiihine  mine,  history  of 46 

Jiimlio,  geology  near 313,316 

K. 

Kakabeka  Falls,  Ontario,  geology  near 149 

Karnes,  formation  of 435 

Kaministikwia  district ,  geology  of 149-150, 599 

Kawishiwi  River,  geology  on 133, 134, 201 

Kearsargelode,  description  of 575 

Keewatin  glacier,  advance  of 428 

Keewatin  series,  age  of 146 

correlation  of 549-600 

deformation  of 624 


634 


INDEX. 


Paj,'<-. 

Keewatln  scries,  distribution  and  cliaracter  of 119-128, 

1 44-145. 148. 150-154. 100. 198. 20&-2CI6. 225. 22f>.  2.';2, 
254-255,   287,   309-310,  322,  330-331,  597,  .59»-li(XI 

extension  ol 623 

gold  in 595 

iron  ores  of 46,149,460-461,501,507,517-518,626 

alteral  ion  of. 554-555 

proiiuction  of 517 

relations  of 504. 506 

relations  of 227,257,260.504.506 

Sc(  also  particular  formalions. 

Kekekaliic.  L.ike,  geology  near 133, 134. 130, 603 

Kettle  Kiver,  geology  near 378-379 

Kettles,  format  ion  of -• 438 

Keweenavvan  series,  age  of 415-416, 420 

area  of 419 

copper  ores  in 574, 580, 581-582 

correlation  of. 305.598. B14-G15 

deposition  of 416-418,424-125, 426, 615. 025 

distribution  and  character  of 137, 

159.177-178.198.201-209.212.215.224-220,2(4- 
235,250,251.300-305,366-420,  597,  602,  614-(il5 

faulting  of. 420-421 ,  620, 622, 6M 

folding  in 622 

grain  of 407-408 

Ustory  of,  r&iunfi  of 424-426. 615 

igneous  rocks  in .395-412, 425. 615 

nomenclature  of 395-407 

source  of 411-412 

intrusions  of 171-172, 197, 215, 278-279, 377-378, 418, 424, 425-426 

iron  horizons  in 460 

jointing  in 420 

metamorphism  of 423-424 

relations  of 202-203, 

234-235, 378-379, 384-386, 388-389, 414-416, 420, 619 

section  of 384 

figure  showing 99 

sediments  in 412-413 

structure  of 376,383,620-625 

figure  showing 419 

subdivisions  of 366-367. 614 

thickness  of 418-419 

topography  on 98, 99-102 

unconformity  at 619 

volume  of 419-120 

See  aho  particular  fonnafions. 

Keweenaw  district,  location  and  area  of 31 

mining  in,  history  of 35-37 

physiography  of. 97, 100, 116 

production  of 36 

topography  of 91-92, 94, 100 

Keweenaw  escarpment,  description  of 115 

Keweenaw  lobe  of  Labrador  glacier,  extent  of 428 

Keweenaw  Point,  copper  ores  of 573-593 

copper  ores  of,  character  of 573 

extent  of 573 

grade  of 574 

minerals  of 673-574 

occurrence  of,  mode  of 574 

production  of 575 

geology  of  and  near 380-385. 408, 409, 412-413, 415, 418, 425 

maps  of 380,574 

section  of,  figure  showing 99,574 

silver  at 575 

structure  on 383 

veins  on 574.575-576 

Kcyes  Lake,  geology  near 321,322 

Kimball  Lake,  geology  near I53 

King,  F.  II.,  on  glaeiation 439 

Kitchi  schist,  correlation  of 598 

distribution  and  character  of 262, 254-255, 260, 287 

Kloos,  J.  II.,  on  Kcwecnawan  series * 397 

Knife  Lake,  geology  near X19 

Knife  Lake  slate,  correlation  of 593 

distribution  and  character  of 132-135 

intrusion  of 133-134 

lithologyof 133-134 

mlner.il  character  of I34 


Page. 

Knife  Lake  slate,  relations  of 135 

structure  of 133 

thickness  of 135 

topography  of 94, 119 

Kona  dolomite,  correlation  of ^ 598.605 

distribution  and  character  of 252-25.3.258.605 

relations  of 258, 200,305 

topography  of 105 

L. 

Laboratory  sTOthesis,  correlation  of  field  work  and 527-529 

Labrador  glacier,  advance  of 428 

Lac  la  Belle  conglomerate,  distribution  and  character  of 381 

Lake  Angeline  mine,  washing  tests  on  ores  from 281 

Lake  basins,  formation  of 431-432 

section  of,  figure  showing '. 432 

Lake  Huron  shore,  correlation  on 598 

Lake  Michigan  lobe,  extent  of 428 

Lake  of  the  "Woods,  geology  at 122 

Lake  of  the  Woods  district,  correlation  of 69<i.  .599.  ooo 

descript  ion  of 144 

geology  of 144-1 47 

iron  absent  in 144 

physiography  of 94 

Lakes,  origin  of 91 

Lakes,  glacial.    See  Glacial  lakes. 

Lake  Shore  trap,  distribution  and  character  of 381.384.425 

Lake  Superior,  east  coast  of,  geology  of 39^-392 

Lake  Superior  basin,  character  of 33. 110-111 .  421 .  423 

escarpments  of 112-116 

age  of 116 

relations  of 116-117 

views  of 112 

formal  ion  of 622-023.626 

map  of 422 

origin  of 1 11-112. 426 

physiography  of 100. 45tv-459 

Lake  Superior  lobe  of  Labrador  glacier,  extent  of 42S.  442 

Lake  Superior  sandstone,  distribution  and  character  of 109, 

225, 2ai,  302, 304, 330. 346, 37S.  379. 456. 61 6 

relations  of 379.415.420.616 

Lakewood,  geology  of 358 

geology  of,  map  showing 358 

Lane,  A.  C,  on  copper  ores 5S1..=>,s8-590 

on  glaeiation 439-440 

on  Isle  Royal 389-390 

on  Keweenawan  series 382.383,385,398.400-403.405.407  414,421 

on  mine  waters 644 

Lane,  A.  C,  and  Seaman,  A.  E.,  on  Lake  Superior  sandstone 616 

Larsen,  E.  S.,  and  \Vright,  F.  E.,  on  quartz  crystallization 549 

Laterite,  association  of,  ^vjth  iron  ores 503 

Laurentian  liighlands.  location  of 355 

Laurontian  peneplain,  extent  of 88 

Laurentian  scries,  batholiths  of 145 

correlation  of 304. 000-601 

deposition  of 023-<i24 

distribution  and  character  of 119, 

128-129,  145-150,  154-155,  160,  198,  205,  225-227,  252, 
255-250.  283-293, 300-302,  306, 330-331,  360, 597, 600-602 

gold  in 595-596 

intnisions  of 145, 600 

relations  of 227, 200 

Lavas,  extension  of.  periods  of 5lt>-517 

variation  in.  relation  of.  to  iron 510-518 

See  also  Eruptive  rocks:  Igneous  rocks. 

Lawson.  .\.  C.  on  glaeiation 450 

on  iron  ores 509-510 

on  Lake  of  the  Woods  and  Rainy  Lake  districts 144-145, 147 

on  Minnesota 371,374 

on  physiography 94, 98, 101, 1 12, 450 

on  subcrustal  fusion,  theory  of 146 

Leaching,  process  of 537-539 

Lee  Hill,  geology  near 119, 122, 123, 127. 131 

Lehmann.  O..  on  liquid  crystals .'V2.5, 572 

Leith.  0.  K..  on  physiography 103-104,432 

work  of 30,44-45 

Lerch  Brothers,  analyses  by 193, 238 


INDEX. 


635 


Page. 

Life,  pre-Cambrian,  existence  of 617 

Lighthouso  I'uint,  Kcology  near 254 

Lime,  relation  of,  to  phosphorus 196, 249, 282, 281 

figure  showing 190. 249, 282 

Limonite,  formation  of 519-520, 571 

natiH-e  of 620-521 

Linear  monadnocks.     See  Monadnocks,  linear. 

Literature,  list  of 73-84 

Little  Falls  district,  geology  of 213, 214, 375 

Little  Presque  Isle  River,  geology  near 227 

Loess,  distribution  and  character  of 438 

Logaa,  W.  E.,  on  Keweenawan  series » 392, 393 

on  Michipicoten  Island 391 

section  by 367 

Logan  sills,  correlation  of 598 

distribution  and  character  of 198, 202, 208, 374, 410 

intrusion  of 208, 420 

relations  of 202-203, 593 

silver  in 593 

topography  on 101, 102 

Long  Lake,  geology  near 119, 132, 153 

Loon  Lake,  Mich.,  geology  near 201, 370 

iron  at 4G,  209 

Sfc  fl/so  Anlmikie  district. 

Loretto  mine,  geology  near 332, 330 

Low,  A.  P.,  on  iron  ores 508 

Lower  Magnesian  limestone,  distribution  and  character  of . .  300,361-3(12 

iron  ores  in 504-565 

Lowlands,  description  of 108-110 

geologj^  of : 108-110 

M. 

McFarlane,  Thomas,  on  Keweenawan  series 397 

on  Maniainse  Peninsula 392 

on  Nipigon  Bay 308 

on  silver 593 

Mclunes,  William,  on  Tlunters Island  and  Thunder  Bay  region.  94-95, 101 

McKays  Mountain,  geology  of 209 

McKenzie,  geology  near 206 

Magma,  iron  from 513-514, 568, 571 

solutions  from.     S(c  Solutions. 

Magnetic  phases  of  ore,  occurrence  of 185. 216-219 

5fc  aZso  Amphibole-magnetite  rocks. 

Magnetic  survey,  prospecting  by 44, 48tV-488 

Magnetite,  deposition  of 527 

occurrence  and  character  of 479, 480, 486 

origin  ol : 562 

Magnetite,  titaniferous,  character  of 561 

origin  of 501 ,  568 

Magpie  Valley,  geology  near 151 

Mahoning  mine,  concretions  in 192-193 

concretions  in,  analyses  of 193 

geology  near 165 

iron  ore  from,  plate  showing 468 

Maniainse  Peninsula,  geology  of 391-393, 418, 425 

section  on 392-393 

Manganiferous  ores,  occurrence  and  character  of 488, 560 

Mansfield,  geology  near -• 295 

Mansfield  slate,  iron  in 295-296, 324 

occurrence  of 294, 295-296, 303 

relations  of 296 

topography  of 107 

Map  of  Lake  Superior  region 86 

showing  topographic  development 87 

showing  topographic  provinces 88 

Maps,  geologic,  accuracy  of 73 

of  Lake  Superior  region Pocket 

Map,  index,  of  region 31 

Marathon  conglomerate,  correlation  of 598 

distribution  and  character  of 356,357 

Mareniscan  series,  name  of 226 .  254 

Marenisco,  geology  near 226 

Mariska,  geology  near 162 

Marquette  district,  acknowledgments  concerning 251 

correlation  in 598, 599, 606, 610-611, 617 

exploration  in 485 

geology  of 251-269,429. 441,605-610,618,620-621 

gold  of 595 


Page. 

Marquette  district,  iron  ores  of,  alteration  of 546, 554, 610-611 

iron  ores  of,  analyses  of 273, 491 

character  of , 274-275. 503 

classification  of 271-272 

composition  of 273-274 

distribution  of 270 

magnetic  phases  of 486 

occurrence  of 270-271 

figure  showing 270 

phosphorus  in 279-283 

concentration  of 281 .  283 

figures  showing 280. 282 

plates  showing 464. 468. 470 

production  of 461 

proportions  of 402 

relations  of 507 

reserves  of 489-492 

secondary  concentration  of .' 275-279, 539 

conditions  of 275 

sequence  of 278, 279 

volume  changes  in 276-277 

figures  showing 276, 277 

section  of,  figure  showing 270 

structure  of 475 

topograph  y  of 476 

location  and  area  of 31-32. 251 

map  of Pocket 

mining  in,  history  of 38-39 

physiography  of 96, 10.'i*-106. 252 

production  from 39. 51-00.69 

structure  of* 252-253. 623 

figure  showing 253 

Marshall  Hill  graywacke,  correlation  of 598 

distribution  and  character  of 356,357 

topography  of 108 

Martin,  L.,  on  Keweenawan  series 420-421 

on  physical  geography 85-117 

on  Pleistocene -127— i59 

Mass  mine,  geology  at 271 

history  of 36 

Mastodon  mine,  geologj'  at 295 

Matawin  district,  geology  of 149-150 

iron  of 150 

Mead,  W.  J.,  diagram  method  devised  by 182-183 

on  iron  ores 137-143, 

179-197, 235-250. 270-283. 286, 323-328, 346-3.54. 3(;2-365. 460-571 

work  of 518 

Meadow  mine,  phosphorus  in 194 

Meeds,  A.  D..  analysis  by 191 

Menominee  district,  correlation  of 59S,  599, 610-611 

cross  sections  in,  plate  shovilng 346 

geology  of 329-346, 605. 007-609, 616 

iron  ores  of.  alteration  of 546 

analyses  of 3.50-351. 491 

character  of 352.  .503 

composition  of 350-352 

magnetic  phases  of 486 

position  of ^59.610-Gll 

production  of 461 

proportion  of 462 

relations  of 501 .  507 

reserves  of '. 489-492 

secondary  concentration  of 3.53-354. 539 

structure  of 346-350. 475-476, 623 

figures  showing 346,347,348,349 

volumetric  diagram  of 353 

location  and  area  of 32. 329 

map  of Pocket 

mining  in,  history  of 39 

physiography  of 96. 105-106. 329. 433 

production  of 39,61-65,(19 

Menominee  River,  geology  on 321,322,344-.345 

Merriam,  W.  N..  map  by 320 

on  Steep  Rock  Lake  district 147-148 

Merritts,  discovery  by 43 

Mesaba,  iron  at  and  near 44, 172, 185 

meaning  of  name 159 

Mesabi  district,  correlation  in 598, 610-611 

definition  of 41, 159 


636 


INDEX, 


Page. 

Mesabi  district,  description  of 159 

exploration  in 47, 485 

geology  of 159-179, 604-605 

glaciation  in,  map  showing 443 

history  of 41-44 

iron  ores  of,  alteration  of 545 

alteration  of,  plate  showing 548 

analyses  of 181, 183, 185, 193, 197, 491 

characteristics  of 183-185,503 

composition  of 180-183, 193, 197, 555 

figure  showing 182 

distribution  of 179-180 

niagnetio  phases  of 185,480 

phosi)horus  in 192-19G 

concentration  of 194-195 

figures  showing 192, 190 

plates  showing 468, 474, 532 

production  of 401 

proportion  of 462, 477 

relations  of 180, 501 

reserves  of 489-492 

rocks  associated  with,  alteration  of 191-192 

secondary  concentration  of 186-191, 475, 538, 539, 558 

figu7  P  showing ISO 

sequence  of 197 

structure  of 180,  475-476,  4S(J 

figure  showing 180 

volume  changes  of 188-191 

figures  showing ' 188. 189, 190 

location  and  area  of 32 

map  of '. Pocket 

mining  in,  history  of 42-44 

magnetic  portion  of,  character  of 43 

development  of 43-44 

mine  waters  in,  analysis  of 543 

mining  in 497-498 

physiography  in : 105 

plate  showing 532 

production  of 05-68, 09 

structjre  in 623 

view  of 180 

Mesas,  distril)ution  and  character  of 100-102 

structure  of,  figure  showing 101 

Mesnard  quartzite,  analj'ses  of 257 

correlation  of 598, 605 

distribution  and  character  of 252-253, 250-258, 605 

relations  of .• 257-258, 260, 305 

topograph  y  of 105 

Metamorphism,  cause  of 545-554, 559, 582 

cycle  of 5a)-561 

effects  of 545-554, 559,582-586 

relation  of,  to  secondary  concentration 552-553 

temperature  of 549 

See  also  Igneous  rocks. 

Michigamme  Lake,  geology  near 262.267 

Michigamme  mine,  geology  at 261 

history  of 38 

Michigamme  Motmtaui  district,  geology  of 293, 295-298 

physiography  of 107 

Michigamme  River,  geology  on 295 

Michigamrn/?  slate,  correlation  of 267. 323, 598, 608-609, 611 

distribution  and  character  of....  251,  2G7-268,  283,  285,  288,  289,  291 
298-299, 306, 307, 309. 31 1-318. 321-323. 330. 340-342. 608-609 

intrusions  in 345 

relations  of 265, 206-267,268,313-314, 338-339, 343 

structure  of 312-313.341-342 

topography  of 100. 329 

Michigsm,  bibliography  for 74-77 

geology  of 380-414 

investigations  in 35. 38. 71-73 

iron  ores  of 461.507 

production  of 401 

physiogniphy  of 100. 433 

Sec  also  pnrticular  districts. 

Michigan  mine,  gold  of 596 

See  aho  Minnesota  mine. 

Michipicoten  district,  copper  ores  of 580 

correlat  ion  in 598-599, 615 

description  of 150 


Page. 

Michipicoten  d istricl,  extensions  of 155-156 

geology  of 150-156,390-391,423,425 

gold  of 505 

iron  ore  of,  analyses  of 156 

distribution  and  character  of 156-157 

reserves  of 492 

secondary  concentration  of 157-158 

location  and  area  of ' 32 

map  of 88 

mining  in,  history  of 45-40 

physiography  of. 95,150,431,456 

section  in . . 391 

Michipi(roten  Harbor,  geology  near 151, 154, 155 

Middle  cont^lorncrate,  distribution  and  character  of 381-387 

Middle  River,  geology  on 379.415 

Mikado  mine,  geolog>'  at 230.238 

Minerals,  source  of 509 

Mine  waters,  analyses  of 543.579 

composition  of 540, 543-544. 579 

Mining,  history  of 35-09 

See  also  Copper;  Iron:  Silver;  Gold. 

Minnesota,  bibliography  for 78-^ 

copper  ores  of 580 

geology  of 307-379, 425, 429 

investigations  in 72 

iron  ores  of *     401 

production  of 461 

lowlands  of 110 

map  of 212 

physiography  of 91-92, 99, 110-117 

titaniferous  ores  of 501 

See  also  particular  districts. 
Minnesota  lobe  of  Labrador  glacier.    See  Red  River  lobe. 

Minnesota  mine,  history  of 36 

ores  of 36, 576 

See  also  Michigan  mine. 

Minnesota  River  valley,  geology  of 224 

Mirmpsota  tax  commission,  aid  of 30 

Misquah  11  ills,  elevations  in 92 

Mississippi  River,  drainage  to 34 

pond  ing  of 438 

Mixer.  C.  T..  and  Dubois,  H.  W.,  analysis  by 281 

Mohawk  mine,  geology  at 178 

Moissan.  H.,  on  metamorphism 549-550 

Moisture,  relation  of,  to  cubic  contents  of  ore 483-484 

relation  of.  to  cubic  contents  of  ore.  figure  showing 480 

Mokoman.  Ont..  geology  near 149 

Monadnocks,  character  and  cause  of 90 

Monadnocks,  Unear,  descriptions  of 98-106 

structure  of.  figure  showing 101 

Mona  schist.  coiTelation  of 598 

distribution  and  character  of 252. 254-255. 287 

Monoclinal  ridges,  distribution  and  character  of 99-100, 102-108 

structure  of.  figure  showing 101 

Monopoly  in  mining,  effects  of 41 

Monroe  mine,  folding  at,  view  of 180 

Montreal  River,  geology  near 413,414 

Moose  Lake,  geology  near 119.127.129,131 

Moraine,  ground,  formation  of 433 

topography  of 433 

view  of 436 

Moraine,  recessional  and  interlobate,  formation  of 435 

Moraines,  terminal,  formation  of. 434 

view  of 434 

Morrison  Creek,  geology  on 313. 316-317 

Morton,  geology  near 224 

Mosinee  conglomerate,  correlation  of. 598 

distribution  and  character  of 356, 357 

Motley,  geology  near 215 

Mountain  Iron  mine,  geology  near 160,162,164 

history  of 43 

views  in 180. 432 

Mount  Houghton,  geology  at 382 

Mount  f  Tough  ton  felsi  tc.  correlation  of '. 382 

distributiim  and  character  of 381 

Mud  Lake,  geologj'  near 254, 257 

Musse.v.  IT.  R..on  Marquette  district 38 

on  Vermilion  district 41 


INDEX. 


637 


N.  Page. 

Namakon  Lake,  geology  at 146 

Nashwauk,  geology  near IfiO 

National  mine,  history  of 3li 

Necedah,  correlation  at 598 

geology  near 358 

maps  showing 358, 35<J 

Negaunee,  geology  near 259, 261, 263, 269, 270, 271 

hematitic  chert  from,  plate  showing 466 

Negaunee  formation,  altera!  ion  of 276-270 

alteration  of,  figure  showing 276 

analysis  of 273 

composition  of 273,  505 

correlation  of 126, 132, 598, 607, 60S 

deposition  of 501 

distribution  and  character  of 252. 262-264, 

270-272, 287-289, 291, 296-298, 300-301 , 607 

intrusions  in 264 

iron  ores  in 263-264,270-271,278,279.400 

alteration  of. 554-555 

phosphorus  in 279 

relations  of 262,264,265,269,296,297,305,506-507 

structiu-e  of 262-263 

topography  on 105 

Nemadji,  Lake,  formation  of 443-444 

map  of 444 

Newport,  geology  near 227, 236, 243 

New  Ulni,  geology  near 224 

Niagara  limestone,  relations  of,  to  ore 567 

topography  on 109 

Nicollet,  J.  N.,  map  by 41-42 

Nicollet  Lake,  fonnation  of 442 

Nipigon  Lake,  geology  near 368-370,421,423 

physiography  near 95 

Nipigon  Bay,  geology  near 367-370, 393-394, 422 

sections  near 367 

physiography  near 95 

Nipigon  series,  nanie  of 207 

Nipissing  Great  Lakes,  description  of 447-448 

map  of 448 

Nonesuch  shale,  copper  ores  in 574. 579 

deposition  of 426 

distrilmtion  and  character  of 382-384, 385, 413-414. 426 

Norrie  m  ine,  geology  at 236. 238 

iron  ore  of,  analyses  of 238 

section  at,  figure  showing 237 

North  Bliill,  correlation  near 598 

geology  near 358 

North  Mound  quartzite,  correlation  of 598 

distribution  and  character  of 356, 357 

topography  of 108 

Norway,  cross  sections  near,  plate  showing 346 

geology  near 334, 335, 339 

Norwood,  J.  A.,  on  Mesabi  district 42 

Nunataks,  effects  of 436 


Ogishke  conglomerate,  correlation  of 598 

distribution  and  character  of 129-132 

intrusions  in 131 

lithology  of 130-131 

metamorphism  of 131 

relations  of 130, 132 

structure  of 129-130 

thickness  of 132 

topography  on 1 19 

Ogishke  Lake,  geology  at 119.129.130 

Oliver  mine,  phosphorus  in 193 

Oliver  Mining  Co., aid  of 30 

area  controlled  by 47 

Ontario,  bibliography  for 81-83 

gold  of 595-596 

map  of 58 

pre-Animikie  districts  of 144-158 

•Ontonagon  district,  copper  ores  of 574,576 

mining  in,  history  of 36 

silver  in 575 


Page. 

Ordovician  system ,  distribution  and  character  of 283, 

285 , 309, 319-320, 329-330, 345-340 

fossils  of 320 

Ore-bearing  districts,  list  of 31-32 

Ore  bodies,  fonn  of 475-476, 529 

structure  of 474-475 

outcrops  of 476-477 

topography  of 47(5 

Ore  deposits,  knowledge  of,  growth  of 73 

Ore  docks,  list  of 495 

view  of 490 

Ores.    Sec  Iron  ores;  Copper  ores;  Silver  ores. 

Ortonville,  geology  near 224 

Osceola  amygdaloid ,  distribution  and  character  of 30 

Osceola  mine,  history  of. ., 30 

Other  Mans  Lake,  geology  near 132 

Otter  Track  Lake,  geology  near ug^  127 

Outer  conglomerate,  deposition  of 426 

distribution  and  character  of 381-387, 413-414 

Outwash  plains,  formation  of 436-437 

pitting  of 438 

view  of 434 

P. 

Pabst  mine,  geology  of 236  237 

Paint  River,  geology  near 299 

Paint  rock,  analyses  of 191 

distribution  and  character  of 171 

phosphorus  in 195 

Paleozoic  rocks,  distribution  and  character  of lOS-llO,  615-616 

Palmer,  relations  at 265 

Palmer  gneiss,  correlation  of 593 

distribution  and  character  of 252, 255-256 

Palms  formation,  correlation  of 598,608 

distribution  and  character  of 225, 229-230, 608 

relations  of 230 

Palms  mine,  geology  at 229, 236 

Paragenesis  of  copper  ores,  data  on 585 

of  iron  ores,  data  on 570-572 

Paulson  mine,  geology  near 199, 200, 202 

iron  of 204 

Pegmatitic  origin  of  ore,  evidence  of 502 

nature  of 569 

Pelites.  deposition  of 625 

occurrence  of 603 

Peneplain,  age  of 8»-89, 116-117 

map  showing gg 

modification  of 85-87 

origin  of. 85, 90, 559 

relations  of,  figure  showing hq 

See  also  Highlands. 

Penobscot  mine,  iron  ore  of.  plate  showing 468 

Penokee  Gap.  geology  at 229,231 

slate  from,  photomicrograph  of 548 

Penokee-Gogebic  district,  correlation  of 598 

description  of 32.225 

geology  of 225-235, 414-415,  Oil 

iron  ores  of,  analyses  of. 238, 239, 240, 241 ,  244 

character  of 240-242 

composition  of 238-240. 244 

figures  showing 239, 246 

distribution  of 235 

magnetitic  phases  of 241-242 

analyses  of 241 

phosphorus  in 247-249 

figure  showing 248, 249 

plate  showing 472 

production  of 461 

proportion  of. 462 

reserves  of 492 

secondary  concentration  of 242-250, 475 

conditions  of 242-243 

sequenceof 2507 

rocks  associated  wi  th ,  alteration  of 245-247 

analyses  of 246 

structure  of 235-238 

figures  showing 236,23 


638 


INDEX. 


Pago. 
T'enokee-Oogebic  d istrict ,  map  of 226 

physiography  of 102-103.225-226 

section  of.  figure  showing 237 

Sec  also  Gogeliic  district. 

Penolcee  Range,  description  of 102-103 

Perch  Lake  district,  description  of 288 

geology  of 288-290 

map  showing 289 

map  of Pocket. 

Pewabic  location,  history  of 36 

Phosphorus,  concentration  of 194-195,249.281,283 

distribution  and  character  of 143,192.220.224.247.279-281 

figures  showing 192.190.248,249,280 

relation  of  lime  and 196. 249, 282 

figure  showing , 196. 249, 282 

source  of 195-190.248-249,281 

Physical  geography,  account  of 85-117 

Physiography,  influence  of,  on  development 48 

Pie  Island,  geology  of 209, 426 

Pigeon  Point  district,  geology  of 204-205 

map  of 204 

physiography  of ^^^ 

Pine  Creek,  geology  near 3.36 

Pioneer  mine,  phosphorus  in 143 

section  of,  figure  showing 138 

Pitching  troughs.    See  Iron  ore;  Drag  folds. 

Pleistocene  deposits,  distribution  and  character  of 179,211,216,269, 

283, 285, 287, 288. 290, 323, 355. 360. 362. 427-459, 359-560, 617 

map  showing 453 

discussion  of 441 

modification  of 455-459 

provinces  of 453-455 

Tlewsof 432,434.436 

See  also  Glaciation:  Glaciers;  Drift;  etc. 
Pleistocene  history,  chapter  on 427-4.59 

simunary  of 459 

Plucking,  explanation  of 431 

Point  airs  Mines,  geology  at 392.393 

Pokegama  Lake,  iron  near 44 

Pokegama  quartzite,  correlation  of 598 

distribution  and  character  of 164, 177, 178 

Porcupine  Mountain  district,  copper  in 579 

geology  of 380, 385, 421 

physiography  of. .'. 97 

Porosity,  relation  of,  to  cubic  contents  of  ore 482-483 

relation  of,  to  cubic  contents  of  ore,  figure  showmg 480 

Porphyry,  age  of 128 

distribution  and  character  of 128 

Portage  Lake,  copper  near 577 

geology  at 380. 408, 425 

section  at 387 

view  at 434 

Port  .\rthur,  geology  near 205, 206, 209 

iron  near 209 

Potato  River,  geology  at 229.230.378,394,413-414 

Potsdam  sandstone,  distribution  and  character  of 251, 

269,307,355,356,360 

iron  ores  in , 564 

Powers  BUift  quartzite,  correlation  of 598 

distribution  and  character  of 356 

topography  of 108 

Pre-Cambrian  rocks,  investigation  of 29 

Presque  Isle,  geology  near 255,609,617 

Presque  Isle  River,  geology  near 229 

Princeton  mine,  geology  at 283,285 

Prospecting,  glacial  drift  in 432 

methods  of 484-486 

Puck\vunge  conglomerate,  distribution  and  character  of 370-371,394 

relations  of 372 

Purapelly,  U.,on  copper  ores 580-581 

on  Keweenawan  series 397, 400-403 


Quartz,  crystallisation  of 549, 552 

Quincy  amygdaloid,  distribution  and  character  of 36 

Quincy  mine,  history  of 36 

ores  in 576,577 

Quinnesec,  geology  near ; 334, 343 


Page. 

Quinnesec  schist,  correlation  of 344-345,598 

distribution  and  character  of 322, 329, :):«), 344-.345 

intrusions  in 323 

relations  of 311, 342 

R. 

Rabbit  Lake,  geology  near 213, 214 

iron  of 220 

composition  of.  figures  showing 221,222 

Rabbit  Mountain,  silver  at 594-595 

Railroads,  ore-carrying,  list  of 495 

Rainy  Lake,  formation  of 442 

Rainy  Lake  district,  correlation  in 598 

description  of 144 

elevations  in 94 

geology  of 122, 146-147 

gold  in 46,595 

physiographj*  of 94 

R.uny  Lake  lobe,  extent  of 427.428-429, 441 

Randall,  geology  near 215 

Randville  dolomite,  correlal  ion  of 598,605.607 

distribution  and  character  of 291, 

293. 300-303,  .306, 330, 333-334, 342, 605, 6O7-C08 

relations  of 305. 333. 334, 342, 343 

topography  on 107,328 

Ransomc,  F.  L. ,  on  ellipsoidal  structure 511 

on  iron  ores 509-510 

Rat  Portage,  geology  near 145 

Red  Cedar  River,  geology  on 357 

Red  Lake,  formation  of 442 

Red  River,  lakes  in 442 

Red  River  lobe  of  Labrador  glacier,  extent  of 429. 430, 441-442 

Red  Rock  mine,  geology  at 297, 300 

Redstone,  geology  near 224 

Reid,  Clement,  on  lavas 511 

Relief,  character  of 33.90-91 

measure  of 89 

Republic  mine,  geology  at 2?2 

history  of 38 

iron  ores  of 480, 552-553 

Republic,  chert  from,  plate  showing 470 

Repulilic  trough,  physiography  in IWi 

Rib  Hill,  map  of 90 

Rib  Hill  quartzite,  correlation  of 598 

distribution  and  character  of 356 

topography  on lOS 

Roberts,  II.  M.,  on  production  of  iron  ore 490-491 

Robinson  Lake,  geology  of 127 

Rock  flowage,  alteration  by 554-555 

Rocks,  depths  of  formation  of 89-90 

Rominger,  Carl,  on  Marquette  district 96,105- 

on  Menominee  district 346 

Ropes  mine,  gold  of 595, 596 

production  of 46 

Rove  slate,  correlation  of 598. 611 

distribution  and  character  of 198,200-201 

metamorphism  of 201 

topography  on 102 

Russell,  I.  Con  ellipsoidal  structure 5U 

on  glaciation 430,435,437.440 

on  physiography 1 10 

S. 

Sabawe  Lake,  iron  near 149 

Saganaga  Lake,  geology  at 119,1.30 

Saganaga  Lake  granite.     See  Basswood  Lake,  granite  of. 

St.  CroLx  River,  geology  near 376,378-379, 409, 415, 421 

section  on,  figure  showing 379 

St.  Ignace  Island,  geology  on 368 

St.  Lawrence  River,  drainage  tributary  to 34 

St.  Louis  Lake,  fonnation  of 443 

St.  Louis  plain,  character  of 92 

St.  Louis  River,  channel  of.  changes  in 112-113, 457  458 

channel  of,  maps  showing 113,457,458- 

geology  near 371, 372, 379,41.'* 

St.  Louis  slate.    See  Virginia  slate. 

St.  Marys  River,  description  of •« 


INDEX. 


639 


PaRp. 

St.  Peter  sandstone,  distribution  and  cliaracter  of 300, 361-3(12 

Salt  waters,  distribution  and  character  ol 5M 

Sault  Ste.  Marie,  physiography  north  of 95-9B 

Saunders,  geology  near 310 

Saunders  formation,  correlation  of 598, 005 

distribution  and  character  of 309, 310-311, 605 

relations  of 311,318,319 

Savoy  mine,  section  of,  figure  showing 138 

Sawteeth  Mountains,  physiography  of 99 

Sayers  Lake,  geology  near ]  52 

Scotty  Islands,  geology  on 145 

Seaman,  A.  E.,  on  Iron  River  district ."...      319 

on  Keweenawan  series 389 

on  Marquette  district 251, 260 

Seaman,  A.  E.,  and  Lane,  A.  C,  on  Lake  Superior  sandstone 616 

Sea  water,  reaction  of,  on  hot  igneoios  rocks 515-516 

Sebenius,  J.  U.  aid  of 159 

Secondary  concentration,  alteration  due  to 186-188, 529 

character  of 529-539 

alterations  due  to 180-188 

condition  of 186 

depth  of 186 

methods  of 529-545 

quantitative  study  of. 545 

relations  of  contact  alteration  and S52-553 

sequence  of. 557-560 

volume  changes  due  to 188-191 

figure  showing 188, 189, 190 

See  also  paTtkvlaT  districts. 

Section  16  inine,  section  of ,  figure  showing 270 

Section  30  mine,  geology  of 139 

Sections,  geologic,  of  Lake  Superior  region,  map  showing Pocket. 

Sedimentation,  conditions  of 600-518 

Seeley  slate,  correlation  of 598 

distribution  and  character  of 360-361 

Sellers  mine,  iron  ore  from,  analyses  of 193 

Shebandowan  River,  iron  on 150 

Shenango  mine,  view  in 1,S2 

Sheridan  Ilill.  geology  of 310, 319 

Siamo  slate,  correlation  of 598, 607 

deposition  of 501 

distribution  and  character  of 252,261-262,287,288,289,607 

relations  of 261 ,  262, 264 

topography  of IO5.  loo 

Sibley  mine,  section  of,  figure  showing 138 

Siderite,  alteration  of 53O  537 

alteration  of,  chemistry  of 550-551 

plates  showing 532,534 

oxidation  and  hydration  of. 530. 537 

plates  showing 472,  ,^24 

Silica,  deposition  of 527 

leaching  of 537-539 

transportation  of 505-506, 537-539 

SiUcate  ores,  distribution  and  character  of 571-572 

Silver  Islet,  geology  at 37O 

silver  ores  of 46, 591-592, 593-594, 626 

analysis  of 594 

Silver  Lake,  iron  near 207 

Silver  ores,  distribution  and  character  of 593-595 

mining  of 575 

history  of 46 

origin  of. 595 

production  of 593 

source  of 5(i9 

Slate,  composition  of 513, 612 

composition  of.  diagram  showing 612 

correlation  of 611-612 

iron  associated  with 502,515.011 

Slate,  ferruginous,  distribution  and  character  of 462 

plates  showing 408, 470 

Smelting,  by-products  from 48 

history  of 47-48 

Smith,  W.  H.  C,  on  Hunters  Island .• 94-95 

Smith  mine,  history  of 39 

Smyth,  H.  L.,  on  copper  ores 580, 581, 585 


Page. 

Smyth,  II.  L.,  on  folding ; 555 

on  physiography igg 

on  Steep  Roelc  Lake  district i4g 

Smyth,  H.  L.,  and  Clements,  J.  M.,  on  Crystal  Falls  district, . . .  39-40, 107 
Smyth,  U.  L.,  Bayley,  W.  S.,  and  Clements,  J.  M.,  on  Crystal 

Falls  district 96-97  107 

Snowbank  granite,  distribution  and  character  of 136 

intrusions  by .■ 131^  135 

topography  of 94 

Snowbank  Lake,  geology  near 119, 129, 131, 132-134, 136 

Soils,  character  of 91 

Solutions,  hot,  metasomatism  by 513-514, 582-586, 614 

source  of 586-588 

Soret,  C.  A.,  law  of 590 

Soudan,  geology  near 126 

phosphorus  at 143 

Soudan  formation,  age  of 127-128 

analogy  of,  to  Negaunee  formation 126 

breccias  in 126-127 

correlation  of 593 

defonnation  of 12^124 

figure  showing 123 

distribution  and  character  of 118, 122-128 

intrusions  in 129 

iron  ores  of.  analyses  of 139-140 

character  of : 140-141.460 

secondary  concentration  of ." 141-142 

structure  of 137-139 

jaspilite  from,  plate  showing 466 

origin  of 126 

relations  of,  to  Ely  greenstone 124-128 

structure  of 123-124 

topography  of 93,119 

See  also  Jasper. 

Soudan  Hill,  geology  near 119,122,123,127,137 

iron  ores  near 137-138 

South  Range,  geologj'  of 385-386, 389 

Spain,  iron  ores  of 495 

Split  Rock,  geology  at 374 

Spring  mine,  history  of 44 

Spring  Valley,  iron  ores  at 46O,  564-666 

Spurr,  ,T.  E. ,  on  greenalite 167-168 

Spurr  mine,  history  of 33 

Stambaugh  Hill,  geology  at 315, 317-318 

Stannard  Rock,  geoiogj-  of 42s 

Stanton,  T.  W.,  fossils  determined  by 179 

Steep  Rock  Lake  district,  Ontario,  geologj^  of 147-149 

iron  of ? 46, 148, 149 

Steep  Rock  group,  distrilmtion  and  character  of 148 

Stegmiller  mine,  geologj-  at : 283,285 

Steidtmann,  Edward,  on  copper  ores 573-593 

Steiger,  George,  analyses  by 166, 173, 191, 405-406 

Stevens,  H.  .1. ,  on  copper  mining 35-36 

Stevenson  mine,  excavation  of,  views  of 495 

ores  from,  analyses  of igg 

figure  showing 188 

Stillwater,  geology  at 375 

Stokes,  H.  N.,  analyses  by 191,689 

Stone\'ille,  geology  near 268 

Straw  Hat  Lake,  geology  near 148 

iron  near 149 

Stream  capture,  description  of u^ 

figure  showing ii3 

Stream  systems,  character  of 91 

Stremme,  H. ,  on  hydration ; 557 

Streng,  \.,  on  Keweenawan  series 397,402 

Stria?,  evidence  of 430 

formation  of 431 

Structure,  development  of 621 

elements  of 621-622 

figures  showing 99, 101, 112 

relation  of.  to  iron  ores 544-545 

Stuntz  Bay,  geology  near 128, 129, 131 

Stuntz  conglomerate.    See  Ogishke  conglomerate. 

Sturgeon  quartzite,  correlation  of 59s,  605 

distribution  and  character  of . . .  291, 293, 300-302, 306, 330, 332-333, 605 


640 


INDEX. 


Page. 

Sturgeon  quartzito,  intrusions  in 345 

relations  of 305,332-333,334 

topography  on 107,328 

Sturgeon  Uiver  district,  correlation  of 599 

description  of 300 

geology  of 300-301,333 

Subcrustal  fusion,  tiieory  of 146 

Sudbury  district,  copper  and  nickel  ores  of 592,626 

Sullivan,  E.  C.  work  of 589 

Sulphur,  metamorphism  and : 550 

occurrence  of 477 

Sunday  Lake,  geology  near 227,229,230-232.236,238,242,385.517 

Sunday  quartzite,  correlation  of 227, 598, 605 

distribution  and  cliaracter  of 225,227-228, 605 

relal  ions  of 228 

Superior,  geology  near 452 

Superior  escarpment,  description  of 115 

Surprise'  Lake,  geology  at 370 

Surveys,  progress  of 29 

Swamp  Lake,  geology  at 131 

Swauzy  district,  description  of . . .  i 283 

geology  of 283-2S0 

iron  ores  of 286 

analysis  of 286 

reserves  of 492 

secondary  concentration  of 286 

structure  of 475 

history  of 39 

map  of 284 

production  of 39 

Swanzy  mine,  geology  at 283,  285 

Sweden,  iron  ores  of 495 

Sweet,  E.  T.,od  glaciation 439 

on  Keweenawan  series 402,  403,  404 

Swineford,  A.  P. ,  on  Menominee  district 39 


Taconite,  alteration  of 187-188 

analyses  of 181 

composition  of 181-183 

distribution  and  character  of 207,  462 

utilization  of 44 

See  also  Iron  ore. 

Talbott  Lake,  geology  near 151 

Tamarack  njine,  history  of : 37 

Taylor,  F.  B.,  on  glaciation 450 

Taylors  Falls,  geology  near 379,  409,  425,  616 

section  at,  figure  showing 379 

Teal  Lake,  geology  at 253,  256,  258,  259,  261 ,  262,  271,  275,  279,  618 

geology  at,  map  showing. ..' 254 

Temperance  River  group,  distribution  and  character  of.  371-372, 375-376 

Terrestrial  sediments,  character  of 501-502 

That  Mans  Lake,  geology  near 132 

Thessalon  group,  correlation  of 598 

This  Mans  Lake,  geology  near .' 132 

Thompson,  Lake,  description  of 442 

Thunder  Bay.  geology  near 209,  410,  426,604 

physiography  near 94,  100-101 

silver  at 594-595 

See  also  Animikie  district. 

Thunder  Cape,  geolog>'  of 209,  426 

Thwaites,  F.  T.,on  Wisconsin  geology 376,379 

Tilden  mine,  geology  at 236 

Titanium,  occurrence  of 477,  533,  561 

Topographic  development,  history  of 85,  90 

map  showing 87 

Topographic  provinces,  types  of 85 

map  showing 88 

See  filto  Pleistocene,  provinces  of. 

Topography,  character  of 33-34 

modification  of 455-459 

relation  of,  to  iron  ores. ..; 544-545 

Tower,  gcologj-  near 119, 122, 123, 124, 131, 133, 134 

Tower  Hill,  geology  near 122, 123 

iron  ores  near 137 

Traders  mcrabor,  correlation  of 598 

distri\)ution  and  character  of 335-3^0, 346-347 

iron  ore  of 346 

relation  of 342 


Page. 

Traders  mine,  geology  near 33(^337 

Transportation,  methods  and  cost  of 495-197 

Trap  Range.  gcoIog>-  of 226-226.  .385 

Trenton  limestone,  distribution  and  character  of 360.361-362 

Two  ltarl)ors  Bay.  geology  near 373 

Tyler  slat^-,  correlation  of 598, 608, 611 

distribution  and  character  of 225-226,227,232-233.(308 

relations  of 233, 385 

U. 

Ulrich,  E.  O. ,  fossils  determined  by 320 

on  Iron  Mountain 319-320 

Unconformities,  description  of 617-620, 62Mi26 

United  States,  iron  reserves  of 492 

United  States  Gcologicjil  Survey,  work  of 72-73 

l^pham.  Lake,  description  of 443. 453 

Upham,  W.,  on  glaciation 440.442 

Uplands,  development  of 89-90 

districts  of,  description  of 91-98 

glaciation  of 91 

monadnocks  on 90 

position  of ; 89 

relief  in 89 

soil  of 91 

valleys  in 90-91 

Upson,  geology  near 230 

V. 

Valleys,  character  of 90-91 

Van  Ilise.  C.  R.,  on  copper  ores 581 

on  formation  of  ILmonite 519 

on  Keweenawan  series 402 

on  metamorphism 551 

on  phj'siography 116 

work  of 29. 92-93 

Van  llise,  C.  R..  and  Bayley,  W.  S.,  on  Marquette  district 96, 10.V106 

Van  Ilise,  C.  R.,  and  Irving,  R.  D.,  on  physiography 103 

Veins,  copper.    Sec  Eagle  River;  Ontonagon;  Keweenaw  Point. 

Vermilion  district,  correlation  in 598.599-600.611 

exploration  in 47. 485 

geology  of 118-137, 603.61 1. 618 

intrusive  rocks  of 135-136 

iron-bearing  rocks  of 118 

iron  ores  of 137-143. 4Sfi 

alteration  of 554-555 

characteristics  of 140-141 

composition  of 139-140 

origin  of '. 570 

plates  showing 466.564 

production  of ; 461 

proportion  of 462 

relations  of 506. 507 

reserves  of 1 4S9 

secondary  concentration  of 141-143 

changes  in 142-143 

figure  showing 142 

structiue  of 137-139 

topography  of 476 

location  and  area  of 32.118 

map  of 118 

mines  in,  sections  of,  figures  showing 138 

mining  in,  history  of 40-41 

phosphorus  in 143 

production  of 68, 69 

physiography  of 92-94. 431 

structure  of 123-124 

figure  showing 123 

topography  of 119 

Vermilion  Lake,  formation  of 442 

geology  near 119.122,129,131,132 

gold  near 40 

Vermilion  range,  mining  on.  history  of 42 

Virginia,  greenalite  from  vicinity  of,  plate  showing 474 

Virginia  slat  e,  analyses  of 1 73 

correlation  of 598.611 

distribution  and  character  of 159, 172-174. 212-213 

phosphorus  in 194 

relations  of 174 


INDEX. 


641 


Volcanic  vents,  distribution  and  character  of • 411-412 

Volcanism,  occurrence  of *''17 

Volume  changes.    Sec  Secondary  concentration. 

Volunteer  mine,  geology  at 259,2fj5 

Vulcan  formation,  alteration  of. ■ 354 

correlation  of 598, 60S,  OlO-Gll 

distribution  and  character  of 302-304,.10G-307, 327, 330, 008 

iron  ore  in 303-304, 335-340, 346, 351 ,  460, 503 

relationsof. . 305,334,338-339,342-343 

structure  of 314,338 

topographyon .  .■.  107,329 

Vulcan  member,  correlation  of 59S,G10-G11 

distribution  and  character  of 291 ,  297-299, 312-318. 321-322. 323 

iron  ore  in 299, 314-315, 323 

magnetic  phase  of 317-318 

relations  of 313-314 

structure  of 315-317 

W. 

Wadsworth,  M.  E.,  on  copper  ores 580-582 

on  iron-bearing  rocks 500 

on  iron  ores 570 

on  Iveweenawan  series 397-398, 400-403, 404 

Walcott,  C.  D.,  on  fossils  of  Lake  LSuperior  sandstone 340 

Wall  rock,  alteration  of 582-585 

alteration  of.  analyses  showing 583 

"Warping,  elTects  of. ._ 44s_449 

evidence  of _ 87,450 

Water,  circulation  of,  in  rock 186 

circulation  of,  figure  showing 186 

Waterloo  district,  correlation  in * 598 

geology  of 3G4 

map  of 359 

Waterloo  quartzite,  correlation  of 598 

distribution  and  character  of 364 

Waucedah.  geology  near 333^  340 

Wausau  district,  geology  of 355-357 

Wausaugraywacke.  correlation  of 593 

distribution  and  character  of 356 

topography  on 108 

Waushara  district ,  map  of _ 359 

Wawa  tuff,  coiTelation  of 598 

distribution  and  character  of 150, 151-152, 153 

petrography  of 151-152 

Wawa,  Lake,  geology  near 151,152 

Weathering,  absence  of 502 

effect  of,  on  concentration _ '. 500 

processes  of 585-586 

relation  of,  to  iron  ores 503-505, 516, 539-540 

Weidman.  S.,  on  age  of  peneplain , .       88 

on  drainage 91 


Pago. 

Weidman,  S.,  onglaciation 4.37.439 

on  iron  ores :  564,566,570-571 

on  physiography . .  98, 107-108, 116 

on  Wisconsin  geology 35S-365, 377 

West  I*ond,  geology  near 382, 3a3, 385, 410 

West  Seagull  Lake,  geology  at 131 

West  Vulcan  mine,  section  of,  figure  showing 349 

Wewe  slate,  correlation  of 598,605 

distribution  and  character  of 252-253,258-259,605 

relations  of 259, 2C0 

topography  on 105 

Weyerhauser.  geology  near 357 

While  Iron  Lake,  geology  near 119, 122 

Whitney,  J.  D..  and  Foster.  J.  W.,  on  Marquette  district 96 

Whittlesey,  Charles,  on  Mesabi  district 42 

Willmott,  A.  B.,  and  Coleman,  A.  P.,  on  Michipicoten  ranges 95 

Wilson,  A.  W.  G.,  on  Minnesota  geology 367-369,374 

Winchell,  A.  N.,  on  ICeweenawan  series 360, 395-410 

on  nomenclature 395-407 

Winchell,  N.  H.,  pnglaciation 438 

on  iron  ores 56^-570 

on  Iveweenawan  series 397, 399 

on  Mesabi  district 42 

on  Minnesota  geology 370 

on  physiography 92. 98, 104 

Wirmebago,  Lake,  formation  of 443 

Wisconsin,  bibliography  for „ 77-78 

correlation  in 598 

Clinton  ores  of 45 

copper  ores  of 580 

geology  of 355-365, 376-380, 413-414, 429 

hematite  in'. 45 

investigations  in 72, 73 

iron  ores  of 461 

production  of 461 

physiography  of 97-98, 100, 107-108 

figure  showing 116 

See  also  particular  disfiicts. 

Wisconsin  stage,  glaciers  of 427, 454 

Wolverine  mine,  history  of 36 

ores  of 576, 577 

Woman  River,  geology  near 126,507,555 

Wood  alcohol,  recovery  of 48 

Woodward,  R.  W.,  analyses  by.' 404 

Wright,  F.  E.,  on  igneous  rocks 410-411,424,582 

Wright,  F.  E  ,  and  Larsen,  E.  S.,  on  quartz  crystallization 549 

Wurtz,  H.,  on  silver  minerals 593 

Z. 

Zenith  mine,  sections  of,  figures  showing 138 

Zapffe,  Carl,  on  Cuyuna  district 216-224 


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ALCONKIAN 


U.   S.   GEOLOGICAL  SURVEY 


MESABI    DISTRICT 


KEWEENAWAN  SERIES 


MINNESOTA 

ByC.Iv-Leith 

(Corrected  toJamiaryl.  1911) 
Scale  esTfiro 


Conlour  interval  20  fcei 
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LEGEN  D 


ALGONKIAN 


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UPPER  HURONIAN  (ANIMIKIE  GROUP) 


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MONOGRAPH   Lll      PLATE   ' 


U.    S-    GEOLOGICAL  SURVEY 
GEORGE    OTiS  SMITH.    DIRECTOR 


GEOLOGIC  MAP  AND  SECTIONS  OF  THE  MARQUETTE  IRON-r 

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QUATERNARY 


CAMBRIAN 


ALGONKIAN 


Contour  interval  60  feel 

LEGEND 


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ERUPTIVE 


UPPER  HURONIAN   (ANIMIKlE  GROUPJ 


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W'rwe  slule 


MONOGRAPH     Lll         PLATE     XVII 


;  OF  THE  MARQUEITE  IRON-BEARING  DISTRICT,  MICHIGAN 

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UNCLASSIFIED 


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} 


OUTCROP  MAP  OF  THE  FLORENCE  IRON  DISTRICT,  WISCONSIN 

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GEOLOGIC  MAP  OF  THE  CRYSTAL  FALLS  DISTRICT,  INCLtTDING  PARTS  OF  THE  FELCH  MOUNTAIN  AND  MARQUETTE  DISTRICTS,  MICHIGAN 

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MONOGRAPH  Lll     PLATE  XXVI 


SEDIMENTARY  ROCKS 


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